Mapping groundwater-surface water interactions on a national scale for the stream network in Denmark

Study region: Denmark.Study focus: Groundwater-surface water interaction (GSI) occurs when groundwater discharges into surface water, or surface water seeps into the groundwater. Understanding GSI aids in understanding the quality and quantity of water and the ecosystem's health, assisting water resource management and the protection of land and aquatic environment. This paper presents a classification mapping routine to classify the Danish stream network into an established GSI typology framework. We mapped three classes for GSI contact types: connected clayey, connected sandy, and disconnected, and three classes for riparian flow paths: diffuse and/or overland, direct, and artificial drainage.New hydrological insights for the region: Results revealed that about 85% of Denmark's streams are connected to groundwater with approximately equal portions in sandy and clayey subsoil areas. Also, about 87% of these streams exchange water with groundwater, either as the only flow path or combined with other flow paths. Furthermore, 41% and 19% of stream sections receive riparian flow water through artificial drainage and diffuse/overland flow paths as the dominant flow path or combined. These numbers correspond to Denmark's land use, which is approximately 60% agricultural, restricting runoff potential, and half of which is artificially drained. We anticipate that the maps will assist policymakers in controlling water and pollutant transfer from the uplands to streams to attain good environmental status for surface and groundwater bodies.