Resistance‐energy Balance Method for Predicting Evapotranspiration: Determination of Boundary Layer Resistance and Evaluation of Error Effects1

Abstract

AbstractAccurate estimates of evapotranspiration (ET) over large areas are needed for hydrologic studies, irrigation planning and scheduling, and other practices related to efficient utilization of water resources. One approach is the use of a resistance‐energy balance model of ET, the data for which can be supplied, in part, by remote sensing and in part from easily accessible records of National Weather Service observations. This model requires measurement of boundary layer resistance, crop and air temperature, net radiation, and soil heat flux. The objectives of this paper are i) to test the utility of the proposed model, ii) to determine boundary layer resistance at varying stages of crop growth, and iii) to evaluate the relative influence of errors in measurement of the meteorological and crop input parameters used.Measurements were made in fields of sorghum [Sorghum bicolor (L.) Moench] and millet (Panicum meliaceum L.) grown at Mead and Mitchell, Neb. Boundary layer resistance was estimated from friction velocity measurements used in a stability‐corrected aerodynamic method. Friction velocity was computed by means of the Deacon‐Swinbank approach. Evapotranspiration rates estimated by the resistance model compared well with results of lysimetric and energy balance measurements, on both a short‐period and a daily basis.The error analysis, in conjunction with field measurement, indicates that the resistance model evapotranspiration estimates are quite sensitive to errors in crop temperature measurement, especially in nonadvective conditions, but are less strongly affected by errors in the estimation of boundary layer resistance.