Evaluation of canopy temperature-evapotranspiration models over various crops
Hatfield, J.L. | Reginato, R.J. | Idso, S.B.
Canopy temperatures, when measured remotely, offer a method of estimating evapotranspiration with surface energy balance models. Equations which were developed by others were evaluated only at a limited number of locations and with a few crops. The study was conducted at several locations with weighing lysimeters with a variety of crops (cotton, grain, sorghum, soybean, alfalfa and tomato) around the USA: Brawley, California; Temple, Texas; Lincoln, Nebraska; St. Paul, Minnesota; Fargo, North Dakota; Kimberly, Indiana and Davis, California, to evaluate evapotranspiration utilizing canopy temperature as an input into the surface energy balance. Evapotranspiration calculated from the aerodynamic resistance form of the surface energy balance was well correlated with lysimeter measurements at all locations. The errors using the surface energy balance were < 10% in all cases for full ground cover. The Bartholic Namken Wiegand method was more closely coupled to net radiation than canopy temperature. Under partial canopy cover, differences between the 2 models were apparent. The Bartholic Namken Wiegand model overpredicted when the actual evapotranspiration was > 200 W m-2 because of its insensitivity to surface temperature. The surface energy balance model exhibited only a slight overprediction above 200 W m-2 when a weighed composite surface temperature (representative of bare soil and crop temperature) was used. This small overprediction could be overcome by considering the soil heat flux term. There was no location bias in the surface energy balance model, which shows that it shoould work well at other locations.
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