9
Inltration, Chapter Streamow, and Groundwater
A watershed is the area of land that contributes ow to a stream, lake, or river. Because water ows downhill, the watershed is the area that lies uphill of the waterbody of concern. A watershed can be small, i.e., the area of land that drains into your neighborhood stream. A watershed can also be large, such as the watershed of the Altamaha River, which is the largest in Georgia. The watershed divide is the boundary line that separates two watersheds - water on one side of the divide ows into one watershed, while water on the other side ows into another watershed. Can you draw the watershed divide for the Altamaha River on the drainage map of Georgia? We can divide the entire state into distinct geographic units, dened by their watershed boundaries. We can subdivide each of the larger watersheds into subwatersheds. For example, we can divide the Altamaha into three zones; the Oconee River watershed, the Ocmulgee River watershed, and the area downstream of where these two watersheds join. What hydrologic features do you notice within a watershed? There are obvious features such as rivers and streams, lakes, springs, seeps, and wetlands. These are surface water features. There are also a number of less obvious subsurface hydrologic features, related to ground-water. Ground water is important in many ways and, although not visible, must be considered when describing a watershed. stand the hydrology of a basin. An annual basin water budget denes how much rainfall evapotranspires, how much becomes streamow, how much leaves the basin as ground water ow, etc. A water budget shows the relative magnitudes of water withdrawals and gives insight into the hydrologic processes at work in a basin and the eventual fate of precipitation. In most cases, precipitation either evapotranspires or becomes streamow. Where else could it go? One place it could go is into long-term ground water storage. If climate is not changing and if ground water is not being overpumped, however, most stream systems are in quasi-equilibrium with their associated ground water aquifers, and ground water storage changes little from year to year. Therefore, one can compute the mean annual runo leaving a watershed (in inches per year) by subtracting the evapotranspiration from the precipitation: Water Budget Equation: R = P ET
where R is the mean annual streamow, P is the mean annual precipitation, and ET is the mean annual evapotranspiration. This equation should only be used with mean values estimated over a relatively long period of time because, on a year to year basis, soil and aquifer storage of water may change because of unusually high precipitation or because of drought. The assumption used in the equation above is that changes in soil and ground water storage are equal to zero. 9.1 Water Budgets The percentage of rainfall that becomes streamow Hydrologists use water budgets to account for the varies considerably across geoclimatic regions (areas eventual distribution of precipitation and to under- with similar climate, geology, and soils). The fol1
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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Figure 9.1: River Systems of Georgia.
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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logic outputs, such as streamow and evapotranspiTable 9.1: Water budgets in selected U.S. cities, ration. inches per year If the outputs of a system are less than the inputs, Location P ET R then the internal storage in the system must be inAthens, GA 50 35 15 creasing. Likewise, if the outputs exceed the inputs, Seattle, WA 40 20 20 internal storage in the system must be decreasing. Olympic Mts, WA 120 20 100 Consider a lake for example. If the lake level is Tucson, AZ 12 35 0 rising, one knows that water is entering a lake faster than it is leaving, and if the lake level is falling, water is leaving the lake faster than it is entering. This is lowing annual water budgets from basins around the precisely why people build reservoirs - so water can U.S. demonstrate this variability. be stored during high ow periods for use during low Note that Tucson has more ET (from irrigated ow periods. cropland) than there is precipitation! Also note that Combining the parts of the hydrologic cycle yields there is very little, if any streamow! Where does the other water come from? Can you guess? What do a more complicated water balance equation: we have to do to x this? More in just a minute... Annual precipitation and evapotranspiration are usually expressed as a depth, such as centimeters or inches. The above equation yields runo expressed as a depth also. Usually, it is desirable to express the mean runo as a ow with units of L3 /T or volume/time. The mean annual ow, expressed as a volume over one year, can be expressed as: Depth-Volume Equation: Q = A R Mass Balance Equation: S = P R ET which says that the inputs (precipitation) less the outputs (inltration or recharge, evapotranspiration, and streamow), must be oset by a change in water storage. More detail can be added to this basic equation to account for other types of inows and outows that may be important in some basins and to partition storage into dierent types. For instance, ground water may enter or exit a basin across the topographic divides, and humans may withdraw water from a basin for residential or industrial use. The following is a more complete water budget that a water resources manager might use to evaluate a watersheds potential as a water source for human supply. Reservoir managers use water budgets to decide whether to hold or release water from a reservoir. Because changes in storage are accounted for, this water budget can be run on any time scale; yearly, seasonally, monthly, weekly, or daily.
