Climate change projections with the National Hydrological Model of Denmark reveal an intensified seasonality of the hydrological cycle

Climate change is expected to substantially alter the hydrological cycle in temperate regions. Denmark, where groundwater and surface water are tightly coupled, provides an ideal test case to investigate the propagation of climate change signals through the hydrological system, due to good data availability and an existing well-established national-scale hydrological model. The Danish National Hydrological Model (DK-model) is a physically based, distributed model that has previously been shown to reproduce the observed propagation of drought and hydrological anomalies throughout the hydrological cycle.

Here, we analyse the climate change impact on the Danish hydrology, as simulated by the DK-model forced by an ensemble of 17 downscaled and bias-corrected climate models (RCP8.5) until the end of the 21st century. The climate projections indicate an increase in net precipitation at the annual scale, driven primarily by wetter winters, while summers experience increased net precipitation deficits. The resulting climate change signals across the hydrological cycle are assessed using both absolute values and standardized indices for soil moisture, streamflow, and shallow and deep groundwater.

Model results show that increased net precipitation and increased seasonality are translated into different hydrological responses. Fast reacting, surface-near compartments (soil moisture) shift towards drier conditions during summer and wetter conditions during winter. In contrast, deep groundwater shows a consistent rise across all seasons, reflecting the overall increase in precipitation. Shallow groundwater and streamflow show intermediate behaviour dominated by large increases in winter and a mixed signal for summer. Despite these differences, all compartments experience an increase in seasonality, expressed as larger amplitudes between annual minimum and maximum states. Notably, climate models show stronger agreement on increasing seasonality than on the direction of absolute changes.

Moreover, the increased seasonality is also reflected in the hydrological drought indices which indicate increased soil moisture droughts during summer and, at the same time, an increase in wet anomalies during winter. The development of streamflow and groundwater droughts is more complex due to the partial buffering of drier summers by wetter winters. Yet, results clearly indicate a similar trend towards increased seasonality. Overall, the results demonstrate that, despite significant precipitation increases, climate change in Denmark is projected to amplify seasonal extremes rather than uniformly shift hydrological states towards wetter conditions, with important implications for water resources management, agriculture, ecosystems, and infrastructure.