Evaluation of actual and potential evapotranspiration in global products across Iran's sub basins

The purpose of the research is to evaluate the global gridded products database for two variables of actual (ETa) and potential evapotranspiration (ETp) across Iran's sub-basins. For this purpose, the daily meteorological data were collected from 100 synoptic stations around the Iran during 1987-2019. The six products including TERRA, CRU, ERA5, GLEAM, GLDAS and MERRA with 0.5×0.5 spatial resolution were compared to ETa and ETp in 30 sub basins. To assess the performance of six popular ET products, across Iran's sub basins on monthly scale, the Mean Bias Error (MBE), Pearson correlation coefficient (R) and Normalized Root Mean Square Error (NRMSE), of each product were calculated at the basin scale. The results show that more accuracy of products in ETp in comparison with ETa especially in July, August and September months. The more underestimate of ETa was accrued in hyper arid sub basins (e.g. Baluchistan with MBE equal to -34 mm per month). While, the more underestimate of ETp products were achieved in humid basins e.g. Haraz sub basin with MBE= -13 mm per month. The high underestimate of ETp were calculated to GLEAM product in June, July and August with -100 mm month-1. The high correlation between ETa observed and reanalysis are belonging to TERRA and ERA5 in Winter and Spring-Summer period, respectively. Based on NRMSE, results indicated that more accuracy of ETp comparison with ETa products with 0.19 (Excellent and Good) and 0.89 (poorly weak), respectively.