where Q (acre-feet/year) is the mean annual streamow volume, A (acres) is the area of watershed basin, and R (feet/year) is the mean annual streamow depth. This means that the mean annual ow increases linearly with basin area. As will be demonstrated later in this chapter, this is one of the few useful relationships in hydrology that is linear. The equation presented above is an example of a water budget. Specically, it is an average annual water budget for a watershed. Water budgets can be conducted on a variety of spatial and time scales. A water budget is a specic usage of the principle of continuity or conservation of mass. This is a basic Inows: I = P + GW (in) principle of physics that says that matter cannot be created or destroyed and that, for any system, the Outows: O = Q + ET + GW (out) + W inputs must equal the outputs plus positive changes Storage: S = S(SW ) + S(GW ) + S(soil) in storage. Mass Balance Equation: S = I O
where I are the inputs, SW and GW are the surface and ground water ows into or out of the basin, W where S is the change in water storage, I are the are the withdrawals to meet water demand, and S hydrologic inputs, such as rain, and O are the hydro- are the storages in the system.
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER Table 9.2: Water Budget Examples
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1. Calculate the mean annual discharge of the Oconee River below Athens in cubic feet per second (cfs). Average annual precipitation is 50 inches; average annual evapotranspiration is 35 inches; and the basin area is 783 mi2 . (a) Calculate the mean streamow discharge: Q = P ET = 50/yr 35/yr = 15/yr So, of the 50 inches of rainfall that fell on the watershed, 15 became streamow. Some of that ow came rapidly after specic rainfall events while some of it drained slowly to the streams as ground water discharge. (b) Determine the mean annual ow in cubic feet per second: We have to convert that convert the depth per year of runo to a volume per year of runo. The volume is simply the depth times the watershed area (using consistent units, of course). We then divide the annual runo volume (cubic feet) by the number of seconds in a year to get cubic feet per second as an average annual ow. (c) Calculate annual runo volume from depth: volume = depth x area We can think of the watershed as a large bucket, and we can determine the volume of the bucket by multiplying the area by the depth. So the annual volume (in f t3 /yr) =
15 1 yr
1 12
783 mi2
1 yr 365 d
640 ac 1 mi2
43,560 f t2 1 ac
= 27.3 109 f t3 /yr
(d) Convert f t3 /yr to f t3 /s (also called cfs):
27.3109 f t3 1 yr
1 day 24 hrs
3600 sec 1 hr
= 858 f t3 /s
Just imagine that you had a pool that held 27.3 billion cubic feet of water, and you wanted to release that pool at a constant rate and empty the pool in a year. You would have to drain the pool at the rate of 858 cfs. Hydrologists make this calculation faster by remembering that: 1 cf s 2 acre f eet/day (AF/day) 2. We can now ask what is the mean annual ow in the North Oconee River at a place where the basin area is 391.5 mi2 or exactly one half of the basin area used above? Because everything else is the same, and this area is 1/2 the area above, we can simply divide the answer by 2. So, the mean annual ow is 429 cfs. 3. Typically, water budgets are used the other way. That is, we know the mean annual ow and we know the precipitation, and we use the water budget to calculate annual actual evapotranspiration. For example, A 100 square mile basin in the Pacic Northwest has a mean annual ow of 176.8 f t 3 /s. The average precipitation is 44 inches per year. What is the average annual evapotranspiration in inches? (a) Convert the mean annual ow into an annual volume:
176.8 f t3 1 s
3600 s 1 hr
24 hr 1 day
365 day 1 year
= 5.58 109 f t3
12 1 ft
(b) Divide the volume of runo by the watershed area to get a depth of runo:
5.58109 f t3 100 mi2
640 ac 1 mi2
43,560 f t2 1 ac
= 24
(c) Calculate the mean annual ET: ET = P R = 44/yr 24/yr = 20/yr
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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gravity, or soil layers. Percolation is the downward vertical movement of water through the vadose zone to the water table. The rate at which water enters the soil surface is called the inltration rate, and the maximum inltration rate is obviously the maximum rate at which the soil can accept water. The maximum inltration rate is controlled by a variety of factors, but in general, the maximum inltration rate decreases over time during rainfall. The maximum inltration rate is highest for dry soils, and it decreases as the soil becomes wetter. When the soil surface becomes saturated, the maximum inltration rate equals the permeability of the soil. If the rainfall rate exceeds the maximum inltraFigure 9.2: Subsurface ow components. tion rate, then rainwater starts to pond on the soil surface. If the soil surface is sloped, as it usually is, water begins to ow downhill over the soil sur9.2 Inltration face. Therefore, water that does not inltrate becomes overland ow, also called direct runo or sur9.2.1 Denition and Importance face runo. A water budget for the soil surface during Inltration is the movement of water into the soil a rain storm would take the form surface when water is applied or ponded on the soil surface, and it usually occurs during rainfall. To be Stormwater Budget: P = F + I + O completely general, we could also speak of inltration where P is the precipitation, F is the inltration, I is of irrigation water, inltration of river overow water the canopy interception, and O is the overland ow. into oodplain soils, or inltration of water ponded This means that, during a storm, all precipitation on the soil surface after rainfall ceases. either inltrates or becomes overland ow. Overland Inltration is an extremely important process be- ow is the cause of most storm ow in streams and cause it determines whether surface runo and ero- is also a cause of soil erosion along with raindrop sion occurs. If the inltration rate is higher than energy. the rainfall rate, all the rainfall is absorbed by the ground. 