2012 Volume 122 Pages 15-21
The proper estimates of evapotranspiration （ET）are critical for better animal manure and/or wa-ter management practices in farmlands. Micrometeo-rological methods, e.g., the Bowen ratio method and the eddy covariance method, to estimate ET have been popularly used. Those methods, however, require specially‒designed instruments or the multiple num-bers of instruments. Estimates of ET based on the en-ergy balance at the canopy surface, on the other hand, need only routine weather data that were easily avail-able. Evapotranspiration estimated by the energy bal-ance method along with routinely‒obtained weather data was evaluated by comparing with estimates of ET by the Bowen ratio method. Weather data, i.e. shortwave irradiance, Rs（W m‒2），air temperature, Ta（K），relative humidity, and wind speed, required for the energy balance method were measured in a reed canarygrass field located near Morioka in north‒east-ern Japan. A Bowen ratio system was installed along-side in the same grass field. An energy balance on a grass canopy is expressed as a function of canopy sur-face temperature, Ts（K）．Using the Newton‒Raphson method, the energy balance equation was numerically solved for Ts , to obtain an evapotranspiration rate, E （kg m‒2 s‒1）．Values of net irradiance, Rn（W m‒2）and latent heat flux, LE （W m‒2），were overly estimated with the energy balance method when Rn>500W m‒2. For Rn<500W m‒2, however, Rn, LE, and sensible heat flux, H（W m‒2），estimated with the energy balance method agreed well with those estimated with the Bowen ratio method. Daily ETs between July 1 and 28, 2003 were calculated by accumulating E between sunrise and sunset. The daily ET values obtained by the two methods agreed very well with the correlation coefficient, r = 0.96（P<0.001）．The energy balance method to estimate daily ET could be promising.
