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United States Department of Agriculture

Agricultural Research Service

Title: Calculation of water evapoation in arid climates)

item Lascano, Robert
item Van Bavel, C
item Evett, Steven - Steve

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2010
Publication Date: 11/11/2012
Citation: Lascano, R.J., Van Bavel, C.H., Evett, S.R. 2012. Calculation of water evapoation in arid climates[abstract]. 3rd International Conference on Deserts, Drylands, and Desertification. November 8-11, 2011, Sede Boqer, Israel.

Interpretive Summary:

Technical Abstract: The concept of maximum rate of water evaporation, i.e., potential evapotranspiration (ETp, mm d-1) was introduced by Charles Thornthwaite in 1944 and defined as the water loss from vegetation with wet soil. Methods to calculate ETp are divided into empirical and theoretical, which combine physical concepts and empirical observations. Theoretical methods relate the flux of water vapor from the evaporating surface to the process of turbulent diffusion, i.e., aerodynamic, and a second method considers the energy balance calculating the energy used in vaporizing water. A third method, introduced by Howard Penman in 1948, combined the aerodynamic and energy balance approaches eliminating the surface temperature (Ts, °C) from the relevant equations and is known as the combination method. In 1951, a similar and independent approach to calculate ETp was given by Mikhail Budyko who termed his technique the complex method. The main distinction between the combination and complex methods is that the assumption made by Penman regarding Ts and humidity of the evaporating surface was not required by Budyko. The complex method finds the Ts of the evaporating surface that satisfies the energy balance by iteration. The application of ETp to estimate crop ET (ETc, mm d-1) was recognized by introducing a “stomatal factor” equated to a bulk canopy resistance (rc, s m-1) and considered in the so-called “big leaf” model. Calculation of ETc with a rc using the Penman model is known as the Penman-Monteith (PM) method and the two well-known adaptations are the FAO-56 and American Society of Civil Engineers equations. We have termed the use of rc with the Budyko method as the Recursive Combination Method (RCM). We compared calculations of ETp and ETc using the PM and the RCM for a 45-day summer period in Lubbock, TX. Results showed that daily values of ETp and ETc were underestimated by as much as 25% when using the PM rather than the RCM method. Further, underestimation of ETp and ETc is more pronounced in arid environments. The RCM can also be used to calculate rc provided radiometric (Ts)_ or lysimetric (ETc) values along with air temperature and humidity, wind speed, and global irradiance values are available. The RCM is based on the same physical principles of the PM method but uses iteration to find an accurate answer of ETp and ETc. The RCM should be used under conditions of high ambient and low dewpoint temperatures, i.e., arid climates.

Last Modified: 8/24/2016
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