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Single- and Dual-Surface Implicit Energy Balance Solutions for Reference ET

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  5th National Decennial Irrigation Conference Proceedings, 5-8 December 2010, Phoenix Convention Center, Phoenix, Arizona USA  IRR10-8347.(doi:10.13031/2013.35821)
Authors:   Steven R Evett, Robert J Lascano, Terry A Howell, Judy A Tolk, Susan A O’Shaughnessy, Paul D Colaizzi
Keywords:   Irrigation scheduling, Evapotranspiration, Penman-Monteith, Crop coefficient, Implicit solution

The concept of a reference evapotranspiration (ETr) calculated from daily or hourly weather data, multiplied by a crop coefficient, Kc, in order to estimate crop water use, ETc, is widely established in agricultural science and engineering. To find region and variety-specific values of Kc from field-measured ETc values, the equation is inverted to: Kc = ETc/ETr. Forms of the Penman-Monteith (PM) formula for calculation of reference alfalfa or grass evapotranpsiration (ETr and ETo, respectively), have been promulgated by ASCE in 1990, FAO in 1998 and ASCE in 2005. The PM formulations are sensitive to climatic conditions, producing estimates of ETr and ETo that are more or less close to measured values depending on regional climate, and yielding values of Kc that vary from region to region and so are not transferrable. Theoretical shortcomings may be the basis of some of these problems, including the explicit nature of the calculation, which relies on the implied assumption that canopy and air temperatures are equal. We tested two surface energy balance formulations that stipulated different air and canopy temperatures, one a two-layer (soil and canopy) and one a one-layer (big leaf) approach but with soil heat flux included. Since canopy temperature is implicit in these formulations, they must be solved iteratively. Iterative solutions of ETr were compared with the FAO and ASCE PM formulations and against lysimeter-measured ETr. All three methods of ETr estimation produced ET values that compared very well with field-measured ET for alfalfa grown under reference ET conditions. Errors may occur with any of the three approaches to ETr estimation when stomatal resistance changes due to weather conditions; and assumptions of constant daytime and nighttime surface resistances thus cause mis-estimation of surface energy fluxes. It appears that a surface resistance value of 200 s m-1 at night for alfalfa grown under reference ET conditions is too large. It also appears that assuming constant daytime surface resistance of 30 s m-1 is probably not ideal, and that presenting daytime surface resistance as a function of vapor pressure deficit might improve ETr prediction.

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