Methods to estimate evapotranspiration in humid and subtropical climate conditions

Selecting appropriate reference evapotranspiration (ETo) methods is crucial for managing water resources efficiently. Statistical criteria commonly used to assess the performance of empirical ETo models on a station level may produce inconsistent results, making ranking approaches a complex process. This study was conducted in India's Humid and Subtropical region, considering 11 years of mean daily data from 2009 to 2019. We evaluated thirty empirical ETo models, which were categorized into four groups based on the input parameters, namely, temperature-based (10), radiation-based (10), mass transfer-based (9), and combination model (1). The results show that the observed ETo reached maximum magnitude during Monsoon followed by Spring, Summer, Autumn, Winter, and pre-Winter season; the average observed ETo from 2009 to 2019 was ≈ 1163 mm. Among temperature-based and radiation-based models, the Hargreaves model with RMSE of 1.45 mm/day and the Turc model with RMSE of 1.01 mm/day yielded the best ETo predictions under the humid, sub-tropical climate conditions. The radiation-based models demonstrate higher accuracy in the prediction of ETo than the temperature-based and mass transfer-based models. The FAO56-PM technique, Turc model, Hargreaves model, Makkink model, and Papadakis model were ranked as the five best models among all 30 tested models. Overall, the FAO56-PM method outperformed among all 30 selected models. Thus, the exact calculation of ETo is essential for many agricultural water engineering applications, particularly in developing countries with a lack of meteorological data records and limited resources to conduct long-term in-situ observation of evapotranspiration. The methodological approach proposed in this work applies to any other location for a simple yet rigorous evaluation of evapotranspiration empirically.