δe = vapor pressure deficit (kPa) λv = latent heat of vaporization (MJ kg−1) Note: this formula implicitly includes the division of the numerator by the density of water (1000 kg m−3) to obtain evaporation in units of mm d−1 Some useful relationships δe = (es - ea) = (1 – relative humidity) es es = saturated vapor pressure of air, as is found inside plant stoma. ea = vapor pressure of free flowing air. es, mmHg = exp(21.07-5336/Ta), approximation by Merva, 1975 Therefore , mmHg/K Ta = air temperature in kelvins See also Pan evaporation Evapotranspiration Thornthwaite model Blaney–Criddle equation Penman–Monteith equation Notes References Jarvis, P.G. (1976) The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field. Phil. Trans. R. Soc. Lond. B. 273, 593–610. Neitsch, S.L.; J.G. Arnold; J.R. Kliniry; J.R. Wolliams. 2005. Soil and Water Assessment Tool Theoretical Document; Version 2005. Grassland, Soil and Water Research Laboratory; Agricultural Research Service. and Blackland Research Center; Texas Agricultural Experiment Station. Temple, Texas. https://web.archive.org/web/20090116193356/http://www.brc.tamus.edu/swat/downloads/doc/swat2005/SWAT%202005%20theory%20final.pdf Penman, H.L. (1948): Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. London A(194), S. 120–145. Agronomy Equations Hydrology