Poschlod et al. (2026) Climate change effects on river droughts in Bavaria using a hydrological large ensemble

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-03-02
• Authors: Benjamin Poschlod, Laura Sailer, Alexander Sasse, Anastasia Vogelbacher, R. Ludwig
• DOI: 10.5194/hess-30-1165-2026

Research Groups

• Research Unit Sustainability and Climate Risk, Earth and Society Research Hub (ESRAH), Universität Hamburg, Hamburg, Germany
• Department of Geography, Ludwig-Maximilians-Universität München, Munich, Germany
• Institute of Geo-Hydroinformatics, Hamburg University of Technology, Hamburg, Germany
• United Nations University Hub on Engineering to Face Climate Change at the Hamburg University of Technology, United Nations University Institute for Water, Environment and Health (UNU-INWEH), Hamburg, Germany

Short Summary

This study investigates the impact of climate change on rare and extreme river droughts in two Bavarian catchments using a unique hydrological large ensemble. It projects a drastic increase in the frequency and intensity of summer droughts, with historical 100-year events becoming significantly more common by the far future (2070–2099) under a high-emission scenario.

Objective

• To investigate the impact of climate change on rare and extreme river droughts in a pluvial (Wörnitz) and a nivo-pluvial (Ammer) catchment in Bavaria.
• To analyze the behavior of very rare hydrological droughts (up to 1000-year return periods) and the importance of internal climate variability (ICV) for low-flow events.
• To identify shifts in dominant hydroclimatic drivers and climate change effects on drought seasonality, intensity, and frequency.
• To illustrate the value of a hydrological Single Model Initial-condition Large Ensemble (SMILE) for robustly assessing changes and extreme events, including the construction of bivariate return periods for peak low flow and duration.

Study Configuration

• Spatial Scale: Two headwater river catchments in southern Germany: Wörnitz (1570 km²) and Ammer (608 km²). Climate model simulations at 0.11° (approximately 12 km) resolution, statistically downscaled to 500 m.
• Temporal Scale:
  • Reference (REF): 1980–2009 (30 years)
  • Current (CUR): 2010–2039 (30 years)
  • Near Future (NF): 2040–2069 (30 years)
  • Far Future (FF): 2070–2099 (30 years)
  • Total simulated years: 1500 years per 30-year period (50 ensemble members × 30 years).

Methodology and Data

• Models used:
  • Hydrological Model: WaSiM (Water balance Simulation Model), a spatially distributed physically-based model.
  • Regional Climate Model: Canadian Regional Climate Model version 5 (CRCM5), dynamically downscaling global climate.
  • Earth System Model: Canadian Earth System Model version 2 large ensemble (CanESM2-LE), providing 50 initial-condition ensemble members.
  • Emission Scenario: Representative Concentration Pathway 8.5 (RCP8.5).
• Data sources:
  • Sub-Daily Climate Reference (SDCLIREF) dataset: 3-hourly, 500 m resolution, based on interpolated meteorological point observations for bias adjustment and statistical downscaling of CRCM5-LE.
  • Observed discharge data from the Bavarian State Water Authority (LfU) for WaSiM calibration and validation.

Main Results

• Temperature and Precipitation Changes: Annual temperatures are projected to increase by +1.0 °C (CUR) to +4.0 °C (FF) in Wörnitz, and +1.1 °C (CUR) to +4.3 °C (FF) in Ammer. Annual precipitation sums change by less than 6%, but seasonality shifts to increased winter and decreased summer precipitation.
• Soil Moisture and Snow: Summer soil moisture conditions are projected to be drier, especially for drought years. In the Ammer catchment, snowfall and snow storage are projected to decrease strongly.
• Wörnitz Catchment (Pluvial Regime):
  • Summer low-flow regime is projected to extend further into autumn.
  • A typical bivariate 100-year event (7-day peak low flow = 1.96 m³/s; duration = 171 days) in the reference period is projected to occur every 30 years (CUR), 17 years (NF), and 6 years (FF).
  • Autocorrelation of peak low flows from one summer to the next increases, emphasizing lagged effects.
  • The catchment's climate aridity is projected to shift towards arid conditions (evaporative fraction around 1.0) by the far future.
• Ammer Catchment (Nivo-pluvial Regime):
  • The low-flow regime is projected to transition from winter-dominated to summer-dominated conditions during the current climate, with more intense summer river droughts in the near and far future.
  • The most probable bivariate 100-year summer low-flow event (7-day peak low flow = 4.9 m³/s, duration = 60 days) in the reference period is drastically altered to occur every 34 years (CUR), 8 years (NF), and 2.5 years (FF).
  • Future winter low flows shift towards November/December and are triggered by hot and dry antecedent summer conditions.
  • Autocorrelation of peak low flows from one summer to the next increases.
• Main Drivers: Hotter and drier summer seasons are identified as the main driver, with an intensifying positive interdependency between heat and drought exacerbating extremes.
• SMILE Utility: The hydrological SMILE provides a large sample size (1500 simulated years per 30-year period), enabling robust assessment of very rare events and generation of bivariate design values, thereby narrowing uncertainties in extreme value statistics.

Contributions

• Employs a unique physically-based modeling chain driven by a 50-member Single Model Initial-condition Large Ensemble (SMILE) of a regional climate model (CRCM5) under RCP8.5, allowing for robust assessment of very rare hydrological droughts up to 1000-year return periods.
• Quantifies the drastic impact of climate change on the seasonality, intensity, and frequency of river droughts in two contrasting Bavarian catchments (pluvial and nivo-pluvial).
• Illustrates the value of a hydrological SMILE for detecting changes and robustly assessing extremes, particularly by constructing bivariate return periods of peak low flow and duration, which reveal significant shifts in typical design events.
• Highlights the increasing importance of lagged effects and antecedent summer conditions for subsequent low-flow events due to increased autocorrelation of peak low flows.
• Recommends distinguishing between summer and winter low flows and analyzing seasonality, duration, and bivariate return levels for comprehensive drought risk assessment, informing water management and adaptation planning.

Funding

• Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC 2037 “CLICCS – Climate, Climatic Change, and Society” – Project Number: 390683824.
• ClimEx II project, funded by the Bavarian Ministry for the Environment and Consumer Protection.
• ClimEx Project, funded by the Bavarian Ministry for the Environment and Consumer Protection.

Citation

@article{Poschlod2026Climate,
  author = {Poschlod, Benjamin and Sailer, Laura and Sasse, Alexander and Vogelbacher, Anastasia and Ludwig, R.},
  title = {Climate change effects on river droughts in Bavaria using a hydrological large ensemble},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-1165-2026},
  url = {https://doi.org/10.5194/hess-30-1165-2026}
}