Evapotranspiration2

# Evapotranspiration2 - WAYS TO MEASURE ACTUAL...

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1/25/2012 1 WAYS TO MEASURE ACTUAL EVAPOTRANSPIRATION Water Loss Methods: 1. Pan Evaporation, 2. Lysimeter Energy Budget Methods: 3. Bowen Ratio Vapor Flux Method: 4. Dalton Law Formula Combined Energy and Vapor Flux Methods: 5. Penman Equation Combined Energy/Vapor Flux/Plant Characteristics: 6. Penman-Monteith Equation Combined Energy and Vapor Flux with Wind: 7. Eddy Correlation Others: 8. Water Balance Technique 9. Groundwater levels 10. Geochemical Methods 11. Remote Techniques: Sattelite Imagery 3. Energy Budget Recognizes that the amount of ET possible is limited by the available energy. R net - G - W = H + λ v E •R net is the net solar radiation, in W/m 2 G is the amount of energy absorbed by the ground surface, (involved in changing the temperature of the soil), in W/m 2 W is the amount of energy involved in changing the temperature of water standing on land, in W/m 2 H is the sensible heat flux (heat transported by convection), W/m 2 λ v is the latent heat of vaporization of water in J/g, W/m 2 E is the evaporation rate of water in g/m 2 -s and λ v E is the latent heat flux, or heat involved in vaporization or condensation of water, W/m 2 Bowen Ratio is H/ λ vE This method is applicable to areas of standing water, dry soil, or vegetation areas. The principle is that there is a limited amount of energy available for vaporization of water and it doesn’t matter if the water vapor is produced from evaporation or transpiration processes. Major variable measured include: solar radiation, temperature of air, water, and soil, 4. Mass-Transfer Equation or Vapor Flux Method (aka Dalton Law Formula) • Uses the transfer of water vapor due to change in potential energy as well as wind speed. E = (a+bV)(e s -e a ) • where, E is the evaporation rate in m/d, a and b are empirical constants that depend on the height at which the wind speed and air vapor pressure are measured.

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Evapotranspiration2 - WAYS TO MEASURE ACTUAL...

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