& Soils Hydrology II
1. 1. 1. 2. 3. 4. 5. Soil Water Precipitation and Evaporation Infiltration, Streamflow, and Groundwater Hydrologic Statistics and Hydraulics Erosion and Sedimentation Soils for Environmental Quality and Waste Disposal Issues in Water Quality
Causes of Air Movement - Solar energy doesn't heat Planet Earth uniformly
- Air rises near the Equator and Polar Fronts - Air sinks near the Horse Latitudes and Poles
Sensible Heat: heat used to raise temperature Latent Heat of Fusion: heat used to melt ice Latent Heat of Vaporization: heat used to evaporate water Absolute Humidity: mass of water vapor in a unit volume of air (mg/L) Relative Humidity: ratio of actual vapor pressure to saturation vapor pressure
Vapor Pressure: partial pressure of water vapor (mb) Saturation Vapor Pressure: maximum vapor pressure (mb)
Dewpoint Temperature: temperature at which the air is saturated, RH=100%
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Dewpoint Temperature
The temperature to which a parcel of air with a given vapor pressure has to be cooled in order to reach saturation. Air warmer than the dewpoint, RH < 100% Air cooler than dewpoint, RH > 100%, causes clouds and rain!
Saturation Vapor Pressure
The pressure is like a tea kettle, the more water in the air, the higher the pressure The saturation vapor pressure is the maximum amount of water that can be held Warm air holds more water than cold air
Example:
Actual vapor pressure = 17.1 mb Air temp = 30C Saturated vapor pressure = 42.6 mb Relative Humidity = 17.1 / 42.6 = 40% Dewpoint = 15C mb = millibar, a unit of pressure
Lapse Rate: Change in temperature with altitude
Dry air lapse rate:
Holds for a clear, cloudless day. Air cools because the pressure drops with altitude This can be blamed on the ideal gas law: P V = n R T
Wet air lapse rate:
Holds for cloudy conditions. Wet air does not cool as quickly as dry air because water vapor gives off heat as it condenses, just like water absorbs heat when it evaporates.
Average, or environmental, lapse rate:
The actual change in temperature with altitude. The average rate is more typical for partly cloudy conditions.
Dry air: 1C / 100 m Wet air: 0.50C / 100 m Average: 0.65C / 100 m
5.5F / 1000 ft 2.7F / 1000 ft 3.5F / 1000 ft
Lapse Rate Examples
If the temperature in Athens is 40F on a relatively dry fall day, what is the likely temperature at an elevation 2000 feet higher in the Georgia mountains?
Use the dry adiabatic lapse rate of 5.5F/1000 feet. For an elevation that is 2000 ft higher, this gives a temperature that is 11F cooler, or 29F.
If the temperature in Athens is 90F on a humid summer day, what is the likely temperature at 2000 feet in the Georgia mountains?
Use the wet adiabatic lapse rate of 2.7F per 1000 ft. This gives a temperature that is 5.4F cooler, or 84.6F.
What does the temperature in Athens have to be on a rainy day for there to be snow falling at an elevation of 2000 feet in the Georgia mountains?
Use the wet adiabatic lapse rate This gives 32F + 5.4F = 37.4F
You now have a great job in Tucson, Arizona (elev = 2000').
Unfortunately, it's often 110F during the summer. You see Mt. Lemmon, which rises to 9,000 feet right outside of town. What is the temperature at the summit on a clear, dry, summer day? During the winter, find the temperature at the ski lodge if the temperature in Tucson is 50F.
Why is the wet lapse rate less than the dry rate?
As wet air rises, the atmosphere becomes saturated and the relative humidity reaches 100%. To cool further requires that the atmosphere release some of it's moisture as precipitation - rain if the air is above freezing, snow or ice if it's below freezing. The condensation of water releases heat - just as evaporation cools. This release of heat warms the air slightly, so the air does not cool as fast as dry air would.
Why Does it Rain?
Air is forced to rise (reasons described below!) Rising air cools because the ideal gas law says that the temperature falls when the air pressure decreases. The air cools at the dry lapse rate until it reaches its dewpoint. Once the air reaches its dewpoint, the relative humidity reaches 100%, and clouds form. As the air continues to rise, the air cools at the wet lapse rate, causing precipitation to form because the colder air can not hold the excess moisture. The condensing water generates heat, causing the air to warm slightly, so that the wet air lapse rate is less than the dry rate. The excess heat generated by the condensing water causes the air to rise faster (because warmer air rises through colder air).
Raingages
Thiessen Polygons
Used to estimate watershed precipitation Individual raingages are assigned the area closest to them The area is found by:
drawing lines between gages bisecting the lines and drawing perpendiculars the volume of runoff is the depth for the gage times the area.
