Evaluating water provision service at the sub-watershed scale by combining supply, demand, and spatial flow

•The spatial flow was included in assessing the sub-basin importance for water supply.•The beneficial population was based on the direction and path of spatial flow.•The importance index was proposed to explore the contribution of sub-basins.•Considering the service delivery, the upstream sub-basin maybe not most important.•The importance ranking provided information for ecological compensation. Water provision service is crucial for human society to survive and develop. It is essential to evaluate the importance of sub-watersheds, based on their different capacities to provide water resources to manage an entire watershed. Previous studies have assessed the importance of the sub-watershed by analyzing its supply and/or demand of water provision service. However, few studies have considered the influence of the spatial flow. In this study, we proposed an assessment framework that combined supply, demand, and spatial flow of water provision service. The Qiantang River Basin in China was selected as our study area. The importance of the sub-watershed was evaluated using the importance index, which was calculated using the spatial flow and the accumulative beneficial population of the water provision service. The spatial flow was simulated using a simplified water flow model, and the accumulative beneficial population was based on the direction and path of water provision service spatial flow. The results indicated that 60% of sub-watersheds with “very high” importance were located in the middle and lower reaches of Xin'an River and Lan River. Due to the internal consumption and sink of the water provision service, the upstream area may provide less spatial flow and may have a lower level of importance. When limited by the direction and scope of water provision service delivery, some sub-watersheds with high surpluses and without external beneficiaries were of low importance. Our study emphasizes the quantity and routing of water provision service delivery and enhances the understanding of the capacity of the sub-watershed for providing water provision service.