Estimation of Reference Evapotranspiration using Regionally Calibrated Hargreaves-Samani Equation

Evapotranspiration (ETO) is a significant module in water-balance, irrigation scheduling and estimation of crop water requirement models. ETO can be adequately assessed when meteorological data are accessible to implement robust and strong models such as FAO-56 Penman-Monteith (PM). However, due to data insufficiency, substitute methodologies are essential. In this context, this study aims to calculate ETO from regionally calibrated Hargreaves-Samani (HSCAL), Hargreaves-Samani (HS) and Hargreaves methods which base on Land Surface Temperature (LST) and Solar Radiation (SR). SR was calculated from empirical formulas and Shuttle Radar Topography Mission (SRTM) 30m Digital Elevation Model (DEM). HSCAL uses SR which calculated from empirical formulas as an input, whereas HS and Hargreaves uses SR which calculated from the SRTM 30m DEM. LST was calculated from Landsat8 (LS8) thermal band for all three methods. Furthermore, ETO obtained from the HSCAL (ETO,HSCAL) was compared with standard FAO-ETO values and after verification HSCAL treated as standard for the verification of the remaining two methods on various Land Use Land Cover (LULC) types. Results of comparison between ETO,HSCAL and standard FAO-ETO shows that mostly values are within the range but lower side. Comparison also disclose that vegetation and built-up LULC are the best and worst case respectively. Further, ETO,HSCAL values are mostly fall within lower class of the ranges during the monsoon season (August-September). Further, the performance of the HS and Hargreaves are evaluated based on statistical indicators; Root Mean Square Error (RMSE), Mean Bias Error (MBE), Mean Absolute Error (MAE) and Correlation Coefficient (R2). ETO values of HS (ETO,HS) and Hargreaves (ETO,H) are underestimated in the sami-arid climate zone. The mean values of all statistical indicators are lower for ETO,HS in comparison to ETO,H when ETO,HSCAL is used to compare ETO,H with ETO,HS. It indicates that, in comparison to ETO,H, ETO,HS is close to ETO,HSCAL.