Global Evapotranspiration from High-Elevation Mountains Has Decreased Significantly at a Rate of 3.923%/A Over the Last 22 Years

Abstract

Changes in the water cycle will have a significant impact on human activities and ecosystems in the context of global warming. Therefore, the actual evapotranspiration change and its attribution analysis in the study area during the period of 2001-2022 were analyzed with the global high-elevation mountains as the study area. Eighteen methods for calculating reference evapotranspiration based on four methods, namely the method of comprehensive, temperature, radiation and mass transfer, were selected to simulate the change of reference evapotranspiration in our study. Meanwhile, we try to improve the evapotranspiration simulation model based on the selected optimal model, and simulate the future evapotranspiration changes based on the scenario simulation data. Our results show that: 1) The global evapotranspiration at high altitude decreases significantly at a rate of 3.923%/a on annual scale from 2001 to 2022. And the evapotranspiration varies in the range of 31.179-33.652 mm·month-1, with a mean value of 32.646 mm·month-1; 2) The model of Irmark-Allen based on the radiation is more suitable for the study of evapotranspiration in high-altitude mountains, and the results of precision analysis and Taylor diagram also show that the simulation effect of evapotranspiration based on the radiation method is the best. After the improvement of the model by the model of least-squares, the simulation formula of the original Irmark-Allen model is modified to ET0=0.710+0.289*Rn+0.023*T. The fitting R2 values of evapotranspiration before and after optimization are 0.633 and 0.853, respectively. 3) Scenario simulation results showed a significant upward trend in evapotranspiration under both the scenary of SSP245 and SSP585. Attribution analysis showed that VPD is the most important positive driver affecting evapotranspiration changes at the global high-altitude mountain scale. Meanwhile, structural equation modeling indicates that changes in net radiation and precipitation also significantly affect the changes of evapotranspiration. The water balance shows a decreasing trend in evapotranspiration between 2001 and 2022. Therefore, the highly significant decrease in VPD, the increase in the NDVI index of vegetation, and the decrease in shallow soil water content during 2001-2022 were the underlying causes of the significant decrease in evapotranspiration at high altitude mountains.

Keywords: Alpine mountains, evapotranspiration, Modelling, Water balance, Environmental changes