Improving Actual Evapotranspiration Estimation Integrating Energy Consumption for Ice Phase Change Across the Tibetan Plateau

摘要

Permafrost thawing over the Tibetan Plateau (TP) is a consequence of climatic warming, which will change local hydrological processes remarkably. Evapotranspiration (ET) is an important local hydrological process indicator that needs to be well quantified. Several methods have been applied to estimate the ET. However, energy consumed by thawing was neglected in ET estimation on TP. Here a simple but effective method was introduced to represent the energy consumption due to ice phase changes in the generalized nonlinear complementary principle. Our method improved ET estimation by 4.60-106.67% in the nonlinear complementary model, validated at five eddy flux observation sites. With the new formulation, we analyzed the spatiotemporal patterns of ET and their driving factors during 1961-2014. The spatial averaged ET was 294.21 mm/year and decreased from southeast to northwest areas, cocontrolled by precipitation (Pre) and net radiation (Rn); dominated by the Rn in the warm-humid areas while by the Pre in the cold-dry areas. The temporal pattern of ET over the TP showed an increasing trend during 1961-2014, with a rate of 0.38 mm/year. The variations in air temperature (Tair) and Rn could explain 79.1% of the temporal variations in ET over the TP during the past 54 years, indicating atmosphere demand is the dominant factor on ET temporal variation. We also found that permafrost thawing accelerated the ET increases in the last 15 years over the transitional permafrost and seasonal permafrost areas, suggesting that degradation and ablation of permafrost under climatic changes will lead to accelerated ET.