Evapotranspiration

1
Evapotranspiration

• Returning water to the atmosphere

2
Evapotranspiration (ET)

• Composed of two subprocesses
• Evaporation occurs on surfaces of open water or
  from vegetation and ground surfaces.
• Transpiration is the removal of water from the
  soil by plant roots, transported through the
  plant into the leaves and evaporated from the
  leafs stomata.
• Typically combined in mass balance equations
  because the components are difficult to partition.

Evapotranspiration
Transpiration
Evaporation
Open Water
Plants
Vegetation Surfaces
Soil
3
Potential vs. Actual ET

• Potential ET (PET) is the amount of evaporation
  that will occur if an unlimited amount of water
  is available.
• Actual ET (AET) is the actual amount of
  evaporation that occurs when water is limited.
  For large areas can use a mass balance approach
  to calculate (Eq. 4.5).

4
Some definitions

• Saturation vapor pressure (es)is the vapor
  pressure at which a liquid-vapor system is in a
  state of equilibrium. It increases exponentially
  with temperature (Fig. 4.11).
• Actual vapor pressure (ed)is the amount of
  pressure the water vapor in the air exerts on the
  surface it contacts (Eq. 4.7).
• Vapor pressure deficit (es-ed) is the difference
  between saturation and actual vapor pressures.
  The book presents three ways to determine the
  vapor pressure deficit.
• Relative humidity is the ratio of the amount of
  water present in the air to the amount required
  for saturation of the air at the same dry bump
  temperature and barometric pressure, expressed as
  a percentage.

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Evaporation

• Phase change of water from a liquid to a gas.
• Latent heat of vaporization is the energy needed
  by a molecule to penetrate the water surface (540
  cal/g of water evaporated at 100C.
• Rate of evaporation is driven by the vapor
  pressure deficit. Function of
• The ability of air to hold water based on air
  temperature and relative humidity.
• The energy in the water largely based on
  temperature.
• Net evaporation ceases when the air has reached
  the saturation vapor pressure.
• For evaporation to continue, some mechanism is
  needed to remove water vapor from the air above
  the evaporating surface (wind).

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Evaporation From Open Water

• Gives good estimation of PET rates.
• Effected by 4 (minor) factors
• Barometric pressure
• Dissolved matter
• Shape, site and situation of evaporating body.
• Relative depth of evaporating body.
• Monthly evaporation from lakes or reservoirs can
  be calculated using the formula developed by
  Meyer, based on Daltons law. (Eq. 4.17)

7
Evaporation from bare soil

• Similar to open water evaporation when soil is
  saturated.
• Divided into two stages.
• Stage 1 Soil is at or near saturation
• Evaporation is controlled by heat energy
• Approximately 90 of maximum PET
• Stage 2 Falling stage
• Surface starts to dry and evaporation occurs
  below the soil surface.
• Controlled by soil properties rather than weather
  conditions.

8
Evaporation from Vegetative Surfaces

• Interception is the water retained on plant
  surfaces during and after precipitation.
• 10 to 25 of annual precipitation is intercepted.
  
• Plant transpiration is reduced by the amount of
  intercepted water to be evaporated.

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Transpiration

• Transpiration is the loss of water in the form of
  vapor from plants
• Factors that affect transpiration rates
• Type of plant
• Wind
• Plant Available Water the portion of water in a
  soil that can readily be absorbed by plant roots.
  Amount of water released between field capacity
  (amount of water remaining in the soil after
  gravitation flow has stopped) and wilting point
  (amount of water in the soil at 15 bars of
  suction).

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Transpiration Ratio consumptive use

• Transpiration ratio is the ratio of the weight of
  water transpired to the dry weight of the plant.
  
• Measure of how efficiently crops use water.
• Examples Alfalfa (900), Wheat (500), Corn (350)
• Consumptive Use is the amount of water needed to
  grow a crop (ET requirement water stored in
  plant tissues).

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Measuring Evaporation and ET

• Several methods
• Evaporation Pans
• PET Gages-acts as surrogates for plants
• Soil Water Depletion
• Lysimeters
• Energy Balance and Mass transfer-measure average
  gradient of water vapor above the canopy.

