## 4 Discussion  
### Attribution Analysis of the Spatiotemporal Variations in WUEc in Xinjiang  
This study demonstrates that crop water-use efficiency (WUEc) in Xinjiang exhibited an overall declining trend from 2002 to 2022, with a marked inflection point in 2007--rising before 2007 and decreasing thereafter. This pattern is consistent with previous findings; however, the underlying causes remain under debate. For example, [PERSON] et al. [55] applied the CASA model to estimate NPP and actual ET, and analyzed the spatiotemporal variations in WUE across Xinjiang from 1990 to 2020, identifying key driving forces of WUE changes. Their results indicate that precipitation and evapotranspiration were the primary contributors to the decline in WUE across the region. In contrast, [PERSON] et al. utilized MODIS data to explore the dominant factors influencing WUE in Central Asia (including Xinjiang) from 2001 to 2020, and found that interannual variation in WUE was mainly driven by GPP, with WUE dynamics closely mirroring those of GPP [56]. Considering the climatic changes and agricultural policy shifts in Xinjiang during 2002-2022, these discrepancies may be attributed to the following reasons: before 2007, a warmer and wetter climate, combined with policies such as the \"grain-to-cotton\" transition and land consolidation, led to a 35% increase in cotton planting area in southern Xinjiang, thereby enhancing WUEc. After 2007, rising temperatures, decreasing precipitation, expansion of low-productivity saline-alkali lands, and diminished water availability jointly contributed to a sharp decline in WUEc [57, 58].  
The conclusions of existing studies regarding the spatial differences in WUE between southern and northern Xinjiang remain inconsistent. For instance, [PERSON] et al. calculated WUE in Xinjiang from 1990 to 2020 using MODIS data and found that WUE in the southern plains was higher than in the mountainous areas of northern Xinjiang [55]. In contrast, [PERSON] et al. employed a fuzzy comprehensive evaluation model and reported that the average WUE in northern Xinjiang (e.g., Urumqi, Karamay) was 3.32 gC\(\cdot\)m\({}^{-2}\), which was higher than the 2.39 gC\(\cdot\)m\({}^{-2}\) observed in southern regions such as Aksu and Kashgar [59]. This discrepancy may stem from the different data sources used in WUE estimation. The conclusions of the present study are consistent with those of [PERSON], and the observed differences may primarily result from variations in regional climate conditions and cropping structures. Southern Xinjiang is rich in thermal and solar resources and benefits from glacial meltwater and well-developed irrigation infrastructure, which ensures a stable water supply and, thus, enhances WUEc. In contrast, although northern Xinjiang receives slightly more precipitation, it experiences lower temperatures and shorter growing seasons, with WUEc being constrained by thermal limitations [60]. Therefore, we recommend adopting region-specific strategies to implement the \"water and land red line\" policy, which includes reducing low-yield, inefficient farmland in northern Xinjiang and steadily improving agricultural productivity in the south, thereby enhancing the overall WUEc level across the region.