Ghaus et al. (2025) Effects of Forest Thinning on Water Yield and Runoff Components in Headwater Catchments of Japanese Cypress Plantation

Identification

• Journal: Water
• Year: 2025
• Date: 2025-12-05
• Authors: Ibtisam Mohd Ghaus, Nobuaki Tanaka, Takanori Sato, Moein Farahnak, Yuya Otani, Anand Nainar, Mie Gomyo, Koichiro Kuraji
• DOI: 10.3390/w17243461

Research Groups

• Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo 113-8657, Japan
• The University of Tokyo Hokkaido Forest, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Furano 079-1563, Japan
• Faculty of Regional Environmental Science, Tokyo University of Agriculture, Setagaya 156-8502, Japan
• Ecohydrology Research Institute, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Seto 489-0031, Japan
• Faculty of International Studies, Setsunan University, Osaka 572-8508, Japan
• Faculty of Tropical Forestry, Universiti Malaysia Sabah, Kota Kinabalu 88-400, Malaysia
• Forest GX/DX Co-Creation Center, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo 113-8657, Japan
• Executive Office, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo 113-8657, Japan

Short Summary

This study investigated the hydrological impacts of 40% forest thinning with contour-aligned log placement in Japanese cypress headwater catchments, finding a temporary increase in annual water yield and enhanced low-flow discharge without increasing stormflow risk.

Objective

• To investigate how forest thinning influences rainfall–runoff characteristics in a Japanese cypress headwater catchment.
• Hypothesized that thinning with minimal disturbance (felled logs parallel to contour lines) would increase annual water yield and enhance baseflow contributions while producing negligible changes in storm quickflow and peak discharge.

Study Configuration

• Spatial Scale: Two headwater catchments (A1: 2.27 hectares, A2: 1.45 hectares) at Obora Experimental Watershed, Toyota City, Aichi Prefecture, Japan. Elevation ranges from 577 meters to 644 meters above sea level.
• Temporal Scale:
  • Pre-thinning (calibration) period: March 2016 to December 2019.
  • Thinning operation: January to March 2020 (40% tree density reduction in A1).
  • Post-thinning period: April 2020 to December 2023.

Methodology and Data

• Models used:
  • Simple linear regression for annual water yield estimation and flow duration curve analysis.
  • Straight-line method for hydrograph separation into quickflow and baseflow.
  • Analysis of covariance (ANCOVA) for comparing regression line slopes.
  • Mann–Whitney U test for comparing distributions of storm event characteristics.
• Data sources:
  • Streamflow: 90° V-notch gauging weirs equipped with Thalimedes shaft encoder data loggers, recording at 5-minute intervals.
  • Rainfall: OW-34-BP tipping bucket rain gauge (0.5 mm per tip), recording at 10-minute intervals.

Main Results

• Annual runoff in the thinned catchment (A1) increased by 108.7 mm in the thinning year (2020), followed by smaller increases of 99.7 mm (2021), 43.7 mm (2022), and 0.4 mm (2023), indicating a strong but short-lived enhancement in water yield that returned to pre-thinning levels by 2023.
• Peak discharge and storm quickflow metrics remained within the pre-thinning range, with negligible changes in quickflow (Qquick), peak discharge (Qpeak), and time to peak (Tpeak).
• Flow duration curve analysis revealed a sustained enhancement of low-flow discharge and baseflow throughout the post-thinning period, with observed-to-estimated runoff ratios generally above 100% over the 64–100% exceedance range.
• Initial discharge (Qinitial) and baseflow volume (Qbase) exhibited significant slope increases of 19% and 21%, respectively (ANCOVA, p < 0.001), after thinning, suggesting enhanced subsurface contributions.
• The combination of reduced evapotranspiration and interception following canopy opening, along with the stabilizing influence of contour-felled logs, contributed to maintaining overall hydrological stability and promoting infiltration.

Contributions

• Provides a comprehensive assessment of minimal-disturbance thinning (40% tree density reduction with contour-aligned felled logs) effects on annual water yield, flow duration curves, and storm event hydrograph components in Japanese cypress headwater catchments.
• Demonstrates that this specific thinning practice can enhance water availability (increased annual yield and strengthened low-flow/baseflow) without exacerbating stormflow risk (negligible changes in peak discharge and quickflow).
• Highlights the utility of combining annual water yield, flow duration curve ratio analysis, and storm event hydrograph separation for a detailed understanding of thinning impacts on different runoff components.
• Confirms the transient nature of annual water yield increases and the more persistent benefits to low-flow conditions under this management strategy.

Funding

• Project of monitoring the water conservation function granted by the Toyota City government.

Citation

@article{Ghaus2025Effects,
  author = {Ghaus, Ibtisam Mohd and Tanaka, Nobuaki and Sato, Takanori and Farahnak, Moein and Otani, Yuya and Nainar, Anand and Gomyo, Mie and Kuraji, Koichiro},
  title = {Effects of Forest Thinning on Water Yield and Runoff Components in Headwater Catchments of Japanese Cypress Plantation},
  journal = {Water},
  year = {2025},
  doi = {10.3390/w17243461},
  url = {https://doi.org/10.3390/w17243461}
}