Natkhin et al. (2026) Impact of extreme precipitation events on summer deep seepage below temperate forests in the Northeastern Germany lowlands

Identification

• Journal: Annals of Forest Science
• Year: 2026
• Date: 2026-01-20
• Authors: Marco Natkhin, Maximilian Strer, Tanja Sanders, Tobias Schad, Kai Schwärzel
• DOI: 10.1186/s13595-025-01318-2

Research Groups

Institute of Forest Ecosystems, Johann Heinrich von Thünen-Institute, Eberswalde, Germany

Short Summary

This study quantifies the contribution of extreme summer precipitation events to deep seepage below temperate forests in the northeastern German lowlands, revealing that these events are increasingly important for annual deep seepage, with distinct patterns influenced by tree species and stand structure.

Objective

• To quantify the deep seepage fed by extreme summer precipitation events using large-scale lysimeters below canopies of European beech, sessile oak, and Scots pine.
• To examine whether the observed abundance of extreme summer precipitation events was increasing.
• To identify and quantify the impact of extreme summer precipitation events on deep seepage by applying a seepage hydrograph separation method to site monitoring data.
• To analyse whether the influence of tree species and stand structure on the water balance was also evident under extreme summer precipitation events.

Study Configuration

• Spatial Scale: Three large-scale lysimeters (100 m² each) located at approximately 5 m depth below canopies of European beech, young sessile oak, and Scots pine pure stands at the Intensive Forest Monitoring Site Britz (IFMS Britz) in Brandenburg, Germany (52°52′41″ N 13°50′2″ E). The groundwater table was approximately 15.6 m beneath ground level.
• Temporal Scale: Analysis of four extreme summer precipitation events from 2018 to 2021. Long-term precipitation trend analysis from 1 January 1993 to 31 December 2022. Water balance components calculated for the period 2013–2023.

Methodology and Data

• Models used:
  • Seepage hydrograph separation method (adapted from baseflow separation) to distinguish "slow deep seepage due to winter precipitation" ((Qw)) and "rapid deep seepage due to extreme summer precipitation events" ((Qe)). This involved fitting an exponential function to model the decrease in winter seepage.
  • Mass balance equation for long-term water balance calculation: (P = ET + Q + \Delta S).
  • Generalized Extreme Value (GEV) distribution fitted to block maxima of daily precipitation for return period analysis, using a pseudo-nonstationary approach with a 20-year moving window.
• Data sources:
  • Lysimeter outflow measurements: Registering tipping gauges (0.1 L per tip) providing daily deep seepage time series.
  • Precipitation: Tipping bucket (0.1 mm resolution, 15 min aggregation) installed 1 m above ground, supplemented by other gauges for gap filling and long-term trend analysis.
  • Interception losses: Monthly manual gauges under canopy (12 per stand, 6 in open field).
  • Soil water content: Capacitive Sentek EnviroSCAN tubes at 10 depths to 4.6 m (measurements from 2010).
  • Stand properties: Leaf area index (LAI), tree height, and diameter at breast height (DBH) measured in 2023.
  • Soil parameters: Particle size distribution, field capacity (Fc), and permanent wilting point (PWP) estimated by pedotransfer function.
  • Data used in the paper is accessible at Natkhin et al. (2025).

Main Results

• Extreme summer precipitation events contributed substantially to annual deep seepage: 71% below pine stands, 22% below young oak stands, and 15% below beech stands.
• The highest ratio of deep seepage from extreme summer precipitation to annual deep seepage occurred below pine, while the highest absolute quantities of deep seepage from extreme summer precipitation were found under young oak (accumulated sum of 163 mm over the 4 analyzed years).
• Extreme summer precipitation events have become more frequent over the observed period (1993–2022); the return period for high daily precipitation amounts decreased (e.g., 80 mm daily precipitation decreased from 90 years to 30 years).
• While no trend was identified in total summer precipitation, there was a significant increasing trend in the maximum single event per summer and its contribution to total summer precipitation (ranging from 25% to nearly 50%).
• The onset of rapid deep seepage after an extreme event varied, with the maximum deep seepage reached after 6 to 27 days.
• Mean annual evapotranspiration accounted for 72% (young oak) to 97% (pine) of annual precipitation. Annual deep seepage was approximately 10% for beech, one-third for young oak, and very low for pine.
• Pre-event soil moisture was highest under young oak and lowest under pine, influencing the magnitude of deep seepage from extreme events.

Contributions

• This study pioneers the application of a seepage hydrograph separation method to directly measured lysimeter data, enabling the attribution and quantification of deep seepage fractions to specific precipitation events.
• It provides novel quantitative evidence for the significant and increasing role of extreme summer precipitation events in deep seepage and groundwater recharge in temperate lowland forests, a mechanism previously considered minor compared to winter precipitation.
• It highlights the critical influence of tree species and stand structure (age, canopy architecture, rooting depth) on the magnitude and dynamics of deep seepage response to extreme events, particularly the high efficiency of pine stands in converting extreme summer rainfall into deep seepage.
• The findings suggest that adapted forest management, including tree species selection, could be a crucial strategy for promoting groundwater recharge and mitigating water scarcity in regions facing changing climate patterns.

Funding

Open Access funding enabled and organized by Projekt DEAL. The research described in this manuscript was not supported by external funding.

Citation

@article{Natkhin2026Impact,
  author = {Natkhin, Marco and Strer, Maximilian and Sanders, Tanja and Schad, Tobias and Schwärzel, Kai},
  title = {Impact of extreme precipitation events on summer deep seepage below temperate forests in the Northeastern Germany lowlands},
  journal = {Annals of Forest Science},
  year = {2026},
  doi = {10.1186/s13595-025-01318-2},
  url = {https://doi.org/10.1186/s13595-025-01318-2}
}