C1308_508_60pages - Spine. Width varies. Printer is to...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Spine. Width varies. Printer is to determine final width and adjust spine and covers. Healy and others—Water Budgets: Foundations for Effective Water-Resources and Environmental Management Water Budgets: Foundations for Effective Water-Resources and Environmental Management USGS/Circular 1308 Circular 1308 U.S. Department of the Interior U.S. Geological Survey Printed on recycled paper Back cover 8-1/2" 11" Spine. Width varies. Printer is to determine final width and adjust spine and covers. Front cover 8-1/2" 11" 1 2 3 Photograph credits: 1. 4 5 U.S. Department of Energy Atmospheric Radiation Measurement Program 2. U.S. Geological Survey 3. U.S. Geological Survey 4. U.S. Fish and Wildlife Service 5. U.S. Geological Survey Water Budgets: Foundations for Effective Water-Resources and Environmental Management By Richard W. Healy, Thomas C. Winter, James W. LaBaugh, and O. Lehn Franke Circular 1308 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director U.S. Geological Survey, Reston, Virginia: 2007 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS and its products: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Healy, R.W., Winter, T.C., LaBaugh, J.W., and Franke, O.L., 2007, Water budgets: Foundations for effective waterresources and environmental management: U.S. Geological Survey Circular 1308, 90 p. iii Foreword W ater availability is an important concern in the 21st century. Ensuring sustainable water supplies requires an understanding of the hydrologic cycle—how water moves through Earth’s atmosphere, land surface, and subsurface. Water budgets are tools that water users and managers use to quantify the hydrologic cycle. A water budget is an accounting of the rates of water movement and the change in water storage in all or parts of the atmosphere, land surface, and subsurface. Although simple in concept, water budgets may be difficult to accurately determine. It is important for the public and decisionmakers to have an appreciation of the uncertainties that exist in water budgets and the relative importance of those uncertainties in evaluating how much water may be available for human and environmental needs. As part of its mission, the U.S. Geological Survey (USGS) provides information that describes the Earth, its resources, and the processes that govern the availability and quality of those resources. This Circular provides an overview of the hydrologic cycle and a discussion of methods for determining water budgets and assessing the uncertainties in those determinations. Examples illustrate the importance of water budgets to humans and the environment and demonstrate how water budgets can be incorporated into management practices. Through this Circular, the USGS seeks to inform the public and decisionmakers about a scientific basis for water-resources and environmental management and to broaden awareness and understanding of water budgets and the hydrologic cycle so as to promote wise use and management of a most precious resource—water. Robert M. Hirsch Associate Director for Water iv Helen H. Richardson, Denver Post Preface How long can the water needs of a growing urban area be sustained by an aquifer that contains a finite amount of water? Water is the essence of life. Its availability determines where and how animals and plants exist on Earth. Humans need water for consumption, for producing food, and for manufacturing; we also are attracted to water for its esthetic value and for the recreational opportunities it offers. At the same time, all other life forms on Earth require water for their sustenance. Native plants in grasslands and forests; wheat and corn crops in agricultural fields; insects, amphibians, and birds in wetlands; fish in streams and lakes; wild mammals and reptiles; and domesticated pets and livestock—all depend on water. Competition for water among humans and between humans and other life forms is the unavoidable outcome of burgeoning populations and a limited resource. Resolution of competing needs requires decisions based on science as well as societal values. Informed decisions are developed with an understanding of the hydrologic cycle—the process by which water moves from the atmosphere to land surface as precipitation, infiltrating the subsurface or flowing along land surface to the oceans, and eventually returning to the atmosphere by evaporation. All water on Earth resides in one of the three compartments of the hydrologic cycle: the atmosphere, the land surface, and the subsurface. A water budget is an accounting of water stored within and water exchanged among some subset of the compartments, such as a watershed, a lake, or an aquifer. Throughout history, humans have managed water for their own needs. Ancient Mayan and Egyptian cultures prospered on crops produced with intricate irrigation systems. Remains of aqueducts built almost two thousand years ago by the Roman Empire can still be found throughout Europe. Early How will droughts affect agricultural and domestic water supplies? Will increased diversions reduce storage in surface-water reservoirs to the point where recreational uses are limited? U.S. Army Corps of Engineers What are the ecological effects of withdrawing water from an aquifer that naturally discharges to a wetland? Will the withdrawal result in reduced discharge to and subsequent drying of the wetland? How will plants and animals be affected? v explorers of the American West, such as John Wesley Powell, realized that civilization could flourish in this arid region only if water could be stored and distributed as needed. Today, population centers and agriculture thrive in the West, mainly because of the dams and reservoirs constructed on rivers such as the Colorado and Columbia. Design and operation of large reservoir projects rely on detailed water-budget analyses, examination of precipitation and evaporation rates, discharge rates of streams, rates of exchange between surface water and ground water, and factors such as climate, geology, vegetation, and soils that affect those rates. The story of water development in the Western United States is a story that has been repeated in various forms all over the Earth. Reservoirs and ground-water wells are key features of the Nation’s water supply infrastructure. They both provide great benefits in terms of the reliable delivery of water to users. However, it is well-recognized that they can also have adverse impacts on aquatic ecosystems. The needs and values of society determine whether or not the benefits of these systems outweigh their negative consequences and determine if changes in the design or operation of these systems should be made. Water needs of ecosystems have become an integral part of water management. Operators of reservoirs now take into account the health of downstream riparian ecosystems. Managers of aquifers are likely to consider the effects of groundwater withdrawals on the interactions between ground water and surface water and the organisms that depend on that interaction. These are but a few of the myriad issues that arise in balancing the water needs of humans and the environment. Water budgets form the foundations of informed management strategies for resolving these issues. Can crops be matched to climate so as to minimize irrigation requirements? Can streamflow in arid regions be increased by the removal of non-native phreatophytes that line channels, thus reducing evapotranspiration? How much water will be used by replacement vegetation? Will dewatering of a surface mine have an effect on surface-water expressions many miles away? vi Contents Foreword ........................................................................................................................................................iii Preface ...........................................................................................................................................................iv Introduction ....................................................................................................................................................1 Hydrologic Cycle ............................................................................................................................................3 Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle ....8 Water in the Atmosphere ....................................................................................................................8 Water on Land Surface .....................................................................................................................12 Snow and Ice ..............................................................................................................................12 Lakes .........................................................................................................................................16 Wetlands .....................................................................................................................................18 Streams........................................................................................................................................19 Water in the Subsurface....................................................................................................................24 Unsaturated Zone ......................................................................................................................24 Saturated Zone ...........................................................................................................................28 Exchange of Water Between Compartments of the Hydrologic Cycle ..............................................36 Precipitation.........................................................................................................................................36 Infiltration and Runoff ........................................................................................................................40 Evapotranspiration..............................................................................................................................41 Exchange of Surface Water and Ground Water ............................................................................43 Water-Budget Studies.................................................................................................................................46 Water Budget for a Small Watershed: Beaverdam Creek Basin, Maryland ............................46 Soil-Water Budgets for Prairie and Farmed Systems in Wisconsin ..........................................48 Water Budget of Mirror Lake, New Hampshire .............................................................................50 Water Budget at a Waste Disposal Site in Illinois .......................................................................52 Humans and the Hydrologic Cycle ............................................................................................................55 Water Storage and Conveyance Structures ..................................................................................55 Land Use ...............................................................................................................................................56 Ground-Water Extraction ..................................................................................................................57 Water Budgets and Management of Hydrologic Systems ..................................................................61 Large River System: Colorado River Basin .....................................................................................61 Watersheds and Reservoir Management..............................................................................63 Aquifers in Arizona ....................................................................................................................64 Large Aquifer System: High Plains Aquifer ....................................................................................66 Water Budgets and Governmental Units: Lake Seminole ............................................................71 Agriculture and Habitat: Upper Klamath Lake ...............................................................................74 Water for Humans and Ecosystems: San Pedro River Ecosystem .............................................77 Urban Water Supply: Chicago ..........................................................................................................81 Concluding Remarks....................................................................................................................................84 References Cited..........................................................................................................................................86 vii Boxes A. B. C. D. E. F. G. H. I. J. The Water-Budget Equation ................................................................................................................6 Water Budgets are Intimately Linked to Energy and Chemical Budgets ...................................10 Chemical, Isotopic, and Energy Tracers Provide Insight into Hydrologic Processes .............14 Models—Important Tools in Water-Budget Studies. ....................................................................22 Lysimeters—Water-Budget Meters .................................................................................................26 Ground-Water Recharge .....................................................................................................................32 Estimating Aquifer Hydraulic Conductivity ......................................................................................34 Uncertainty in Water-Budget Calculations .....................................................................................54 Water Use and Availability ..................................................................................................................58 Water Budgets of Political Units .......................................................................................................60 Rain is grace; rain is the sky condescending to the earth; without rain, there would be no life. John Updike (1989) viii Conversion Factors and Datums Multiply By To Obtain Length centimeter (cm) 0.3937 inch millimeter (mm) 0.03937 inch meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) Area square meter (m2) 0.0002471 2.471 hectare (ha) square kilometer (km ) acre acre 247.1 2 square centimeter (cm ) acre 0.001076 2 square meter (m2) square foot (ft2) 10.76 square foot (ft2) square centimeter (cm ) 0.1550 square inch (ft2) hectare (ha) 0.003861 square mile (mi2) square kilometer (km2) 0.3861 square mile (mi2) 2 Volume cubic meter (m3) 264.2 gallon (gal) cubic centimeter (cm ) 0.06102 cubic inch (in3) cubic meter (m3) 0.0008107 acre-foot (acre-ft) 3 Flow rate cubic meter per year (m /yr) 3 0.000811 acre-foot per year (acre-ft/yr) cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s) cubic meter per second (m /s) 22.83 million gallons per day (Mgal/d) 3 Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F–32)/1.8 Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). American Geological Institute, Bruce F. Molnia I came where the river Ran over stones My ears knew An early joy. And all the waters Of all the streams Sang in my veins That summer day Theodore Roethke from “The Lost Son” (1948) Water Budgets: Foundations for Effective Water-Resources and Environmental Management By Richard W. Healy, Thomas C. Winter, James W. LaBaugh, and O. Lehn Franke Introduction National Aeronautics and Space Administration Water budgets provide a means for evaluating availability and sustainability of a water supply. A water budget simply states that the rate of change in water stored in an area, such as a watershed, is balanced by the rate at which water flows into and out of the area. An understanding of water budgets and underlying hydrologic processes provides a foundation for effective water-resource and environmental planning and management. Observed changes in water budgets of an area over time can be used to assess the effects of climate variability and human activities on water resources. Comparison of water budgets from different areas allows the effects of factors such as geology, soils, vegetation, and land use on the hydrologic cycle to be quantified. Human activities affect the natural hydrologic cycle in many ways. Modifications of the land to accommodate agriculture, such as installation of drainage and irrigation systems, alter infiltration, runoff, evaporation, and plant transpiration rates. Buildings, roads, and parking lots in urban areas tend to increase runoff and decrease infiltration. Dams reduce flooding in many areas. Water budgets provide a basis for assessing how a natural or human-induced change in one part of the hydrologic cycle may affect other aspects of the cycle. “Only from space can you see that our planet should not be called Earth, but rather Water, with speck-like islands of dryness on which people, animals, and birds surprisingly find a place to live.” Oleg Makarov (1988) Water Budgets: Foundations for Effective Water-Resources and Environmental Management Natural Resources Conservation Service This report provides an overview and qualitative description of water budgets as foundations for effective water-resources and environmental management of freshwater hydrologic systems. Perhaps of most interest to the hydrologic community, the concepts presented are also relevant to the fields of agriculture, atmospheric studies, meteorology, climatology, ecology, limnology, mining, water supply, flood control, reservoir management, wetland studies, pollution control, and other areas of science, society, and industry. The first part of the report describes water storage and movement in the atmosphere, on land surface, and in the subsurface, as well as water exchange among these compartments. Our ability to measure these phenomena and inherent uncertainties in measurement techniques also are discussed. The latter part of the report presents a number of case studies that illustrate how water-budget studies are conducted, documents how human activities affect water budgets, and describes how water budgets are used to address water and environmental issues. Natural Resources Conservation Service 2 Hydrologic Cycle Hydrologic Cycle Earth’s water exists on land surface in oceans, ice fields, lakes, rivers, streams, and wetlands; it also exists in the subsurface as soil water and ground water and in the atmosphere (fig. 1). More than 97 percent of the Earth’s water is in oceans (table 1). Of the inland water that resides on and beneath land surface, 77 percent is contained in icecaps and glaciers and for practical purposes is inaccessible. The remaining inland water is stored primarily in the subsurface as ground water. Water is constantly moving within the hydrologic cycle, and that movement takes place over many pathways (fig. 1). Water moves quickly through some pathways; for example, rain falling from the atmosphere to a field of corn in summer may return to the atmosphere in a matter of hours or days by evaporation. Traveltimes over other pathways are measured in years, decades, centuries, or more—ice fields in Greenland contain water that fell from the atmosphere thousands of years ago. Precipitation Surface-water inflow, imported water (pipelines, canals) Evapotranspiration Ground-water inflow Wa ter tab 3 s Un le at ur a d te zo ne Bedrock Aquifer Surface-water outflow, exported water (pipelines, canals) Figure 1. The hydrologic cycle for part of a watershed. Ground-water outflow “The central concept in the science of hydrology is the so-called hydrologic cycle—a convenient term to denote the circulation of the water from the sea, through the atmosphere, to the land; and thence, with numerous delays, back to the sea by overland and subterranean routes, and in part, by way of the atmosphere; also, the many short