Improved Framework for Attribution Analysis of Contributing Climatic Factors Influencing Reference Evapotranspiration

Reference evapotranspiration (ETo) quantifies the atmospheric water demand from a hypothetical reference crop under standardized conditions. Analyzing the changing dynamics of the climatic drivers, this study rigorously assesses their individual contributions to the trend of ETo. The spatial and temporal variations of ETo, and influencing climatic drivers are studied based on data from 87 conventional weather stations (CWS) from 1990 to 2020 over different climatic zones in India. Gap-filling was performed to overcome the data discontinuity, and inhomogeneity was identified and corrected. The Penman–Monteith (PM) method was used to compute ETo. This study proposes the use of a second-order attribution model to account for the interaction among the climatic drivers and reduce the approximation error. The mean annual ETo was found to be 1,836.956 mm/year, with a declining trend observed over India from 1990 to 2020. The second-order attribution model helped improve the attribution analysis over the first-order attribution model by considering second-order independent and interactive components. Attribution analysis over the climatic zones showed that decreasing solar radiation (dimming) and wind speed (stilling) are the major contributors to the declining trend of ETo. Vapor pressure is also found to be a major contributor, which is attributed to the increasing irrigation practices causing an increasing trend of vapor pressure over India. Temperature contributes to increasing trends, which gets offset by solar radiation, wind speed, and vapor pressure, eventually leading to an overall decline of ETo. The existence of the evaporation paradox over India is also discussed. Assessment of the factors contributing to the fluctuations of ETo, is valuable for informed irrigation planning and water resource management practices.