Quoted from:[1] Hargreaves G H ,  Samani Z A . Estimating Potential Evapotranspiration[J]. J.irri. & Drain.div.asce, 1982, 108(3):225-230.

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Increasing population and needs for an augmented food supply give greater importance to improved procedures for estimating agricultural water requirements both for irrigation and for rain-fed agriculture. Four methods for estimating potential evapotranspiration are compared and evaluated. These are the Class A evaporation pan located in an irrigated pasture area, the Hargreaves equation, the Jensen-Haise eguation, and the Blaney-Criddle method. The evaporation pan is rated as superior to the other methods. However, the difference in reliability between the pan and the Hargreaves method are not considered to be very significant. Both the Jensen-Haise and the Hargreaves methods require either measured or estimated solar radiation. Methods are presented for estimating solar radiation from the difference between maximum and minimum temperature, from the percentage of possible sunshine, and from relative humidity. These procedures have some limitations, but provide improved reliability and make the estimates more universal.