In semi-arid agro-pastoral ecotones, the conflict between agricultural water demand and ecological water security is intensifying under changing climate conditions. Understanding the driving mechanisms of runoff depletion is critical for sustainable agricultural water management. Focusing on the Xiliao River Basin, a typical semi-arid region in China, this study quantified the contributions of climate change and human activities to runoff reduction from 1980 to 2022 using a Budyko-elasticity framework. The results revealed an abrupt decline in runoff occurring in 2002, with significant spatial heterogeneity characterized as "upstream climate-driven, middle-downstream human-driven". In the upstream areas, precipitation reduction was the dominant driver (contribution: 52.6%–64.1%). Crucially, in the middle and lower reaches, where agriculture is concentrated, changes in underlying surface parameters were the primary factors (contribution: 75.7%–78.9%). By decomposing these parameters, this study confirmed that high-intensity direct water withdrawal for agricultural irrigation, rather than the indirect ecological effects of vegetation restoration, was the fundamental cause of runoff depletion in these sections. Furthermore, a superimposed effect was identified, where upstream climate-induced drought exacerbates downstream water shortages. These findings highlight that controlling agricultural irrigation quotas and implementing precise zoned management are imperative for reconciling agricultural development with river ecosystem health in semi-arid basins. • The annual runoff sequence in the Xiliao River Basin underwent a mutation in 2002 and has since entered a significant decline period. The runoff in different sub-basins has decreased by as much as 51.59%–98.08%. • It reveals the significant spatial differentiation feature of "climate dominance in the upper reaches and human activity dominance in the middle and lower reaches" within the basin. The reduction in upstream runoff is mainly driven by the decrease in precipitation, while in the middle and lower reaches, it is mainly driven by the changes in underlying surface parameters. • It clearly points out that the fundamental cause of the depletion of runoff in the middle and lower reaches is not the indirect ecological effect brought about by vegetation restoration, but rather the high-intensity direct water withdrawal, correcting the previous general understanding of the impact of underlying surface changes. • It was found that the reduction in downstream runoff is the result of the dual superposition of reduced inflow due to arid climate in the upstream and intense water withdrawal in the downstream, and this superimposed effect has exacerbated the shortage of water resources.
Yan et al. (Sat,) studied this question.