Hi - I’m working through the he ‘modified Hargreaves’ method as described under Urban Cooling model guidance. When I Google RA (extraterrestrial radiation), I’m told this is a constant i.e. “the current accepted value for this “solar constant” (now referred to as the total sky irradiance) is 1366 W/m2.”
Does this sound right?
Cheers
Richard
Hi @Minton71 -
I’m not familiar with the 1366 W/m2 value, but when I calculate Modified Hargreaves, I usually use an RA value more related to the location of my study, and the values need to have units of MJ/m2/day. One general source for this is the FAO document “Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56”. Radiation values can be found in Annex 2. I select the value that’s in the latitude closest to my area of study.
Another option could be to use a GIS tool to calculate solar radiation, and use that spatial result as input to the Modified Hargreaves calculation.
~ Stacie
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Hi Stacie - many thanks for getting back to me on this. I obtained the RA from GIS data quite easily. I’m now in the process of obtaining the remaining precipitation and temperature variables required by Modified Hargreaves. But when I look at the Urban Cooling model input table, what is actually asked for is a Reference Evaporation raster. Does this mean converting the result of Modified Hargreaves into a raster? Or should we be able to obtain a raster (from the same public sources we can generally find climate data) showing ‘Reference Evapotranspiration’?
Thanks for clarifying.
Regards
Richard
Hi Richard -
Yes, you will need to create a raster of Reference ET. This just means doing the Hargreaves equations in GIS using something like Raster Calculator. One example, based on global data, is to download monthly precipitation and min/max temperature rasters from WorldClim, and use those, along with the RA raster you’ve created.
If you’d rather use a prepared layer, the User Guide notes CGIAR as one global source for Reference ET, which I have used. I think that there has been a post or two on this forum where people have mentioned other sources as well.
~ Stacie
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