Abstract: Compared with the quantification of service supply and the degradation risk assessment of supply capacity, research on the balance of supply and demand of ecosystem services and the risk of supply and demand is relatively weak. It is important to clarify the relationship between supply and demand of ecosystem services, spatial matching and the temporal and spatial evolution of supply and demand risks for the management of watershed ecosystem and the effective allocation of resources. With Bailongjiang watershed in Gansu Province as an example, based on InVEST model, ArcGIS and GeoDa, we evaluated the supply and demand of soil conservation services, and revealed the temporal and spatial change characteristics of supply and demand risks under the research framework of supply and demand risks of ecosystem services at the basin scale. The results showed that: (1) From 1990 to 2016, the spatiotemporal variations of soil conservation service supply and demand in Bailongjiang watershed were significantly different. The total supply of soil conservation services in the basin showed a trend of decreasing first and then increasing. It decreased from 8.79×10⁸ t in 1990 to 7.74×10⁸ t in 2002, and then increased to 2016. The total demand showed a decreasing trend, from 7.06×10⁸ t in 1990 to 5.69×10⁸ t in 2016. However, there was a trend of increasing risk in some areas. (2) In terms of spatial matching of supply and demand, the supplydemand ratio was less than 0 in some areas of the northern mountainous Diebu, Wudu, and Wenxian, indicating that the supply exceeded demand. Other areas basically maintained a balance. The main spatial matching types of watershed soil conservation services were highhigh and lowlow types. (3) In the aspect of supply and demand risk, the risk levels of supply and demand of soil conservation services in the basin were mainly fragile (V) and safe (VI) low risk, accounting for 21% and 64% of the total area of the basin respectively. The medium risk (IV and III) areas accounted for 15%. There was no high risk area (II and I). The ecological environment of the river basin in 2002-2016 was better than that in 1990-2002, with the level of supply and demand risk being reduced. The proportion of high-risk areas had been reduced from 8% to 0. Our results would provide theoretical support for the research and management of ecosystem service risk assessment.

Key words: ecosystem services, soil conservation, spatiotemporal pattern, supply and demand risk.