Daily reference evapotranspiration modeling by using genetic programming approach in the Basque Country (Northern Spain)

► We use Genetic Programming (GP) technique to model daily reference evapotranspiration. ► The GP results are compared with those of the ANFIS and empirical models. ► Two approaches were followed: Each station approach and Pooled data approach. ► Comparison results show that the GP models perform better than the others. Evapotranspiration, as a major component of the hydrological cycle, is of importance for water resources management and development, as well as for estimating the water budget of irrigation schemes. This study presents a Gene Expression Programming (GEP) approach, for estimating daily reference evapotranspiration ( ET 0) in four weather stations in Basque Country (Northern Spain), for a 5-year period (1999–2003). The data set comprising air temperature, relative humidity, wind speed and solar radiation was employed for modeling ET 0 using FAO-56 Penman Monteith equation as the reference. The GEP results were compared with the Adaptive Neuro-Fuzzy Inference System (ANFIS), Priestley–Taylor and Hargreaves–Samani models. Based on the comparisons, the GEP was found to perform better than the ANFIS, Priestley–Taylor and Hargreaves–Samani models. The ANFIS model is ranked as the second best model.