Quantifying water fluxes and storage in a degraded peatland using a fully integrated hydrological model

Peatland rewetting is an important measure for climate-change mitigation in northern Europe, but its effectiveness and plausibility require a detailed understanding of hydrological processes in degraded systems. To address this need, this study employs a fully integrated HydroGeoSphere (HGS) model of a drained fen peatland in Brandenburg, Germany, using nine years (2015–2023) of field measurements including groundwater (GW) dynamics, ditch water levels, eddy-covariance evapotranspiration (ET), and in-situ vegetation (LAI) observations. The model represents three-dimensional surface–subsurface flow, spatially distributed vegetation and management units, and a vertically heterogeneous peat profile. Evapotranspiration is parameterized using site-specific eddy-covariance data and in-situ measurements of seasonal leaf area index and management practices. Model performance was evaluated against GW levels and ET using a multi-metric approach for calibration (2016–2020) and validation (2021–2023) periods. Simulated GW dynamics and ET are in agreement with observations (GW: NSE = 0.83–0.86, KGE = 0.80–0.85; ET: RMSE ≈ 1.0 mm d⁻¹). Results show pronounced seasonal reversals in hydraulic gradients with evapotranspiration-driven groundwater drawdown leads to lateral inflow from ditches and the surrounding aquifer during summer, and recharge-driven outflow and surface inundation in winter conditions. Seasonal and interannual water-storage analysis shows that wet years (e.g., 2017, 2023) generate positive storage, whereas consecutive drought years (2018–2020) produce cumulative deficits, highlighting the vulnerability of degraded peat to climatic water imbalance. The presented modeling framework provides a robust basis for assessing peatland vulnerability to climate extremes and for evaluating rewetting and water-management strategies aimed at enhancing water retention and reducing CO₂ emissions from drained fen peatlands.