Hydrological responses in water loss due to thinning of forested watersheds in Japan using the short-term water balance method

Abstract:

We examined the changes in seasonal patterns of catchment-scale evapotranspiration (i.e., water loss) using Short-Term Water Balance Model (STWBM) developed. STWBM is applied to estimate the value of water loss based on precipitation minus discharge volume during　short-periods(8 to 80 days). This method can be applicable for examining seasonal characteristics of water loss that relets to ET. We applied STWBM for investigating the effects of 50% thinning in nested headwater catchments draining Japanese cypress (Cryptomeria japonica) and cedar (Chamaecyparis obtusa) forests. Study areas is located to 70 km north of Tokyo with 1250 mm annual precipitation and 14℃ mean annual temperature. 50% of the stems (46% of timber volume) were removed by strip thinning in 17 ha treatment catchment, 9 ha catchment remained untreated as a control. We installed 4 nested gauging stations in treated and control catchments with 3 to 10 ha of drainage areas. Runoff in each nested gauging station was measured in the pre- (from April, 2010 to June 2011) and the post-thinning periods (from January 2012 to December 2012). Total runoff coefficient in treated and control catchment was 54% and 26%, respectively. , . Estimated annual water loss by STWBM was 585 mm in treated and 969 mm in control catchments. Because annual evapotranspiration of Japanese cypress and cedar was about ranging from 400 to 800 mm in this catchment, our estimated water loss mostly associated with ET and partially by water loss by deep bedrock percolation. Estimated water loss after thinning in growth season (May to October) decreased 45 to 60 (in 2012) % and 51 to 60 (in 2013) % for all nested gauging station, while estimated water loss in control catchment was consistent. This result suggested that 50% of thinning decreased water loss by ET but changes can be varied among nested gauging station.