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Soils

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Holds for a clear, cloudless day. Air cools because the pressure drops with altitude ... soil moisture to next day. say from end of day 1 to beginning of day 2 ... – PowerPoint PPT presentation

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Title: Soils


1
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

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

5
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
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.

8
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).

9
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.

10
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

11
Types of Air Masses
  • Fronts occur at the boundary of air masses
  • The types of air masses are

12
Frontal Storms
Cold Front
13
Warm Front
14
Occluded Front
15
Convective Storm
16
Orographic Precipitation
Rain Shadow
17
Hurricanes CyclonesTyphoons
18
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

19
Lake Effect Storms
20
arid lt 10/year semi-arid 10 - 20/yr humid 20
- 60/yr moist gt 60/yr
21
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.

22
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23
Precipitation Intensity
24
Effect of Area on the Maximum Precipitation
25
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26
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

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!

29
Weighing Lysimeter
30
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

31
Variation of the Crop Factor
32
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)

33
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)

34
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
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

37
Streamflow Depletion
38
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.

39
Paired Watershed Studies
40
Energy Budget Approach
41
Effect of Shelterbelt Harvesting
Effect of Sun Angle
42
Effect of Forest Harvesting on Stream Temperatures
43
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!!
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