Spatial transformation of river runoff under hydroclimatic change and agricultural water use in the Shu River basin, Central Asia

Abstract

Understanding the spatial differences in runoff response to hydroclimatic variability and anthropogenic water use is essential for sustainable water resource management in arid transboundary river basins. This study evaluates spatial differences in runoff–hydroclimatic relationships along the Shu River basin using long-term river runoff observations and key hydroclimatic indicators. Hydroclimatic changes were assessed using potential evapotranspiration (ET), climatic water balance, and the Standardized Precipitation Evapotranspiration Index (SPEI) for the upper and lower parts of the basin. Long-term trends were assessed using the Modified Mann–Kendall test (Yue and Wang modification) and Sen’s slope estimator, while relationships between runoff and hydroclimatic variables were examined using Spearman correlation analysis. The results reveal pronounced spatial differences in the mechanisms governing runoff variability across the basin. In the upper reaches, runoff remained strongly associated with long-term moisture availability (SPEI-12; ρ = 0.73, p < 0.01), while a significant drying trend was detected at the Kordai station (Modified Mann–Kendall, Z = −5.93, p = 3.07 × 10 −9 ). Climatic water deficit increased markedly downstream, from approximately 520 mm yr −1 at Kordai to −1130 mm yr −1 at Moiynkum. In contrast, runoff in the lower reaches exhibited weak and statistically insignificant relationships with hydroclimatic indicators, suggesting that intensive agricultural water withdrawals likely contribute to modifying the natural climate–runoff relationship. The results suggest that the Shu River basin is undergoing a transition from a hydrological system strongly associated with hydroclimatic conditions in the upper reaches to a human-modified water-limited system downstream.