Analysis and Calibration of Empirical Relationships for Estimating Evapotranspiration in Qatar: Case Study

Knowledge of evapotranspiration (${\mathrm{ET}}_o$), which is the process of water loss from vegetated soils due to evaporation and transpiration, is important in real-time irrigation management and water-resource allocation, particularly in water-scarce regions. In this study, several methods used in estimating evapotranspiration, including the Blaney-Criddle, Hargreaves-Samani, Jensen-Haise, Linacre, and Turc methods were calibrated and validated against the Penman-Monteith model, which is considered as the standard method of estimating evapotranspiration. The paper utilizes data from the Doha International Airport meteorological station over a period of 30 years (January 1985–December 2014). ${\mathrm{ET}}_o$ values were estimated using the different methods. These values were then compared to those obtained by the Penman-Monteith method. Using appropriate indicators, the Turc method was found to be the best for estimating ${\mathrm{ET}}_o$ over Doha ($R^2=0.9519$, $\mathrm{RMSE}=1.4511\mathrm{mm}{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}$, and $\mathrm{MAE}=1.1633\mathrm{mm}{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}$). The Turc method comes in handy for estimating ${\mathrm{ET}}_o$ over Qatar as it utilizes only three meteorological parameters (mean temperature, relative humidity, and solar radiation), which are easily measurable over that area.