| 10 Jun 2026
Climate change intensifies hydrological seasonality in Denmark: Insights from an integrated model assessment
Abstract. Temperate regions across Europe, such as Denmark, are projected to be subjected to substantial changes in the hydrological cycle due to climate change. Changes in climate can materialize as general changes in long term means, extremes, or in seasonal patterns, e.g., dampening or intensification of the seasonal contrasts. Changes in seasonal patterns can affect the hydrological cycle in various ways, due to the interlinkage between hydrological compartments. To detect, track and quantify the impact of changes in climate and seasonal patterns, integrated hydrological modelling is needed. This makes Denmark an ideal test case due to the established integrated and physically based National Hydrological Model of Denmark (DK-model). Utilizing climate projections from 17 RCP8.5 climate models, downscaled and bias-corrected for Denmark, we calculate climate change impacts on both overall values and seasonality for the variables soil moisture, streamflow, shallow and deeper groundwater to the end of the century. Moreover, standardized hydrological drought indices are calculated for the same variables. Climate change projections point towards a future with higher annual precipitation, mainly due to wetter winters, while climatic water balance deficits increase during summer; thus, intensifying the seasonal contrast. The increased contrast is reappearing in the fast-responding hydrological variable, soil moisture; while streamflow and shallow groundwater clearly reproduce increase during the wetter winter, the summer signal differs. The deep groundwater systems experience higher future groundwater heads across the entire year. Common for all variables is a larger seasonality, defined as contrast between intra-annual low and highs. Notably, the ensemble of hydrological projections is more in agreement regarding the seasonal contrast than on the direction of absolute change, with results agreeing for 85 % to 99 % of the area of Denmark on increased seasonality, whereas only agreeing for 50 % to 98 % on the absolute direction of that change. The drought indices exhibit a similar seasonal change, with more droughts during summer and more wet anomalies during winter for soil moisture, while summer droughts for streamflow and shallow groundwater partially are buffered by wetter winters and the related recharge increase. In summary, the results indicate that despite considerable increases in precipitation, projected climate change for Denmark is expected to enhance hydrological seasonality instead of producing a uniform transition to a wetter regime, potentially impacting the climate adaptation and mitigation effort, agricultural yields, and water supply.
The authors investigate how projected climate change propagates through the interconnected hydrological system of Denmark, using the physically-based and integrated National Hydrological Model of Denmark (DK-model, MIKE SHE based, 500 m resolution, 24h time steps). They force the model with 17 EURO-CORDEX RCM projections (CMIP5-driven, RCP8.5), downscaled and bias-corrected for Denmark, and compare a mid-century (2041-2070) and an end-century (2071-2100) period against a 1991-2020 reference. The analysis covers four compartments of the hydrological cycle: root-zone soil moisture, streamflow, depth to the phreatic surface (shallow groundwater), and the deeper aquifer groundwater head. In addition to changes in monthly mean conditions, the authors compute standardized hydrological indices to quantify changes in the frequency and severity of dry and wet anomalies, and explicitly quantify changes in seasonality.
This is a careful, well-written, and policy-relevant study with an appealing visual presentation of results. The integrated modelling approach, and the multi-compartment treatment of seasonality is a nice contribution that distinguishes it from the more common single-compartment assessments. However, there are still some points, where I see the need for improvement. My major comments are below, while I provide an annotated version of the PDF attached for the minor stuff. In sum I'd still consider it a "minor revision".
Major comments:
1. RCP8.5 framing and the emphasis on end-of-century
The study relies exclusively on RCP8.5 ("business-as-usual storyline", L197-199). RCP8.5 is now widely regarded as an implausible rather than a business-as-usual trajectory (https://doi.org/10.5194/gmd-19-2627-2026), and this is especially consequential for the end-of-century period, where RCP8.5 diverges most strongly from more plausible pathways. As written, the abstract, the headline figures (Figs. 4, 5, 6, 7, 9), and most of the narrative are anchored on the 2071-2100 results, which risks overstating the magnitude of the projected changes relative to a realistic emissions future. I recommend one of the following two options:
(a) Shift the framing, figures, and discussion towards the mid-century (2041-2070) period as the primary result, relegating end-century to a secondary/illustrative role. The mid-century results already exist in the appendix (Fig. B1, Fig. B4) and could be moved to the main text. The "business-as-usual" wording should be removed or corrected regardless.
or preferably
(b) Re-structure the analysis in terms of global warming levels (GWLs, e.g. +1.5, +2, +3 °C above 1850-1900) rather than fixed time slices under a single RCP. This decouples the results from the RCP8.5 trajectory and also reduces the climate model uncertainty, where the different equilibrium climate sensitivity (ECS) of the driving GCM is a major uncertainty source for the diverging 17 simulations. GWLs further make the results transferable to other scenarios and to the GWL framing used in IPCC AR6. The years at which each driving GCM reaches a given GWL are e.g. provided by Hauser et al. 2022 and are openly available: https://doi.org/10.5281/zenodo.3591807
The authors could still use 30-year periods instead of Hauser's standard 20 years.
At minimum (option a), the RCP8.5 limitation and its implication for the end-century numbers should be stated explicitly (at best briefly in the methods when describing the data and later in detail in Sect. 5.2.). The main conclusion (increased seasonality is a more robust signal than the mean change) will very likely hold in either case.
2. "Depth to phreatic" label vs. the sign of the reported change
There is a sign/labelling inconsistency around the shallow groundwater variable throughout the paper. "Depth to phreatic surface" is to my understanding defined as depth in metres below ground surface (L263, L339-340: "depth to phreatic (below surface [m])"). By that definition, a larger value means the water table is deeper, i.e. shallow groundwater is lower / further from the surface. However, the results consistently treat positive changes in "depth to phreatic" as more/higher groundwater (e.g. L324-326: "the groundwater levels generally rise. For the depth to phreatic, climate change impacts show a clear seasonal signal, with highest increases during late winter (up to +0.27 m)"; Fig. 4 and Fig. 6 colour positive changes blue = increase = wetter).
I'd suggest to fix this by re-naming the label to "shallow groundwater level" consistently.