9.2.2 Maximum Inltration Rates Water that inltrates into the soil may be stored temporarily in the root zone and be used later by Lets consider an experiment in which we apply water plants, or ow laterally as saturated or unsaturated to the soil surface to a constant depth of two centimeow down the slope, or it may percolate vertically ters, and we continue to replace the ponded water as down to the water table and then ow with the ground it is lost to inltration. In this case, there is always water. The soil zone between the soil surface and the water on the soil surface available for inltration. water table is called the unsaturated zone or vadose If we measure how quickly we have to add water zone. to maintain the water ponded on the surface, we can Soils in the vadose zone contain water adhered to determine the maximum inltration rate. If we plot the soil particles, but these soils are typically not sat- this rate over time, we nd that it decreases over urated except for short periods of time during heavy time and nally reaches a steady rate equal to the rainfall. In the upper part of the vadose zone, re- saturated hydraulic conductivity, or permeability. distribution of soil water may occur as it moves upWhy does the inltration rate decrease over time? ward, downward, or sideways due to evaporation, or The two main drivers of water ow in soil are gravity
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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than the maximum inltration rate, then not all of the rainfall can inltrate, and the remainder contributes to overland ow. The Cropland Case In bare soils, inltration rates of rainfall decrease more quickly over time, because the energy of the raindrops impact on the bare soils breaks up the soil aggregates. This creates ner particles that clog the macropores (large, visible pores, such as root and worm holes) at the soil surface, thus forming a crust (a hard layer on the soil surface). This crust reduces the maximum inltration rate and the permeability of the soil at the surface. Raindrops form hundreds or thousands of feet above the ground surface where the drops have a lot of potential energy. As these drops fall, they trade potential energy for kinetic energy. The drops also aggregate with other drops to create bigger drops. Much of this kinetic energy is released to the soil when the raindrops hit the ground, and this energy tears apart the soil particles at the surface and causes crusting. Kinetic energy of raindrops is especially large during high intensity storms (thunderstorms) because the drops tend to be large and drop velocities are high. The impact of the raindrops on bare soil coupled with rapid wetting of soil results in aggregate breakdown and macropore collapse. The nal result is low inltration rates and high overland ow rates. Surface-limited inltration, or crusting, is aected by certain soil properties. Soils dier in their aggregate stability, which is a measure of soil aggregates resistance to breakdown by raindrops and rapid wetting. Some soils have low aggregate stability while others have very high aggregate stability. Humus is the key to aggregate stability because it stabilizes aggregates to a large degree. Texture also aects aggregate stability in complicated ways. Loams and clay tend to suer from low aggregate stability, but loamy sands and sands do not crust. Sodium increases crusting due to clay dispersion (repulsion of clays on the microscale). Mulch, slash, or any organic cover prevents crusting from occurring by absorbing the raindrop energy and letting the rain drip into the soil from the mulch. Crusting, overland ow, and erosion on croplands can
Figure 9.3: Reduction of inltration rates with time. and soil capillary forces, or in other words, the force of gravity, and the wicking eect of dry soil. Gravity does not change over time during inltration, but the wicking eect does. At the beginning of inltration, the soil below the ground surface is dry, while the soil contacting the ponded water is wet. Initially, there is a very high pressure gradient that imbibes (sucks or wicks) water into the ground. After the lower soils become wet, however, the wicking effect decreases. When the upper part of the soil prole becomes saturated, the wicking eect drops to zero, and the only force causing inltration is gravity. At this point the maximum inltration rate is equal to the permeability. The maximum inltration rate is determined by the soils permeability and by the pressure driving inltration. Inltration does not always occur at the maximum inltration rate, however. The rainfall rate may be less than the maximum inltration rate, and therefore the actual inltration rate equals the rainfall rate. On the other hand, if the rainfall rate is greater