circuits of the water that is returned to the atmosphere without reaching the sea***. The science of hydrology is especially concerned with the second phase of this cycle—that is, with the water in its course from the time it is precipitated upon the land until it is discharged into the sea or returned to the atmosphere. It involves the measurement of the quantities and rates of movement of water at all times and at every stage of its course***.” O.E. Meinzer (1942, p. 1) 4 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Table 1. Estimated global water supply (from Nace, 1967). [km3, cubic kilometers] Water storage Volume, in thousands of km3 Ocean water 1,320,000 Atmosphere 13 Percentage of total water 97.1 0.001 Water in land areas 37,800 Freshwater lakes 125 0.009 Saline lakes and inland seas 104 0.008 Icecaps and glaciers Soil root zone Ground water (to depth of 4,000 meters) 1.25 29,200 67 8,350 0.0001 2.14 0.005 0.61 The atmosphere receives water through evaporation and loses it as precipitation, mostly in the form of rain or snow. The average residence time for water in the atmosphere is about 10 days. A drop of rain can have a multitude of fates, depending on where and when it falls. Some rainfall never reaches land surface; instead, it evaporates as it falls (a phenomenon known as virga) and returns to the atmospheric reservoir. A falling raindrop could land on a leaf of a tree, from where it might fall to the ground, evaporate, or perhaps be imbibed by the plant. Another drop might land directly on the ground. That water could puddle in a depression, travel over the surface to a lower elevation (runoff), or enter the subsurface (infiltrate). Water in a puddle will likely evaporate or infiltrate. Water that runs off may infiltrate at a more favorable location or travel to a stream and ultimately be transported to an ocean; at any point on this journey, that water can evaporate. The average residence time for water in free-flowing rivers ranges between 16 and 26 days (Vorosmarty and Sahagian, 2000). Streams that run through reservoirs can have substantially longer residence times. Not all surface water flows to oceans. Some lakes and wetlands have no surface drainage. They lose water to evaporation and to ground water. Humans withdraw water from streams and reservoirs, thus interrupting its migration to the ocean. Water moves much more slowly in the subsurface than in the atmosphere or on land surface. Water that infiltrates the subsurface can remain in the unsaturated zone where it will most likely be returned to the atmosphere by evaporation or plant transpiration; it can discharge to the surface in a channel or depression, thus becoming surface flow; or it can traverse the unsaturated zone to recharge an underlying aquifer. Most water that infiltrates the subsurface is returned to the atmosphere by evaporation from bare soil or by plant transpiration (table 2). That returned water typically resides in the subsurface for less than a year. Discharge to land surface of unsaturated-zone water, sometimes referred to as interflow, D.H. Campbell Rivers 2.8 “It is the sea that whitens the roof. The sea drifts through the winter air. It is the sea that the north wind makes. The sea is in the falling snow.” Wallace Stevens from “The Man With the Blue Guitar” (1937) may occur days to months after that water has infiltrated, depending on the distance between the points of infiltration and discharge. Infiltrated water that travels downward past the depth of the root zone may eventually reach the saturated zone, thus becoming aquifer recharge. Traveltimes of water through the entire thickness of the unsaturated zone span a very large range: from hours, for thin unsaturated zones in humid regions (Freeze and Banner, 1970), to millennia, for thick unsaturated zones in arid regions (Phillips, 1994). Water that reaches the saturated zone may reside there for days to thousands of years (Alley and others, 2005). Under natural conditions, ground water discharges to surface-water bodies such as streams, wetlands, lakes, or oceans, or it is extracted by plants and returned to the atmosphere by transpiration. Humans also extract ground water for agricultural, domestic, and industrial uses; such water is ultimately reapplied to land surface, returned to the subsurface, or discharged to surfacewater bodies. Hydrologic Cycle 5 Table 2. Water budget for global land mass (from Lvovitch, 1973). Evapotranspiration is the sum of evaporation and plant transpiration. Annual rate, in millimeters Percentage of annual precipitation Precipitation 834 100 Evapotranspiration 540 65 Total discharge to oceans 294 35 Discharge to oceans from surface runoff 204 24 Discharge to oceans from base flow 90 11 Infiltration of precipitation 630 76 Water-budget component Precipitation in the form of snow can follow several courses. In many environments, snow accumulated on land surface melts in a few days or less. In other areas, a seasonal snowpack exists throughout winter and melts in the spring. Still other areas, such as Greenland and Antarctica, have snow and ice fields that are thousands of years old. In any of these cases, the melting water flows to a surface-water body, infiltrates into the subsurface, or is evaporated back into the atmosphere. It is evident from the preceding discussion that water moves within the hydrologic cycle along many complex pathways over a wide variety of time scales. The challenge for humans is to monitor the hydrologic cycle for some geographic feature of interest, such as a watershed, a reservoir, or an aquifer. Such a feature will be referred to as an accounting unit. A water budget states that the difference between the rates of water flowing into and out of an accounting unit is balanced by a change in water storage: Flow In – Flow Out = Change In Storage. Spring at head of Paris Canyon, Bear Lake County, Idaho 1912. Rainbow Falls on the Missouri River near Great Falls, Montana, 1904. Simple, yet universal, the water-budget equation is applicable over all space and time scales, from studies of rapid infiltration in a laboratory soil column to investigations of continental-scale droughts over periods of decades or centuries. A 1-m2 soil column in the middle of an agricultural field, the entire field itself, or the watershed in which the field lies—these are all examples of water-budget accounting units. “And you, vast sea, sleepless mother, Who alone are peace and freedom to the river and the stream, Only another winding will this stream make, only another murmur in this glade, And then shall I come to you, a boundless drop to a boundless ocean.” Kahlil Gibran (1923) 6 Water Budgets: Foundations for Effective Water-Resources and Environmental Management A The Water-Budget Equation The water-budget equation is simple, universal, and adaptable because it relies on few assumptions on mechanisms of water movement and storage. A basic water budget for a small watershed can be expressed as: P + Qin = ET + ∆S + Qout where and (A1) P Qin ET ∆S is precipitation, is water flow into the watershed, is evapotranspiration (the sum of evaporation from soils, surface-water bodies, and plants), is change in water storage, Qout is water flow out of the watershed. The elements in equation A1 and in all other water-budget equations are referred to as components in this report. Water-budget equations can be written in terms of volumes (for a fixed time interval), fluxes (volume per time, such as cubic meters per day or acrefeet per year), or flux densities (volume per unit area of land surface per time, such as millimeters per day). Typically, water budgets are tabulated in spreadsheets or tables such as that shown in table A–1, which contains monthly and yearly data for Seabrook, New Jersey, from Thornthwaite and Mather (1955). With the approach used by those authors, it is assumed that Qin is zero and Qout is equal to runoff. Equation A1 can be refined and customized depending on the goals and scales of a particular study. Precipitation can be written as the sum of rain, snow, hail, rime, hoarfrost, fog drip, and irrigation. Water flow into or out of the site could be surface or subsurface flow resulting from both natural and human-related causes. Evapotranspiration could be differentiated into evaporation and plant transpiration. Further refinement could be based on the source of the water that is evapotranspired. Evaporation can occur from open water, bare soil, or snowpack (sublimation); plants can extract ground water or water from the unsaturated zone. Such refinements must be balanced with available measurement techniques, which often are not designed, or lack sufficient resolution, to distinguish among subcomponents. Most methods for measuring evapotranspiration, for example, quantify the flux of water from the land/vegetation surface to the atmosphere and do not distinguish between different water sources. Fashioning a viable water-budget approach for estimating evapotranspiration or other water-budget components requires analysis of available measurement techniques. Water storage occurs within all three compartments of the hydrologic cycle. The amount of water stored in the atmosphere is small compared to that on land surface and in the subsurface. Surface water is stored in rivers, ponds, wetlands, reservoirs, icepacks, and snowpacks. Subsurface storage can be categorized into various subaccounting units, such as the root zone, the unsaturated zone as a whole, the saturated zone, or different geologic units. An expanded form, but certainly not an exhaustive refinement, of the water budget appropriate for many hydrologic studies can be written as (Scanlon and others, 2002): P + Qswin + Qgwin = ETsw + ETgw + ETuz + ∆Ssw + ∆Ssnow + ∆Suz + ∆Sgw + Qgwout + RO + Qbf (A2) where the superscripts refer to surface water (sw), ground water (gw), unsaturated zone (uz); RO is surface runoff; Qgwout refers to both ground-water flow out of the site and any withdrawal by pumping; and Qbf is base flow (ground-water discharge to streams). It is unlikely that all elements in equation A2 will be of importance at any one site; some will be of negligible magnitude and can be ignored. Indeed, when selecting an accounting unit for developing a water budget, judicious selection of boundaries can greatly facilitate the accounting process. Consider, for example, a small watershed and associated shallow ground-water system. Watershed boundaries are well defined: there is no surface flow in, and surface flow out occurs only in a stream channel, where discharge can be readily measured. If watershed boundaries correspond to ground-water divides, there is also no subsurface inflow. Suppose all ground water that is not lost to ET eventually discharges to the stream; an appropriate water budget for the watershed could be stated as: P = ET + ∆S + RO + Qbf (A3) If the annual change in storage is small, evapotranspiration can be estimated as the difference between precipitation and streamflow out of the watershed. Table A–1. Monthly and yearly water budget, in millimeters, for Seabrook, New Jersey (Thornthwaite and Mather, 1955). Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year total 87 93 102 88 92 91 112 113 82 85 70 93 1,108 Storage change 0 0 0 0 0 –38 –35 –17 –10 32 51 17 0 Evapotranspiration 1 2 16 46 92 129 147 130 92 53 19 3 730 61 76 81 61 31 15 8 4 2 1 1 37 378 Precipitation Runoff Hydrologic Cycle 7 National Aeronautics and Space Administration Earth’s energy budget is directly coupled to its water budget. 8 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle Water in the Atmosphere Hurricane Katrina. At any one time, the atmosphere holds only a small fraction of the Earth’s water (table 1, fig. 2), the equivalent of a layer about 25 mm thick over all of the Earth’s surface. Yet this compartment is a vital part of the hydrologic cycle in terms of water storage and transport. Water flows to the atmosphere in a gaseous form as it evaporates from water, plant, and soil surfaces. This water will eventually condense, and possibly freeze, and be returned to the Earth’s surface as precipitation. Between the times of entry and departure from Surface-water inflow, imported water (pipelines, canals) Evapotranspiration Ground-water inflow Wa ter tab le Aquifer Surface-water outflow, exported water (pipelines, canals) Figure 2. National Aeronautics and Space Administration The atmosphere, the land surface, and the subsurface are the three compartments that hold the Earth’s water. Each compartment acts as a storage reservoir within which water moves from its point of entry to the compartment to its point of outflow. Water also moves between compartments. A water-budget accounting unit may consist of a single part of one compartment, such as a lake, or an accounting unit may comprise parts of all three compartments, such as a watershed. This section discusses storage and movement of water within individual compartments. The following section discusses exchange of water between compartments. The atmosphere within the hydrologic cycle. the atmosphere, a water molecule can be transported rapidly over long distances. The atmosphere is part of an amazing water-distribution system, carrying water from where it is plentiful (primarily oceans) and depositing it in regions where it is less plentiful (land surfaces). Water in the atmosphere is also important in Earth’s energy balance and climate. Evaporation and subsequent condensation of water require transfers of energy. As water moves from the liquid to gaseous state, it absorbs energy; as it condenses, that energy is released. Thus, the transport of water in the atmosphere is accompanied by a large transport of energy, effectively distributing energy across the Earth. Atmospheric water also affects radiation Precipitation transfer at land surface. The formation of clouds limits the amount of solar radiation that reaches land surface. Long-wave radiation emitted by the Earth is absorbed and reflected back by gases, including water vapor, in the atmosphere (the greenhouse effect). e on dz te Global climate and water storage in the ra tu sa atmosphere are linked. As the Earth’s Un temperature changes, so does the ability of the atmosphere to store water. In Bedrock cyclic fashion, changes in the amount of water stored in the atmosphere can alter the Earth’s energy balance and thus affect surface temperatures. Ground-water outflow Movement of water within the atmosphere occurs over a range of space and time scales. Movement occurs both by convection (watervapor transport by moving air masses) and molecular diffusion (the natural Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle within the atmospheric boundary layer can be as high as 50 to 100 km/day (Oke, 1978). Atmospheric transport of water is driven by gradients in pressure, temperature, and humidity. Predictions of moisture storage and movement are integral parts of weather forecasts. These forecasts are based on large-scale computer models that rely on data collected at National Weather Service surface monitoring sites across the United States. These surface sites provide point measurements of temperature, pressure, and humidity. Radar and satellite imagery provide additional data that are integrated over large areas. U.S. Department of Energy Atmospheric Radiation Measurement Program tendency of water vapor to move from areas of high concentration to areas of low concentration). The lower part of the atmosphere, called the atmospheric boundary layer, is the part of the atmosphere that is most influenced by the Earth’s surface. The layer varies in height between about 500 and 2,000 m and typically holds about one-half of all atmospheric water. It is characterized by turbulent mixing generated as warm, moist air pockets move up from the heated surface and by frictional drag as the atmosphere moves over the Earth’s surface. Horizontal transport rates of water vapor U.S. Department of Energy Atmospheric Radiation Measurement Program Cumulus clouds. Anvil cloud. 9 Clouds in Hawaii. Water Budgets: Foundations for Effective Water-Resources and Environmental Management B Water Budgets are Intimately Linked to Energy and Chemical Budgets Energy Budget The global water budget is intrinsically linked to the global energy budget. When water changes among its different phases (solid, liquid, and gas) energy is absorbed or released, thus affecting the energy budget. A simple energy budget for the Earth is (Sellers, 1965): Rn = G + LE + H (B1) Rn Net radiation LE Latent heat flux H Sensible heat flux where Rn is net radiation (the sum of incoming solar and longwave radiation minus reflected solar and emitted longwave radiation); G is surface-heat flux (that is, the energy used to warm soil, or water in the case of a surface-water body); LE is latent heat flux (that is, the energy used to evaporate water); and H is sensible heat flux, or the energy used to warm air. The equation states that available energy at the Earth’s surface goes to heating the surface, warming the air, and evaporating water (fig. B–1) . Latent heat flux is the product of latent heat of vaporization (λ) and evapotranspiration rate (ET); that is, LE = λET. Evapotranspiration provides a direct link between Surface Energy Budget G the energy-budget and the water-budget equations because it Rn − G = H + LE Surface heat flux appears in both equations. These equations form the basis of general circulation computer models that are used to predict Figure B–1. Schematic of the energy budget at the climate trends. Estimation of ET rates can be addressed from both Earth’s surface. energy-budget and water-budget perspectives. The movement of heat in ground and surface waters may be materially affected by the movement of water. An important component of energy transport is convection, or the movement of heat by the movement of water. The transport of energy by surface water is important in studies of powerplant or dam discharges in rivers where the health of natural fish populations is affected by heat loads or changing temperatures. Ground-water flow has been shown to be an important controlling factor on the occurrence and severity of volcanic eruptions (Matsin, 1991). The interdependence of water and energy movement has proved useful for estimating rates of exchange between ground and surface waters (Lapham, 1989; Stonestrom and Constantz, 2003). Geothermal Education Office, Tiburon, California 10 Geothermal plant in California. Box B Water Budgets are Intimately Linked to Energy and Chemical Budgets 11 Water vapor rises from hot springs in Yellowstone National Park. Chemical Budget National Aeronautics and Space Administration Chemical fluxes are important to our environment. For example, fluxes and storage of carbon in the ocean, on land, within inland waters, and in the atmosphere have vital implications for ecosystems and climate. Water movement within and among the atmosphere, surface, and subsurface is an important mechanism for transport of chemicals through the environment. The water budget provides a foundation for understanding chemical fluxes and balances. As water contacts rocks, sediment, and organic materials, its chemistry is altered by reactions such as dissolution, precipitation, ion exchange, and oxidation/reduction. Groundand surface-water flows sustain many wetlands, lakes, and ponds. In addition to supplying water, these inflows also provide nutrients and chemicals that support biogeochemiThe Mississippi discharging water and sediment to the Gulf cal processes within these bodies. of Mexico. Chemicals are transported to the atmosphere naturally (by diffusion and wind advection, and through plants, fires, and volcanic activity) and as a result of human activities (combustion of fossil fuels, application of agricultural chemicals, and production of chemical compounds). Some chemicals become dissolved in atmospheric water and fall back to Earth in precipitation. Sulfate-bearing precipitation has been implicated as a major cause for acidification of some lakes in the Adirondack Mountains of New York (Driscoll and others, 2003). Surface waters are reservoirs and conveyance mechanisms for chemicals and sediment. Sediment and contaminants can be washed off of streets and fields during rainfalls and be carried through storm drains to streams. It is estimated that, in one year, the Mississippi River discharged 900,000 tons of nitrate and 35,000 tons of orthophosphate to the Gulf of Mexico (Antweiler and others, 1995). Severe rainfalls can lead to flooding, which can Floodwaters can transport debris. greatly enhance the transport capabilities of surface water. Floods are capable of transporting not only sediment and chemicals but also pathogens, animals, cars, and even houses. Water moves more slowly through the subsurface than it does through surface-water bodies or the atmosphere. Hence, removal of subsurface contaminant plumes may take much longer than cleanup of surface plumes. Long residence times in the subsurface allow more time for reactions to occur and, in some instances, may promote natural remediation of contaminants by indigenous microbes (Lahvis and others, 1999). Water Budgets: Foundations for Effective Water-Resources and Environmental Management National Aeronautics and Space Administration 12 Ice fields in Antarctica, such as the Ross Ice Shelf, store about 70 percent of Earth’s freshwater. Water on Land Surface Freshwater is present on the Earth’s land surface in solid and liquid forms. Solid forms include snow and ice; liquid water is stored in lakes, surface-water reservoirs, some wetlands, and streams. Snow and Ice The largest amount of freshwater on Earth (29.2 million km3) is stored in glaciers and polar ice (Nace, 1967). Most of this ice is present in Antarctica and Greenland and is largely inaccessible to humans. Solid water present as glaciers and snow in more temperate regions may be available for humans (fig. 3). Here, snow and ice serve as seasonal storage receptacles that contribute to water supplies upon melting. Melt from the annual snowpack, especially that captured in reservoirs, is the primary source of water for humans and aquatic ecosystems in many parts of the world. Glaciers represent a more permanent form of water storage. Residence time of water stored in glaciers can be decades to centuries. Meltwater from glaciers can sustain streamflows throughout the year. Snowpits are dug to determine water content and chemistry of snowpacks. Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 13 Toboggan Glacier, Alaska, photographed by S. Paige on June 29, 1909 (left), and by Bruce F. Molnia on September 4, 2000 (right). Atmospheric water, primarily in the form of snow, is the source of water to glaciers and snowfields. Water moves from these bodies to the atmosphere (as ablation), to the subsurface (as infiltration), and to streams. Measurement of water storage in seasonal snowpacks generally is done by conducting snow surveys, where snow depth and the water content of snow are determined in designated areas or along snow courses that transect an area. Measurement of changes in water stored in glaciers has historically been difficult because high mountain terrain is often inaccessible. Storage changes were determined by repeated detailed surveys of the ice surface topography. In recent years, remote sensing from aircraft or satellite, used in conjunction with high-resolution digital-elevation models, has greatly enhanced the accuracy of these measurements. Accurate determinations of water budgets of glaciers are rare. Only a few studies of glaciers have resulted in detailed, long-term monitoring of their water budgets (Mayo and others, 2004). However, in a general way, comparative photographs (a form of remote sensing) of glaciers show that many glaciers have been shrinking over the last few decades. Precipitation Surface-water inflow, imported water (pipelines, canals) Evapotranspiration Ground-water inflow Wa ter tab Un le sa tu t ra ed zo ne Bedrock Aquifer Surface-water outflow, exported water (pipelines, canals) Ground-water outflow Figure 3. Snow and ice within the hydrologic cycle. 14 Water Budgets: Foundations for Effective Water-Resources and Environmental Management C Chemical, Isotopic, and Energy Tracers Provide Insight into Hydrologic Processes Direct physical measurements of water-budget components may at times be inconvenient, problematic, or impractical. In such cases, indirect methods may provide estimates of water-budget components or act to reduce the uncertainty associated with those estimates. Chemical, isotopic, and energy (heat) tracers are commonly used to provide insight into processes such as ground-water recharge, ground-water discharge to lakes and wetlands, and base flow. A tracer is simply a chemical or isotope (or property, in the case of heat) that is transported by water. Analysis of spatial or temporal patterns of tracer concentrations can be used to identify trends in water movement and therefore can provide insight for shaping conceptual models of water budgets.The ideal hydrologic tracer is one that moves with water, is conservative (that is, not altered by reactions or other processes in water, porous media, or atmosphere), and is easily and accurately detected. Tracers can be categorized as environmental, historical, and applied. Environmental tracers are those that occur naturally in the environment. Isotopes of oxygen and hydrogen have been used for decades to distinguish sources of water and to examine water balances (Gat and Gonfiantini, 1981). These isotopes are well suited as tracers because they are part of the water molecule itself. Carbon isotopes, chloride, sulfate, and nitrate are other useful environmental tracers. Historical tracers are those that were released to the environment continuously or or at specific times during the past. Radionuclides (including tritium, 3H, Tracers are used to determine the age of subsurface water (that is, the time since that water last had contact with the atmosphere), velocities and traveltimes for ground and surface waters, and travel paths of water in the subsurface. Applying dye tracer to land surface at research site in Minnesota. Dye tracer is visible in trench excavated beneath application area. Tracers are used in streams to measure traveltimes. Box C Chemical, Isotopic, and Energy Tracers Provide Insight into Hydrologic Processes 1987 Cl 36 100 CFC–11 36 200 Cl FALLOUT, IN ATOMS PER SQUARE METER PER YEAR 400 0 1940 1960 1980 2000 0 FEET 150 WELL LOCATION–Number is the year that ground water at that location was recharged Water ta ble 100 Lake Barco 1987 1986 1973 1986 1967 1963 50 50 1962 1964 Surficial sand 1981 SEA LEVEL CFC–12 H 3 YEAR ORGANIC-RICH SEDIMENTS 150 200 600 Figure C–1. Atmospheric concentrations for historical tracers, including 3H, 36Cl, CFC–11, and CFC–12 (after Scanlon and others, 2002). EXPLANATION FEET 100 CONCENTRATION, IN PARTS PER TRILLION BY VOLUME (CFC–11 and CFC–12) OR TRITIUM UNITS (3H) and chlorine-36, 36Cl) released to the atmosphere from testing of nuclear bombs in the 1950s and 1960s fall into this class (fig. C–1). Chlorofluorocarbons (CFCs) and sulfur hexafluoride were released to the atmosphere by industrial processes over the last 50 years and are common hydrologic tracers (http://water.usgs.gov/lab/). For example, Katz and others (1995) used concentrations of CFCs to estimate the ages of ground water near Lake Barco in Florida (fig. C–2). Applied tracers include those introduced intentionally (for example, chloride, bromide, and dyes) and those inadvertently introduced to the environment, such as through a chemical spill. Applied tracers commonly are used to determine velocities of streamflow and ground-water flow, to identify subsurface flow paths, and to quantify exchange rates between surface and ground waters. Properties and uses of common hydrologic tracers are given in table C–1. 15 1959 1958 1978 SEA LEVEL Bedrock –50 VERTICAL SCALE GREATLY EXAGGERATED 0 500 FEET –50 Figure C–2. Lake Barco, in northern Florida, is a flow-through lake with respect to ground water. The dates when water in different parts of the groundwater system was recharged indicate how long it takes water to move from the lake or the water table to a given depth (after Katz and others, 1995). Table C–1. Examples of tracers used in water-budget studies. Use Ground-water age — Time since recharge water became isolated from the atmosphere Naturally occurring in the environment Historical — Added to the environment from human activity in the past Applied — Added to the environment in the present H, 36Cl, 85K, chlorofluorocarbons, herbicides, caffeine, pharmaceuticals 3 S, C, H/ He, 39 Ar, 36Cl, 32Si 35 14 3 3 Temperature of recharge Tracing ground-water flow paths 18 Plummer and others (2001) N2/Ar solubility Exchange of surface water and ground water Surface-water discharge and traveltime Example study O, 2H, 13C, 87Sr O, 2H, 3H, 14C, 222 Rn 18 Plummer (1993) Chlorofluorocarbons, herbicides, caffeine, pharmaceuticals Cl, Br, dyes Renken and others (2005) Cl, Br, dyes Katz and others (1997) Cl, Br, dyes Kimball and others (2004) 16 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Lakes Lakes are the fourth largest reserve of water in the global water budget. The volume of water in natural lakes is estimated to be about 229,000 km3 (table 1; fig. 4). Of this total volume, 125,000 km3 are in freshwater lakes and 104,000 km3 are in saline lakes; the Caspian Sea alone contains about 95 percent of the total volume of water in saline lakes. For this report, surface-water reservoirs are considered to be lakes. Lvovitch (1973) estimated the total volume of water in reservoirs to be about 5,000 km3. The largest reservoir in the United States, Lake Mead, contains about 38 km3 of water at full pool elevation; Lake Powell contains about 33 km3 at full pool elevation. Lakes interact with the atmosphere, the subsurface, and other surface-water features. They gain water from precipitation, streamflow, and ground water and lose water by evaporation, surface outflow, and seepage to ground water. However, all these interactions do not occur for every lake. Some topographically high lakes have no stream or groundwater inflows, gaining water only from precipitation. At the other topographic extreme, some lakes, called terminal lakes, receive water from precipitation, streams, and ground-water inflow and lose water only by evaporation. The volume of water in a lake may be determined by preparing a bathymetric map of the lake bottom and by calculating the volume of water present at a given lake stage (lake level). Lake stage is measured by reading a staff gage, or it can be continuously monitored by a recording gage. A stage-volume relation is then established that can be used to determine the volume of water at any given stage. This approach can produce accurate results if the bathymetry is well defined. Residence times of water in lakes span a wide range. Residence time is calculated by dividing the volume of a lake by the rate of outflow. For very large lakes, like Lake Superior, residence time is nearly 200 years. Lake Powell, much smaller but still a large surface-water reservoir, has a residence time of about 2.3 years. Lakes with no stream outlet, like many in glacial terrain, can have residence times of several years to a decade, and small lakes with outlet streams commonly have residence times of days to weeks (Winter, 2003) Precipitation Surface-water inflow, imported water (pipelines, canals) Evapotranspiration Ground-water inflow Wa ter tab Un le sa r tu at ed zo ne Bedrock Aquifer Surface-water outflow, exported water (pipelines, canals) Ground-water outflow Figure 4. Lakes, wetlands, and streams within the hydrologic cycle. Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 17 National Aeronautics and Space Administration Crater Lake, Oregon. Lake country in northern Wisconsin. The Great Lakes. Water Budgets: Foundations for Effective Water-Resources and Environmental Management “Wetlands are lands where saturation with water is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the soil and on the surface. The single feature that most wetlands share is soil or substrate that is at least periodically saturated with or covered by water.” U.S. Fish and Wildlife Service 18 Lower Klamath National Wildlife Refuge, California. Cowardin and others (1979, p. 3) Wetlands Depressional wetlands, North Dakota. Wetlands in depressions generally contain standing water and in many respects are much like lakes. Many types of wetlands do not contain standing water, however, or contain it for only brief periods each year. Such wetlands consist mainly of saturated soils. Most wetlands receive surfacewater inflow at some time of the year, some are fed by both surface and ground water, and others are supported solely by ground-water flow. Like lakes, some wetlands located high in the landscape gain water only from precipitation; others, low in the landscape, like terminal lakes, lose water only to the atmosphere. A major difference between wetlands and lakes is that wetlands lose water to the atmosphere largely by transpiration from plants, whereas lakes lose water to the atmosphere mostly by evaporation. Determining the volume of water in a wetland and the change in that volume over time is more difficult than it is for lakes because, other than the open-water portion, water is present in wetland soils. Measurement of water storage in soils is addressed in the section “Unsaturated Zone.” Red Rock Lakes National Wildlife Refuge, Montana. Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle 19 “A river seems a magic thing. A magic, moving, living part of the very earth itself—for it is from the soil, both from its depth and from its surface, that a river has its beginning.” Laura Gilpin (1949) Streams The volume of water in the Earth’s streams at any given time (about 1,250 km3 according to Nace, 1967) represents only a small part of the total volume stored on land surface. Streams, then, are generally not important in terms of global water storage. Streams function mainly to transport water, conveying it from higher to lower altitudes on the land surface and, in most cases, ultimately to the oceans. Streams also facilitate water exchange between the surface and the subsurface, and to a lesser extent between the surface and the atmosphere. Sources of water in streams can be surface-water bodies, surface runoff of precipitation (as well as direct precipita- tion on a stream), interflow (shallow subsurface flow usually associated with hillslopes), and base flow (ground-water discharge). Along their course, streams can lose water to other surface-water bodies, to the subsurface, and to the atmosphere (by evaporation). Streams range in size from small rivulets in headwater areas that flow only after precipitation events to large rivers, such as the Mississippi and the Amazon. Magnitudes of velocities in streams are variable; 30 cm/s may be typical, whereas 300 cm/s is quite high. Because they are confined to channels on the Earth’s surface, streams are visible and relatively accessible for measurement of discharge and are therefore the part of the hydrologic cycle that can be measured most accurately. 20 Water Budgets: Foundations for Effective Water-Resources and Environmental Management American Geological Institute, Michael Collier Large streams in the United States tend to show seasonal trends (fig. 5A); highest discharges generally occur in spring, a time when snow melts, soils thaw, and soil moisture contents are high. Small streams are usually more dynamic than large streams and they show rapid rises and falls in response to storms (fig. 5B). The source of water in a stream also influences discharge patterns. Streams dominated by snowmelt or base flow follow a more predictable pattern than those dominated by surface runoff. For major streams, the U.S. Geological Survey maintains a network of thousands of stream gages across the United States (http://water.usgs.gov). Stream level (stage) is DAILY DISCHARGE, IN CUBIC FEET PER SECOND Stream network in arid region, Organ Pipe National Monument, Arizona. 20,000 10,000 A 1,000 100 60 Oct Jan Apr DAILY DISCHARGE, IN CUBIC FEET PER SECOND 2000 1,500 July Oct Jan Apr July Oct Jan 2002 2001 Apr July Oct 2003 B 1,000 500 0 Oct 2000 Jan Apr July 2001 Oct Jan Apr July 2002 Oct Jan Apr July Oct 2003 Figure 5. Streamflow hydrographs for two gaging sites: (A) Kishwaukee River near Perryville, Illinois (U.S. Geological Survey station number 05440000; drainage area 1,099 square miles) and (B) South Branch Kishwaukee River at DeKalb, Illinois (U.S. Geological Survey station number 05439000; drainage area 77.7 square miles). Red indicates estimated values. Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle monitored continuously at these sites, and a stage/discharge relation is developed using periodic discharge measurements. Discharge is the product of stream velocity and cross-sectional area integrated over that area. Velocity has historically been measured manually at many locations along a cross section by using a current meter. Recently, acoustic velocity meters have reduced the need for manual measurements (Yorke and Oberg, 2002). By establishing good stage/discharge relations, stream Discharge is determined with measurements of stream depth and velocity. 21 discharge can be determined from measurements of stage. Typical errors in stream discharge measurements are 10 percent (Rantz and others, 1982). For small streams, more accurate measurements of discharge can be obtained by installing a flume or a weir and a stage recorder in the channel. Flumes and weirs are carefully calibrated in hydraulic laboratories, so measurements of discharge commonly have errors of about 5 percent. USGS gaging station. Flumes can be used to measure discharge in small streams. 22 Water Budgets: Foundations for Effective Water-Resources and Environmental Management D Models—Important Tools in Water-Budget Studies Hydrologic computer-simulation models Solar contribute substantially to our understanding of radiation the hydrology of watersheds, rivers, and aquiPrecipitation fers. They are integral tools for managing water Evaporation Sublimation Air temperature resources in many areas. Using calculations that are too cumbersome to be performed by hand, these models allow detailed investigation Plant canopy interception of complex hydrologic processes and provide predictions of responses within a specific Rain Throughfall Rain Evaporation water-budget accounting unit to external or and Snowpack Evaporation Transpiration internal stresses. Most hydrologic computerSurface runoff Transpiration Snowmelt simulation models are derived from some to stream variant of equation A2 and thus are truly waterImpervious-Zone Reservoir Soil-Zone Reservoir budget models. As water-budget equations vary Recharge zone greatly in complexity, so do the models that are Lower zone based on them. A simple model may provide a quick view of the water budget for an accountSubsurface recharge ing unit but is unlikely to provide insight into the Ground-water Interflow or processes that drive water movement within Subsurface recharge subsurface Reservoir that unit. A more complex model may provide flow to stream that insight but at substantially greater expense. Ground-water recharge Watershed models are perhaps the most Ground-Water complete form of a water-budget model. They preReservoir Ground-water flow to stream dict stream discharge within a basin in response to precipitation and snowmelt, usually accounting for Ground-water processes such as evapotranspiration, groundsink water/surface-water exchange, and surface-water routing (fig. D–1). Watershed models are widely Figure D–1. Shematic diagram showing various reservoirs and processes used for watershed management and planning. For that are considered in a watershed model example, they can be used to predict the effects of (R.S. Regan, written commun., 2007). land-use changes (such as urban development) on streamflow (fig. D–2). 