Evolution Characteristics of Water Use Efficiency and the Impact of Its Driving Factors on the Yunnan–Guizhou Plateau in China

Water use efficiency (WUE) of ecosystems plays a crucial role in balancing carbon storage and water consumption. The Yunnan–Guizhou Plateau, a karst landscape region with relatively fragile ecosystems in China, requires a better understanding of the evolution of WUE and the factors driving it for the region’s ecological sustainability. This study employs Theil–Sen slope estimation and Mann–Kendall significance analysis to investigate the temporal trends and spatial patterns of WUE in the study area. Additionally, a machine learning model, XGBoost, is used to establish driving relationships, and the SHAP model is applied to interpret the importance of the driving factors and their specific relationship with WUE. The results show that (1) WUE exhibits an increasing trend, with a slope of 0.002, indicating improved water absorption and utilization capacity of vegetation in the region. (2) The spatial distribution of WUE follows a “high–low–high” pattern from southwest to northeast, with 6.68% of the area showing a significant increase, 50.80% showing a weak increase, 4.60% showing a significant decrease, and 37.92% showing a weak decrease. (3) The importance of the driving factors is ranked as follows: normalized difference vegetation index (NDVI), maximum temperature (TMAX), shortwave radiation (SRAD), Palmer drought severity index (PDSI), vapor pressure deficit (VPD), and precipitation (PRE). The NDVI has a linear positive relationship with WUE; SRAD has a decreasing effect on WUE, with this effect weakening at higher values; and TMAX, PRE, the PDSI, and VPD show a non-monotonic relationship with WUE, increasing and then decreasing. The findings of this study are significant for ecological civilization construction and sustainable development in the region.