Long-term relative decline in evapotranspiration with increasing runoff on fractional land surfaces

Abstract

Evapotranspiration (ET) accompanied by water and heat transport in the hydrological cycle is a key component in regulating surface aridity. Existing studies documenting changes in surface aridity have typically estimated ET using semi-empirical equations or parameterizations of land surface processes, which are based on the assumption that the parameters in the equation are stationary. However, plant physiological effects and its responses to a changing environment are dynamically modifying ET, thereby challenging this assumption and limiting the estimation of long-term ET. In this study, the latent heat flux (ET in energy units) and sensible heat flux were retrieved for recent decades on a global scale using a machine learning approach and driven by ground observations from flux towers and weather stations. This study resulted in several findings; for example, the evaporative fraction (EF) - the ratio of latent heat flux to available surface energy - exhibited a relatively decreasing trend on fractional land surfaces. In particular, the decrease in EF was accompanied by an increase in long-term runoff as assessed by precipitation (P) minus ET, accounting for 27.06% of the global land areas. The signs are indicative of reduced surface conductance, which further emphasizes that surface vegetation has major impacts in regulating water and energy cycles, as well as aridity variability.