Multi-scale fractal characteristics and driving attribution of water use efficiency across China's agricultural zones.

Water use efficiency (WUE) in ecosystems is a key indicator for assessing regional resource utilization efficiency and characterizing the water-carbon balance. However, there exists a lack of quantitative metrics to assess the heterogeneity levels and spatial structural complexity of different agricultural regions. This study applied an improved the Triangular Adaptive Differential Box-Counting (TADBC) method to calculate the fractal dimension (FD), which is used to quantify the spatial complexity of WUE and reveal the spatiotemporal variation characteristics and driving mechanisms of 38 sustainable agricultural regions in China. The results showed that the national average WUE across China's agricultural regions was 2.19 gC·kg-1H2O, with a distribution pattern of high in the east and low in the west. Approximately 84% of the WUE in these agricultural zones showed an upward trend, FD showed a distribution pattern of high in the west and low in the east, and the Hurst index of the eastern ecosystems was generally higher than 0.5, which reflected the strong spatial stability of agricultural ecosystems. Although temperature (TEM) and precipitation (PRE) play crucial roles in driving WUE, their influence on the spatial structure of WUE is primarily mediated indirectly through the interaction between Vapor pressure deficit (VPD) and Normalized Difference Vegetation Index (NDVI), rather than acting directly. The proposed framework extends WUE assessment from magnitude-based evaluation to spatial-complexity diagnosis, providing methodological support for agricultural water-resource management in agricultural landscapes.