Providing accurate information of potential evapotranspiration (PET) is mandatory for arid regions (such as Egypt) for assessing the crop water requirements. Such precision is limited by the dynamical downscaling options and the physical settings used in regional climate models (like RegCM4). To address these issues, four simulations were run as part of the current study. The first two simulations take direct (DIR) and one-way nesting (NEST) into account, while the other two use two boundary layer techniques (HOLTSLAG; HOLT) and (University of Washington; UW). All simulations were driven by ERA-Interim reanalysis of 1.5 degrees. The simulated PET was evaluated with respect of high-resolution reanalysis gridded derived product of the ERA5-Land (hereafter ERA5).

The findings revealed that while there is no discernible difference between DIR and NEST in terms of the global incident solar radiation (RSDS). Also, NEST has a higher mean air temperature (TMP) than DIR. Additionally, UW has a lower TMP than HOLT, but switching between HOLT and UW did not impose a considerable impact on the simulated RSDS. Concerning PET, it is affected neither by switching between DIR and NEST or between HOLT and UW. Such results suggest that the RSDS is the main driver in controlling the PET variability followed by TMP. Therefore, using the DIR downscaling option and HOLT/UW boundary layer scheme throughout the 1980–2010, as recommended by the World Meteorological Organization, the RegCM4 model can be used to develop a regional PET map of Egypt.