1. Lecture 11 Evapotranspiration (4)
ET Measurements
Lysimeters
Evaporation Pans
Pan size effects

2. Lysimeter Allow determination of water balance of a block of soil
Maintain wet soil to measure PET
Allow soil moisture to fluctuate to measure AET
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3. Weighing Lysimeters Use a balance to assess changes in storage
Small lysimeters can be removed and weighed
Measure both PET and AET

4. Draining Lysimeters
Soil block is isolated but must be undisturbed and in the same surroundings
Can’t measure change in storage, only inputs and outputs
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5. Evaporation Pan (US Class A) standard of the National Weather Service (NWS)

6. Sunken Colorado Pan

7. Evaporation Pans
Solve a simplified water-balance equation for a given period (t):
E = W – (V2 – V1)
E = Evaporation
W = Precipitation during t
V1 = Volume at beginning of t
V2 = Volume at end of t

8. Estimating the Effect of Pan Size
Empirical equations to account for energy exchange through the side the pan, in order to estimate free water evaporation:
+ when Tspan > Ta
- when Tspan < Ta
Efw = Evaporation from free-water surface
Epan = Pan evaporation
P = Atmospheric pressure (mb)
vpan = Avg. wind speed 15 cm above pan (km/day)
Tspan = Water-surface temperature (C)
Ta = Air temperature (C)
pan = Proportion of energy lost to evaporation
Kohler et al. (1955)

9. Evaporation Pans Free-water evaporation
Expose a cylindrical (or rectangular) pan of water to the atmosphere
Provide estimates of potential evapotranspiration (PET) – can be more accurate than formulae
Must be related to vegetation canopies using other methods (coefficients etc.)
Precipitation is measured in an adjacent rain gauge
Pan volume is measured by recording water height (which is maintained by adding or removing measured amounts of water)
Pans can be placed on land in clearings or on floating platforms on open water and usually surrounded by fences

10. Plot Studies
Water balance approach applied to small undisturbed plots of land
Analogous to small lake for simple evaporation
Must select suitable sites where inputs/outputs can be measured
Spatial uncertainty increases with size of plot
Artificial rain (sprinklers) can be used to assess PET
Soil moisture assessed by spot sampling and measurement by:
Gravimetric methods
Soil block moisture
Neutron probe
Time domain reflectometry

11. Catchment Studies Water balance of entire drainage basin
Even greater spatial uncertainties
  Variable precipitation, soil water capacity
Ground water seepage
Better for long term estimates
On a yearly basis, change in storage is approx. zero