Hi, I'd like to know what method WEAP applies to estimate solar radiation when using the soil moisture method (not MABIA). Is it the hargreaves-samani equation?

WEAP uses equations from Chapter 4 of the Handbook of Hydrology for calculating net solar radiation, specifically equations 4.2.5, 4.2.6, 4.2.7 and 4.2.13.

You can view pages in the book on Amazon. Equations for solar radiation start on page 114 (search for "solar"): https://www.amazon.com/Handbook-Hydrology-David-Maidment/dp/0070397325#reader_0070397325

Jack

Thanks a lot Jack. I checked the handbook and the equations are very clear now.

Denny

Hi Jack,

The Soil Moisture Method uses Angstrom equation to calculate incoming shortwave radiation (Rs) from cloudiness fraction (CF). That equation considers that Rs is always between 25% and 75% of extraterrestrial solar radiation (Ra) since coefficients a and b in that equation are 0.25 and 0.5 respectively.

I am modelling the Santa River basin in the Peruvian Andes and I am using observed Rs to calculate cloudiness fraction (CF) as an input for WEAP by inverting the Angstrom equation. However, in that basin Rs/Ra is sometimes (many times in fact) larger than 0.75 and that results in CF values above 1. I know if I use those CF values I will get the right Rs that WEAP uses to then calculate ETo, but WEAP does not run with CF>1. Do you have any ideas as to how I can overcome the problem? The only way I see at the moment would be removing the CF>1 condition from WEAP.

Thanks,
Andrea

To provide clarity on cloudiness fraction and solar radiation-

Cloudiness fraction can only be a maximum of 1 because it represents the fraction of time that overcast skies are present, which cannot go above 100%.