Brauer et al. (2026) Rain-on-wet-soil compound floods in lowlands: the combined effect of large rain events and shallow groundwater on discharge peaks in a changing climate

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-01-19
• Authors: Claudia Brauer, Ruben Imhoff, Remko Uijlenhoet
• DOI: 10.5194/hess-30-249-2026

Research Groups

• Hydrology and Environmental Hydraulics Group, Wageningen University, Wageningen, the Netherlands
• Department of Operational Water Management & Early Warning, Deltares, Delft, the Netherlands
• Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands

Short Summary

This study investigates the importance of initial groundwater depth on flood peaks in 12 lowland catchments under current and future climate scenarios, finding a strong relationship where shallower groundwater exacerbates flood severity, leading to projected increases in flood frequency and magnitude by 2085 despite mitigating effects of deeper summer groundwater.

Objective

• To determine the importance of initial groundwater depth (as a proxy for catchment wetness) on flood peaks in lowland catchments and to examine its effect on flood magnitude and timing under future climate change scenarios.

Study Configuration

• Spatial Scale: 12 lowland catchments in the Netherlands, Germany, and Belgium, ranging from 6.5 square kilometers to 2821 square kilometers.
• Temporal Scale: 109 years of hourly data (1906-2014) for current climate, extended to 2050 and 2085 using four climate scenarios.

Methodology and Data

• Models used: Wageningen Lowland Runoff Simulator (WALRUS) rainfall-runoff model.
• Data sources: Hourly precipitation and potential evapotranspiration (Makkink method) from De Bilt (KNMI) for 1906-2014, detrended and transformed to 2050 and 2085 using four KNMI’14 climate scenarios (based on CMIP5, EC-Earth, and RACMO2 downscaling). Groundwater level observations from Dinoloket database and Hupsel Brook catchment for model validation.

Main Results

• A strong relationship exists between initial groundwater depth and flood peaks in lowland catchments, with flatter catchments showing greater sensitivity to groundwater depth.
• Climate change projections indicate reduced summer precipitation and increased evapotranspiration, leading to deeper groundwater in summer and autumn, which mitigates flood frequency and magnitude during these seasons.
• Conversely, increased precipitation in autumn, winter, and spring is projected to cause more frequent and severe floods specifically in winter and spring.
• Averaged across all scenarios and catchments, effective rainfall sum is projected to increase by 1.5 % in 2050 and 5.6 % in 2085. Initial groundwater depth is projected to increase by 0.7 % in 2050 and 0.3 % in 2085.
• This combined effect leads to an overall increase in flood frequency and severity: 1 % more floods and 3 % larger peak volumes by 2050, escalating to 9 % more floods and 21 % larger peak volumes by 2085.
• The mitigating effect of deeper initial groundwater tables in summer and autumn prevents even more frequent and severe floods from higher rainfall sums.
• More extreme flood events (e.g., 0.1 floods per year) are projected to increase disproportionately, with up to 57 % more floods and 60 % larger peak volumes by 2085.

Contributions

• Provides a comprehensive analysis of the compound effect of initial groundwater depth and rainfall on flood peaks specifically in lowland catchments, an area previously understudied compared to sloping regions.
• Utilizes a dedicated lowland rainfall-runoff model (WALRUS) with long-term, high-resolution (hourly) simulations to overcome limitations of short observational data series for rare flood events.
• Quantifies the trade-off effect of groundwater and rainfall on flood severity in lowland environments, demonstrating its significant role in mitigating or aggravating future flood risks.
• Offers actionable insights for climate-robust water management strategies in lowland delta areas globally, emphasizing the potential for flexible groundwater and surface water management to mitigate flood and drought risks.

Funding

• Joint Cooperation programme for Applied scientific Research on flood and drought risk management in regional river basins (JCAR ATRACE)
• Perspectief research programme “Future Flood Risk Management Technologies for rivers and coasts” (project number P21-23), financed by the Domain Applied and Engineering Sciences of the Dutch Research Council (NWO)

Citation

@article{Brauer2026Rainonwetsoil,
  author = {Brauer, Claudia and Imhoff, Ruben and Uijlenhoet, Remko},
  title = {Rain-on-wet-soil compound floods in lowlands: the combined effect of large rain events and shallow groundwater on discharge peaks in a changing climate},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-249-2026},
  url = {https://doi.org/10.5194/hess-30-249-2026}
}