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Computes the PET with the method of Hargreaves and Samani [Hargreaves_Samani_1985], modified by Droogers and Allen [Droogers_Allen_2002].

ET_o = 0.0013 (R_a/\lambda_H) (T_{avg}+17) (\Delta T - 0.0123 P_{mm})^{0.76}

where P_{mm} is the cumulative rainfall [mm].


rainfall : TimeSeries

Monthly cumulative rainfall [mm], as a TimeSeries object.


This relation was derived for monthly data, assuming that total precipitation is to some extent representative of relative humidity A warning will be issued if the frequency of the PotentialEvapoTranspiration is not monthly.

[Droogers_Allen_2002] compared their estimate of ET_o with the estimates calculated with the ASCE version of Penman-Monteith and the Hargreaves model [Hargreaves_Samani_1985] to assess the effiency of each model in the presence of inaccurate climatic data. They concluded that their model was better at approximating the Penman-Monteith ASCE equation, and that it was more accurate than this latter when random errors were introduced in the dataset.