55 Average annual basin water budget, in inches, for water years 1993–98 45 Precipitation 35 INCHES 35.2 25 2.8 Overland flow 15 Interflow 0.4 5 –5 23.9 Evapotranspiration 1.5 Base flow 1993 1994 1995 1996 1997 1998 EXPLANATION Ground water to regional system Base flow Interflow Evapotranspiration Overland flow Interception Change in local ground-water storage 6.0 Ground water to regional system (not captured by stream) Budget not balanced because of change in ground-water storage. Figure D–2. Steuer and Hunt (2001) used a watershed model to simulate water fluxes in the Pheasant Branch Creek watershed near Middleton, Wisconsin, for the period 1993 to 1998. The model was subsequently used to predict the effects of urban development in the watershed. Box D Models—Important Tools in Water-Budget Studies 89˚32'30" 89˚35' 43˚07'30" 43˚05' Pheasant Branch Basin Figure D–2. Steuer and Hunt (2001) used a watershed model to simulate water fluxes in the Pheasant Branch Creek watershed near Middleton, Wisconsin, for the period 1993 to 1998. The model was subsequently used to predict the effects of urban development in the watershed.—Continued Big Big Ground-water-flow models predict how water levels 97˚ 97˚ in an aquifer will be affected by changes in withdrawals A B or in recharge rates (fig. D–3). They are used in studies of ground-water supply and ground-water contaminant 98˚ 98˚ 10 transport. Most of these models simulate flow only in 41˚ 41˚ 41˚ 41˚ 0 20 2 15 the saturated zone (that is, the region beneath the water 5 table). Other more complex models simulate water move15 10 5 15 ment within both the unsaturated and saturated zones. 30 20 25 20 Streamflow routing models predict stream dis20 charge and velocity. Managers use these models to 5 15 20 5 1 0 ittle Bl 10 5 estimate where, when, and at what stage flood waves will ittle Bl 1 5 5 ve r ve r crest, allowing them to adjust release rates from reserNEBRASKA NEBRASKA 98˚ 98˚ 40˚ 40˚ 40˚ 40˚ voirs to mitigate adverse effects of flooding. KANSAS KANSAS 97˚ 97˚ General circulation models forecast weather and 0 20 MILES climate trends at the continental scale over periods of 0 20 KILOMETERS EXPLANATION days to centuries. NEBRASKA Line of predicted equal 20 Soil–vegetation–atmospheric transport models water-level decline, in feet 20 Line of measured equal are used to study the movement of water from the water-level decline, in feet MAP AREA atmosphere to the soil through plants and back into the atmosphere. Coupled models combine water-budget models with Figure D–3. Contours of model predicted (A) and measured (B) mass or energy transport models and are useful for simuground-water levels for the Blue River Basin, Nebraska (Alley and lating contaminant transport in surface or ground water. Emery, 1986). Statistical techniques (such as regression, nonparametric statistics, and geostatistics), while not water-budget models, are important in many water-budget studies. They can be used for quantifying uncertainty in simulation results, determining which types of data can improve simulation results, and interpolating and integrating point measurements (from a rain gage, for example) over entire watersheds or basins. Blue Blue L ue ue L 5 15 25 er Riv er Riv 30 25 23 Ri Ri Water Budgets: Foundations for Effective Water-Resources and Environmental Management American Geological Institute, Bruce F. Molnia 24 Heterogeneity of subsurface sediments complicates study of water movement in the subsurface. Water in the Subsurface (fig. 6). The thickness of this zone varies spatially and temporally and may range from 0 to more than 1,000 m. In general, thicker unsaturated zones are found in more arid regions. No known estimates exist for the amount of water stored in unsaturated zones at the global or continental scales. The importance of the unsaturated zone as a storage reservoir is often overlooked because the water held there generally is not extractable for human use. The unsaturated zone, however, is the primary source of water for vegetation and therefore plays a critical role in the hydrologic cycle. An estimated 76 percent of precipitation infiltrates the subsurface (table 2). Because water moves through the unsaturated zone at a relatively slow rate, plants are able to extract that water over extended periods of time. About 85 percent of the water that infiltrates the soil surface returns to the atmosphere either by evaporation from soil or by plant transpiration. Water storage within the unsaturated zone is determined by measuring moisture content at different depths between the land surface and the water table. Repeated measurements over time can be used to infer rates of storage change. Moisture content can be measured directly by collecting samples in the field and weighing the sample before and after oven drying. Indirect techniques, which are more conducive to automatic recording, take advantage of electrical or physical properties of the sediment-water continuum (for example, time domain reflectometry and neutron moderation). Infiltrated water moves predominantly in a downward direction through the unsaturated zone toward the water table. Water also can move upward (in response to evaporative demand) or laterally (in the case of impeding layers of soil). Rates of water movement are notoriously difficult to measure The Earth’s subsurface consists of solid rock, mineral grains, organic matter, and varying amounts of water and other liquids and gases that occupy open spaces or voids. The subsurface serves as the major reservoir of extractable freshwater, accounting for more than 95 percent of worldwide storage. On the annual global scale, change in storage of water in the subsurface is negligible. At smaller scales, changes in subsurface storage can be substantial and significant. Groundwater levels in the San Joaquin Valley of California declined as much as 100 m between 1920 and 1970 as a result of pumping for irrigation. In addition to a reduction in the amount of water stored in the subsurface, the declining water levels resulted in land-surface subsidence of more than 9 m in some areas (Galloway and others, 1999). Surface-water inflow, imported water (pipelines, canals) A principal difficulty in quantifying the movement and storage of water in the subsurEvapotranspiration face is the natural variability in the physical and hydrologic properties of earth materials at all spatial scales. For convenience, discussion Ground-water inflow of subsurface hydrology is divided into the Wa ter unsaturated zone (where open spaces or voids tab le in the earth materials are partly filled with water and partly filled with air) and the satuAquifer rated zone (where voids in the earth materials are completely filled with water). Precipitation Un at ed zo ne Bedrock Surface-water outflow, exported water (pipelines, canals) Unsaturated Zone The unsaturated zone, sometimes referred to as the vadose zone or zone of aeration, encompasses the earth materials that lie between the land surface and the water table sa r tu Ground-water outflow Figure 6 . The unsaturated zone within the hydrologic cycle. 25 Forest Service Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle Water in the unsaturated zone sustains most vegetation. directly because of problematic measurement techniques and the variable nature of the fluxes. Lysimeters (see Box E— Lysimeters: Water-Budget Meters) can provide accurate, albeit expensive, measurements of these fluxes. More commonly, flux rates are inferred by using indirect approaches such as the Darcy approach or unsaturated-zone water-budget methods. The Darcy approach requires measuring depth profiles of pressure head (sometimes referred to as matric potential or soilwater tension, measured with tensiometers, heat-dissipation or electrical conductivity probes, or thermocouple psychrometers) and unsaturated hydraulic conductivity. Unsaturatedzone water-budget methods are based on measurement of changes in water storage in the unsaturated zone over time (for example, the zero-flux plane method) or analysis of fluctuations in water-table elevations (Scanlon and others, 2002). Moisture content profiles within the unsaturated zone typically display seasonal trends (fig. 7). Largest fluctuations occur near land surface; the magnitude of the annual fluctua- 0 Installing moisture content sensors through the wall of a trench; the trench was later backfilled. tions decreases with depth. At some depth, moisture contents may show no measurable change throughout the year. This does not mean that there is no flow occurring at these sites; rather, this implies a constant flux of water (usually small in magnitude). Residence times of water within the unsaturated zone depend upon factors such as climate, geology and soils, depth to water table, and vegetation. In most areas, the residence time of water in the root zone ranges from days to months (although some water is maintained in small pores over much longer periods; this is referred to as immobile water, and its presence has been identified through tracer tests). For the region below the root zone, residence times can be estimated as the amount of water stored there divided by the estimated flux through that region. In humid areas with thin unsaturated zones, residence times are usually a year or less. In arid regions, residence times may be millennia. DEPTH, IN METERS 2 4 6 8 0.20 Winter Spring Summer Fall 0.25 0.30 0.35 MOISTURE CONTENT, DIMENSIONLESS 0.40 Figure 7. Hypothetical moisture-content profiles at four different times of the year. As depth increases, the variation in moisture content decreases. The unsaturated zone at Yucca Mountain, Nevada is as thick as 500 meters. Water Budgets: Foundations for Effective Water-Resources and Environmental Management E Lysimeters—Water-Budget Meters Lysimeters are instruments specifically designed for measuring one or more components of the water budget, such as evapotranspiration or ground-water recharge. Most lysimeters consist of containers filled with soil, hydrologically isolated from the surrounding undisturbed environment but intended to mimic the hydrologic behavior of that environment. Lysimeters vary in design from simple collection vessels with a surface area on the order of 100 cm2 to units constructed on sensitive weighing balances with surface areas of several square meters (Young and others, 1996). Some instruments are capable of resolving fluxes of less than 1 mm/d. When properly constructed and maintained, lysimeters provide perhaps the most sophisticated approach for studying water budgets at a small scale. Assuming that there is no surface or subsurface flow to it, the water budget for a lysimeter is: where and ∆S = P – ET – RO – D ∆S P ET RO is change in storage within the lysimeter and is determined on a weight basis, is precipitation and irrigation, is evapotranspiration, is runoff, D (E1) is drainage out the bottom of the lysimeter. Installations with weighing lysimeters typically are also equipped with precipitation gages and runoff collectors. In addition, most lysimeters permit measurement and collection of drainage, either by having a free-draining base or by having a porous plate base across which a tension can be imposed by means of a vacuum or wick system. With independent measurements of P, RO, and D, the lysimeter provides a direct measurement of ET: ET = P – ∆S – RO – D (E2) During periods when precipitation, runoff, and drainage are all zero, changes in weight of the lysimeter are due solely to evapotranspiration. GSF – National Research Center for Environment and Health 26 The GSF National Research Center for Environment and Health of the Institute for Soil Ecology, Neuherberg, Germany, operates 32 lysimeters for studies of water budgets, ground-water recharge, and nutrient uptake by plants. (http://www0.gsf.de/eus/index_e.html, accessed on February 26, 2007) Box E Lysimeters—Water-Budget Meters Pan lysimeter. Installing pan lysimeter through trench wall with a hydraulic jack. Large drainage lysimeters are expensive to construct and often problematic to maintain. As such, they are rarely used in hydrologic studies. Figure E–1 shows measurements of rainfall and drainage at one such lysimeter at Fleam Dyke, England (Kitching and Shearer, 1982). Small, simple lysimeters are easier to install and maintain and are practical for evaluation of spatial variability of evaporation and irrigation, for example. With any lysimeter, careful design and installation are required to avoid altering the natural hydrologic conditions of the system under study. 120 1978 1979 Precipitation Lysimeter drainage 80 60 40 20 Au g Se ust pt em be r Oc to be r No ve m be De r ce m be r ne Ju ly Ju il ay M Ap r Ja nu ar y Fe br ua ry M ar ch Au g Se ust pt em be r Oc to be r No ve m be De r ce m be r ne ay Ju ly Ju M Ap ril 0 Ja nu ar y Fe br ua ry M ar ch WATER DEPTH, IN MILLIMETERS 100 Figure E–1. Monthly rainfall and drainage from lysimeter at Fleam Dyke (Kitching and Shearer, 1982). 27 28 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Saturated Zone Ground water, water stored within the saturated zone, constitutes the largest reservoir of extractable freshwater on Earth (table 1, fig. 8). More than 1.5 billion people worldwide, including about 50 percent of the population of the United States, rely on ground water for their drinking water. The importance of ground water is sometimes overlooked simply because the subsurface is hidden from our view. There are no windows through which we can view the vastness and complexities of the saturated zone. The saturated zone is bounded above by the water table or by the fixed interfaces at the bottom of surface-water bodies. The lower boundary of the saturated zone is difficult to define. There is a tendency for pores in earth materials to become smaller and fewer with depth, thus limiting the availability of the stored water to humans. Saline ground water underlies fresh ground water in most areas. Precipitation Surface-water inflow, imported water (pipelines, canals) Evapotranspiration Ground-water inflow Wa ter tab Un le sa tu t ra ed zo ne Bedrock Aquifer Surface-water outflow, exported water (pipelines, canals) Ground-water outflow Figure 8. The saturated zone within the hydrologic cycle. Artesian well in Boyd County, Nebraska, circa 1900. Inflow to the saturated zone, often referred to as groundwater recharge, occurs when water from precipitation (and perhaps irrigation) percolates downward through the unsaturated zone or when water moves from surface-water bodies to the water table (see Box F—Ground-Water Recharge). Outflow from the saturated zone occurs naturally to surfacewater bodies (for example, through seeps or springs) and to the atmosphere by evapotranspiration. In humid regions, ground-water discharge to streams is typically the dominant outflow mechanism and can account for more than 90 percent of annual flow in some streams. In arid regions, there may be essentially no ground-water discharge to streams but high rates of ground-water evapotranspiration. In some regions, human extraction of ground water for domestic, agricultural, and industrial uses constitutes the major portion of outflow. The subsurface is composed of geologic materials of varying chemical and physical properties. Conceptualization of the geologic features and how they affect ground-water flow (fig. 9) is a difficult but fundamental part of groundwater investigations. Insight on boundaries of ground-water flow systems, rates of water movement, amounts of water in storage, and rates and locations of recharge and discharge must be inferred from sources such as geologic maps, geophysical tests, ground-water levels, physical and chemical properties of water and rock, spring and streamflow records, and ground-water-flow models. An aquifer is a body of earth material that contains sufficient permeable material to yield significant quantities of water to wells. “Significant quantities” is a relative term: pumping rates of 2 m3/min or greater are considered large rates; 0.04 m3/min is considered a small rate even though it is more than sufficient to supply the needs of most households. Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle Figure 9. Ground-water flow systems in complex geological terrain. Ground water in the uppermost part of the ground-water system flows from surface-water body to surface-water body in the high hydraulic conductivity zone (characteristic of glacial outwash) because the water table slopes uniformly from one to the other (1). However, where the hydraulic conductivity of the deposits is lower (characteristic of glacial till), water-table mounds beneath the land-surface highs cause the local flow systems to discharge to contiguous surface-water bodies, such that there is no flow from one surface-water body to the next lower surface-water body (2). In both settings, intermediate-scale ground-water flow systems pass at depth beneath the local flow systems (3), and a regional ground-water flow system passes at depth beneath both local and intermediate flow systems (4). EXPLANATION High hydraulic conductivity Moderate hydraulic conductivity Low hydraulic conductivity Direction of ground-water flow 1 1 2 2 3 3 4 Bedrock WATER LEVEL, IN FEET BELOW LAND SURFACE 18 WATER LEVEL, IN FEET BELOW LAND SURFACE Aquifers in direct connection to the atmosphere are unconfined or water-table aquifers. Confined aquifers are separated from the atmosphere by a confining unit, which consists of materials with a hydraulic conductivity much lower than that of the aquifer. Hydraulic conductivity is a measure of a geologic material’s ability to transmit water (see Box G—Estimating Aquifer Hydraulic Conductivity). Wells are important in ground-water studies. They provide direct access to the subsurface environment and make it possible to measure ground-water levels, to obtain water samples for chemical analysis, to conduct aquifer tests to estimate aquifer properties, and to apply geophysical techniques to estimate physical and chemical properties of earth materials. Long-term measurements of ground-water levels provide data for evaluating trends over time, calibrating ground-water-flow models, and assessing resource-management schemes. Systematic water-level measurements in networks of monitoring wells across the country are conducted by a wide array of organizations. Measurements are made electronically or manually at frequencies ranging from hourly to annually. Water levels in many aquifers fluctuate seasonally in response to recharge and discharge patterns. Levels tend to rise from fall through early spring, when precipitation rates usually exceed evapotranspiration rates, and decline in late spring and summer when evapotranspiration rates are high (fig. 10A). The magnitude of fluctuations varies from aquifer to aquifer and year to year depending on geology, water use, and climate. In addition to seasonal patterns, long-term trends occur in some aquifers as a result of changes in climate (for example, droughts) or stresses imposed by humans. Figure 10B shows water levels for a period of almost 70 years from a well located in Memphis, Tennessee. Water levels declined by about 70 ft between 1928 and 1975 as the rate of pumping from the aquifer increased. After 1975, pumping rates stabilized and the long-term decline abated (Taylor and Alley, 2001). 29 A 20 22 24 26 50 Oct. 1986 B Oct. 1988 Oct. 1990 Oct. 1992 Oct. 1994 Oct. 1996 70 90 Missing record 110 130 150 1925 Well Sh:P-76 at Memphis Land-surface altitude: 287 feet Well depth: 488 feet 1935 1945 1955 1965 1975 1985 1995 Figure 10. (A) Hydrograph of daily water-level measurements over a 10-year period for a well in Vanderburgh County, Indiana; and (B) water-level trends in a long-term observation well in Memphis, Tennessee (Taylor and Alley, 2001). 