For the Water Yield model, I need to compute reference evapotranspiration. I am using a modified Hargreaves Equation for the computation of reference evapotranspiration. For the equation, we need a value of RA which is extraterrestrial radiation. As mentioned on InVEST user guidelines, I checked FAO irrigation drainage paper 56 which has values for radiation in its Annex 2.
The values are for integer latitude degrees. As my stations are on decimal latitude degrees, can I interpolate it from two values? For a clear illustration, for Feb month on 34-degree latitude, RA value is 39.2 and for 36-degree latitude, RA value is 38.9. My station is on latitude 34.5 degrees. So I basically interpolate it between 34 and 36 degree or is there any other way to find value for 34.5 degree Lat?
When I use the Annex 2 values, honestly I just use the value in the table that is closest to the latitude of my study area, which is generally within 1 degree so it’s an ok approximation. It seems reasonable to me to interpolate between the table values if you want to get more precise, keeping in mind that the table values are already somewhat coarse (as is the very simple Annual Water Yield model.)
Depending on how you’re calculating ET, you could also try a GIS tool for creating a solar radiation layer. I haven’t tried these in a long while, but I believe that ArcGIS and QGIS both have them, which would allow for a much more localized result.
Hi Shikxya
Now I need RA data as well, but after I downloaded this PDF file, Ifound that the 2.6 and 2.7 tables of Annex 2 are blank.So if it is convenient, could you please provide me with a form? (Email : m15510761127@163.com )
Thank you very much!
Sunsiqi