Independent testing for assessing the calibration of the Hargreaves-Samani equation: New heuristic alternatives for Iran

There is multitude of models for estimating daily reference evapotranspiration (ET0) using meteorological parameters. Among others, the temperature-based Hargreaves-Samani (HS) model is one of the frequently applied models for estimating ET0 when meteorological parameters in the studied station are limited. However, this method tends to require a preliminary local calibration. Most calibration procedures usually apply the same data sets for calibrating and testing. At the most, some studies reserve an independent test set for evaluating the calibrated model, but considering a single data set assignment. In the present study, the HS model and its calibrated version were assessed using meteorological parameters from 29 weather stations in Iran, through complete temporal and spatial data scanning, using a k-fold testing approach. A similar procedure was also repeated using the Gene Expression Programming (GEP) technique relying on the same input variables of the HS model. The results showed the importance of adopting k-fold based independent testing approach in order to avoid problems related to the influence of selected test period on the performance of the GEP models.