30 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Water-level contour maps of aquifers provide a means by which ground-water movement and storage can be assessed. These maps can be constructed from water-level measurements obtained from multiple wells at about the same time (fig. 11). Alternatively, maps can be based on water levels generated by ground-water-flow models. In general, ground water moves from areas of higher water-level altitudes to areas of lower water-level altitudes. As shown in figure 11C, lines drawn perpendicular to water-level contours indicate direction of ground-water flow. The amount of water stored in the subsurface changes as pores (voids between soil grains) drain or fill and as water and the geologic material compress or expand. Change in aquifer storage between any two points in time is calculated as the product of the difference in water levels at the two times and a storage coefficient, Sc. For unconfined aquifers, gravity drainage and filling of pores is the dominant mechanism for storage change, and the storage coefficient, called specific yield (Sy), has values that range from about 0.02 for fine-grained sediments to 0.35 for very coarse grained sediments. Storage B A 152.31 138.47 131.42 152.31 138.47 150 131.42 145.03 145.03 140 132.21 132.21 126.78 137.90 128.37 13 0 121.34 0 12 121.34 126.78 C 137.90 128.37 EXPLANATION 152.31 138.47 150 152.31 131.42 145.03 140 140 132.21 126.78 13 0 0 12 121.34 137.90 128.37 Location of well and altitude of water table above sea level, in feet Water-table contour— Shows altitude of water table. Contour interval 10 feet. Datum is sea level Ground-water flow line Figure 11. By using known altitudes of the water table at individual wells (A), contour maps of the water-table surface can be drawn (B), and directions of ground-water flow along the water table can be determined (C) because flow usually is approximately perpendicular to the contours (Winter and others, 1998). Well installation requires special equipment. Storage and Movement of Water Within the Principal Compartments of the Hydrologic Cycle RECHARGE AREA Stream DISCHARGE AREA PUMPED WELL Da ys Unconfined aquifer D rs Ye ars Water table a Ye changes for confined aquifers are dominated by water and sediment compression and expansion, and storage coefficient values are much less, typically in the range of 10–5 to 10–4. Values of storage coefficient are best determined with aquifer tests that integrate over a fairly large area. Laboratory and empirical methods for determining specific yield (Healy and Cook, 2002) are easier to apply but test only a very small sample of an aquifer. Ground-water flow simulations and chemical, isotopic, and energy tracers are useful tools for identifying groundwater flow paths and estimating traveltimes (fig. 12). Magnitudes of ground-water velocities vary widely. A value of 1 meter per day or greater is considered high. A value of 1 meter per decade is considered low but not unusual for a confining unit. Thus, even for water-table aquifers, the time needed for small parcels of ground water to traverse the aquifer along the longest flow paths from point of recharge at the water table to point of discharge can be decades or longer. 31 ay s Confining bed Confined aquifer Centuries Confining bed Confined aquifer Millennia Figure 12. Ground-water flow paths vary greatly in length, depth, and traveltime from points of recharge to points of discharge in the ground-water system. Measuring water levels in an observation well in Colorado. Water Budgets: Foundations for Effective Water-Resources and Environmental Management F Ground-Water Recharge Ground-water recharge is an important component in aquifer water budgets. Information on recharge rates is useful in assessing the sustainable yield of aquifers, but recharge rates are difficult to quantify accurately because they vary widely in space and time. How, where, and when ground-water recharge occurs depends on factors such as climate, geology, soils, land-use practices, and depth to the water table. Annual rates of recharge in Minnesota (fig. F–1) tend to increase from west to east, similar to the trend in annual precipitation. In humid areas, recharge generally occurs as the widespread movement of water from land surface to the water table as a result of precipitation infiltrating and percolating through the unsaturated zone. This type of recharge generally occurs in winter and spring when evapotranspiration rates are low (fig. F–2A). In arid regions, focused recharge, or water that percolates down to the water 96º CANADA 94º 92º 90º NORTH DAKOTA 48º or ri ke e up S La EXPLANATION Mississippi Water Recharge―in centimeters per year WISCONSIN 46º SOUTH DAKOTA 32 Greater than 30 25.01 to 30 20.01 to 25 15.01 to 20 10.01 to 15 M inn 5.01 to 10 es ota Ri River ve r 0 to 5 Unclassifiable 44º MINNESOTA IOWA 0 0 Figure F–1. 20 40 60 80 M ILES 20 40 60 80 KI LOMETERS Estimated annual rates of recharge in Minnesota (Lorenz and Delin, 2007). Box F Ground-Water Recharge R = ∆S + Q + ET + ∆Q gw bf gw gw (F1) Recharge arriving at the water table augments ground-water storage (∆Sgw), discharges to the surface as base flow (Qbf), is extracted by plant transpiration (ETgw), or moves out of the accounting unit as ground-water flow (∆Qgw). Often, one of these processes dominates the others. So, for example, measurements of base flow or changes in storage are sometimes used to infer recharge rates. Other methods for estimating recharge are based on physical or chemical data on ground water, water in the unsaturated zone, or surface water (Scanlon and others, 2002). 160 A. Indiana 140 120 DEPTH, IN CENTIMETERS table from streams, is usually the dominant mechanism of natural recharge. If those streams drain mountainous areas, then their flow may be mostly snowmelt, and recharge would be expected in spring. Irrigation also can be a source of ground-water recharge during the growing season (fig. F–2B). The water budget of an aquifer provides some insight into what becomes of water that enters an aquifer as recharge (R): 100 80 60 40 20 0 −20 1/1/04 160 4/1/04 7/1/04 10/1/04 1/1/05 7/1/04 10/1/04 1/1/05 B. California 140 DEPTH, IN CENTIMETERS 120 100 80 60 40 20 0 Local depressions in land surface may be areas of enhanced recharge. −20 1/1/04 4/1/04 Central Arizona Project EXPLANATION Cumulative water input Cumulative evapotranspiration Cumulative recharge Daily change in storage Natural recharge to aquifers can be augmented by induced recharge through spreading basins such as these in the Avra Valley of Arizona. Figure F–2. Water budgets for (A) an unirrigated agricultural field in central Indiana, where most recharge occurs in winter and spring, and (B) an irrigated agricultural field in central California, where most recharge occurs during the growing season (after Fisher and Healy, 2007). Water input refers to the sum of precipitation and irrigation. 33 34 Water Budgets: Foundations for Effective Water-Resources and Environmental Management G Estimating Aquifer Hydraulic Conductivity Estimation of hydraulic conductivity is an essential, yet problematic, activity in ground-water hydrology. Values of hydraulic conductivity vary over more than 10 orders of magnitude for common earth materials (fig. G–1). Very permeable material such as karst limestone, fractured basalts, and coarse gravel can have hydraulic conductivities as large as 1,000 meters per day. Shale, marine clay, and glacial tills, on the other hand, may have conductivities on the order of 10–6 meters per day. Difficulties in estimating hydraulic conductivity arise from the highly variable manner in which geologic material was formed and deposited, the limited accuracy with which this parameter can be measured, and the small spatial scale over which measurements are made. IGNEOUS AND METAMORPHIC ROCKS Unfractured Fractured BASALT Unfractured Fractured Lava flow SANDSTONE Fractured Semiconsolidated SHALE Unfractured Fractured CARBONATE ROCKS Fractured CLAY Cavernous SILT, LOESS SILTY SAND CLEAN SAND Fine Coarse GLACIAL TILL 10-8 10-7 10-6 10-5 10-4 GRAVEL 10-3 10-2 10-1 1 10 102 103 104 HYDRAULIC CONDUCTIVITY, IN METERS PER DAY Figure G–1. Approximate ranges in hydraulic conductivity for selected earth materials. A total range of 13 orders of magnitude is shown, which is indicative of the range for more common earth materials. In general, the average hydraulic conductivity of earth material in the same hydrogeologic terrain can vary by orders of magnitude (after Heath, 1983). Roadcuts and outcrops provide insight to the geologic complexities of the subsurface. Box G Estimating Aquifer Hydraulic Conductivity Methods for estimating hydraulic conductivity fall into two categories: direct and indirect. Direct methods are based on hydraulic tests done in the laboratory or the field. Laboratory tests are conducted on sediment cores obtained during drilling of boreholes. The cores are generally several centimeters in diameter and may be a few centimeters to more than a meter in length. Exacting laboratory procedures can produce very accurate measurements of hydraulic conductivity. However, because of the small size of the sample, the representativeness of the measured values to the aquifer as a whole is largely unknown. Field methods, using single or multiple wells, provide estimates that are integrated over larger volumes than those of laboratory tests. The most common single well test is the slug test, whereby a known volume of water is instantaneously withdrawn from (or injected into) the well, and the resulting change in water level in the well is monitored over time. This test samples the aquifer material within perhaps a meter of the well screen. Multiple well tests, sometimes referred to as aquifer or pumping tests, are labor intensive and expensive. One well is pumped and water-level changes are monitored in observation wells at various distances from the pumped well. These tests can run for days or weeks and produce values of hydraulic conductivity integrated over the distances over which drawdown is measured, typically tens to hundreds of meters. Field data are processed by using analytical or numerical mathematical models, models that have inherent assumptions on aquifer boundaries and uniformity, to produce estimates of hydraulic conductivity. Indirect methods for estimating hydraulic conductivity are generally less expensive than direct methods, although they may not provide the same level of accuracy. The simplest such approach is to consult the literature; many reports contain tables of hydraulic conductivity for various consolidated and unconsolidated earth materials, such as shown in figure G–1. Geophysical measurements, made within boreholes, on land surface, or from aircraft or satellites, provide information that can be used to infer values of hydraulic conductivity on the basis of correlations developed at specific sites. Inverse ground-water-flow modeling uses best-fit algorithms to determine parameter values (including hydraulic conductivity) that produce simulated results that most closely match measured water levels and fluxes. All methods are tied to a distinct spatial scale (fig. G–2). Direct measurements, in particular, are made over a relatively small volume. If the aquifer is heterogeneous, many measurements Performing slug tests to determine aquifer hydraulic may be required to adequately describe that variability. Indirect conductivity. approaches may be applied over much larger spatial scales. 10–1 HYDRAULIC CONDUCTIVITY, IN METERS PER SECOND 10–3 10–5 10–7 Permeameter tests Piezometer tests Pumping tests High capacity pumping tests Regional flow model 10–9 10–11 10–13 –5 10 10–3 10–1 10 103 105 VOLUME OF TESTED MATERIAL, IN CUBIC METERS 107 109 Figure G–2. Hydraulic conductivity as a function of sample size (after Schulze-Makuch and others, 1999). 35 36 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Water moves from the atmosphere to the surface through the process of precipitation. Water in the subsurface is obtained from land surface either as infiltration of direct precipitation or as seepage from streams or other water bodies. Subsurface water discharges to the surface naturally at springs, streams, lakes, or wetlands and in response to human activities such as a pumping well. Subsurface water also discharges directly to the atmosphere by evapotranspiration. Surface water discharges to the oceans, infiltrates the subsurface, or returns to the atmosphere by evaporation. An understanding of these exchange processes is useful in discussions of water budgets, especially when considering how changes to one process may affect other exchange rates. Figure 13 shows annual rates for North America of water movement from the atmosphere to the land surface (precipitation), from the land surface to the subsurface (infiltration), from the subsurface back to the land surface (base flow), and from the subsurface and land surface back to the atmosphere (evapotranspiration). Precipitation 670 Evapotranspiration 380 Rain storm. Precipitation Precipitation, in the form of rain, snow, dew, or fog drip, is the ultimate source of all water on the Earth’s landmasses. Average annual precipitation rates vary across the world (table 3). Within the United States, annual rates exceed 10 m in parts of Hawaii and are as low as 50 mm in Death Valley. Figure 14 shows how annual precipitation rates are distributed across the conterminous United States. Precipitation patterns vary with location and season. Although some areas of the United States (such as coastal areas in the Northwest) experience steady, predictable precipitation patterns during some months, precipitation in most of the United States is episodic with no precipitation on most days and rainfall rates up to 100 mm/hr for short periods on other days. Local, convectivecell type thunderstorms can produce several centimeters over an hour in one locale, while no rain may be falling a short distance away. Such variability complicates efforts to determine precipitation rates for any study area. Base flow Agricultural Research Service 80 J. C. Koch Exchange of Water Between Compartments of the Hydrologic Cycle 460 Infiltration Figure 13. Average annual water exchange rates for North America (from Lvovitch, 1973) in millimeters of depth per unit area of land surface. Weighing-bucket precipitation gage. Exchange of Water Between Compartments of the Hydrologic Cycle Table 3. Average annual precipitation rates for various locations (BBC Weather, http://www.bbc.co.uk/ weather, accessed on February 12, 2007). Precipitation, in millimeters per year Atlanta, Ga. 1,500 Nashville, Tenn. 1,220 Cleveland, Ohio 1,040 Denver, Colo. 400 Death Valley, Calif. 50 Hilo, Hawaii 3,200 Fairbanks, Alaska 250 Tokyo, Japan 1,570 Singapore, Singapore 2,400 London, UK 580 Cairo, Egypt 25 Barcelona, Spain 580 Sydney, Australia 1,200 Buenos Aires, Argentina 940 The largest source of precipitation data in the United States is the National Climatic Data Center (http://www.ncdc. noaa.gov). It holds daily precipitation data for thousands of sites across the country. Precipitation at these sites is measured with a weighing-bucket gage, a cylindrical container with an opening at the top that is 20.32 cm in diameter. Accumulated 37 water in the gage is measured either manually or with an automated sensor that monitors the weight of the container. A standard, manually read gage measures the total precipitation between readings. Another widely used gage is the tippingbucket gage. Snowfall is difficult to measure directly. Instead, snow accumulation, or snow depth, is measured. At fixed reporting stations, such as those operated by the National Weather Service or the Natural Resources Conservation Service (NRCS), snow depth is determined by manual observation or by sonic sensors. Some stations are equipped with snow pillows. These are electronic balances that determine the weight of accumulated snow; they can provide hourly information on equivalent water content of the snow. SNOTEL is a network of remote snowpack stations maintained by NRCS in the western United States (http://www.nrcs.usda.gov). Using satellite communications, near real-time data on snow accumulation is available for more than 600 sites. A limitation to measuring snow depths at a fixed location is that wind may move the snow around after it has fallen. To compensate for this, agencies such as NRCS make repeated manual measurements of snow depth at multiple points along set courses throughout the snow season. Uncertainty in precipitation estimates arises from inaccuracies in gage measurements and a limited number of gages. High winds or heavy rainfalls can lead to underestimation of rainfall rates. Gages can get clogged with debris or freeze over. Proper placement of precipitation gages is critical. Nearby objects, such as trees and buildings, may produce a shadow effect, essentially blocking rainfall from the gage. EXPLANATION 0–559 560–945 946–1,303 Agricultural Research Service Precipitation, in millimeters 1,304–2,020 2,021–7,089 Figure 14. Average annual precipitation in the conterminous United States for the period 1980–1997 as determined by the DAYMET model (http://www.daymet.org). Snow gage in Idaho. 38 Water Budgets: Foundations for Effective Water-Resources and Environmental Management With a collection area of about 324 cm2, a rain gage samples only a very small part of a landscape’s surface area. To generate an average depth of precipitation for a specific area, data from gages within that area can be combined using one of several methods. Results from using three methods (arithmetic mean, Thiessen method, and isohyetal method; Linsley and others, 1982) for a simple example are within 18 percent of each other (fig. 15). Geostatistical techniques, such as kriging, can also be used to integrate precipitation over an area (Seo, 1998). Hevesi and others (1992) made use of the correlation between elevation and precipitation rates to improve estimates of rainfall rates for an area in southern Nevada. The density of rain gages within a watershed affects the accuracy with which the average rainfall for the entire watershed can be estimated. Increasing the number of gages should increase the accuracy of that estimate, especially for short periods of time. At a site in Illinois, Huff and Schickedanz (1972) found that a gage density of 50 mi2/gage resulted in a 17-percent sampling error for a 3-hour sampling period and a 6-percent sampling error for a monthly period. National Weather Service weather stations have an average density of 250 mi2/gage. Studies in mountainous terrain in Idaho indicate that a gage density of about 2 mi2 per gage is needed to obtain reasonably accurate estimates of precipitation (Molnau and others, 1980). Fog drip collector in Hawaii. In some areas, fog drip is the primary form of precipitation. 