The Evaporation Paradox in India

Abstract

Evapotranspiration is pivotal in the hydrological cycle, significantly influencing agricultural practices, water balance and irrigation. An increase in reference evapotranspiration (ETo) is anticipated because of global warming; however, a decrease in pan evaporation/reference evapotranspiration is observed in various regions worldwide– known as the “evaporation paradox”. We study the existence of the evaporation paradox in India from 1990 to 2020 using data from 87 ground stations spanning across various climatic zones and its causative factors. Out of the four climatic drivers influencing ETo – solar radiation, wind speed, relative humidity, and air temperature; solar radiation was the major contributing factor leading to a decline of ETo over India. The reason being - ETo was observed to be highly sensitive to solar radiation, along with a decreasing trend of solar radiation (or sunshine hours). A seasonal analysis of the contributions analysis shows that - solar radiation is the major contributor in winter (Dec-Feb) and post-monsoon season (Oct-Nov) and the second major contributor after relative humidity during the summer season (Mar-May), all leading to a decline in ETo. During the monsoon (June-Sept), solar radiation does contribute to the declining trend of ETo. However, wind speed and vapour pressure are the major contributors during the monsoon season. Global dimming, which prevailed from the 1950s until late 1980 in most parts of the world, continues to persist over India without any signs of abating.