Energy Balance Approach to Evapotranspiration from Crops

Abstract

The vertical energy balance yields reliable estimates of evapotranspiration on an hourly (or halfhourly) basis under large variations of thermal stratification provided the measurements are made close to a reasonably homo geneous surface and provided that suitable spacial and timesampling procedures are followed. The energy bal ance method is relatively insensitive to incorrect assump tions concerning the eddy coefficients and to estimates of the Bowen ratio (ratio of sensible to latent heat) except during the unusual, and most often uninteresting, condi tion where the evaporation approaches the sensible heat flux to the surface (E = -A). The energy balance meth od, which measures the radiation exchange at the surface, places reasonable limits on evaporation estimates and thus is a promising method for daily estimates provided either periods of positive and negative net radiation are con sidered separately or that a reasonable estimate of the 24hour Bowen ratio can be developed. In humid regions there is little vertical transfer of sensible heat to the sur face, so that when "potential evapotranspiration" condi tions are obtained, the evapotranspiration will approximate the daily net radiation. In arid regions the evapotranspira tion from a wellwatered field may be, under extreme con ditions, almost twice the net radiation.Though the vertical energy balance is not adequate for estimating evapotranspiration from small plots, considera tion of the complete energy balance indicates serious limitations of smallplot experiments for evaluation of field practice. Energy balance measurements on corn, including the measurement of the radiation transmitted to the soil surface, indicates that the heat exchange at the soil surface is an appreciable fraction of the total heat exchange even in mature corn at high populations. Because of the rela tively high heat exchange at the soil surface, an under 'Contribution from the Department of Soils, University of Wisconsin, Madison. Published with the permission of the

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