A network perspective for mapping freshwater service flows at the watershed scale

Spatial mismatch often occurs between high-supply and high-demand regions of ecosystem services (ES), as human dwellings and workplaces are usually distant from natural ecosystems. In this regard, studies have increasingly focused on ecosystem service flows (ESF) from ecosystems to humans. However, current mapping methods for ESF are rarely able to visually display the flow relationships between supply and demand regions. Network models that commonly used for dealing with relational data show great potential in mapping ESF. Here, we demonstrated the effectiveness of the network model in mapping ESF by constructing a freshwater service flow network for the Yanhe watershed in China. We concluded that from 2000 to 2015 freshwater demand was approaching the limit of freshwater supply in the Yanhe watershed, with the freshwater demand continuously increasing. Due to the shortage of freshwater service, the deficit nodes and stop-flowing edges in the freshwater service flow network increased, and the network density decreased. These results can provide useful information to guide watershed freshwater resource management (e.g. optimizing the allocation of freshwater resources across the watershed and designing payment schemes for freshwater service) in the Yanhe watershed, particularly in a time of population growth, climate change, and threats of water scarcity. Finally, we pointed out that applications of the network model in ESF deserves more attention and further study and analyzed the wide applicability of the network model in mapping ESF and the possible future efforts in this field.