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Pan Evaporation

• Oldest / simplest method to measure evaporation
• Measure water depths in a pan
• U.S. Weather Bureau has standard Class A pan
• Cylindrical container made of galvanized steel
• 10 inches deep and 48 inches in diameter
• Pan placed on a 6 inch wooden platform
• Site should be flat and free of obstructions
• Water filled to 8 inches deep
• Refill when water drops to 7 inches deep
• Water level measurements made using a hook gage
• Measurements to 0.01 inch

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Determining Pan factors

• Etrkp Epan
• Lake evaporation
• Typically taken as 70 of pan evaporation
• PET
• Pan evaporation times a coefficient ranging from
  0.6 to gt 1.0.

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Pan Evaporation / Example Problem

• Given
• Set up below with a class A pan
• Average wind speed 4.3 km/hr to the east
• Average relative humidity 67
• Measured water change in pan on July 1 7.5 mm

200 m
N
Class A Pan
200 m
Turfgrass (4 in.)
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Pan Evaporation / Example Problem

• Required
• Calculate the PET for July 1
• Solution
• Fetch
• Wind speed
• Set up
• Kp
• PET Kp x depth change
• PET

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Pan Evaporation estimates for Texas
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Lysimeters

• Allow an area to be isolated from the rest of the
  field while carefully measuring the individual
  components of the water balance.
• Weighing
• Non-weighing-measure drainage from the bottom

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Estimating ET

• SCS Blaney-Criddle Method
• Estimates seasonal AET.
• Can be used for monthly estimates if monthly crop
  coefficients are locally available (Table 4.8)
• Assumes mean monthly air temperature and annual
  day time hours can be used as an substitute for
  solar radiation to estimate the energy received
  by the crop.
• Monthly consumptive factor (f)
• Where t is the mean monthly air temperature in F
  and p is the mean monthly percentage of annual
  daytime hours (Table 4.6).

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Monthly Percentage of daytime hours, p
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Blaney-Criddle Equation

• U is the seasonal consumptive use in in/season
• K is the seasonal consumptive use coefficient for
  a crop with a normal growing season (Table 4.7)

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Seasonal consumptive Use Factors

• Mean monthly temperatures are available on the
  web at a variety of places. For example
• http//cdiac.esd.ornl.gov/r3d/ushcn/statemean.html
  
• http//www.weatherbase.com/

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PET estimation methods

• Simple models require measurement of only 1
  weather variable
• Temperature methods
• Relates PET rates to air temperature
• Thornthwaite Method (good only for east-central
  U.S.)
• Requires average monthly air temperature
• Latitude which is related to the length of day
• Radiation methods
• Relates PET rates to solar radiation
• Jensen-Haise method

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Penman methods

• Penman equations
• Equations to account for energy required to
  sustain evaporation
• Solar radiation
• sunshine
• Humidity
• Wind
• Long equations with many variables (Eqn. 4.30)
• Problems
• Complex equation
• Need to keep units consistent
• Need lots of data as inputs

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PET in Texas

• Daily PET (MM) January

• Daily PET (MM) August

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Daily evaporation in Belgium
Daily evapotranspiration (mm/day). June 26, 1996.
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Long Term Water Balances

• Basic equation for a control volume
• I - O DS
• Inputs Outputs Change in Storage
• Control volumes in hydrology
• Pond, cultivated field, subdivision, watershed,
  river basin, etc.
• Example1 Control volume is a pond
• Inputs (I)
• precipitation, runoff, water pumped in
• Outputs (O)
• Discharges, seepage losses, evaporation
• Change in Storage (DS)
• Change in volume of water stored in pond

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Long Term Water Balances

• Example 2 Control volume is a vegetated plot
• Inputs precipitation, irrigation
• Outputs evapotranspiration (ET), infiltration,
  runoff
• D S change in volume of water stored in the
  soil profile
• 2 conditions exist for vegetated plots
• If the soil profile is kept very wet ET is
  maximized.
• If the soil profile dries naturally ET is
  limited by available water in the soil profile

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BAEN 460 and AGSM 335

• Homework 3