Soils

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Soils 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

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• 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

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• 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

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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 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

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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 5.5F / 1000 ft Wet air
0.50C / 100 m 2.7F / 1000 ft Average 0.65C
/ 100 m 3.5F / 1000 ft
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• 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

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• 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.

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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).

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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.

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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

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Types of Air Masses

• Fronts occur at the boundary of air masses
• The types of air masses are

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Frontal Storms
Cold Front
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Warm Front
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Occluded Front
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Convective Storm
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Orographic Precipitation
Rain Shadow
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Hurricanes CyclonesTyphoons
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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

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Lake Effect Storms
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arid lt 10/year semi-arid 10 - 20/yr humid 20
- 60/yr moist gt 60/yr
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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.

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Precipitation Intensity
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Effect of Area on the Maximum Precipitation
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Evapotranspiration

• 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

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• 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
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• 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!

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Weighing Lysimeter
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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

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Variation of the Crop Factor
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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)

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Water 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)

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Depth 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
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• 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

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• 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

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Streamflow Depletion
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Paired 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.

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Paired Watershed Studies
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Energy Budget Approach
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Effect of Shelterbelt Harvesting
Effect of Sun Angle
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Effect of Forest Harvesting on Stream Temperatures
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Chapter 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!!