Subject: Solar radiation estimation using the soil moisture method Posted: 10/25/2016 Viewed: 9033 times
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?
Mr. Jack Sieber
Subject: Re: Solar radiation estimation using the soil moisture method Posted: 10/27/2016 Viewed: 8986 times
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.
Subject: Re: Solar radiation estimation using the soil moisture method Posted: 10/27/2016 Viewed: 8975 times
Thanks a lot Jack. I checked the handbook and the equations are very clear now.
Dr. Andrea Momblanch Benavent
Subject: Re: Solar radiation estimation using the soil moisture method Posted: 11/10/2021 Viewed: 885 times
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.
Topic "Solar radiation estimation using the soil moisture method"