Estimation of Reference Evapotranspiration during the Irrigation Season Using Nine Temperature-Based Methods in a Hot-Summer Mediterranean Climate

The FAO-56 Penman-Monteith (PM) equation is regarded as the most accurate equation to estimate reference evapotranspiration (ETo). However, it requires a broad range of data that may not be available or of reasonable quality. In this study, nine temperature-based methods were assessed for ETo estimation during the irrigation at fourteen locations distributed through a hot-summer Mediterranean climate region of Alentejo, Southern Portugal. Additionally, for each location, the Hargreaves-Samani radiation adjustment coefficient (k(Rs)) was calibrated and validated to evaluate the appropriateness of using the standard value, creating a locally adjusted Hargreaves-Samani (HS) equation. The accuracy of each method was evaluated by statistically comparing their results with those obtained by PM. Results show that the calibration of the k(Rs), a locally adjusted HS method can be used to estimate daily ETo acceptably well, with RMSE lower than 0.88 mm day(-1), an estimation error lower than 4% and a R-2 higher than 0.69, proving to be the most accurate model for 8 (out of 14) locations. A modified Hargreaves-Samani method also performed acceptably for 4 locations, with a RMSE of 0.72-0.84 mm day(-1), a slope varying from 0.95 to 1.01 and a R-2 higher than 0.78. One can conclude that, when weather data is missing, a calibrated HS equation is adequate to estimate ETo during the irrigation season.