Types of Precipitation Events
Frontal
when a cold air mass collides with a warm air mass. At least one of the air masses must be maritime.
Convective
when moist, warm (maritime tropical) air heats near the ground surface, it warms, rises, cools, and releases its moisture as rain, hail, etc.
Orographic
when moist (maritime) air is forced upward over mountains, it cools, releasing its moisture as rain or snow.
Cyclones (hurricanes)
when a self-sustaining (non frontal) low pressure system develops in the tropics.
Mesoscale Convective Complex
Mid-latitude storm complex covers large area, but does not persist
Lake Effect Storms
Downwind of warm lake, lake evaporation increases rain and snow
Types of Air Masses
Fronts occur at the boundary of air masses The types of air masses are:
Dry = Continental, c Wet = Maritime, m
Warm = Tropical, T
cT = Continental Tropical
mT = Maritime Tropical
Cold = Polar, P
cP = Continental Polar
mP = Maritime Polar
Frontal Storms
Cold Front
Warm Front
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Occluded Front
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Convective Storm
Orographic Precipitation
Rain Shadow
Hurricanes Cyclones Typhoons
Mesoscale Convective Complex
Occurs over a large area Persists for many hours, then dies away Not long-lived like a front or hurricane Associated with heavy rains and flooding Affects Midwest and sometimes Georgia
Lake Effect Storms
arid: < 10"/year semi-arid: 10 - 20"/yr humid: 20 - 60"/yr moist: > 60"/yr
Seasonal Distribution of Temperature and Rainfall Return Period, Tr = 1 / P A 100-yr flood has a 1% probability each year
P = 1/ Tr = 1/100 = 0.01 = 1%,
A 10-yr flood has a 10% probability
P = 1/ Tr = 1/10 = 0.10 = 10%
A median flood has a 50% probability, Tr = 2 years
Tr = 1/P = 1 / 0.5 = 2 yrs
An average flood happens every 2.5 years or so.
Precipitation Intensity
Effect of Area on the Maximum Precipitation
Evapotranspiration
Evaporation from soil & water surfaces:
Loss of water to the atmosphere by abiotic processes Large if soil is moist and there's no mulch or leaf cover!
Transpiration through plant tissue:
Loss of water to the atmosphere by biotic processes
Plant Factors: Leaf area, root depth, plant type. Pumping of water through roots to leaves through stomata Soil Factors: Plant Available Water
Interception:
Precipitation falling on plant surfaces that then evaporates About 10-20% of total precip for hardwoods, more in pines
Evaporation, a function of:
wind speed vapor pressure deficit (VPD)
The VPD is how dry it is, a large deficit means the air is dry VPD = es - ea = es ( 1 - RH )
RH = ea / es is the relative humidity ea is the actual vapor pressure es is saturated (maximum) vapor pressure, f(temp)
Seattle, WA = 30"/yr Massachussetts = 35"/yr Minnesota = 30 to 45"/yr Pennsylvania = 40"/yr Rocky Mountains = 45"/yr North Georgia = 55"/yr Los Angeles, CA = 60"/yr East Texas = 70 to 80"/yr Tucson, Arizona = 95"/yr West Texas = 100 to 120"/yr Imperial Valley, CA = 120"/yr
Potential Evapotranspiration, PET
The maximum possible transpiration by plants with unlimited soil moisture. We usually take a percentage (e.g., 70%) of pan evaporation to estimate PET.
Actual Evapotranspiration, AET
The actual amount of evapotranspiration loss per time for given area Depends on the type of plant, stage of growth, soil moisture, and climatic variables.
AET is less than PET
If no moisture in soil, then plants run out of water Plant responds by
wilting twisting the petiole so leaves are perpendicular to sun flutter to help dissipate heat close their leaves
Pan has plenty of water, soil doesn't!
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Weighing Lysimeter
AET Equation
AET = Kc Ks PET
AET is actual evapotranspiration PET is potential (max) evapotranspiration
From evaporation pans or models
Kc is a crop factor - changes with time
See next slide
Ks is a soil factor - changes with soil moisture
Ks = F / S, where
F is how much water in soil S is how much water the soil can hold
Variation of the Crop Factor
Soil Factor
We use a very simple approach:
Ks = F / S
S = FC - WP is the Maximum Available Water F = - WP is the Actual Available Water
This means:
If F = S then Ks = 1 and AET = PET If F = 0 then Ks = 0 and AET = 0
We calculate the soil storage using:
F = P - (Q + AET)
Water Budget Procedure
Find the initial water storage in the root zone
set equal to the field capacity, F(1) = S ...