## 3 Results
### Spatial Drivers and Synergistic Mechanisms of Irrigated Cropland CWUE
#### 3.3.3 Synergistic Effects of Drivers on Irrigated Cropland CWUE  
Figure 12 presents the SHAP dependence of the key drivers affecting irrigated cropland in Henan Province from 2001 to 2019. When the sunlight exposure was low (1400 h \(<\) Sun \(<\) 1550 h) and precipitation was also high (Pre \(>\) 800 mm), SHAP values were negative, indicating that both sunlight and precipitation negatively impacted the irrigated cropland \(\mathrm{WUE_{GFP}}\) within this range. Conversely, at higher sunlight levels (1550 h \(<\) Sun \(<\) 2200 h) with varying precipitation, SHAP values remained mostly positive,  
Figure 11: The dominant factors of spatiotemporal changes in CWUE and their spatial distribution in irrigated cropland across Henan Province from 2001 to 2019. The ranking of importance indices of the impact of drivers on irrigated cropland CWUE in Henan Province (**a1–a3**) and dominant factors of spatiotemporal variation in irrigated cropland CWUE and their spatial distribution (**b1–b3**). The histograms in the figure show the area share of every factor serves as the main driving force throughout the research area.  
suggesting a generally positive influence of sunlight and precipitation on the WUE\({}_{\text{GFP}}\) (Figure 12(a4)). For the photosynthetically active radiation absorption ratio (Rad), values in the lower range (0.25 \(<\) Rad \(<\) 0.35) combined with high precipitation (Pre \(>\) 800 mm) corresponded to negative SHAP values, indicating a negative effect on the WUE\({}_{\text{NPP}}\). As the Rad increased and precipitation decreased, SHAP values declined further, reflecting a growing negative impact (Figure 12(b3)). Similarly, as the LAI and precipitation increased, SHAP values for CUE showed a declining trend. This value gradually decreases from its positive peak to zero and then further declines below zero to its minimum negative value. This indicates that the influence of the LAI and precipitation on the CUE of irrigated cropland shifts from a gradually weakening positive effect to a progressively intensifying negative effect (Figure 12(c2)). Initially, moderate increases in the LAI and precipitation improved CUE by easing light and water limitations, thereby enhancing photosynthetic carbon assimilation. However, beyond optimal thresholds, an excessive LAI caused leaf shading, increasing respiratory carbon loss and reducing the nutrient allocation per leaf area. At the same time, excessive precipitation led to soil anoxia, impairing the root respiration and nutrient uptake, while promoting nutrient leaching and unproductive soil evaporation. Together, these factors transformed the LAI and precipitation from facilitators of carbon assimilation into metabolic stressors, ultimately reducing CUE.  
Figure 12: A shape-dependent graph of the importance of driving factors of CWUE of irrigated cropland in Henan Province from 2001 to 2019 (the abscissa axis represents the driving factor value, and the ordinate axis represents the SHAP value). The red to blue bars on the right side of the graph indicate the high to low values of precipitation, respectively. Similarly, the reddest points and the bluest points in the figure indicate the highest and lowest precipitation values, respectively. Shape dependency plots of the importance of WUE\({}_{\text{GFP}}\) drivers (**a1–a5**), the importance of WUE\({}_{\text{NPP}}\) drivers (**b1–b5**), and the importance of CUE drivers (**c1–c5**).