Assessing Changes in Hargreaves Evapotranspiration Model Accuracy Across Time and Altitude

Modeling evapotranspiration is an increasingly relevant topic in scientific discussions, as its volume and trends are essential for identifying climate change. However, there is still no accepted method use as a reference for evapotranspiration modeling. The most preferred method is the FAO65 Penman-Monteith (P-M) model, which is widely used as a reference method for calculating reference and crop evapotranspiration and is recommended by scientific authorities. The aim of the research was to regionalize the Hargreaves model for calculating reference evapotranspiration under Central European conditions, aiming to achieve accuracy as close as possible to the P-M model.

A significant finding of the study is that the model coefficients are not stable over time, and therefore the accuracy of any modification to the Hargreaves model must be regularly validated.  Our results revealed a decrease in the accuracy of the modified Hargreaves model as the altitude of the climatological station increased. When altitude was incorporated into the Hargreaves equation, the model's accuracy significantly improved for stations at higher elevations, achieving a consistent level of accuracy across all stations, regardless of their location or altitude. Additionally, the results suggested that the optimal values for the model coefficients vary over time, with the B coefficient showing a decreasing trend of -0.5 and the C coefficient declining by -0.1 between the periods 1981-2000 and 2001-2020. This issue is particularly pronounced in the analysis of shorter time periods, where model may lead to substantial accuracy reductions.