UBC Faculty Research and Publications

Test of an Equation for Evaporation From Bare Soil Water Black, T. Andrew; Novak, M. D.


An empirical equation developed by Idso et al. (1979) at Phoenix, Arizona, to calculate daily average evaporation rates during all three drying stages of a bare soil was tested using measurements made at Agassiz, British Columbia, and is discussed on the basis of available evaporation theory. The results show that their expression for potential evaporation rate did not apply at Agassiz due to differences in the advection regimes at the two locations. The Agassiz potential evaporation rate data was well represented by the Priestley-Taylor equation with aPT (‘alpha’) = 1.27 ± 0.1. It was concluded that the Idso et al. equation for potential evaporation rate has no greater generality than the Priestley-Taylor or other such semiempirical approaches. The concept of expressing the stage III rate as proportional to the expression for potential evaporation rate worked marginally well at a culti-packed site and quite well at a disc-harrowed site. It was concluded that for soils with stage III rates much greater than 50% of potential evaporation rate, more complete procedures are necessary for calculating evaporation rates during extended drying periods. An edited version of this paper was published by AGU. Copyright 1982 American Geophysical Union.

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