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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Figure 9.5: Breakdown of soil structure. be reduced or eliminated by leaving crop residues on soil surface or by practicing no-till planting. On construction sites, you often see that hay has been strewn over the graded soil or that a ber netting has been laid down on the bare soil. This is done to absorb the raindrop energy and to protect the soil from crusting. The Forested Case Inltration rates in forest soils tend to be high. The litter layer protects the ground surface and prevents crusting. Macropores at the soil surface are created by roots, burrowing animals, earthworms, and invertebrates. Raindrop energies are lower on forest oors because the canopy breaks up the rain drops before they hit the ground. As a result of these conditions, overland ow in forests with intact O horizons is quite rare, and it is usually small when it does occur. Timber harvest may or may not reduce inltration of forest soils, depending on the harvest and site preparation practices. If the soils are not extensively compacted by heavy equipment and if the organic cover is not burned o, then inltration rates are hardly aected. If the slash is burned hot to prepare the site for planting, however, the organic layer may be destroyed and the resulting erosion may be extensive.
9.2.3
Other Factors
So far we have considered only the rate at which waFigure 9.4: Reduction of inltration rates with time. ter enters the surface of the A horizon as if the properties of the A horizon were the only determinant of inltration. If the A horizon is shallow, however, and is underlain by a low permeability Bt horizon, then inltrating water may back up in the A horizon as it tries to enter the Bt. If the inltration rate in the Bt horizon is low
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER enough, the entire A horizon may saturate, creating a perched water table which is a saturated zone overlying a low permeability layer with an unsaturated zone below. When this happens, the inltration rate at the surface is determined by the rate in the Bt horizon. Overland ow occurs because the rain is falling faster than the Bt horizon can accept water. Inltration can also be limited in areas where the water table is just below the ground surface. As the water table rises during rainfall, the soil becomes completely saturated and the inltration rate drops to zero. In fact, some ground water ows out of the soil to the surface. In this case, all rainfall becomes overland ow. This situation often occurs in oodplains, in and around wetlands, and in the lower Coastal Plain. Temperature also aects inltration by changing the water viscosity (stickiness - cold honey is viscous). Inltration is faster at higher temperatures because warm water is less viscous (more slippery) than cold water. In the extreme case where the ground is frozen, inltration rates become very low.
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9.3
Sources of Streamow
Where does water in a stream come from? Why is there water even during a long drought? Hydrologists divide streamow into two types of ow, baseow and stormow. Baseow is that component which provides streamow during low ow periods, while stormow refers to streamow that occurs quickly in response to precipitation events. If a stream was owing before the rainfall (a typical situation), stormow is the ow that occurs in addition to the baseow that would have occurred if it had not rained. There are many ways to separate streamow into stormow and baseow.
9.3.1
Overland Flow
The most obvious example of Hortonian ow is on streets and parking lots. In Georgia, Hortonian ow is also common on plowed elds and bare soils, but it rarely occurs in forests unless it rains hard, such as during a hurricane. Even then, however, forest runo is more likely to occur due to saturation of the underlying soils, rather than due to low inltration rates across the soil surface. The dierence between the rainfall rate and the inltration rate is the amount of rain that runs o the landscape. Potential inltration rates tend to decrease over time. When rainfall begins, the relatively dry soils below the soil surface absorb water faster than it can be carried by gravity alone. This uptake (also called imbibition) of water is the result of capillary forces (like how a paper towel absorbs water) in the soils. As soil moisture contents become uniform with depth near the surface, the inltration rate becomes equal to the soils hydraulic conductivity (the permeability of the soil) for that moisture content (as we shall examine in great detail in a subsequent chapter!). As inltration occurs, there is typically a sharp moisture dierence between the newly wetted soils and the drier soils below them. This sharp break in the moisture contents is called the wetting front, and it moves downward during the storm. Because of the change in potential inltration rates during rainfall, the occurrence of surface runo depends not only on the intensity of the rainfall, but also on the timing of the intensity. For example, a rainfall rate of 2 inches/hour is much more likely to cause surface runo if it occurs after two days of light rain than if it occurs at the beginning of a storm. Two fundamentally dierent models of stormwater generation have been proposed. The rst posits that runo occurs when the rainfall intensity exceeds the soil inltration rate. In this model, high intensity rains are more likely to cause stormow than low intensity storms.
