Examining responses of suspended sediment transports after intense thinning in a forested headwater catchment using nested monitoring

Friday, 19 December 2014
Sooyoun Nam1, Takashi Gomi1, Yuichi Onda2, Marino Hiraoka1, Bui Xuan Dung3 and Hiroaki Kato2, (1)Tokyo University of Agriculture and Technology, Tokyo, Japan, (2)Univ Tsukuba, Tsukuba Ibaraki, Japan, (3)Vietnam Forestry University, Ha Noi, Vietnam
We examined responses of suspended sediment (SS) transports after 50% intense thinning in a 17 ha forested headwater catchment using nested 4 to 10 ha gauging stations. Strip thinning of 13- to 45-yr Japanese cedar (Cryptomeriajaponica) and cypress (Chamaecyparisobtusa) was conducted by cable dragging to skid trails in the entire watershed including nested sub-catchments. The skid trails density varied from 0 to 134 m/ha, followed by 4 m/ha. Peak SS yields monitored by turbidity sensor at the catchment outlet was 10-folds greater after the first year of thinning by comparing the pre-thinning conditions, while these values became 2-folds in the third year after thinning. This changes of peak SS concentrations suggested that recovery of vegetation cover on disturbed soil reduced peak concentrations of SS. We also confirmed changes in SS yields and recovery using paired-catchment analysis. Integrated SS yields including periods during thinning operation from August to October, 2011 varied from 0.03 to 0.06 kg. Integrated SS yields in the catchment outlet (0.05 kg) were corresponded to 5.0 kg/ha in SS yields. Organic matter content of integrated SS samples in one sub-catchment was low (0%), while the other catchments contained 0.08 and 0.13% of organic matter. 210Pbex activity in the sub-catchment (181 Bq/kg) with low organic matter content was the highest among the other nested catchments from 36 to 59 Bq/kg. Because mean 210Pbex activity from < 10 cm soil surface depth along hillslopes transects was 124 ± 83 Bq/kg, soil surface erosion from < 10 cm depth on the soil surface can be contributed to fine sediment supplies for the sub-catchments with high 210Pbex activity. In contrast, 210Pbex activity on the skid trails was low, because soil disturbance became deep with > 10 cm. Therefore, fine sediment with low 210Pbex activity on the skid trails can be transported to the catchment outlet. These characteristics were also confirmed by 137Cs activity with various potential sources. We further show how nested catchments responded in terms of radionuclides tracers during the recovery processes from the first to the third years after intense thinning. Nested observations permit us understanding for spatial and temporal variability of soil disturbance and recovery processes from intense forest thinning.