Mahmoodi et al. (2026) Integrating coupled surface–subsurface modelling and field measurements in a degraded fen: water-balance dynamics and a framework for evaluating rewetting measures

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-09-25
• Authors: Nariman Mahmoodi, Christoph Merz, Jürgen Pickert, Ottfried Dietrich
• DOI: 10.5194/hess-30-6019-2026

Research Groups

• Lowland Hydrology and Water Management Group, Leibniz Centre for Agricultural Landscape Research (ZALF), Muencheberg, Germany
• Department of Hydrogeology, Faculty of Geology, Freie University Berlin, Berlin, Germany
• Sustainable Grassland Systems Group, Leibniz Centre for Agricultural Landscape Research (ZALF), Muencheberg, Germany

Short Summary

This study integrates coupled surface–subsurface modeling with field measurements in a degraded fen peatland to analyze water-balance dynamics. It provides a robust framework for evaluating rewetting measures by accurately quantifying hydrological fluxes and their sensitivity to climate and land use.

Objective

• Provide optimized parametrization for a better understanding of peatland hydrological processes under different climate conditions and land management pressures.
• Consider all relevant water fluxes between groundwater, surface water, and atmosphere within a fully-integrated modeling approach.
• Quantify the specific contributions of precipitation, evapotranspiration, inflow, outflow, and water storage change to the water balance of the peatland site.

Study Configuration

• Spatial Scale: A degraded fen peatland site covering approximately 11.6 hectares (116,000 square meters) in Brandenburg, Germany. The vertical model domain extends to a depth of nearly 12 meters, representing two peat layers (approximately 1 meter total thickness) overlying sand and till.
• Temporal Scale: Simulations used daily meteorological inputs and detailed field measurements from 2015 to 2023. The year 2015 served as a spin-up phase, 2016–2020 for calibration, and 2021–2023 for validation.

Methodology and Data

• Models used: HydroGeoSphere (HGS), a fully integrated, physically based model that simultaneously solves the three-dimensional Richards equation for variably saturated subsurface flow and two-dimensional surface flow equations.
• Data sources:
  • Field observations: Eddy covariance measurements for actual evapotranspiration (AET), net radiation, soil heat flux, temperature, wind speed, and relative air humidity. In-situ Leaf Area Index (LAI) measurements collected seasonally. Daily groundwater levels from piezometers and ditch water levels.
  • Meteorological data: Daily precipitation and potential evapotranspiration (ET₀) calculated using the FAO Penman–Monteith method.
  • Land use data: Detailed management schedules (mowing, grazing) for eight distinct management units.
  • Geological data: Brandenburg geological database for subsurface layering.

Main Results

• The model successfully reproduced seasonal water-table fluctuations and ditch–peatland interactions, including evapotranspiration-driven hydraulic gradient dynamics between summer and winter.
• Simulated actual evapotranspiration (AET) closely matched eddy covariance measurements, with Root Mean Square Error (RMSE) values of 0.064 meters per year, 0.0102 meters per month, and 0.00101 meters per day.
• Groundwater variations were reproduced with high accuracy, achieving Kling–Gupta Efficiency (KGE) values of 0.80–0.85, Nash–Sutcliffe Efficiency (NSE) of 0.83–0.86, and RMSE of 0.15 meters during calibration and validation.
• Analysis of seasonal and interannual water-storage changes showed pronounced shifts between hydrological surplus and deficit, demonstrating that drained fens are highly sensitive to evapotranspiration demand and prolonged drought.
• Transpiration dominated AET during the growing season, often reaching 0.08–0.10 meters per month, while evaporation contributed a larger share during cooler/wetter months and immediately following rainfall (0.005–0.030 meters per month).
• Ditch water levels were consistently higher than peatland groundwater levels during summer, leading to inflow, while the gradient reversed in winter, causing outflow.
• Annual water balance showed positive storage changes in wet years (e.g., 2017, 2023) and negative storage changes in dry years (e.g., 2016, 2018, 2019), indicating a cumulative moisture deficit.

Contributions

• This study presents the first application of a fully coupled surface–subsurface hydrological model (HydroGeoSphere) for a degraded fen peatland, integrating extensive, long-term (9 years) field measurements.
• It provides an optimized parametrization of evapotranspiration processes by incorporating field-measured Leaf Area Index (LAI) and detailed land management schedules, enhancing the realism of vegetation-water interactions.
• The explicit representation of the layered peat structure (degraded upper peat and less degraded lower peat) and its distinct hydraulic properties was crucial for accurately simulating rapid water-table fluctuations and storage changes.
• The research offers a comprehensive, process-based quantification of all water balance components and their seasonal and interannual variability, highlighting the vulnerability of degraded fens to climatic water deficits.
• The established modeling framework provides a robust hydrological baseline and a physically-based tool for future assessment of management interventions aimed at sustainable peatland rewetting and climate-change adaptation strategies.

Funding

• WetNetBB project (Management and Biomass Utilization of Wet Fens: Network of Model and Demonstration Projects in Peatland Regions of Brandenburg)
• Federal Ministry of Food and Agriculture through the Climate and Transformation Fund (FNR – Fachagentur Nachwachsende Rohstoffe 100619639)

Citation

@article{Mahmoodi2026Integrating,
  author = {Mahmoodi, Nariman and Merz, Christoph and Pickert, Jürgen and Dietrich, Ottfried},
  title = {Integrating coupled surface–subsurface modelling and field measurements in a degraded fen: water-balance dynamics and a framework for evaluating rewetting measures},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-6019-2026},
  url = {https://doi.org/10.5194/hess-30-6019-2026}
}