Evapotranspiration

1
Evapotranspiration

• Watershed Hydrology NREM 691
• Week 3
• Ali Fares, Ph.D.

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

3
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(Wsws)(1- a)Ia-Ig
• Rn is determined by measuring incoming outgoing
  short- long-wave rad. over a surface.
• Rn can or
• If Rn gt 0 then can be allocated at a surface as
  follows
• Rn (L)(E) H G Ps

4

• 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.

5
(No Transcript)
6
ET Available Soil Water
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.

8
Yw -1.3 MPa
Yw -1.0 MPa
Yw -0.8 MPa
Yw -0.75 MPa
Yw -0.15 MPa
Ys -0.025 MPa
9
Measuring ET Pan-Evaporation
10
(No Transcript)
11
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

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

13
Methods to Calculate PETPhysically 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

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

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

16
Evapotranspiration
ET P Q ?S - ?D

• ?S watershed storage variation (mm)
  SendSbeginning
• P Precipitation (mm)
• Q Stream flow (mm)
• ?D Seepage out seepage in (mm)
• ET evaporation and transpiration (mm)

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.
18
Rain
Transpiration
Evapo-transpiration
Irrigation
Evaporation
Runoff
Root Zone
Water Storage
Below Root Zone
Drainage
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
Effects of Vegetative Cover
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
ET / Potential ET