Effects of Forest Thinning on Water Yield and Runoff Components in Headwater Catchments of Japanese Cypress Plantation

Forests play a key role in sustaining global water cycles by regulating precipitation partitioning, which in turn influences both water yield and ecosystem stability. Thinning is a silvicultural tool used to improve forest plantation productivity, but it is increasingly recognized as a means for water resource management. This study investigated hydrological changes following 40% thinning of tree density with contour-aligned log placement in paired headwater catchments of a Japanese cypress forest. Annual runoff in the treated catchment was 108.7 mm above the pre-thinning baseline in the thinning year (2020), followed by smaller increases of 99.7 mm, 43.7 mm, and 0.4 mm in 2021 to 2023, after which annual yields effectively returned to pre-thinning levels. Despite these temporary increases, peak discharge and storm quickflow metrics remained within the pre-thinning range. Flow duration curve analysis revealed a sustained enhancement of low-flow discharge and baseflow throughout the post-thinning period, indicating improved low-flow resilience without increased stormflow risk. These findings demonstrate that moderate thinning combined with contour felled logs can enhance water availability in plantation forests while maintaining flood protection. They also highlight the need for long-term, multi-site studies to test the persistence and generality of these low-flow benefits under varying forest and climate conditions.