If the rate of rainfall exceeds the soil inltration rate (the rate at which soil absorbs water), then water ponds on the soil surface. If the soil surface is sloped, the ponded water ows downhill toward the channel system. This is referred to as overland ow, sheet ow, or surface runo. It is also called Hortonian ow after R.E. Horton, the hydrologist who rst described this process in the 1930s.
9.3.2
Interow
Interow is lateral, shallow, subsurface ow that occurs on hillslopes with shallow permeable soil layers overlying low permeability layers. Interow can occur as either saturated (soil pores are lled with water) or unsaturated (soil pores are only partially lled with water) ow. Interow begins in a soil layer as soon as
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
9
Figure 9.6: Components of ow on a hillslope.
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
10
the wetting front crosses that layer and reaches the surface of the layer below. Obviously, interow does not reach stream channels as quickly as surface ow, but interow is fast enough to generate part of the stormow response. In some forested areas, interow dominates stormow response. Interow continues between storms, transporting soil water from higher portions of the landscape to lower portions of the landscape. Interow is one of the processes that create variable source areas - saturated areas near streams. Interow does not occur in all landscapes. Interow is more important when soil layers are thin and when slopes are relatively large. The Bt horizon can Figure 9.7: Excess precipitation resulting from limited soil moisture storage. cause interow because of its low permeability.
9.3.3
Direct Precipitation on Channels
Some rainfall lands directly on the surfaces of streams, rivers, wetlands, and lakes, and obviously becomes stormow immediately. This is usually a small percentage of the stormow, however, because surface waters usually cover a small part of the landscape. This is not true in swamps, however, such as the Okefenokee, where large areas are covered with 9.3.5 Baseow - Groundwater Discharge water. Streamow between storms comes from ground water discharge (water stored in underground aquifers), interow (hillslope drainage), and the draining of wa9.3.4 Variable Source Areas ter stored in lakes and wetlands. Baseow is not An alternative way of thinking about runo is to say constant. It steadily but slowly decreases between that soil storage limits the volume of water that the rainfall events as water drains from the watershed soil can absorb. In this model, storms with more (like how a bathtub drains more slowly as it emprainfall depth, as opposed to greater intensities, result ties). Baseow is a critical determinant of habitat in greater runo. Areas that are saturated contribute conditions in streams and rivers. to stormow, while areas that are unsaturated absorb When ows are lower, there is less dilution of polall the rainfall and do not contribute to stormow. lutant inputs resulting in higher contaminant concenSome parts of the landscape tend to stay wetter trations during low-ow periods. Also, there is less than others because water continually drains toward buering - attenuation - for solar and atmospheric these areas between storms or because saturated soils heating of the water. Thus, stream temperatures can lie near the surface. Hillslope hollows, low areas be a problem for sh during summer low ow pearound streams and rivers, wetlands, and the mar- riods. The ground water characteristics of a basin gins of wetlands are examples of these areas. largely control the quantity, quality, and temperature During rainfall, soils in these areas may become of baseow. completely saturated, and their inltration rates may fall to zero. When this occurs, overland ow occurs 9.3.6 Land Management on these saturated areas. This runo generation process is also called the variable source area concept, How water moves through the landscape has imbecause these saturated areas expand during rain- portant consequences for water chemistry, stream
storms or during wet seasons because larger areas become saturated. Variable source areas typically comprise from between ve to fteen percent of a humid landscape, and are largely responsible for stormows from forested areas.
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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Figure 9.8: Map of saturated areas showing expansion during a single rainstorm of 46 mm.
The basin has steep, well-drained slopes and a narrow valley oor. The solid black shows the saturated area at the beginning of rain; the lightly shaded area is saturated by the end of the storm.
