Coastal zones are particularly vulnerable to flooding. Several climatic and state variables may drive the occurrence of such events, e.g., storm surges, sea level rise, heavy rainfall, and high river and groundwater levels. The co-occurrence of such events, i.e. compound or cascading effects, has been shown to escalate flooding impacts and extent, but the contribution of groundwater is routinely overlooked. Here, we apply an integrated hydrological/hydrodynamic/groundwater model to investigate underlying causes and compound effects in a Danish Wadden sea catchment. Two models were developed: a long-term model and an overbank-spilling model. The long-term model was calibrated and used to simulate 30-year periods. Extreme value analyses were carried out for sea levels, precipitation, simulated river water stages, and groundwater levels. The co-occurrence of extremes was used to identify compound effects on high river-stage incidents (as a flood proxy). The overbank-spilling model was then used for simulating flooding for a subset of the largest river stage events identified from the long-term model. The analysis showed that the river-stage events were closely correlated to the sea level extremes, but that the largest river-stage events were almost exclusively compounded by precipitation or groundwater, or both. High groundwater tables seem to correlate to the flooding events with the largest spatial extent, as well as prolonged extreme events where either precipitation or sea level were elevated during long periods. Thus, this study shows that there is a general need to acknowledge the potential effect of groundwater levels on the resulting flooding on terrain in coastal zones.

Changes in the quantity and quality of groundwater and water in the hydrological cycle in general have important implications for the evolution of water resources, the built environment, and terrestrial and aquatic ecosystems, globally. Exploitation of groundwater and other subsurface resources may lead to e.g. land subsidence, salt water intrusion, loss of important terrestrial and aquatic ecosystems and hence biodiversity. Together with biogeochemical flows of nitrogen and phosphorus and changes in the land-system and climate, these are currently considered the main environmental problems of the planet, which are breaching or close to breaching planetary boundaries. Changes in the hydrological cycle including groundwater is closely related to and affecting these changes. It is the ambition of the four GeoERA groundwater projects studying aspects of groundwater quantity and quality issues related to natural processes and human activities to further develop the European Geological Data Infrastructure as a leading information platform for groundwater data in Europe and one of the leading platforms, globally. Here we briefly present the contents and objectives of the four groundwater projects: HOVER - Hydrogeological processes and geological settings over Europe controlling dissolved geogenic and anthropogenic elements in groundwater of relevance to human health and the status of dependent ecosystems; RESOURCE - Resources of groundwater, harmonized at cross-border and Pan-European Scale; TACTIC – Tools for assessment of climate change impact on groundwater and adaptation strategies and VoGERA - Vulnerability of shallow groundwater resources to deep sub-surface energy-related activities. The four projects will deliver “FAIR” (Findable, Accesssible, Interoperable and Reusable) data and information via the European Geological Data Infrastructure easily accessible for all relevant endusers. This will improve our understanding of the subsurface and support common efforts for developing geoethical uses of the subsurface.