Equilibrium and Actual Evapotranspiration from a Very Dry Vegetated Surface

Abstract

Energy balance measurements of evapotranspiration from very dry seeded rangeland grass (Agropyron desertorum) near Kamloops, British Columbia, are compared with daily equilibrium evapotranspiration estimates. The equilibrium model was found to seriously overestimate evapotranspiration from all sites at rates much less than potential. The data indicate by extrapolation that the accepted α value of 1.26, as reported by Priestley and Taylor, was confirmed at the lower limit of potential evapotranspiration. An attempt to select values of the proportionality factor α1, for nonpotential conditions, as a function of surface soil moisture proved to be inadequate. A relationship between α1 and soil moisture in the entire root zone was also unsuccessful.