EVALUATION OF CANOPY TEMPERATURE--EVA POTRANSPIRATION MODELS OVER VARIOUS CROPS*

Hatfield, J. L., Reginato, R. J. and Idso, S. B., 1984. Evaluation of canopy temperatureevapotranspirati on models over various crops. Agric. For. Meteorol., 32: 41--53. Canopy temperatures, when measured remotely, offer a method of estimating evapotranspiration with surface energy balance models. Equations which have been developed by others have been evaluated only at a limited number of locations and with a few crops. Our study was conducted at several locations with weighing lysimeters with a variety of crops around the United States: Brawley, CA; Temple, TX; Lincoln, NE; St. Paul, MN; Fargo, ND; Kimberly, ID; and Davis, CA, to evaluate evapotranspirati on utilizing canopy temperature as an input into the surface energy balance. The results show that evapotranspirati on 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 less than 10% in all cases for full ground cover. The Bartholic--Namke n--Wiegand method was more closely coupled to net radiation than canopy temperature. Under partial canopy cover, differences between the two models were apparent. The Bartholic--Namke n--Wiegand model overpredicted when the actual evapotranspirati on was above 200 W m -2 because of its insensitivity to surface temperature. However, 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 should work well at other locations.