Dear Sir/Madam,

Good evening. Hope this e-mail finds you well. It is know that potential evapotranspiration (PET) is computed in the RegCM model from version 4.7 and upwards. However, calculating PET using PM method gives unreasonable estimates of PET and such calculation is not fully tested on a variety of time scales from daily to seasonal and annual.

Therefore, may I suggest to incorporate the Hargreaves and Samani (HS) method (Hargreaves and Samani, 1985) in the RegCM4 code to compute the PET as an alternative method (as recommended by FAO) based on the following recent literature reviews:

2 - https://www.researchgate.net/publication/361014193_Future_droughts_in_northern_Italy_high-resolution_projections_using_EURO-CORDEX_and_MED-CORDEX_ensembles

3- https://www.researchgate.net/publication/363350089_Projecting_the_Potential_Evapotranspiration_of_Egypt_Using_a_High-Resolution_Regional_Climate_Model_RegCM4

4 - https://www.researchgate.net/publication/354619329_On_the_Influence_of_Vegetation_Cover_Changes_and_Vegetation-Runoff_Systems_on_the_Simulated_Summer_Potential_Evapotranspiration_of_Tropical_Africa_Using_RegCM4

P.S. Calculation of PET (using HS) can be included in the STS output file because it requires daily values of the global incident solar radiation (rsds), daily air temperature extremes (tasnax, tasmin and tas).

Looking to hear from you.

Best Regards

Samy