Is climate change or abstraction patterns driving historic changes in Danish groundwater levels?

Media, citizens, and insurance companies in Denmark increasingly report problems and concerns related to rising groundwater levels and groundwater flooding. However, the primary governing causes of these issues remain unclear. The driving factors are believed to include climate-driven changes in precipitation and evapotranspiration, changes in groundwater abstraction, and local anthropogenic alterations of water pathways, such as the renovation of old, leaky sewer systems and the implementation of sustainable drainage systems.

Using groundwater observations from the national well database Jupiter, we investigate the relative impacts of historical climatic changes and changes in groundwater abstraction patterns on observed groundwater levels over the past 33 years in Denmark. This is achieved by selecting long, consistent groundwater time series suitable for trend analysis. Based on these time series, we identify two subsets of monitoring wells: (1) climate-controlled wells and (2) anthropogenically influenced wells, using Transfer Function Noise time-series analysis as implemented in Pastas. The identified groundwater-level trends are compared with trends simulated by the National Hydrological Model of Denmark (DK-model), run with fixed abstraction rates to represent a purely climate-driven signal.

The analysis reveals a pronounced east–west contrast in climatic drivers (precipitation and net precipitation), with increasing trends (wetter) in western Denmark and decreasing trends (drier) in eastern Denmark over the last 33 years. Both the climate-controlled wells and the DK-modelled groundwater levels reproduce this pattern with rising groundwater levels in west (+20 cm over 33yr) and lowering in east (-3 cm over 33yr). Groundwater abstraction patterns in Denmark have changed since the early 1990s, when abstraction levels were significantly higher than today. Wells classified as anthropogenically influenced generally exhibit much larger changes in groundwater-level (often exceeding several meters of increase over the 33 years period) and do not consistently follow the climatic signal, especially for the eastern regions. The analysis shows that while climate impacts can explain moderate increases in groundwater levels in Western Denmark, large increases observed in Eastern Denmark are contributed to changes in abstraction patterns or other anthropogenic factors. This indicates that the most likely drivers behind the growing concerns related to hazards from high groundwater-levels in Denmark are direct anthropogenic changes with climate change playing a secondary role.