This document discusses various methods for calculating actual evaporation from land and vegetation. It introduces concepts like pan evaporation, reference evapotranspiration, and actual evapotranspiration. Methods covered include long-term water balance, Budyko curves, soil moisture functions, complementary relationship approaches, and turbulent transfer models like Penman-Monteith. Measurement techniques involve lysimeters and remote sensing of factors like leaf area index and fractional canopy coverage.

1. Methods of Calculating Actual Evaporation
实际蒸散发的计算方法
水文学原理
李红 (挪威奥斯陆大学)
lihong2291@gmail.com

2. 2
Concepts
Epan (pan evaporation) is the evaporation measured
by a evaporation pan.
ETp (reference crop evapotranspiration or reference
evapotranspiration, or potential evapotranspiration) is
the evapotranspiration rate from a reference surface,
not short of water. PE
ETa (actual evapotranspiration) describes all the
processes by which liquid water at or near the land
surface becomes atmospheric water vapour under
natural condition. AE

3. 3
Controlling factors

4. 4
Controlling factors
Water
- Amount, depth, temperature, quality, …
Energy
- Solar radiation, net radiation, temperature, ...
 Dynamics
- Vapour pressure, wind speed, humidity, …

5. 5
1. Lysimeter
2. Remote sensing (?)
Measurement

6. 6
Input: Rainfall and Additional water
Output: Percolated water PE = R + A – P
R A
P
Measurement: Lysimeter

7. 7
Input: Rainfall and Additional water
Output: Percolated water
R
P
Measurement: Lysimeter

8. 8
Input: Rainfall and Additional water
Output: Percolated water AE = R – P – ΔS
R
P
Measurement: Lysimeter

9. 9
Measurement: Remote Sensing

10. 10
Measurement: Remote Sensing
PE
AE













 ）（ cc
k
ff
LAI
LAI
w
w
11,min
max

W: water content in soil, Wk: water capacity
LAI: leaf area index, LAImax: maximum LAI of the whole year,
fc: the fractional canopy coverage.

11. 11
Methods of Calculating Actual Evaporation

12. 12
– In arid case, P <<PE, water limited
AE = P
– In humid case, P >>PE
AE = PE, energy limited
Methods of Calculating Actual Evaporation

13. 13
1 . Long-term water balance
2. Budyko types
3. Use of soil moisture functions
4. Complementary relationship approach
5. Turbulent-transfer/energy-balance methods
Methods of Calculating Actual Evaporation

14. 14
1. Long-term water balance method
AE = P – Q
Input: P, Q
Application: Annual or longer time steps
Methods of Calculating Actual Evaporation

15. 15
AE = P – Q – ΔS
Input: P, Q
Methods of Calculating Actual Evaporation

16. 16
AE = P – Q – ΔS
Input: P, Q
Methods of Calculating Actual Evaporation
http://www.nasa.gov/mission_pages/Grace/index.html

17. Methods of Calculating Actual Evaporation
17
2. Budyko types

18. Methods of Calculating Actual Evaporation
18













P
PE
PAE exp1







PE
P
PEAE tanh
 2
1
PE
P
P
AE


2. Budyko types
Input: P, PE
Application: Annual or longer

19. Methods of Calculating Actual Evaporation
19
3. Use of soil moisture functions
)( relfPEAE 
pwpfc
pwp
rel






Input: PE,
Application: Monthly, daily or hourly


20. 20







 1,
LPFC
minPA

EE
Soil moisture (mm)
0
1
act.evaporation/pot.evaporation
FCFC * LP
AE/PE

Methods of Calculating Actual Evaporation
3. Use of soil moisture functions
)( relfPEAE 
pwpfc
pwp
rel







21. 21
Methods of Calculating Actual Evaporation
3. Use of soil moisture functions
)( relf 

22. 22
Methods of Calculating Actual Evaporation
3. Use of soil moisture functions
AE/PE
rel

23. Increase of wetness
evapotranspiration
Based on heuristic arguments of Bouchet (1963)
AE + PE = 2 ETw
23
Methods of Calculating Actual Evaporation
4. Complementary relationship approach

24. 24
CRAE (Morton, 1983)
Advection-aridity (AA) (Brutsaert and Stricker, 1979)
GG (Granger and Gray, 1989)
Methods of Calculating Actual Evaporation
4. Complementary relationship approach
AE = 2 ETw - PE

25. 5. Turbulent-transfer/energy-balance methods
25
Methods of Calculating Actual Evaporation
FAO: http://www.fao.org/docrep/X0490E/x0490e00.htm
Penman-Monteith equation

26. 5. Turbulent-transfer/energy-balance methods
26
Methods of Calculating Actual Evaporation
FAO: http://www.fao.org/docrep/X0490E/x0490e00.htm
Penman-Monteith equation