1. Watershed Hydrology NREM 691 Week 3 Ali Fares, Ph.D.
Evapotranspiration
Watershed Hydrology NREM 691
Week 3
Ali Fares, Ph.D.
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Watershed Hydrology Lab. Fall 2006

2. Objectives of this chapter
Explain and differentiate among the processes of evaporation from a water body, evaporation from soil, and transpiration from a plant
Understand and be able to solve for evapotranspiration (ET) using a water budget & energy budget method
Explain potential ET and actual ET relationships in the field.
Under what conditions are they similar?
Under what conditions are they different?
Understand and explain how changes in vegetative cover affect ET.
Describe methods used in estimating potential and actual ET
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Watershed Hydrology Lab. Fall 2006

3. Watershed Hydrology Lab. Fall 2006
Energy Budget
L is latent heat of vaporization, E evaporation, H energy flux that heats the air or sensible heat, G is heat of conduction to ground and Ps is energy of photosynthesis.
LE represents energy available for evaporating water
Rn is the primary source for ET & snow melt.
Net radiation:
Rn=(Ws+ws)(1- α)+Ia-Ig
Rn is determined by measuring incoming & outgoing short- & long-wave rad. over a surface.
Rn can – or +
If Rn > 0 then can be allocated at a surface as follows:
Rn = (L)(E) + H + G + Ps
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Watershed Hydrology Lab. Fall 2006

4. Watershed Hydrology Lab. Fall 2006
An island of tall forest vegetation presents more surface area than an low-growing vegetation does (e.g. grass).
The total latent heat flux is determined by:
LE = Rn + H
Advection is movement of warm air to cooler plant-soil-water surfaces.
Convection is the vertical component of sensible-heat transfer.
In a watershed Rn, (LE) latent heat and sensible heat (H) are of interest.
Sensible heat can be substantial in a watershed, Oasis effect were a well-watered plant community can receive large amounts of sensible heat from the surrounding dry, hot desert.
See Table 3.2 comparison
See box 3.1 illustrates the energy budget calculations for an oasis condition.
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Watershed Hydrology Lab. Fall 2006

5. Watershed Hydrology Lab. Fall 2006
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Watershed Hydrology Lab. Fall 2006

6. ET & Available Soil Water
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Watershed Hydrology Lab. Fall 2006

7. Water movement in plants
Illustration of the energy differentials which drive the water movement from the soil, into the roots, up the stalk, into the leaves and out into the atmosphere. The water moves from a less negative soil moisture tension to a more negative tension in the atmosphere.
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Watershed Hydrology Lab. Fall 2006

8. Watershed Hydrology Lab. Fall 2006
Yw~ -1.3 MPa
Yw~ -1.0 MPa
Yw~ -0.8 MPa
Yw~ MPa
Yw~ MPa
Ys~ MPa
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Watershed Hydrology Lab. Fall 2006

9. Measuring ET: Pan-Evaporation
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Watershed Hydrology Lab. Fall 2006

10. Watershed Hydrology Lab. Fall 2006
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11. Watershed Hydrology Lab. Fall 2006
PET Definitions
Penman Definition:
The amount of water transpired in unit time by a short green crop completely shading the ground, of uniform height and never short of water.
Therefore, PET based on atmospheric conditions and a specific vegetation type
11/28/2018
Watershed Hydrology Lab. Fall 2006

12. Methods to Calculate PET
Empirical Estimation Equations for a Reference Crop
Kimberly – Penman Combination Equation
Combines energy and diffusion components
Radiation based equations
Priestley – Taylor equation
Turc equations
Doorenbos and Pruitt (FAO-24) method
Jensen – Haise method
11/28/2018
Watershed Hydrology Lab. Fall 2006

13. Methods to Calculate PET Physically Based Equation
Penman-Monteith Reference Crop Evaporation Equation
The most advanced model of evaporation
Assumes all energy available is accessible by the crop canopy
Uses:
Net radiation (Rn)
Soil heat flux (G)
Air temperature
Wind speed (U)
Vapor pressure deficit
11/28/2018
Watershed Hydrology Lab. Fall 2006

14. Methods to Calculate PET
Empirical Estimation Equations for a Reference Crop
Temperature based equations
Hargreaves equation
Blaney – Criddle (SCS TR-21) method
Thornthwaite method
Pan Evaporation Methods
FAO-24 method
Christiansen – Hargreaves method
11/28/2018
Watershed Hydrology Lab. Fall 2006

15. Water Mass balance Equation
S =(I + R + U) - (D + RO + ET)
ET = Evapotranspiration
R, I = Rain & Irrigation
D = Drainage Below Rootzone
RO = Runoff
S = Soil Water Storage variation
U = upward capillary flow
11/28/2018
Watershed Hydrology Lab. Fall 2006

16. Watershed Hydrology Lab. Fall 2006
Evapotranspiration
ET = P – Q – ΔS - ΔD
ΔS= watershed storage variation (mm): Send–Sbeginning
P = Precipitation (mm)
Q = Stream flow (mm)
ΔD = Seepage out – seepage in (mm)
ET = evaporation and transpiration (mm)
This equation is telling us that if you know the water content in the soil profile now (St) and after a certain period of time (1 hour, 1 day) and if you also know how much irrigation or rainfall that was added into the system you can determine the losses out of the system D and ET.
Thus, this equation has two unknowns: ET and D. If we determine one of them then the equation will be left with only one unknown that can be determined form the equation. ET can be determined from weather data and using mathematical equation such as penman-monteith. It can be determined form lysimeter measurements. For our case we choose to calculate drainage using Darcy’s flow equation.
11/28/2018
Watershed Hydrology Lab. Fall 2006

17. Soil Water Mass Balance
There are different ways to estimate drainage.
The direct method is the use of lysimeters.
Lysimeters have a weighing device and a drainage system, which permit continuous measurement of excess water and draining below the root zone and plant water use, evapotranspiration.
Lysimeters have high cost and may not provide a reliable measurement of the field water balance.
11/28/2018
Watershed Hydrology Lab. Fall 2006

18. Watershed Hydrology Lab. Fall 2006
Rain
Transpiration
Evapo-transpiration
Irrigation
Evaporation
Runoff
Root Zone
Water Storage
Below Root
Zone
Drainage
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Watershed Hydrology Lab. Fall 2006

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25. Effects of Vegetative Cover
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30. Watershed Hydrology Lab. Fall 2006
ET / Potential ET
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Watershed Hydrology Lab. Fall 2006