Wang et al. (2025) The effect of rainfall variability on Nitrogen dynamics in a small agricultural catchment

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-11-10
• Authors: Qiaoyu Wang, Jie Yang, Ingo Heidbüchel, Teng Xu, Chunhui Lu
• DOI: 10.5194/hess-29-6093-2025

Research Groups

• The National Key Laboratory of Water Disaster Prevention, Hohai University, Nanjing, China
• College of Hydrology and Water Resources, Hohai University, Nanjing, China
• College of Water Conservancy and Hydropower Engineer, Hohai University, Nanjing, China
• UFZ – Helmholtz-Centre for Environmental Research GmbH, Department of Hydrogeology, Leipzig, Germany
• Hochschule Koblenz, Koblenz, Germany

Short Summary

This study investigates the effect of inter-annual and intra-annual rainfall variability on nitrogen (N) dynamics and water quality in a small agricultural catchment in central Germany using a coupled hydrological and N transport model driven by a stochastic rainfall generator. It finds that higher annual precipitation enhances N transformation and transport, while lower annual precipitation promotes N retention, with vegetation health critically influencing N dynamics during extreme droughts and rewetting periods.

Objective

• To investigate the effect of rainfall variability on nitrogen (N) dynamics and its potential influence on water quality across inter-annual and intra-annual timescales in a small agricultural catchment.

Study Configuration

• Spatial Scale: A small agricultural catchment (Schäfertal) in central Germany, with an area of 1.44 square kilometers.
• Temporal Scale: Analysis of inter-annual (wet, normal, dry, and extreme dry years) and intra-annual rainfall variability. Rainfall records from 1997 to 2022 were used to select representative years. Simulations were conducted over multi-year periods (e.g., 3 years for scenarios) with 100 stochastic realizations for each scenario. Nitrate concentration data covered 2001–2010.

Methodology and Data

• Models used:
  • Fully coupled surface-subsurface numerical simulator HydroGeoSphere for water flow and solute transport.
  • N dynamics framework modified from the ELEMeNT (Exploration of Long-tErM Nutrient Trajectories) modeling approach for nitrogen transformation and transport.
  • Stochastic rainfall generator (Robinson and Sivapalan, 1997) to generate rainfall time series with specific climatic characteristics.
• Data sources:
  • Meteorological station records: precipitation, air and soil temperatures, radiation, and wind speed.
  • Gauge station data: stream discharge at 10-minute intervals, aggregated to daily means.
  • Nitrate concentration data: measured near the gauge station at 14-day to monthly intervals (2001–2010).
  • Groundwater levels: recorded by automated data loggers in 13 wells.
  • N surplus: estimated as 48.8 kilograms of nitrogen per hectare per year (kg N ha⁻¹ yr⁻¹) for 1997–2010.
  • External N input: fixed at 180 kilograms of nitrogen per hectare per year (kg N ha⁻¹ yr⁻¹).
  • Soil type and aquifer material data.

Main Results

• Higher annual precipitation enhances N transformation and transport, while lower annual precipitation promotes the catchment's N retention capacity.
• The N retention capacity severely declines when vegetation suffers from drought stress, indicating a vital role for vegetation in N dynamics response to extreme droughts.
• The calibrated N transport model showed good performance (Nash-Sutcliffe efficiency of 0.79 for in-stream nitrate concentration) and a modeled N surplus of 51.87 kg N ha⁻¹ yr⁻¹, comparable to the measured 48.8 kg N ha⁻¹ yr⁻¹.
• Soil organic nitrogen (SON) accounts for 92% of the total N in the soil source zone.
• The median in-stream nitrate concentration (CQ) reached nearly 5 milligrams per liter (mg L⁻¹) in wet years (WY) and dramatically elevated to 8 mg L⁻¹ in extreme dry years (EDY) due to reduced plant uptake.
• Wet/dry conditions determined by storm duration distributions significantly affect N loads; droughts prompt SON accumulation, but drying-wetting cycles enhance extensive SON transformation in the warm season.
• Different dry–wet patterns formed by inter-storm period distributions cause noticeable variations in in-stream nitrate concentrations, with prominent elevation during the rewetting period after a drought.
• Elevated mean rainfall intensity contributes to N transformation and promotes plant absorption of inorganic nitrogen (SIN) during the growing season, but has only a small effect on stream water quality.
• The probability of drizzle events, characterizing precipitation intensity patterns, prominently alters N dynamics, with extreme dry–wet patterns enhancing SON transformation and plant uptake more effectively than continuously humid conditions, while inhibiting denitrification and leaching.

Contributions

• Pioneered the application of a stochastic rainfall generator coupled with a flow and transport model to systematically investigate inter-annual and intra-annual rainfall variability effects on N dynamics.
• Clarified the complex effects of various rainfall variability characteristics (e.g., annual precipitation, storm duration, inter-storm period, average intensity, drizzle probability) on N transformation, transport, and water quality.
• Confirmed that reduced plant uptake during extreme droughts leads to significant elevation of in-stream nitrate concentration and a decline in catchment N retention capacity.
• Provided theoretical support for developing improved fertilization strategies and protecting aquatic ecosystems under changing climate conditions.

Funding

• National Natural Science Foundation of China (grant no. U2340212, 42377046, 51879088)
• National Key Research and Development Project, China (grant no. 2024YFC3211600)
• Fundamental Research Funds for the Central Universities, China (grant no. B250201002)
• German Research Foundation – DFG, Germany (grant no. 454619223)
• Natural Science Foundation of Jiangsu Province, China (grant no. BK20190023)

Citation

@article{Wang2025effect,
  author = {Wang, Qiaoyu and Yang, Jie and Heidbüchel, Ingo and Xu, Teng and Lu, Chunhui},
  title = {The effect of rainfall variability on Nitrogen dynamics in a small agricultural catchment},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-6093-2025},
  url = {https://doi.org/10.5194/hess-29-6093-2025}
}