1'' 0.65 2'' 1.92 2.82 1.46 3'' 2.69 4'' 4.50 1.54 2.98 5'' 5.0 1.95 1.75 Arithmetic mean: 3.09 inches Thiessen method: 2.84 inches Isohyetal method: 2.61 inches Figure 15. Estimating average precipitation for an area from precipitation gage records using three methods (after Linsley and others, 1982). National Weather Service Exchange of Water Between Compartments of the Hydrologic Cycle 39 An alternative method for measuring precipitation, one whose usefulness has yet to be fully integrated into water-budget studies, is Doppler radar. A single radar installation can provide virtually instantaneous estimates of rainfall over areas of more than 70,000 km2. The National Weather Service has a radar network that permits estimation of rainfall on a 4-km grid over most areas of the United States, but some biases may impair estimates. A recently developed program called MPE uses a network of real-time standard rain gages and satellite imagery to remove these biases (Seo, 1998; Seo and others, 1999). The program produces hourly to daily total precipitation estimates on a 4-km grid across the conterminous United States (fig. 16). Used with various hydrologic models, these estimates have greatly improved our ability to predict runoff and streamflow, to provide warnings of severe weather and floods, and to manage reservoir systems. These estimates are particularly useful for large river basins and for areas that have few or no standard gages. However, the 4-km grid may not provide sufficient detail for studies conducted on areas of less than a few square kilometers. Doppler radar station. Figure 16. Daily rainfall for August 30, 2005 on a 4-kilometer spatial grid as determined by the National Weather Service MPE program. (National Weather Service, http://www.srh.noaa.gov/rfcshare/precip_analysis_new.php accessed on Feb. 13, 2007) Water Budgets: Foundations for Effective Water-Resources and Environmental Management Infiltration and Runoff Precipitation falling on land surface can evaporate, be stored on the surface, run off to another point on the surface, or infiltrate the subsurface. Surface storage is mainly in the form of snow. Precipitation falling directly on surface-water bodies or on small surface depressions and precipitation intercepted by vegetation also constitute surface storage. Surface storage is relatively short lived, with the exception of glaciers and ice fields. During periods of rainfall, rates of evaporation and storage change are usually much less than those of infiltration and runoff. If these two processes can be ignored, the sum of infiltration and surface runoff is equal to precipitation. A common practice in many hydrologic studies is to measure precipitation and either infiltration or surface runoff and to calculate the third value by difference. Factors such as soil properties, vegetation, land use, slope, climate (especially precipitation rate and temperature), and water-table depth can affect infiltration and runoff rates. Infiltrometer. If the rate of precipitation on bare soil is less than the rate at which the soil can absorb water, then all precipitation will infiltrate. Runoff is initiated once the rate of precipitation exceeds the rate at which the soil can absorb water. The time at which this occurs, the time of ponding, is an important parameter in many hydrologic models. Although there are no theoretical means for determining this time beforehand, empirical equations have been developed for this purpose. Figure 17 shows hypothetical rates of infiltration as a function of precipitation rate and time for a homogeneous soil. When the precipitation rate is greater than the saturated hydraulic conductivity of the soil, the infiltration rate is steady until the time of ponding. It then decreases over time and approaches a magnitude equivalent to the hydraulic conductivity. Direct measurements of natural infiltration rates are not common but can be made with certain kinds of lysimeters. Infiltrometers are used to determine steady-state infiltration rates after the soil has been saturated (these rates should be similar to values for saturated hydraulic conductivity), but 0.009 INFILTRATION RATE, IN CENTIMETERS PER SECOND 40 Ponding Precipitation rate, in centimeters per second 0.008 0.007 0.008 0.004 0.001 0.0008 0.006 0.005 Ponding 0.004 0.003 0.002 0.001 0 10 100 1,000 10,000 100,000 TIME SINCE START OF INFILTRATION, IN SECONDS Figure 17. Infiltration rates generated for a one-dimensional uniform soil column with a variably saturated flow model as a function of time for four precipitation rates. The saturated hydraulic conductivity of the soil is 0.001 centimeter per second. those rates may be substantially less than early time rates of infiltration into initially dry soils. Indirect methods for estimating infiltration may be based on measurement of pressure head and moisture content at different depths below land surface or on measurements of precipitation and runoff. Empirical methods for estimating infiltration also exist (Chow and others, 1988). Measurement of surface runoff, sometimes referred to as overland or Hortonian flow, is difficult but has been accomplished in some studies by building berms around a small area to funnel runoff to a central collector or measurement device. Surface runoff flows overland, eventually entering an established stream channel. As discussed previously in the “Streams” section, runoff is only one component of streamflow. It is sometimes possible to analyze a streamflow hydrograph to estimate rates of runoff and base flow (see, for example, Rutledge, 1998). Empirical equations for estimating runoff, most notably the NRCS Curve Number method, are also in widespread use (Chow and others, 1988). Surface runoff collection apparatus. Exchange of Water Between Compartments of the Hydrologic Cycle Evapotranspiration Evaporation is the conversion of liquid or solid water into a vapor. It is the process by which water is transferred from a surface-water body or land surface to the atmosphere. Evaporation that occurs through the stoma of plants is called transpiration. In a typical terrestrial setting, it is difficult to measure plant transpiration separately from evaporation from bare soil or water bodies. Therefore, it is common for these two processes to be lumped into a single term—evapotranspiration. Evapotranspiration, when averaged over one-year periods, is usually second in magnitude among water-budget components to precipitation, representing about 65 percent of precipitation that falls on global landmasses (about 540 mm/yr, table 2). Evapotranspiration may not vary spatially as much as precipitation, but accurate estimates of evapotranspiration are generally more difficult to obtain. There is no national network of evapotranspiration monitoring sites within the United States such as exist for precipitation and streamflow. However, at select National Weather Service sites, pan evaporation (evaporation from 1.2-m-diameter Class A pan) is measured daily. As the common link between the water and energy budgets, evapotranspiration is dependent upon the availability of both water and energy. In arid regions, water availability is the major limitation on evapotranspiration rates. In humid regions, there is generally an excess of water relative to available energy, so rates are energy limited. Evapotranspiration rates generally follow a trend similar to that of net radiation: highest in the summer and lowest in winter. The importance of energy on evapotranspiration rates is also apparent on a daily time EXPLANATION Potential evapotranspiration (PET), in millimeters per year 0–564 565–709 710–877 878–1,074 1,075–1,662 Figure 18 . Annual potential evapotranspiration rates across the conterminous United States as calculated with the Hamon equation. 41 scale. Rates are essentially 0 during night hours when no solar radiation is arriving at the site and highest during the daylight hours of peak net radiation. Potential evapotranspiration is the evapotranspiration that would occur if water were plentiful. Figure 18 shows estimates of annual potential evapotranspiration rates for the conterminous United States as calculated with the Hamon method using average temperatures from 1961 to 1990 generated by the PRISM model (Daly and others, 1994) . Estimates of potential evapotranspiration are used in the planning and management of irrigation systems. The difference between precipitation and potential evapotranspiration (fig. 19) provides an index of areas where evapotranspiration is water limited (differences less than 0) and energy limited (differences greater than 0). Measurement of evapotranspiration rates at specific locations are made with lysimeters or micrometeorological techniques (Rosenberg and others, 1983). These latter techniques, which include eddy correlation, Bowen ratio/energy budget, and aerodynamic profile methods, measure or estimate the vertical flux of water vapor from land surface to the atmosphere. These methods may provide accurate estimates of evapotranspiration rates, but lysimeters and micrometeorological instrumentation are expensive and delicate and require frequent maintenance. Climatological methods (Rosenberg and others, 1983) provide estimates of potential evapotranspiration. Although not as sophisticated as the micrometeorological techniques, they are much easier to apply, usually requiring only data that are available from National Weather Service stations (for example, daily temperature, relative humidity, or solar radiation). Included in this class are the Thornthwaite, EXPLANATION Precipitation-PET, in millimeters per year –1,600–0 1–318 319–868 869–1,838 1,839–6,656 Figure 19. Difference between annual precipitation and potential evapotranspiration rates across the conterminous United States. 42 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Jensen-Haise, Hamon, and Penman-Monteith methods. Actual evapotranspiration can be estimated from potential rates by application of a correction factor. In the agricultural literature, this factor is referred to as the crop coefficient. The crop coefficient is a related to crop type and maturity and climate; values for different crops and some native vegetation can be found in Jensen and others (1990). Obtaining estimates of actual evapotranspiration from pan evaporation rates requires application of a second correction factor called a pan coefficient (Doorenbos and Pruitt, 1975) as well as a crop coefficient. Other techniques for estimating evapotranspiration deserve mention even though they are not yet as widely used. Light detection and ranging (LIDAR) is a ground-based system capable of making accurate measurements of sensible and latent heat fluxes over areas as large as several hectares. The expense of the LIDAR equipment limits its use to select research studies. Satellite and aerial remote sensing offers no direct method of measuring evapotranspiration. However, progress has been made in correlating point micrometeorological measurements with variables that can be mapped from space, such as vegetation type and cover (Liu and others, 2003), soil moisture, and surface temperature (Quattrochi and Luvall, 1999). Models incorporating these variables can be used to generate regional evapotranspiration estimates from the point measurements. Tower to measure evapotranspiration in a forest. Instrumentation to measure evaporation from a lake. Instruments used in eddy correlation measurements of evapotranspiration. Exchange of Water Between Compartments of the Hydrologic Cycle 43 Exchange of Surface Water and Ground Water Streams and ground-water bodies exchange water in all types of hydrologic settings. Streams gain water from inflow of ground water through the streambed and banks (gaining stream, fig. 20A); they lose water to ground water by outflow through streambed and banks (losing stream, fig. 21A). A stream can be gaining in some reaches and losing in other reaches. For ground water to discharge into a stream channel, the altitude of the water table in the vicinity of the stream must be higher than the altitude of the stream-water surface. Conversely, for surface water to seep to ground water, the altitude of the water table in the vicinity of the stream must be lower than the altitude of the stream-water surface. Contours of water-table elevation indicate gaining streams by pointing in an upstream direction (fig. 20B) and losing streams by pointing in a downstream direction (fig. 21B). Losing streams can be connected to an aquifer by a continuous saturated zone (fig. 21A) or separated from it by an unsaturated zone (fig. 22). Ephemeral streams flow only in response to snowmelt and storms. Generally, ephemeral streams are not directly connected to an aquifer. However, GAINING STREAM A Big Spring, Missouri. LOSING STREAM A Flow direction Unsaturated Flow direction zone Unsaturated zone Water table Water table Shallow aquifer B B 70 60 100 60 e contour Water-tabl Ground-water flow line 50 50 r-ta Wate tour ble con 90 40 80 40 Ground-water flow line Stream Stream 30 30 70 20 20 Figure 20. Gaining streams receive water from the groundwater system (A). This can be determined from water-table contour maps because the contour lines point in the upstream direction where they cross the stream (B). Figure 21. Losing streams lose water to the ground-water system (A). This can be determined from water-table contour maps because the contour lines point in the downstream direction where they cross the stream (B). 44 Water Budgets: Foundations for Effective Water-Resources and Environmental Management DISCONNECTED STREAM Flow direction Unsaturated zone Water table Figure 22. Disconnected streams are separated from the water table by an unsaturated zone. during periods of flow, these losing streams can be important sources of ground-water recharge. In many stream settings, surface water flows through short segments of the streambed and banks and back into the stream. These segments, which may exist along the entire reach of a stream, constitute the hyporheic zone (figs. 23 and 24). Ground water and surface water mix within the hyporheic zone, providing a unique environment for important biological and chemical reactions. The size and geometry of hyporheic zones surrounding streams vary in time and space. Streams A Pool and riffle stream Flow in hyporheic zone flowing over sand and gravel may have hyporheic zones up to 2 m thick. Bank storage, the temporary storage of stream water in the subsurface (fig. 25), occurs when stream stage rises as a result of precipitation, snowmelt, or release of water from a reservoir upstream. As long as the rise in stage does not overtop streambanks, most stored water returns to the stream a few days or weeks after the stage returns to normal level. Bank storage tends to reduce flood peaks and supplement streamflow when stage recedes. If the rise in stream stage is sufficient to overtop the banks and flood large areas of the land surface, widespread recharge to the water table can take place. In this case, the time it takes for the recharged floodwater to return to the stream by ground-water flow may be weeks, months, or even years. Many stream-aquifer systems are in continuous readjustment from exchanges of water related to bank storage and overbank flooding. Lakes and wetlands interact with ground water in a manner similar to that of streams. There are some differences, though. Evaporation is generally a larger component of the water budget for a lake or wetland than for a stream. Bank storage is usually of minor importance because water levels do not fluctuate as much as they do in streams. Important exceptions are surface-water reservoirs in arid and semiarid regions. B Flow in hyporheic zone Meandering stream Figure 23. Surface-water exchange with ground water in the hyporheic zone is associated with abrupt changes in streambed slope (A) and with stream meanders (B). Stream Water ta ble Stream of n ter tio wa c re dDi un low ro f g H Interface of local and regional ground-water flow systems, hyporheic zone, and stream y po rheic zo n e Direction of ground-water flow Figure 24. Streambeds and banks are unique environments because they are where ground water that drains much of the subsurface of landscapes interacts with surface water that drains much of the surface of landscapes. Exchange of Water Between Compartments of the Hydrologic Cycle 45 BANK STORAGE Flow direction Water table at high stage High stage Bank storage Water table during base flow Figure 25 . If stream levels rise higher than adjacent groundwater levels, stream water moves into the streambanks as bank storage. Reservoir stage can change substantially over the course of a year, rising as spring rain or snowmelt fills the reservoir and falling through summer and fall as water is distributed to users. Wetlands may be present in many different parts of the landscape, whereas lakes and streams occupy local topographic low regions. Ground water contributes to many lakes, wetlands, and streams. Ground-water discharge can account for more than 90 percent of total annual streamflow (table 4). These contributions can be critical to the maintenance of diverse ecosystems. Wetlands and riparian zones provide wildlife habitat, mitigate floods, and process nutrients and contaminants; the existence of these areas may depend on a steady discharge of ground water. Understanding the water budget for these areas can aid in assessing how changes to one water-budget component will affect other components. For example, an understanding of the water budget could help determine if diversion of ground water to a domestic water-supply well will reduce the rate of ground-water discharge to a wetland and, if so, what effect that would have on plants and wildlife. Table 4. Base flow as a percentage of total streamflow for selected streams across the United States (Winter and others, 1998). Stream State Percentage of ground-water contribution Dismal River Nebraska North Dakota 13 Sturgeon River Michigan Techniques are available for estimating exchange rates between surface and ground waters over various space and time scales. Seepage meters provide point measurements over an area of about 1 m2 for periods of seconds to days. Discharge measurements can be made at different locations along a reach of stream; the difference in discharge between any two points will be equal to the net stream loss or gain along that reach. At the watershed scale, hydrograph separation methods and streamflow duration curves can be used to estimate base flow in gaining streams. Solute- and energy-budget approaches have been used over a variety of scales to estimate exchange rates of ground water with lakes and streams. 94 Forest River Stream disappearing into sinkhole in karst terrain in Texas. 90 Ammonoosuc River New Hampshire 61 Brushy Creek Georgia 68 Homochitto River Mississippi 36 Dry Frio River Texas 58 Santa Cruz River Arizona 35 Orestimba Creek California 23 Duckabush River Washington 65 Seepage meters are used to measure the exchange of water between surface water and the subsurface. 