It takes input maps of Albedo, surface skin temperature, latitude, day of year, single-way transmissivity and takes input value of the density of fresh water.
DEM is used for calculating min and max temperature for Senay equation.
The "-s" flag permits output map of evaporative fraction from Senay.
i.evapo.senay is highly dependent on the wet pixel being the lowest temperature in the crop pixels to work for non water stressed crops, force it that way, even if it breaks non crop areas. I suggest you reduce your region to the irrigation system boundaries, checking that it includes a bit of dry area for the hot/dry pixel.
Since it is a direct relationship to LST, evaporative fraction can be very sensitive to the kind of pixel sample you feed it with.
Chemin, Y., 2012. A Distributed Benchmarking Framework for Actual ET Models, in: Irmak, A. (Ed.), Evapotranspiration - Remote Sensing and Modeling. InTech. (PDF)
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