Discharge of Suspended Sediments During Line Thinning Across 17 % of a Forested Watershed and Relate Construction of Spur Roads

Abstract

　We investigated suspended sediments (SS) discharges before and during line thinning in a forested watershed located in a national forest in Ibaraki Prefecture, Japan. Line thinning was conducted at a thinning rate of 35 % across 17 % of the watershed area in 2012. Five spur roads were constructed along the streams. The lower slopes in the experimental watershed are covered with conifer plantation of Japanese cedar whereas the upper slopes are covered with deciduous trees. After stream water samples of about 1 L were collected hourly using an automatic water sampler, the water samples were filtered through glass fiber filters (0.5 μm) to measure the SS concentrations. Comparison within the same range of runoff showed that SS concentrations in flood flow during thinning (September-November in 2012) were higher than before thinning (June 2010 - August 2012). We calculated the SS discharges using runoff data (September-November in 2012) and the regression equations between the specific cumulative load of SS and the specific cumulative runoff before and during thinning. The SS discharges before and during thinning were 0.8 kg ha^-1 and 1.9 kg ha^-1, respectively. The SS discharge was 2.4 times greater during thinning than before thinning. Results show that both SS concentrations and SS discharges increased during thinning. The harvesting intensity was low, but results show that thinning influenced on the SS concentrations in stream water.