Title: Soils
1Soils Hydrology II
- 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
2- Causes of Air Movement
- - Solar energy doesnt heat Planet Earth
uniformly - - Air rises near the Equator and Polar Fronts
- - Air sinks near the Horse Latitudes and Poles
3- 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, RH100 -
4- 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 lt 100
- Air cooler than dewpoint, RH gt 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
5Lapse 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 5.5F / 1000 ft Wet air
0.50C / 100 m 2.7F / 1000 ft Average 0.65C
/ 100 m 3.5F / 1000 ft
6- 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
7- You now have a great job in Tucson, Arizona (elev
2000). - Unfortunately, its 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
its 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.
8Why 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).
9Raingages
- 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.
10Types 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
11Types of Air Masses
- Fronts occur at the boundary of air masses
- The types of air masses are
12Frontal Storms
Cold Front
13Warm Front
14Occluded Front
15Convective Storm
16Orographic Precipitation
Rain Shadow
17Hurricanes CyclonesTyphoons
18Mesoscale 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
19Lake Effect Storms
20arid lt 10/year semi-arid 10 - 20/yr humid 20
- 60/yr moist gt 60/yr
21Seasonal 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.
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23Precipitation Intensity
24Effect of Area on the Maximum Precipitation
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26Evapotranspiration
- Evaporation from soil water surfaces
- Loss of water to the atmosphere by abiotic
processes - Large if soil is moist and theres 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
27- 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
28- 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 doesnt!
29Weighing Lysimeter
30AET 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
31Variation of the Crop Factor
32Soil 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)
33Water Budget Procedure
- Find the initial water storage in the root zone
- set equal to the field capacity, F(1) S
- appropriate in the spring after soaking rains
- Calculate the soil factor
- Ks F / S
- Calculate the AET Kc Ks PET
- Subtract AET from the soil storage, F' F - AET
- If rainfall, then add, F'' F' P
- Subtract drainage and runoff if soil is too wet
- if F'' gt S, then Q F'' - S, and F''' S
- Carry over soil moisture to next day
- say from end of day 1 to beginning of day 2
- F(2) F'''(1)
34Depth of rooting zone Ds 20 cm Bulk
density BD 1.70 g/cm3 Field capacity FC
0.20 Wilting point WP 0.08 Crop factor Kc
0.8 Soil factor Ks F / S
AET Kc Ks PET 0.8 (F / S) PET PET Given
in table Max water content S FC - WP BD Ds
0.20 - 0.081.7020 4.1 cm Initial
water content F(1) S Precipitation Given in
table
35- Irrigation Scheduling Procedure
- Find 25 of the maximum available water
- F 0.25 S 0.25 4.1 cm 1.02 cm
- Irrigate when F falls below F
- F lt 1.02 cm on Day 9.
- Determine how much water to add to bring rooting
depth back to FC - I S - F 4.10 - 0.97 3.13 cm
- Determine how long to irrigate
- ?t 3.13 cm / 1 cm/hr 3.13 hours
36- Water Budget Approach
- ET P - R - I
- P is precipitation
- R is runoff
- I is interception
- Mature Hardwoods P 150, R 70, I 18, ET
62 cm/yr - White Pines P 150, R 52, I 36, ET 62
cm/yr
37Streamflow Depletion
38Paired Watershed Studies
- Select two watersheds of approximate equal size,
shape and aspect. - Monitor streamflow for several years and find the
correlation between the two. - Hold one watershed as the control, and alter the
second watershed, in this case by converting to
grass. - Monitor the change in streamflow and compare to
what would have happened if the watershed had not
been treated.
39Paired Watershed Studies
40Energy Budget Approach
41Effect of Shelterbelt Harvesting
Effect of Sun Angle
42Effect of Forest Harvesting on Stream Temperatures
43Chapter 10 Quiz
- 1. If three inches of rain falls on 100 acres,
this is equal to - a. 300 acre-inches of rain
- b. 1,089,000 cubic feet of rain
- c. 30,861 cubic meters of rain
- d. 25 acre-feet of rain
- 2. Name the four precipitation mechanisms
- 3. Which of the following combinations of air
masses will produce the maximum precipitation - a. A continental maritime meeting a tropical
polar - b. A continental polar meeting a continental
maritime - c. A tropical maritime meeting a continental
polar - d. A tropical continental meeting a polar
continental - 4. Is canopy interception more like evaporation
or transpiration? Explain your answer!!