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER Table 9.3: Relationship of hillslope ow processes with land management concerns
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Water Chemistry - Interaction, or lack thereof, between water and soils has a strong inuence on the chemical composition of water entering streams and wetlands. For instance, most microbial activity, nutrient cycling, and plant uptake occur in shallow soils. The longer ow spends in this zone, the purer the water that leaves the hillslope. It also inuences the suitability of groundwater as a supply of drinking water. Floods and Baseows - Soil and vegetative conditions determine how rainfall moves to streams and thus dictate baseows and ood peaks and volumes. Land managers want to maximize inltration and minimize overland ow to minimize ooding and maximize baseows. Productivity and Irrigation Requirements - Soil moisture is a limiting factor for tree and crop in growth much of the U.S., and some parts of the landscape grow trees or crop better because topographic and geologic conditions cause water to accumulate in those areas. At the extreme, subsurface ow conditions may make an area too wet to grow many commercially valuable crops. Stormwater Management - The magnitude of hydrologic alteration caused by development depends on the degree to which soils are disturbed, vegetation is altered, and land is covered with pavement. Appropriate design of stormwater management and treatment facilities depends on the ability to predict this change. Stream, Slope, and Wetland Geomorphology - Geologic conditions are a dominant control of hydrologic processes, but runo patterns and characteristics in turn alter the landscape. Landscapes are never in equilibrium, although some landscapes change much more rapidly than others. Runo patterns and groundwater ow in a basin determine the number and distribution of streams and wetlands as well as other landscape features. Hillslope Stability - The location and timing of landslides is largely driven by subsurface ow conditions. For example, seepage areas on steep hillslopes are high landslide danger areas.
habitat quality, timing and amount of streamow, and other issues of concern in land management. Overland ow is also necessary to cause surface erosion, rill erosion, and gully erosion (to be discussed in Chapter 13) - erosion is insignicant if there is no overland ow. The following table provides an overview of how ow pathways aect land management issues.
9.4
Subsurface Water Movement
water discharge to streams is a principal source of baseow (streamow between rainfall events). Ground water is also important as a water supply for humans. Most rural residents rely on ground water extracted from wells for their consumption. Similarly, many farms rely on ground water to irrigate their crops. Because ground water is connected to surface water, however, heavy use of ground water in a location may detrimentally aect surface water resources.
Wherever you are on a landscape, there is water beneath your feet, and usually this water is in motion. Ground water is the general term for all water beneath the ground surface, although water in unsaturated soils is typically called soil moisture and water in saturated zones in usually called ground water. Ground water is an important water resource. Ground water accounts for two-thirds of the freshwater resources in the world, and if inaccessible waters of glaciers and ice-caps are not considered, it amounts to 95% of the worlds freshwater resources. Ground
9.4.1
Water Levels in Wells
If you dig a hole with a shovel or an auger on most places on a ridge or a hillside, you usually nd the ground beneath your feet to be unsaturated. As you continue to dig downward, eventually you discover wet soils, and then water seeps into the hole from the surrounding soil. The elevation at which water stands in the hole denes the water table at that point. More generally, the water table is the three-dimensional surface
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER dened by the elevation of the water that stands in wells or piezometers in the area. A piezometer is a small diameter well that is used only to observe the water table, not to extract useable water from the ground. Above the water table, water in the ground is held under tension or suction, and the pressure therefore is negative. Below the water table, ground water is under positive pressure. At the water table, the water pressure is zero. Another way to dene water table is the surface at which the water pressure is equal to zero. In aquifer schematics, water tables are noted by lines with upside-down triangles touching the tops of the lines and usually with two short lines beneath the triangle (see schematics below). Hydrologists use the same symbol of an upside-down triangle to denote the water surface in lakes or streams. The total hydraulic head at the water table is equal to the elevation of the water table. Generally, the water table intersects surface water features in a valley, forming a smooth surface that mimics the surface topography. The typical way to map a water table is to drill wells or piezometers throughout an area and interpolate between water table elevations determined from the wells and from surface water elevations. Water tables rise and fall seasonally, but in many landscapes, ground water and surface waters are in quasi-equilibrium such that ground water levels do not change much from year to year, and streams and rivers act as the principal ground water discharge points for watersheds. Like surface water, ground water usually moves towards low spots in the landscape, either wetlands, streams, lakes, or the ocean. Ground water is replenished by the process of percolation, by which water moves downward by gravity towards the water table. Recharge is the water that successfully moves past the root zone and reaches the water table. During long dry periods, water tables fall as ground water is discharged to streams but is not replenished by recharge from rainfall. Relating this back to the basin water budget, during dry periods stream outows exceed rainfall inputs, and ground water storage is reduced to make up the dierence. During wet periods, recharge to ground water exceeds discharge to streams, and water tables rise. During normal precipitation years in basins where ground water is not being extracted, water tables
13
tend be at similar elevations from year to year. During successive dry or wet years, however, water tables may fall or rise, respectively, from year to year. The region of partially saturated soils between the ground surface and the water table is referred to as the vadose zone. Soils in the vadose zone are usually unsaturated, but may be fully saturated for short periods of time during extreme inltration events. There is a thin band of soils in the vadose zone, just above the water table, that are fully saturated although the soil water is held under negative pressure. This is the zone of capillary rise. The height of capillary rise is inversely proportional to the size of the interstitial pore spaces. Therefore, a more correct denition of the vadose zone is an area where soil water is usually held under tension. The zone beneath the water table is called the saturated or phreatic zone, and the pores are entirely lled with water under positive pressure.