46 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Water-Budget Studies Space and time scales associated with water-budget studies largely determine the appropriateness of methods for estimating fluxes and changes in storage. Different methods are applicable over different scales. Some techniques provide estimates at a single point in space, such as the use of standard rain gages. Because rainfall rates vary with location, multiple gages may be needed to determine an average rate for a watershed. Other techniques provide estimates that are integrated over large areas (for example, a stream-discharge measurement provides an estimate of runoff for the entire area that drains to the measurement point). Time scales are also important. For estimating ground-water recharge, the water-table fluctuation method provides an estimate for each recharge event, of which there could be many during a year. Groundwater age-dating techniques, on the other hand, provide a single estimate of recharge that is averaged over several years or decades. Prudence dictates that the time and space scales of measurement and estimation methods match the needs of the water-budget study at hand. Four intensive water-budget studies are presented in this section to illustrate the different approaches and exacting procedures that have been applied in studies of water budgets. Detailed studies such as these are not commonly undertaken, mostly because of monetary and time constraints. The examples convey the level of complexity inherent in conducting such studies and show that results from even the most detailed studies of water budgets in natural hydrologic systems contain some uncertainty. This uncertainty arises from the natural variability in hydrology, geology, climate, and land use and inaccuracies in the techniques used to collect and interpret data. Water Budget for a Small Watershed: Beaverdam Creek Basin, Maryland The Beaverdam Creek basin is situated on the Atlantic Coastal Plain in the Delmarva Peninsula of Maryland (fig. 26). The water budget of the basin was studied for 2 years in the early 1950s (Rasmussen and Andreasen, 1959) to determine the apportionment of precipitation among ground-water recharge, subsurface runoff to ponds and streams, groundwater evapotranspiration, and ground-water storage. The 19.5-mi2 drainage basin ranges in elevation from 10 to 85 ft above sea level and receives on average 43 inches of precipitation annually. In the subsurface, Quaternary-age surficial sands and silts, as much as 70 ft thick, overlie aquifers of Tertiaryage sand. The water table is generally within 12 ft of land surface. The study area was in a natural setting, largely unaffected by human activity. Understanding such a natural hydrologic system is fundamental to evaluating human influences on this and other systems. The amount and kinds of data collected are unusual for a small watershed. Ground-water levels were measured weekly in 25 observation wells. Stream discharge was monitored Typical stream in Delmarva Peninsula, Maryland. by means of a sharp-crested weir at the outlet of the basin. Changes in water storage in the two ponds within the basin were calculated from stage readings and a table (developed by bathymetric survey) that related pond volume to stage. Twelve precipitation gages were deployed across the basin and monitored on a weekly basis. A 4-ft Class A evaporation pan was part of a weather station that also included an anemometer, barometer, wet- and dry-bulb thermometers, and a thermistor for measuring soil temperature. Soil moisture content was determined weekly by electrical resistance (individually calibrated Bouyoucos blocks) at depths of 4, 12, and 39 inches at three locations within the basin. Major components of the water budget are shown in figure 27. Precipitation for the 2-year period totaled 83 inches. Sixty percent, or 50 inches, was returned to the atmosphere through evaptranspiration; 31 inches (37 percent of precipitation) left the basin as streamflow. Two inches remained in the basin, augmenting surface and subsurface storage. The quasilinear nature of the precipitation curve in figure 27 indicates relatively uniform precipitation rates throughout the study Beaverdam Creek, Maryland Figure 26. Location map for Beaverdam Creek basin. Exchange of Water Between Compartments of the Hydrologic Cycle WATER, IN INCHES 80 60 47 Precipitation Evapotranspiration Streamflow Change in storage Surface water Ground water 40 20 0 April 1950 September 1950 March 1951 September 1951 March 1952 Figure 27. Water budget for Beaverdam Creek Basin, April 1950 to March 1952. period. There is no indication of the seasonality in precipitation that is common in many regions. Evapotranspiration, on the other hand, shows a distinct seasonal trend, with highest values occurring in summer months and negligible rates for most winter months. Evapotranspiration was not measured directly. All other water-budget components were measured or estimated independently. Evapotranspiration was then determined to balance the water-budget equation. Ground-water recharge, the only source of inflow to the shallow ground-water system, was calculated by application of the water-table fluctuation method, using weekly average water levels from the 25 observation wells. Recharge was then partitioned into its different components: where: R ∆Sgw Qbf is recharge, is change in ground-water storage, is base flow, ETgw is evapotranspiration from ground water. Change in ground-water storage was calculated from the difference in head between the end and the beginning of the week. Base flow was determined by a stream hydrograph separation method. Evapotranspiration from ground water was then calculated as the residual of the equation. Figure 28 shows plots of calculated recharge components. For the 2-year period, ground-water recharge was 42.6 inches and was partitioned into 21.5 inches of base flow, 1.7 inches of increase in ground-water storage, and 19.5 inches of evapotranspiration of ground water. In summer months, evapotranspiration is the largest draw on ground water. For the rest of the year, base flow is the predominant mechanism of ground-water discharge. Few studies before the Beaverdam Creek watershed study devoted as much effort to the comprehensive examination of the water budget of a small watershed. Measurements of water-budget components can be made more easily and more accurately with the improved instrumentation that has been developed over the decades since the original study was conducted. Even so, conducting a similar study today would require a substantial commitment of funding and manpower. 50 40 WATER, IN INCHES and R = ∆Sgw + Qbf + ETgw Farm pond, Delmarva Peninsula. Recharge Base flow Evapotranspiration from ground water Storage change 30 20 10 0 –10 April 1950 Agriculture is the main industry in the Delmarva Peninsula. September 1950 March 1951 September 1951 March 1952 Figure 28. Ground-water budget for Beaverdam Creek Basin, April 1950 to March 1952. 48 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Soil-Water Budgets for Prairie and Farmed Systems in Wisconsin Water budgets of soil zones are important for management of agricultural fields. They also are used to estimate rates of evapotranspiration and ground-water recharge. Agricultural practices can greatly affect soil-water budgets. Irrigation and cultivation techniques (conventional, minimum, or no tillage) can influence surface runoff, erosion, infiltration, and transport of applied agricultural chemicals. Brye and others (2000) determined water budgets for a 132-week period in 1995–98 at three sites in Columbia County in southern Wisconsin: a restored natural prairie, maize under no-tillage, and maize with chisel-plow tillage (fig. 29). The objectives of the study were to evaluate the usefulness of newly designed instrumentation and to assess the effects of agricultural practices on drainage beneath the root zone. Columbia County, Wisconsin Figure 29. Location map of Columbia County, Wisconsin. ET = P – RO – D – ∆S where: P RO D K.R. Brye and is precipitation, is surface runoff, is drainage below the root zone, ∆S is change in storage (soil and surface). Runoff only occurred immediately after large precipitation events or snowmelt and was estimated by a procedure described by Brye and others (2000). Calculations were made on a weekly basis. Agricultural site. K.R. Brye Precipitation was measured with rain and snow gages. Soil-moisture content profiles were measured weekly during the growing season and at 3-week intervals during winter with a neutron moisture meter. Readings were taken to a depth of 1.4 m in four access holes in each field. Water movement through the unsaturated zone is very difficult to measure directly. In this study, drainage beneath the root zone was measured with equilibrium tension lysimeters (ETLs) (Brye and others, 1999). These novel devices have a porous stainless steel surface that allows collection and measurement of drainage. Soil-matric potential sensors and a vacuum system allowed the tension within the ETL to be set slightly greater than that recorded in the bulk soil surrounding the lysimeter. Thus, flow into the ETLs should be similar to natural drainage rates. The ETLs were installed through a 2-m-deep trench; the 75-cm by 25-cm top surface was set at a depth of 1.4 m. Evapotranspiration (ET) was equal to the differences in inputs and outputs and storage changes: Prairie site. 49 K.R. Brye K.R. Brye Exchange of Water Between Compartments of the Hydrologic Cycle Equilibrium tension lysimeter. The lysimeter requires careful installation. Water budgets for the three sites are shown in figure 30. Precipitation rates were similar for the three sites. The prairie site had greater soil-moisture contents, more evapotranspiration, and less drainage compared to the maize fields at the other two sites. Evapotranspiration rates for the prairie site were slightly greater than those from the no-tillage site, which were slightly greater than those from the chisel-plow site. Drainage occurred from late January to mid-June at all sites. However, drainage totals for the 132-week period were substantially different: 199 mm for the prairie, 563 mm for no-tillage, and 793 for chisel-plow. It appears that infiltration A B Goose Pond Prairie rates increased with increasing disturbance of the land surface. Runoff, because of its episodic nature, is not included in figure 30. For the study period, runoff totaled 197 mm at the Prairie site, 182 mm at the no-tillage site, and –5 mm (due to drifting snow) at the chisel-plow site. Rates of infiltration, drainage, and evapotranspiration are important in terms of plant health and ground-water quantity and quality. It is difficult to accurately determine the effects of different agricultural practices on local water budgets, but new and improved instrumentation, such as that used in this study, can provide valuable insight into complex processes. C No-tillage maize Chisel-plow maize 1,800 CUMULATIVE WATER-BUDGET COMPONENT IN MILLIMETERS Precipitation Winter surface-water storage 1,575 Soil-water storage changes 1,350 Drainage Evapotranspiration 1,125 900 675 450 225 0 –225 26 35 45 1995 3 13 23 33 43 1 1996 WEEK 11 21 31 41 49 1 1997 1998 26 35 45 1995 3 13 23 33 43 1 1996 WEEK 11 21 31 41 49 1 1997 1998 26 35 45 1995 3 13 23 33 43 1 1996 11 21 31 41 49 1 1997 WEEK Figure 30. Water budget for (A) prairie site; (B) no-tillage maize site; and (C) chisel-plow maize site in central Wisconsin (Brye and others, 2000). Imbalance in the water budgets is attributed to runoff for the prairie and no-tillage site and to runon from melting snow at the chisel-plow site. 1998 50 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Water Budget of Mirror Lake, New Hampshire The hydrology and chemistry of Mirror Lake (fig. 31) have been studied since the late 1970s to develop an understanding of the hydrological processes associated with the lake and to examine uncertainties in estimates of water and chemical budgets for the lake (Winter, 1984). To accomplish this, all components of the water budget were determined independently, either by direct measurement or by calculation from measurements of environmental variables. Precipitation was measured at two gages within 400 m of the east and west shores of the lake. Evaporation was calculated by using an energy-budget method with data from meteorological instruments located on a raft in the lake and a land station near the lake. Stream inflow and outflow were measured by using Parshall flumes and weirs equipped with stage recorders. Ground-water inflow and outflow were calculated by using Darcy’s equation with water levels in the lake and in numerous wells near the lake together with measured hydraulic conductivity. Water storage in the lake was estimated from measured lake stage and a stage-volume relation. Monthly and annual water and chemical budgets were determined for Mirror Lake for the 20-year period from 1981 to 2000. Streams provided the largest inflow of water to the lake during this period; seepage to ground water was the largest loss of water from the lake (table 5). The largest uncertainty associated with the water budget is in the ground-water Mirror Lake, New Hampshire Mirror Lake, New Hampshire. fluxes and is related to the complexity of the geologic deposits and the associated variability in hydraulic conductivity. For initial calculations of ground-water fluxes, hydraulic conductivity was determined by single-well aquifer tests, which test only small volumes of the aquifer in close proximity to the wells. The rate of ground-water inflow to the lake was estimated to be 47,000 m3/yr. A second estimate of ground-water inflow was generated by using data on oxygen isotopes (J.W. LaBaugh, U.S. Geological Survey, oral commun., 2006). The ratio of oxygen-18 to oxygen-16 in water can be used to estimate the rate of ground-water discharge to the lake. The isotopic ratio of lake water must be balanced by the ratios in incoming water and the change in the ratio that takes place as a result of evaporation. The rate of ground-water inflow to the lake can be computed by using measured isotopic Table 5. Initial and final water budgets for Mirror Lake in New Hampshire. Values are in 1,000 cubic meters per year. Initial Final Inflows Precipitation 182 182 Surface-water inflow 417 417 Ground-water inflow 47 113 77 77 Surface-water outflow 257 257 Ground-water outflow 281 347 16 16 15 15 Outflows Evapotranspiration Figure 31. Location map of Mirror Lake, New Hampshire. Lake volume change Imbalance Exchange of Water Between Compartments of the Hydrologic Cycle Weather station. ratios of ground water, precipitation, streamflow, and the lake itself, along with independently measured rates of precipitation, streamflow, and evaporation. This approach yielded an estimate of 95,000 m3/yr of ground-water inflow to Mirror Lake, about twice the value originally calculated. A third estimate of ground-water inflow was generated from a separate study of the ground-water basin within which Mirror Lake lies (Tiedeman and others, 1997). As part of this study, a computer model of ground-water movement was constructed, producing a water budget for the ground-water basin (fig. 32); therefore, water budgets were available for two distinct accounting units. One accounting unit, Mirror Lake, is actually nested within another accounting unit, the groundwater basin in which the lake resides. A common component in the two water budgets is inflow to the lake. The mutual benefit of these concurrent studies was that data from one 1,000 CUBIC METERS PER YEAR 200 Inflows could be used to refine the other. For example, the groundwater-flow model was calibrated by using stream discharge and water levels in wells that were part of the lake study. Conversely, ground-water flow to and from the lake could be calculated from the ground-water-flow model. The model results indicated that 133,000 m3/yr were discharged to Mirror Lake, a value 2.8 times greater than the initial calculation. The lack of agreement among the three approaches used to estimate ground-water inflow to the lake clearly illustrates some of the uncertainties in water-budget studies. Which estimate of ground-water inflow is correct? Researchers at the site ultimately settled on a rate of 113,000 m3/yr. To balance the final water budget for the lake (table 5), ground-water outflow from the lake was adjusted to compensate for the change in inflow; estimates of precipitation, stream inflow and outflow, and evaporation measurements were all thought to be acceptable. Results from the Mirror Lake study demonstrate the value of applying multiple approaches to quantify water-budget components. However, it is clear that all water budgets, including the most detailed budgets, contain some degree of uncertainty related to measurement inaccuracies and to our limited ability to make measurements in sufficient spatial and temporal detail. Methods used to calculate the water budget of Mirror Lake were state-of-the-art and the most accurate available. Estimated uncertainties are 5 to 10 percent for precipitation, 10 to 15 percent for evaporation, 5 to 10 percent for streamflow into and out of the lake, and 30 to 50 percent for ground-water inflow and outflow (T.C. Winter, U.S. Geological Survey, and G.E. Likens, Institute of Ecosystem Studies, oral commun., 2006). The overall uncertainty in the water budget is considered to be about 13 percent. Outflows 150 100 50 Pr Pr ec ip ita ec tio ip ita n to tio be n to dr oc gl ac St k ia re ld am ep st os o its gl ac ia ld Gl ep ac os ia its ld ep Gl os ac its ia ld to ep st os re La am its ke s to se M di irr m or en La ts ke to M Be irr or dr oc La kt ke o M irr or La ke 0 Figure 32. Model calculated water budget for the Mirror Lake ground-water basin from Tiedeman and others (1997). Error bars indicate approximate 95-percent confidence interval. 51 Surface-water outflow from Mirror Lake was measured with a combined weir and flume. 52 Water Budgets: Foundations for Effective Water-Resources and Environmental Management The waste disposal site near Sheffield, Illinois, was in operation from 1967 to 1978. Water Budget at a Waste Disposal Site in Illinois A water budget was developed for a radioactive-waste disposal site in Bureau County in northwestern Illinois (fig. 33) for the period of July 1982 through June 1984 (Healy and others, 1989b). The 8-ha site is situated in complexly layered glacial and eolian sediments that range from 10 to 30 m in thickness and overlie a thick sequence (140 m) of lowpermeability shales. The disposal site was in operation from 1967 to 1978. During that time, waste was placed in shallow trenches that were subsequently covered with compacted clay covers. The main objective of the study was to estimate rates of water percolation through the trench covers, an issue of concern for regulatory agencies seeking to minimize potential for ground-water contamination. A secondary objective was to assess the utility of water-budget methods for estimating those rates. Precipitation was measured at three locations (two tipping bucket and one weighing gage). Evapotranspiration rates were estimated on an hourly basis by using a combination Bowen-ratio/aerodynamic profile method. Data collected from the onsite weather station included net radiation, shortwave and longwave radiation, wet- and dry-bulb air temperatures at three heights, soil temperatures and heat flux, and windspeed and direction (Healy and others, 1989a). Flumes and weirs were used to measure runoff in the ephemeral streams that drained the site. Change in water storage in trench covers was measured with nuclear moisture probes at weekly intervals at three locations. At those same locations, percolation through trench covers (which was considered equivalent to recharge) was estimated with the Darcy method by using pressure heads measured with vertical clusters of tensiometers. Tensiometer readings were obtained electronically with pressure transducers and data recorders at intervals ranging from 5 to 60 minutes. Bureau County, Illinois Figure 33. Location map of Bureau County, Illinois. Data from onsite weather station were used to estimate evapotranspiration. Exchange of Water Between Compartments of the Hydrologic Cycle A 1,000 Precipitation Evapotranspiration Runoff Change in storage Recharge WATER, IN MILLIMETERS 800 600 400 200 0 B 1,000 800 WATER, IN MILLIMETERS Results of this study illustrate the day-to-day, season-to-season, and year-to-year variability in individual water-budget components. They also demonstrate the interdependence among all components. Similar seasonal trends are apparent between the 2 years, but there are some distinct differences (fig. 34). Precipitation totals for the 2 years were similar. Flumes and weirs were used to The average of 948 mm measure runoff. is close to the long-term average of 890 mm; however, monthly values varied between years. July of the first year was quite wet, but July in the second year saw very little rain. May and June had little rain in the first year but were extremely wet the second year. This variability in precipitation affected other waterbudget components, especially runoff and recharge. Runoff was episodic, occurring only in response to large precipitation events, and was more likely to occur if the soil-moisture contents were high. About one-half of the average annual recharge of 208 mm occurred during the months of March and April. Recharge was 54 percent of precipitation for those 2 months in the first year and 55 percent the second year. Other months did not display such consistency: July had recharge of 28 mm in the first year and 0 mm in the second, and May and June had 1 mm in the first year and 64 mm in the second. Evapotranspiration showed consistent seasonal patterns over both years, but daily values during peak summer months were influenced by the availability of soil moisture. The residual error in the water budget, defined as the difference between precipitation and all other components of the water balance, was –81 mm/yr on average (table 6). Although the magnitude of this number is small relative to precipitation and evapotranspiration, it is large relative to all other waterbudget components. Therefore water-budget methods may be suitable for estimating evapotranspiration at this site but problematic for estimating other components of the water-budget equation. Given the variability in weather patterns, the 2-year study period may have been too short to adequately determine a long-term average water budget. 