Quantifying Driving Factors of Water Use Efficiency in Ningxia and Its Viticulture Zones Using an Optimal Geographic Detector

Water use efficiency (WUE) is a key indicator of carbon–water coupling in arid and semi-arid ecosystems. This study focuses on Ningxia and its grape-growing areas (2001–2024), integrating multi-source remote sensing with trend analysis, coefficient of variation, Hurst exponent, and geographic detector to examine WUE spatiotemporal evolution, stability, sustainability, and driving mechanisms: (1) The spatial distributions of high-value gross primary productivity (GPP), evapotranspiration (ET), and WUE were highly consistent across both Ningxia and its grape-growing regions; annual mean WUE increased at rates of 0.0059 and 0.0058 g C·m−2·mm−1·a−1, with a decrease-then-increase pattern covering 53.52% and 41.67% of the areas; (2) In Ningxia, ecosystem WUE is predominantly characterized by low-to-moderate volatility, accounting for approximately 89.37% of the area, and its sustainability is mainly represented by sustainability and decrease then increase pattern, covering about 53.06%. In the grape-growing regions, low-to-moderate volatility also dominates, accounting for about 84.25%, while the sustainability level is slightly lower than the average level of the whole Ningxia region; (3) The Normalized difference vegetation index (NDVI) and leaf area index (LAI) constitute the primary factors governing the spatial heterogeneity of WUE across Ningxia and grape-growing zones, with respective q-values of 0.73 and 0.91, whereas the human footprint (PopuFopr) and temperature (Tem) exert the lowest explanatory capacity for WUE spatial differentiation in both regions, with q-values of 0.14 and 0.18; (4) The interaction between NDVI and Tem exerted the strongest influence on WUE in Ningxia, with a q-value of 0.86, whereas the interaction between LAI and precipitation (Pre) played the dominant role in the grape-growing regions, achieving a q-value of 0.93. This study clarifies the synergistic impacts of climate, vegetation, and human activities on regional ecosystem WUE, which delivers theoretical support for refined water resource regulation and climate adaptation frameworks in arid regions, while also supporting the development of targeted ecological restoration and irrigation strategies.