9.4.2
Types of Aquifers
The saturated soil and rock strata beneath the water table are roughly classied as aquifers, aquitards, and aquicludes. An aquifer is a geologic media that is capable of transmitting signicant quantities of water to a well or system of wells. Aquifers tend to have relatively large porosities, hydraulic conductivities, and volumes (either depth, areal extent, or both). Typical aquifer materials are gravels, sands, limestone, or highly fractured rock. At the other end of the spectrum, an aquiclude is a geologic media which entirely prevents uid ow, meaning either the hydraulic conductivity and the porosity are zero. True aquicludes are rare, but an example is an unfractured crystalline rock. An aquitard is a geologic medium with limited ability to transmit water. Aquitards, such as clay layers, have low hydraulic conductivities and/or porosities. There are several types of aquifers with important dierences between them. When drilling from the ground surface, the rst aquifer a hydrologist reaches is most likely to be an unconned aquifer (also called a water table aquifer or a surcial aquifer). An unconned aquifer is one whose water table is connected to the atmosphere through the pores in the overlying vadose zone and which is not overlain by an aquitard or an aquiclude. In an unconned
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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Figure 9.9: Flow in the shallow subsurface. aquifer, water does not fully saturate the aquifer media, meaning that the zone of saturation is free to move up or down. Typically, unconned aquifers are connected directly to surface waters (rivers, lakes, wetlands). A perched aquifer is a saturated zone above the regional water table. Usually a perched aquifer is formed on top of an aquitard, such as a clay layer, and is a temporary feature that forms during high recharge periods. A conned aquifer is conned between overlying and underlying aquitards or aquicludes. Water fully saturates a conned aquifer. Typically, a conned aquifer outcrops somewhere on the landscape, meaning that the geologic strata bearing the aquifer contacts the land surface at some location. These outcrop areas serve as the recharge areas for conned aquifers, because recharge from above the aquifer is limited by the overlying aquitard. The Floridan aquifer located below the coastal plain of Georgia is an example of a conned aquifer. It supplies water to Savannah, several pulp mills, and many farmers and small communities. Most conned aquifers, including the Floridan, are not completely conned because there is some leakage through the conning layers. Water levels in wells drilled into a conned aquifer form a potentiometric surface which is a threedimensional surface dened by the how high water rises in the wells or piezometers. How is this dierent from a water table? All water tables are potentiometric surfaces, but not all potentiometric surfaces are water tables.
In an unconned aquifer, the water table marks a transition between the vadose zone above and the saturated zone below, and the water table is the top of the aquifer. A conned aquifer is fully saturated, and the top of the aquifer is dened by the upper conning layer. The potentiometric surface is well above the top of the aquifer.
In some cases, the potentiometric surface may be higher than the ground surface, and the aquifer is called an artesian aquifer. Water ows freely from a well drilled into an artesian aquifer, and such a well is called an artesian or owing well. Some areas of the Floridan aquifer are artesian, or were artesian prior to large-scale ground water pumping which lowered the potentiometric surface.
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER
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Figure 9.10: Types of aquifers.
