Computing Evapotranspiration by Geostrophic Drag Concept

A procedure for computing regional evapotranspiration has been derived and tested. The assumptions underlying the model are essentially those used in the one dimensional mass transfer methods, but the shear stress is determined by means of τ_o = ρC_g²V_go² in which ρ = the density of the air; C_g = the geostrophic drag coefficient as first proposed by Lettau; and V_go = the surface geostrophic wind. The main advantage of the method is that all data needed in the computation, i.e., the surface temperature, the pressure of the air, the specific humidity at two levels in the atmospheric boundary layer, and the surface geostrophic wind speed can be obtained from standard meteorological data. Generally, good agreement was obtained between mean monthly evapotranspiration computed for a number of stations and the corresponding evaporation data obtained from Class-A pans.