Main question

Reference evapotranspiration estimates are widely used in water resources management, hydrological modeling and irrigation engineering to define crop water requirements. The de-facto standard, the Penman-Monteith equation, is relatively high data demanding and sensitive to data difficult to measure. Is a simpler method less sensitive to errors in data?

Methodology

A global climate dataset was used to evaluate the Penman-Monteith and the Hargreaves reference evapotranspiration equations on sensitivity in data observations. The use of a global dataset ensured that results are valid over a broad spectrum of climate conditions.

The two equations were first compared assuming that no error in data existed and later a random error was added to the dataset.

Figure 1. Comparison between Penman-Monteith and Hargreaves reference evapotranspiration equations on a global scale.

Result and conclusions

The deviation between Penman-Monteith and Hargreaves is for many regions low, except for very arid regions (Figure 1). Including a random error term shows that the Hargreaves equation performs better then Penman-Monteith (Figure 2), despite its simplicity.

The conclusion from this study was that Penman-Monteith is still the recommended method as long as accurate data can be guaranteed. In cases where accurate data collection is less certain the Hargreaves method should be considered as alternative.

Figure 2. Impact of inaccuracy of meteorological observations on the two reference evapotranspiration methods.

Publications

Droogers P. and R.G. Allen. 2002. Estimating reference evapotranspiration under inaccurate data conditions. Irrigation and Drainage Systems, Volume 16, Issue 1, February 2002, Pages 33 – 45 doi:10.1023/A:1015508322413

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