Methodology

Reference evapotranspiration (ETo)

The derivation of ETo had developed over many years with several different equations. The latest and hopefully last variant is derived from the Penman-Montieth equation.

Extensive documentation on the methods and concepts can be found in the UN-FAO 56 paper [1]

Hargreaves

The derivation for the Hargreaves equation can also be found in the UN-FAO 56 paper.

The History and Evaluation of Hargreaves Evapotranspiration Equation [2] is a more detailed description and background of the Hargreaves method.

References

1

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9), D05109.

2

Hargreaves, G. and Allen, R. (2003). History and Evaluation of Hargreaves Evapotranspiration Equation. Journal of Irrigation and Drainage Engineering. Vol. 129, Issue 1 (February 2003).