Evapotranspiration

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Evapotranspiration

• Plan
• ET in Water Resources
• Evaporation physics
• Transpiration Process
• Methods to estimate E and ET

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ET
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Evapotranspiration
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Evapotranspiration in Water Resources

• Recall Homework 1, how much of the annual
  precipitatation was lost to ET in Dry Creek?

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ET in Water Resources

• Agrimet (http//137.77.133.1/pn/agrimet/ )
• Sebal (http//www.sebal.us/
• http//www.idwr.idaho.gov/gisdata/et.htm)
• ET in Idaho (http//www.kimberly.uidaho.edu/ETIdah
  o/)

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ET and Streamflow
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ET in Water Resources

• Water Yield
• http//ag.arizona.edu/oals/watershed/highlands/pon
  derosapine/ppmanagement.html
• http//books.google.com/books?iddHo4nO4ssJYCpgP
  A124lpgPA124dqwatershedETyieldsourceblots
  7WfWdoMS6nsigoZFAVIR8LP1oVM-0PSm0DYflaYohlen
  eiNr6sSayIDcTAnQejl8C2BgsaXoibook_resultresn
  um3ctresult
• http//www.srnr.arizona.edu/nemo/newsitems/NEMO_Ri
  pETBrochure.pdf

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Evaporation Physics

• Basic physical principles
• Conservation of Mass
• Conservation of Energy
• Ideal Gas Law related to water vapor
• Latent heat of phase change
• Turbulent transfer near the ground -
  (diffusion of momentum)Conservation of Mass
• Diffusive process driven by a vapor pressure
  gradient

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Evaporation Physics
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Evaporation Physics

• Daltons Law
• E a (esat(Ts) ea)
• esat(Ts) is the vapor pressure of the liquid
  which is related to the temperature of the liquid
• Evaporation occurs when the above relationship is
  positive.
• Condensation occurs when the above relationship
  is negative.
• What happens if RH is 100, but esat(Ts) gt ea?

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Evaporation Physics
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Evaporation Physics

• Daltons law is a proportionality. We make it a
  equality by adding a conductance term
• E KeVa (esat(Ts) ea))
• UNITS E (LT2M-1)(LT-1)(ML-1T-2) L/T

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Evaporation Physics

• Note the similarity between Daltons Law and our
  equation for LE
• E KeVa (esat(Ts) ea) L/T
• LE KleVa (esat(Ts) ea) Energy/(L2T)
• The relationship between E and LE is embodied in
  the K terms.
• Kle ?w?v Ke

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Evaporation Physics
-Water mass, rather than energy, is being diffused
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Evaporation Physics

• So, there are 4 conditions necessary for
  evaporation to occur.
• Energy available for phase change
• Water available at the surface or in root zone
• Vapor pressure gradient, or dry air
• Wind Capacity of the atmosphere to transport
  away moisture
• Conductance
• Factors that influence evaporation include
  anything that influence the above

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Open Water Evaporation
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Bare Soil Evaporation (pp 291-293
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Bare Soil Evaporation
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Transpiration
Evaporation of water from the vascular system of
plants. Process 1. absorption into roots, 2.
translocation to stomatal cavities, 3. evaporation
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Transpiration
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Transpiration
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Transpiration
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Transpiration

• The physics of evaporation from stomata are the
  same as for open water. The only difference is
  the conductance term.
• Conductance is a two step process
• stomata to leaf surface
• leaf surface to atmosphere

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Transpiration

• Evaporation from the leaf surface is simply open
  water evaporation
• E KeVa(es-ea)
• For transpiration we simply need to add the first
  step
• T KeVa(es-ea) Cleaf(est-es)
• Cleaf is leaf conductance and est is the vapor
  pressure in the stomatal cavity

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Transpiration

• We have to modify the open water K terms to
  account for vegetation
• First lets look at the atmospheric component
• E KeVa(es-ea)
• Ke is related to the ability of water vapor to
  diffuse through the atmosphere.

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Transpiration
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Atmospheric Constant
Atmospheric Conductance/windspeed
Z terms are related to vegetation
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Transpiration

• Leaf Conductance
• See equation 7-52
• Related to
• number of stomata per unit area
• Species particular
• size of the stomatal opening (Table 7-6)
• Controlled by light intensity, CO2, vapor
  pressure difference, leaf temperature, leaf water
  content
• See figure 7-13

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Transpiration

• Canapy Conductance
• Ccan fs(LAI)Cleaf
• fs shelter factor
• LAI Leaf Area Index

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Estimating Evaporation and ET
PET can be considered a drying power
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EstimatingEvaporation, POT, and actual
Evapotranspiration

• No good direct measurement method
• Methods
• Pan
• Water balance
• Mass transfer approach
• Energy balance approach
• Combination Penman Monteith
• Eddy correlation
• Temperate index Thornthwaite
• Radiation index Priestley Taylor

Observe changes in water level
Calculate from weather data
Relate to an index
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Pan method
Class A Pan Standard at advanced weather
stations Must relate actual evaporation to pan
with a coefficient Figure 7-7 Can be used to
estimate open water evaporation or POT Why not
actual evaporation or ET?
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Pan Evaporation
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Water Balance

• Evaporation or Actual Evapotranspiration can be
  calculated as the residual
• in-out ds
• Can be written for any body of open water,
  watershed, soil, pan..
• difficult to apply for large water bodies
• accuracy increases as dt increases

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Soil Water Balance
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Mass Transfer Approach

• apply Daltons law see equations 7-18a,b
• requires measurement of wind speed, surface vapor
  pressure, and air vapor pressure. None of these
  are commonly measured.
• Only good over short periods of time
• Gives instantaneous rate
• Ke is highly variable
• Many problems make this technique difficult to
  apply.

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Energy Balance Approach

• Just like snowmelt calculations
• See equation 7-22. Solve the energy budget
  equation for everything except LE. LE is the
  residual, then convert to E
• Notice the addition of Aw. What is advected water
  energy?
• Again, the technique is difficult to apply
• Data intensive
• Only good over small time intervals
• Need water surface temperature
• Bowen ratio is often used to eliminate the need
  to calculate sensible heat.

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Bowen Ratio
?- psychrometric constant
Using standard values for P and ?v the value is
0.066 kPa/K
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Combination Approach

• A key difficulty to applying the mass-transfer or
  energy balance approach is the need for surface
  temperature. If the two approaches are combined,
  the need to measure Ts disappears.
• See equation 7-33. Evaporation is the weighted
  sum of the rate due to net radiation and the rate
  due to mass transfer.
• Data needed
• K, L, Va, Ta, RH
• Question how do we obtain L?
• Penman is a commonly used approach
• Open water eqn 7-33, page 286
• ET eqn 7-56, page 299
• Whats the difference?

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Combination Approach

• Lets look at the terms in the Penman-Monteith
  equation
• S(Ta) ?
• (KL) ?
• Gamma ?
• Cat Ccan ?
• Is this an actual or potential ET approach?

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Combination Approach
es(Ta)
es(Ts)
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Other Concepts to Know

• Priestly Taylor
• Thornthwaite
• Turbulent transfer
• Eddy correlation, Bowen ratio