9.4.3
Ground Water Flow To Streams
To understand where baseow in rivers and streams comes from, we must rst show that overland ow from precipitation is not enough to account for the source of the water. Most streams ow at an average rate of about 3 to 5 feet per second, or about 2 to 3 mph. Even if a river is 200 miles long, it should take less than 100 hours (4 days) for the river to completely drain. What sustains rivers during a drought is the slow release of ground water. During wet periods, recharge to ground water raises the water levels in wells, and then moves toward streams. As the ground water levels rise, seepage areas next to streams begin to ow, because the water table is intersecting the ground surface at a higher elevation. The water ows out as seeps and springs along the bank of the river, as well as directly into the channel. This is called the source, or contributing, area concept for streamow generation. The gure shows how wet areas contributing to a stream enlarge during a rainstorm, and is the area over which the water table had risen to the ground surface. and then shrink as the water is drained to the stream. Think of the ground-water system as a giant sponge that swells as it gets wet, and shrinks as it dries out. Wells near streams yield more water than wells further from the stream, because there is a greater col-
lection area upstream of the well. The collection area for a well is called the ground-water capture zone. Obviously, any sources of pollution upstream of the well will be captured by the well. It is important to dene a wellhead protection zone that is a conservation area within the capture zone that protects the well from contamination. If there are sources of ground-water contamination, then the streams will also be aected. Seeps and springs may show high levels of pollution in those areas where excessive ground-water contamination is present. We will discuss this more in a later chapter. It is common practice in urban areas to route all stormwater directly to a stream via pipes or ditches. The result of direct stormwater discharge is the reduction of inltration and ground-water recharge. By reducing ground-water recharge, we end up reducing baseow in streams. Thus, we are switching water from baseow to stormow. Stormows have more power, thus causing more erosion. Also, there is a shorter residence time, thus providing less opportunity for water quality improvement by decay and ltration of organic materials. All-in-all, direct stormwater disposal to streams is a big negative for stream health. Can you envision an alternative to big pipes carrying water to streams? Household wastewater systems dispose of their wastes using seepage systems that discharge to ground water. Could stormwater
CHAPTER 9. INFILTRATION, STREAMFLOW, AND GROUNDWATER from streets and roofs be inltrated? Perhaps using mulching to increase soil inltration rates, using seepage lines, or even injection wells. Care must obviously be taken to prevent contamination of the water in any case, whether it goes to a stream directly by a pipe, or indirectly through ground water.
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9.4.4
Ground Water Applications
Lets look at how we can use maps of water tables or piezometric surfaces to determine groundwater ow directions and amounts. Floridan Aquifer The Floridan aquifer is composed of limestone and dolomite deposited when the southeastern Coastal Plain was beneath the ocean approximately 45 million years ago. The top of the aquifer system lies 40 to 100 meters below the ground surface, and the aquifer is usually about 100 to 400 meters in thickness. The aquifer outcrops along the Fall Line (the contact between the Coastal Plain and the Piedmont) where recharge water enters the aquifer. Water ows south and southeast to the Gulf of Mexico and to the Atlantic Ocean. Because of the abundance and high quality of water in the aquifer, the Floridan aquifer is heavily used for irrigation and for industrial water supply (paper and pulp mills). Using data from many wells in the aquifer, the United States Geological Survey (USGS) has developed a potentiometric surface map of the Floridan aquifer (see the page after next). Note the cones of depression around Savannah, Jessup, Brunswick, and Saint Marys. These cones of depression in the potentiometric surface are created by heavy pumping for industrial water supply. This map was developed before the paper mill in Saint Marys closed. Now the cone of depression around Saint Marys has disappeared. As we have learned before, groundwater ows from high head to low head, so the potentiometric surface map can be used to determine groundwater ow paths. Assuming uniform hydraulic conductivities in the aquifer, water will ow from high head contours to low head contours, and the ow lines will be perpendicular to the contours (water will take the shortest path from high head to low head). The potentiometric map of the Floridan aquifer shows ow
paths through the aquifer. Note that groundwater ow in much of southwestern Georgia moves toward the Flint River. Groundwater ow in southeastern Georgia tends to end up in the paper and pulp mill wells. The cones of depression around the pulp mill wells are drawing water from the Ocean toward the coast, causing salt-water intrusion in the aquifer. The potentiometric map, along with Darcys Law, an estimate of saturated hydraulic conductivity, Ks, and an estimate of the aquifer depth, can be used to estimate ow through sections of the aquifer. Consider points A and B on the potentiometric map. They are on the same contour, so water is owing perpendicular to the line between A and B in a southeasterly direction. Using the contour lines, we can estimate a hydraulic gradient. Between the 100 ft and 60 ft contours, the head drop is 40 feet, and the distance between these contour lines is approximately six miles or 32,000 feet (using the map scale). The hydraulic gradient is therefore G = 40 f t/32, 000 f t = 0.00125. Assuming a saturated hydraulic conductivity of 0.003 ft/s and an aquifer thickness of 600 feet, and 25 miles between points A and B, the ow through the aquifer is: Q = KA h = KAG x = (0.003 f t/s) (600 f t) (132, 000 f t) (0.00125) = 300 f t3 /s = 192 mgd (9.1) This is enough water to provide domestic supply for approximately 1.3 million people (assuming a per capita use of 150 gallons/day). If you go bac...