Impacts of evapotranspiration variability on the availability and utility of water resources in the Qilian Mountains

Under the dual contexts of global warming and vegetation greening, clarifying the impacts of evapotranspiration (ET) on water resources in arid regions is of critical importance. Focusing on the Qilian Mountains, this study quantitatively examined the effects of the spatiotemporal dynamics and the ET driving mechanism components—plant transpiration (T) and soil evaporation (E)—on regional availability (WA, WA/Pre) and effectiveness (T/ET, T/Pre) of water resouces from 2000 to 2023. The analysis utilized GLEAM data products, trend analysis, and structural equation modeling. The results showed a significant upward trend in ET (1.58 mm·a−1), primarily driven by increased T (1.79 mm·a−1). The T/ET ratio also increased (0.004 a−1), indicating improved effectiveness of water resources. Pre was the dominant environmental factor regulating ET variation, while T was mainly driven by the leaf area index (LAI; total effect 0.90). The T/ET ratio was most responsive to air temperature (Tem; total effect 1.68), which also acted as the dominant factor regulating E (total effect −0.748). Although vegetation restoration, characterized by increased LAI, enhanced effectiveness of water resources by increasing T, the combined effect of decreased precipitation and increased ET resulted in reduced WA, thus decreasing availability of water resources. This study highlights that while the increase in ET—particularly T—improves water resource use efficiency, it also exacerbates water scarcity. Accordingly, optimizing the trade-off between T and E is critical to alleviating this contradiction. These findings offer a scientific basis for ecological restoration and sustainable water resource management in arid regions under climate change.