Abstract
This work showed the Hargreaves equation (HG) can be modified into a highly efficient computational replacement for the FAO-56 Penman-Monteith equation and its auxiliary functions for the computation of daily reference evapotranspiration (ETo) estimation when only available data are daily temperature data. By modifying all the constants (i.e., '0.0023', '0.5' and '17.8') in HG and adding a new constant, a modified HG (HG1234) gave almost identical estimates of daily ETo as FPM, with modified coefficient of efficiency, 𝐸1 = 0.99, 𝑟2 = 1.00, and MAE of 0.00 mm/d for a weather station in Accra, Ghana. HG1234 and ten other less drastic modifications of HG were compared against FPM at simulated average wind speeds, 𝑢2, of 0.5 m/s, 2.0 m/s and 4.0 m/s for daily estimates of ETo. In general the uncalibrated HG predicted FPM with very low 𝐸1 at 𝑢2 other than the global average of 2 m/s, and the more drastic the modification of HG the higher its efficiency at simulating FPM ETo at all wind velocities. Thus although HG was not originally developed for daily ETo estimation, modifying it to HG1234 can turn it into a very efficient and much faster simulator of FPM daily ETo estimation that would be very useful in applications, such as areal ETo estimation research using satellite images, where fast and frequent re-evaluations of FPM ETo for millions of points are necessary.
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