Enhanced surface water-energy coupling on the Tibetan Plateau over the past six decades (1960-2020).

Long-term changes in surface water and energy budgets on the Tibetan Plateau (TP) are essential for understanding the variability of the Asian summer monsoon and water resources. The water budget and energy budget are inherently coupled, and in this study evaporative fraction (EF, the proportion of the surface available energy used for evapotranspiration) is used to quantify their coupling strength and simultaneously study its response to climate change in the TP. A so-called 'surface flux equilibrium' (SFE) method that uses only air temperature and humidity as inputs to estimate EF is evaluated and adopted for understanding EF variability in the TP during the warm season of the past six decades (1960-2020). The results show an overall increase in EF of the TP, but this increase is not uniform, instead comprising three distinct stages of change, i.e., little change until the 1980s, a significant increase in the next two decades, and a slight but insignificant decrease in the last two decades. Accordingly, evapotranspiration has undergone similar decadal changes over this period. More importantly, there is a spatial pattern of the response to climate change, showing a stronger enhancement of EF in the west TP than in the east TP during 1979-2020. This response pattern shows a weakened contrast in the surface water cycle between the dry west and the wet east TP. The enhanced coupling and diminished spatial contrast may provide a perspective for understanding spatiotemporal changes in environment and ecology. Finally, it is worth pointing out that given the simplicity of the SFE method, it is expected that it can be applied in climate-sensitive regions outside the TP.