Modified Hargreaves-Samani Equation for the Assessment of Reference Evapotranspiration in Alpine River Basins

Abstract

Deriving accurate estimates of reference evapotranspiration is required for water resource management, irrigation water requirement computations, and successful hydrological modeling. The Food and Agricultural Organization of the United Nations (FAO) recommended the Penman-Monteith equation as the standard for estimating reference evapotranspiration. An alternative method for application at sites where only air temperature measurements are available is the Hargreaves-Samani equation. The primary objective of this study is to investigate the possibility for application of the Hargreaves-Samani equation in alpine areas for computing daily reference evapotranspiration. An evaluation of the Hargreaves-Samani equation and its modifications proposed in literature is made by comparing daily estimates with Penman-Monteith results at 51 meteorological stations in the Upper Po River Basin (Italy) and the Rhone River Basin (Switzerland). Significant error was encountered in all methods using the Hargreaves-Samani equation. A relationship for adjusting the Hargreaves-Samani coefficient on the basis of local elevation is proposed, calibrated, and validated. The resulting modified Hargreaves-Samani equation showed a significant reduction of error for estimating daily reference evapotranspiration. The proposed equation is not intended for replacement of the Penman-Monteith method but for application in alpine rivers when only air temperature data are available.