Quantifying the contribution of terrestrial water storage to actual evapotranspiration trends by the extended Budyko model in Northwest China

Actual evapotranspiration (AE) is a key component of water cycle. The arid climate of northwest China makes water supply a major constraint on AE. Water source analysis is of great significance to study the causes of AE change. In this paper, we presented an assessment of AE trends in northwest China based on GLEAM (AE_GLEAM), an ensemble of six reanalyses (AE_reanalyses), and a complementary-relationship-based AE dataset (AE_CR). Both AE_GLEAM and AE_reanalyses increased significantly (p < 0.001) during 1981-2017. AE_CR also increased to a certain extent, but the rate was slower. In general, the increasing trends of AE were mainly found in mountainous areas, such as the Altai, Tianshan, Karakoram, Kunlun, Altun, and Qilian Mountains. For these regions, AE was even higher than precipitation in terms of both the climatological mean and the linear trends. It demonstrated that terrestrial water storage (S) could give supplementary supply for AE. Therefore, we extended the Budyko framework to evaluate the effect of S on AE. Taking the landscape parameter n = 1.8 as a reference, S contributed approximately 27% of total water availability for AE. Overall, S seemed to be a major contributor to the increasing AE in the Altai, Tianshan, Qilian, and Altun Mountains. Precipitation was the main source of evaporated water in the western Tianshan to Karakoram Mountains, where potential evapotranspiration also contributed to the increase in AE. In the case of different n values, the contribution from S had a deviation range of -16.9% to 8.5%. This model provides an opportunity to assess the response of AE to S in the absence of observational data.