600 400 200 0 July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June Figure 34. Water-budget components for (A) July 1982 through June 1983 and (B) July 1983 through June 1984 for a site in northwestern Illinois (Healy and others, 1989b). Soil moisture content and pressure head were measured. Table 6. Annual values of water-budget components in millimeters for a site in northwestern Illinois (Healy and others, 1989b). Precipitation Evapotranspiration Runoff Storage change Percolation into trench Residual July 1982 – June 1983 927 606 206 –11 216 –90 July 1983 – June 1984 969 667 113 60 201 –72 2-Year average 948 637 160 24 208 –81 Year 53 Water Budgets: Foundations for Effective Water-Resources and Environmental Management H Uncertainty in Water-Budget Calculations All water-budget calculations contain some uncertainty. There are two general sources of this uncertainty: natural variability of the hydrologic cycle and errors associated with measurement techniques. Natural variability occurs in all aspects of the hydrologic cycle. Precipitation patterns are affected by altitude; evapotranspiration and runoff are affected by soil properties, vegetation type and density, surface slope and aspect, depth to ground water, and other factors. Temporal variability in storage and fluxes is largely tied to diurnal, seasonal, and long-term trends in weather. On a daily basis, the pattern of solar radiation generally limits evapotranspiration to daylight hours. Evapotranspiration also is affected by seasonal trends in solar radiation; rates are low during winter months when solar radiation is low, and rates are high during summer months. Seasonal patterns in precipitation exist in many regions. Perhaps the most extreme An instrumented field in north-central Oklahoma. example is in South Asia. In Mumbai, India, storms during the monsoon season of June through September account for 94 percent of the average annual precipitation of 180 cm (BBC Weather, http://www.bbc.co.uk/weather, accessed on February 12, 2007). Long-term climate change has a large effect on the hydrologic cycle. Ground water from the middle Rio Grande aquifer is as old as 30,000 years (Plummer and others, 2004), indicating that some recharge to the aquifer occurred when the Southwestern United States was experiencing a much wetter climate. The water-budget equation commonly is used to estimate rates of evapotranspiration or ground-water recharge. A simple analysis of this approach illustrates the importance of considering measurement errors. In this approach, all but one of the water-budget components are measured or estimated independently. The remaining component is assumed equal to the residual of the equation. Consider, for example, an arid region with coarse-grained soils. Typically, there is no surface runoff; precipitation infiltrates the subsurface and is either removed by evapotranspiration or percolates through the unsaturated zone to recharge the underlying aquifer. An appropriate water-budget equation would be: U.S. Department of Energy Atmospheric Radiation Measurement Program 54 Precipitation = Evapotranspiration + Recharge Multiple gages reduce the inaccuracy of precipitation estimates. (H1) Suppose an estimate of evapotranspiration is needed. For one year, precipitation was measured at 25 cm and recharge was measured at 3 cm. An evapotranspiration estimate of 22 cm is then derived. If the recharge estimate were in error by 10 percent (recharge was actually 3.3 cm), the uncertainty in the evapotranspiration estimate would be small, less than 2 percent. Even if the recharge estimate were in error by 100 percent (recharge was actually 6 cm and evapotranspiration was 19 cm), the evapotranspiration uncertainty would be less than 15 percent. Now suppose, instead, that we are interested in estimating recharge as the difference between precipitation and evapotranspiration. If evapotranspiration were independently measured at 24 cm, a recharge estimate of 1 cm would be derived. If measurement uncertainty was 10 percent for evapotranspiration, recharge for that year may have been as high as 3.4 cm, a 240-percent difference from the original estimate. As can be seen from this example, if the magnitude of the water-budget residual is small compared to those of the other components, then small uncertainties in other components can result in very large uncertainties in the residual. Humans and the Hydrologic Cycle 55 Humans and the Hydrologic Cycle Bureau of Reclamation By our very existence, humans, along with all other animals and plants, are a part of the Earth’s hydrologic cycle. Therefore, any human activity affects the natural hydrologic cycle. Drinking a glass of water, taking a bath, washing the car—these activities involve a small amount of water, yet they alter the course of that water within its cycle, though perhaps only in minor ways. The activities of humans that affect the hydrologic cycle can be grouped into three overlapping categories: construction of water storage and conveyance structures, land use, and extraction of ground water. These activities are reflected in the hydrologic cycle as a redistribution of water within the atmosphere, land surface, and subsurface, in changes in rates of water flow within and among these compartments, and in a relocation of points of water inflow to and outflow from them. Alterations to the hydrologic cycle may, in turn, lead to changes in natural environments, such as creation of new habitat for fish or loss of wetlands. The following three sections provide a brief overview of the effects humans can have on the hydrologic cycle. Bureau of Reclamation Glen Canyon Dam and Lake Powell. Canal and irrigated fields. Water Storage and Conveyance Structures Surface-water reservoirs serve many beneficial purposes, providing water for irrigation, domestic use, navigation, hydroelectric power, and recreation. Dams and the reservoirs they create alter the natural movement of water in streams, reducing the number of extreme events, such as floods, and possibly changing stream temperatures (Collier and others, 1996). These alterations may affect downstream ecosystems, benefiting some species but stressing others. Reservoirs create whole new ecosystems, often providing valuable fish habitat. They generally lead to an increase in evapotranspiration and the flow of surface water to the subsurface and to a decrease in total streamflow relative to natural conditions. Reservoirs, along with an infrastructure of pipelines, canals, and ditches, facilitate the transport of water between watersheds. For some basins, this artificial export or import may be the single largest component of its water budget. In Colorado approximately 475,000 acre-ft of water from the Colorado River basin is transferred eastward across the Continental Divide each year for agricultural and domestic use (http://www.water.denver.co.gov/). This water would have originally flowed to the Pacific Ocean; it is now diverted to the Gulf of Mexico. Canals and ditches and other conduits for transporting water provide the opportunity for exchange of water with the subsurface and the atmosphere. Other conveyance structures are designed to remove rather than supply water. The Corn Belt of the United States, running through Indiana, Illinois, and Iowa, boasts some of the most productive agricultural land in the world. Yet less than 200 years ago, much of this fertile farmland was natural wetland. Since the early 1800s, farmers have installed tiles and dug ditches to facilitate drainage of the wetlands. Tile drains effectively lower the water table to about 1 meter below land surface, thus providing an adequate environment for crops to grow. Tile drains are used to lower the water table in some agricultural fields. Image provided courtesy Rain Bird Corp. Water Budgets: Foundations for Effective Water-Resources and Environmental Management Terra Photographics, Bruce F. Molnia 56 Urban development alters water’s course through the hydrologic cycle. Land Use Ground water being pumped for irrigation of rice field. Satellite image of irrigated fields in western Kansas. Natural Resources Conservation Service National Aeronautics and Space Administration Land use is perhaps the most important phenomenon affecting water exchange between land surface and the atmosphere. Conversion of native forests, grasslands, and wetlands to agricultural uses constitutes the largest land-use change (in terms of area, at least) in the United States and most other countries. Replacement of native vegetation with agricultural crops leads to changes in patterns of infiltration, evapotranspiration, and ground-water recharge. Irrigation of crops in arid regions has produced an inflow of water to the atmosphere through evapotranspiration that was absent under natural conditions. Clearcutting of rain forests has reduced evapotranspiration rates in large areas of the Amazon River basin. Because of the importance of this region in global circulation patterns, potential climatic effects could extend beyond South America. Urbanization accounts for the second largest change in land use within the United States. Urban features such as buildings, roads, and parking lots are all impermeable. Thus, they tend to enhance surface runoff of precipitation and reduce infiltration. Runoff from these features may be channeled through storm sewers to streams, leading to increased streamflow and flooding in the worst situations. That runoff also could be funneled to an infiltration gallery leading to an increase in ground-water recharge. Water in urban areas is conveyed to and from users in networks of underground pipes. Invariably, there is some leakage from these pipes. That leakage can be a substantial source of ground-water recharge. Trickle irrigation conserves water. Humans and the Hydrologic Cycle 57 Throughout history, humans relied primarily upon surface water to satisfy their needs for water. Storage reservoirs were constructed, streams were diverted, and canals were built to convey the water to the areas of need, usually agricultural fields or urban areas. Over the past 200 years, humans have become more reliant on ground water to supply their needs. Extraction of ground water, whether for domestic, agricultural, or industrial uses, is balanced by a reduction in ground-water storage, a reduction in natural discharge, or an increase in recharge. For any particular aquifer, all of these phenomena can occur simultaneously, but change in storage (indicated by changing ground-water levels) is usually more easily determined than changes in discharge or recharge. Many aquifers within the United States have experienced widespread declines in ground-water levels over the last several decades. Declining water levels indicate a reduction in subsurface water storage, and they may result in reduced ground-water flow to wetlands and streams. Streams that normally gain water from the subsurface could be transformed into losing streams. Effects such as these can sometimes be seen instantaneously—for example, a stream drying up when a well pump is turned on. More commonly, the effects are prolonged in time and difficult to quantify. Similarly, the effects of reduced ground-water discharge on stream and wetland ecosystems may become apparent only over extended periods of time. Natural Resources Conservation Service Ground-Water Extraction Development of a new irrigation well in west-centrl Florida triggered hundreds of sinkholes over a 20-acre area. (See person in center for scale.) Most ground water that is extracted for irrigation is evapotranspired back to the atmosphere shortly after it is applied to the land surface. However, a percentage of irrigation water, called irrigation excess or return flow, may run off to a stream or may infiltrate, percolate through the unsaturated zone, and eventually become ground water again. Most ground water removed from the subsurface for domestic use eventually ends up returning to the saturated zone through septic leach fields or is discharged to surface-water bodies from wastewater-treatment plants. A 1942 photograph of a reach of the Santa Cruz River south of Tucson, Arizona, shows stands of mesquite and cottonwood trees along the river (left photograph, Arizona Game and Fish Department). A replicate photograph of the same site in 1989 shows that riparian vegetation has largely disappeared (right photograph, Robert H. Webb). Data from two nearby wells indicate that the water table has declined more than 30 meters due to pumping; this pumping appears to be the principal reason for the loss of vegetation. 58 Water Budgets: Foundations for Effective Water-Resources and Environmental Management I Water Use and Availability Humans need water. But just how much water do we need? Every day in the United States 345 billion gallons on average is withdrawn from ground- and surface-water sources for human use (Hutson and others, 2004). This is equivalent to more than 1,000 gal/d for every person in the country—about 40 bathtubs full. We do not usually take that many baths, so how is this water used? The largest use (48 percent) is by thermoelectric power plants, for cooling and steam generation. Other uses, as shown in figure I–1, are for irrigation of agricultural lands, domestic needs, industry, mining, aquaculture, and livestock. Water satisfies a myriad of thirsts; a daily bath is but a few drops in the water-use bucket. Public Supply, 11 percent Public supply water intake, Bay County, Florida Alan M. Cussler, USGS Richard L. Marella, USGS Domestic, less than 1 percent Domestic well, Early County, Georgia Irrigation, 34 percent Gated-pipe flood irrigation, Fremont County, Wyoming Courtesy of Jeff Vanuga, USDA NRCS Courtesy of Jeff Vanuga, USDA NRCS Livestock, less than 1 percent Livestock watering, Rio Arriba County, New Mexico Aquaculture, less than 1 percent World’s largest trout farm, Buhl, Idaho Alan M. Cussler, USGS Courtesy of Clear Springs Foods, Inc. Industrial, 5 percent Paper mill, Savannah, Georgia Mining, less than 1 percent Spodumene pegmatite mine, Kings Mountain, North Carolina Alan M. Cussler, USGS Nancy L. Barber, USGS Thermoelectric Power, 48 percent Cooling towers, Burke County, Georgia Figure I-1. Percent of total water withdrawals for major categories within the United States (from Hutson and others, 2004). Box I Water Use and Availability Total water withdrawals in the United States have been stable since the mid-1980s. However, on a per capita basis, total withdrawals have decreased over the same period (fig. I–2). This is likely the result of improved techniques that require less water for power generation and advances in irrigation efficiency. Technologies such as low-flow bathroom fixtures and water-saving appliances in homes aid in conserving local water supplies. In New York, for example, water use has declined from about 200 gal/d per person in 1990 to less than 140 gal/d in 2003 (City of New York, Department of Environmental Protection, http://www.nyc.gov/html/dep/html/droughthist. html accessed on December 18, 2006). On the national scale, however, the savings realized in the home are minimal when compared to thermoelectric and agricultural use. Conservation and improved efficiency of water use may be driven by economic and water-quality issues as well as by water supply. Energy costs for pumping water continue to rise, and stricter water-quality standards for water discharges have been put in place at the Federal and State levels. Periods of drought may prompt water managers to impose limits on water use. Farmers, ranchers, manufacturers, energy providers, and individual consumers adapt their water use to technological advances, changing regulations, and market forces. 300 Ground water Surface water Total Population 350 300 250 200 250 150 200 150 POPULATION, IN MILLIONS WITHDRAWALS, IN BILLION GALLONS PER DAY 400 100 100 50 50 0 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Figure I-2. Freshwater withdrawal and population in the United States, 1950–2000 (from Hutson and others, 2004). “Compilation of water-use information on a regular basis provides information on the amount of water used by humans, yet there has been no corresponding assessment of water availability within the United States. In addition, the water needs of biota that inhabit waterways, wetlands, flood plains, and other environments are largely unknown” (National Science and Technology Council, 2004). Water budgets of watersheds, aquifers, and surface-water bodies are essential tools for assessing the availability of water for both human and environmental needs. It is useful, therefore, to look at the way water budgets are determined, the uncertainty inherent in those budgets, and the effect of human activity on water budgets. Episodes of drought and floods are reminders that water availability can change substantially over time. It is therefore important to consider the temporal variability in the movement and storage of water in the hydrologic cycle and the relation of that variability to water use. 59 Water Budgets: Foundations for Effective Water-Resources and Environmental Management Water Budgets of Political Units CU 4.7 SWI 1.45 SWO 11.6 Figure J–1. Average-annual water budget for Kansas, in million gallons per day, 1970s and early 1980s. Abbreviations: BRF, boundary-river flow; CU, consumptive use (evapotranspiration related to human activities, mostly irrigation); ET, evapotranspiration from native plants and nonirrigated agricultural fields; P, precipitation; SWI, surface-water inflow; SWO, surfacewater outflow (Carr and others, 1990). er av Blu e n Be Big Cr blica Stra ri k Cree Rive s ansa r Saline River K R M Hill ais ar River Cyg nes tle Lit River des ka Ar as ns River Pawnee Ark ansa ke sna ttle Ra Cr Ark ansa s Walnut R Cr is ok ed igr Cro rd Ve Riv er s r Rive R r Rive n ro ar The Missouri River flows in or along seven States. er r sa Smoky Riv Rive Wakaru Cim Figure J–2. Principal rivers, Kansas (Paulson and others, 1991). Natural Resources Conservation Service State boundaries cross tens, if not hundreds, of streams that flow from Colorado and Nebraska into Kansas and a similar number of streams that flow out of Kansas to Oklahoma and Missouri (fig. J–2). Only a small percentage of those streams have stream gages to monitor flow, and few of those gages are located at State lines. Thus, uncertainty in estimating total surface inflows and outflows for the State of Kansas may be quite high. Conspicuous by their absence in the above equation and in figure J–1 are ground-water inflow and outflow. These processes do occur. At the State level, those rates were deemed insignificant relative to other terms. At a local scale, however, groundwater flow into Kansas from Colorado in the High Plains aquifer (McGuire and others, 2003), for example, may be an important component in the water budget of some western Kansas counties. er mom sou er Riv nger Riv Solo Mis (J1) KANSAS Repu Precipitation + Surface-Water Inflow = Evapotranspiration + Surface-Water Outflow BRF 28 . 7 ET 91 Water in most areas is managed by governmental units, be it countries, States, counties, or water districts. Concerns of these entities include how much water they have, how much water they use, and how, when, and at what rate water supplies are replenished. Throughout this report, the uncertainty inherent in water-budget calculations is demonstrated. That uncertainty is compounded when boundaries of an accounting unit are not aligned with hydrologic boundaries. Such is often the case with governmental units where humans have defined political boundaries that may crisscross natural watershed boundaries and partition aquifers. These boundaries may also follow rivers, resulting in the rivers being shared by competing entities. Measurement of surface- and ground-water flow across political boundaries presents unique and sometimes contentious challenges. The average annual water budget for the State of Kansas is depicted in figure J–1 for a period from the 1970s into the 1980s. Average annual precipitation within the State is about 27 inches. Equating inflow and outflow, with the assumption that annual change in storage is relatively small, the water budget can be expressed as: P 106 J U.S. Army Corps of Engineers 60 The High Plains aquifer lies in parts of eight States. ...
View Full Document

This note was uploaded on 10/19/2011 for the course GLY 3882 taught by Professor Screaton during the Spring '09 term at University of Florida.

Ask a homework question - tutors are online