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Energy Balance Approach to Evapotranspiration from Crops¹

ABSTRACT

The vertical energy balance yields reliable estimates of evapotranspiration on an hourly (or half-hourly) basis under large variations of thermal stratification provided under large variations of thermal stratification provided the measurements are made close to a reasonably homogenoeous surface and provided that suitale spacial and time-sampling procedures are followed. The energy balance method is relatively insensitive to incorrect assumptions 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, condition where the evaporatin approaches the sensible heat flux to the surface (E = -A). The energy balance method, 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 megative net radiation are considered separately or that a reasonable estimate of the 24-hour Bowen ratio can be developed. In hu,id regions there is little vertical transfer of sensible heat to the surface, so that when "potential evapotranspiration" conditions are obtained, the evapotranspiration will approximate the daily net radiation. In arid regions the evapotranspiration from a well=watered field may be, under extreme conditions, almost twice the net radiation.

Though the vertical energy balance is not adequate for estimating evapotranspiration from small plots, consideration of the complete energy balance indicates serious limitations of small-plot 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 relatively high heat exchange at the soil surface, an under-standing of capillary movement of soil water to the soil surface is necessary if the evaporation and transpiration components of consumptive-use are to be evaluated from experiments on corn and other plantings that do not fully intercept the radiation.

¹ Contribution from the Department of Soils, University of Wisconsin, Madison. Published with the permission of the Director of the Wisconsin Agr. Exp. Sta. This work was supported in part by Army Electronic Proving Ground Contract DA-36-039-SC-80063. This paper was presented at the annual meeting of the American Association for the Advancement of Science as part of the symposium on Water and Agriculture, Dec. 30, 1958, at Washington, D. C.

² Associate Professor of Soils, University of Wisconsin, Madison.

Received for publication May 13, 1959. Accepted for publication September 3, 1959.