Event-Scale Responses of Forest Water Balance Components to Thinning Revealed by Soil Moisture Dynamics

Forests regulate hydrological processes and support water resources through rainfall partitioning and evapotranspiration, forming the basis of forest water balance. Previous studies have shown that thinning alters stand structure and affects throughfall, forest floor evaporation, and tree transpiration. However, quantifying individual components of the forest water balance within a stand typically requires multiple measurement systems operating simultaneously, making observations technically demanding and labor-intensive. As a result, systematic assessments of water balance responses to forest management remain limited.

In this study, we evaluated the applicability of a soil moisture balance approach for event-scale estimation of forest water balance components using a single soil moisture time series. Field observations were conducted in a Japanese cedar (Cryptomeria japonica) plantation subjected to row thinning. Multi-depth soil moisture sensors were installed at 20 locations in both thinned and control plots at depths of 5, 15, 25, 35, and 45 cm. In addition, two weighing lysimeters equipped with soil moisture sensors were installed in the thinned plot to directly measure soil water dynamics and forest floor evaporation.

During rainfall events, throughfall was estimated from increases in soil moisture within the 0–50 cm soil layer. The estimated throughfall showed excellent agreement with rain gauge measurements. Moreover, its spatial variability successfully reproduced increasing interception associated with canopy volume derived from LiDAR point cloud data. Forest floor evaporation was quantified during rain-free periods using changes in lysimeter weight and soil moisture depletion within the 0–20 cm layer. Tree water uptake was then estimated as the residual of soil moisture decreases in the 0–50 cm layer after accounting for forest floor evaporation, using an empirical relationship between radiation and forest floor evaporation.

The results revealed clear spatial contrasts in evapotranspiration components within the thinned stand: forest floor evaporation dominated in the center of thinning rows, while tree water uptake was more pronounced near tree stems. These findings demonstrate that the soil moisture balance approach enables separation and quantitative evaluation of forest water balance components at the event scale without reliance on large or complex measurement systems. This method provides a scalable and efficient framework for assessing hydrological impacts of forest management and offers valuable insights for sustainable forest and water resource management.