Assessment of actual evapotranspiration product accuracy using NDVI and GRACE-derived water balance in Fars, Iran

Although several remote sensing products have been developed to estimate actual evapotranspiration (ETa) in recent years, identifying a suitable product is still challenging due to the lack of validation data and the varying performance of ETa products in different regions. This study evaluated the performance of six remote sensing products, namely WaPOR, EEFlux (hereafter referred to as METRIC), SSEBop, GLEAM, FLDAS, and TerraClimate, in arid and semi-arid regions of Fars Province, Iran. These products were evaluated using two indirect methods: the integrated normalized difference vegetation index (iNDVI) and the water balance estimates derived from the GRACE satellite data for the four water years 2018–2019 to 2021–2022. During 2018–2019 and 2019–2020, METRIC showed the best performance, with mean annual ETa values of 370 and 372 mm/year, respectively. In 2020–2021 and 2021–2022, WaPOR performed best, with annual ETa values of 208 and 207 mm/year, respectively.