Calibration and Modification of the Hargreaves–Samani Equation for Estimating Daily Reference Evapotranspiration in Iraq

Abstract

Daily reference evapotranspiration (ETo) is an essential component of the hydrologic processes of surface water generation from precipitation. Iraq suffers from a scarcity of surface water due to the erection of many dams on the Tigris and Euphrates Rivers by neighboring countries on upstream locations along these rivers. Therefore, ETo values should be measured more accurately in Iraq. In this study, the Penman–Monteith (PM) equation was used as the benchmark to calculate the standard values of ETo for 103 stations in Iraq using a daily meteorological data set from 2010 to 2019 (5 years each for calibration and validation periods). Three forms of the Hargreaves–Samani (HS) equation were evaluated for estimating the ETo values. The first two equations have common and optimum coefficient sets (HSC and HSO equations, respectively). The third equation is proposed in this study and modifies the HS equation by incorporating a wind speed variable with an optimum coefficient set (HSW equation). The coefficients of the HSO and HSW equations were optimized in terms of the Nash–Sutcliffe efficiency (NSE) coefficient and the coefficient of determination (R2) using the shuffled complex evolution (SCE) optimization method. The respective success rates of ETo estimation during the calibration and validation periods are 37% and 39% when using the HSC equation, 96% and 95% with the HSO equation, and 100% and 100% with the HSW equation. Results indicated that the HSO and HSW equations enhance the estimation of ETo values relative to the HSC equation, and the HSW equation performs best among the three. Therefore, incorporating the wind speed in the HS equation with the optimum coefficient set increases the robustness of its predictive capacity for all regions of Iraq, including those with high altitudes.