Computing Evapotranspiration by Geostrophic Drag Concept

by John A. Mawdsley, Teaching Asst.; School of Civ. and Envir. Engrg., Cornell Univ., Ithaca, NY,
Wilfried Brutsaert, (A.M.ASCE), Assoc. Prof. of Civ. and Envir. Engrg.; Hollister Hall, Cornell Univ., Ithaca, NY,

Serial Information: Journal of the Hydraulics Division, 1973, Vol. 99, Issue 1, Pg. 99-110

Document Type: Journal Paper


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 τo = ρCg²Vgo² in which ρ = the density of the air; Cg = the geostrophic drag coefficient as first proposed by Lettau; and Vgo = 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.

Subject Headings: Weather forecasting | Evapotranspiration | Shear stress | Terrain models | Mass transfer | Shear resistance | Drag (fluid dynamics) | Computing in civil engineering

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