Interpreting Runoff Process of Suspended Sediment at the Watershed Scale by Observation and Fingerprinting for Improvement of Model

Tuesday, 16 December 2014
Shigeru Mizugaki, Mayumi Kubo, Kazuyoshi Watanabe, Yasuyuki Hirai and Satoshi Hamamoto, PWRI Public Works Research Instituite, CERI Civil Engineering Research Institute for cold region, Sapporo, Japan
Information of source and runoff process of suspended sediment is crucial for better sediment management at the watershed scale. To understand the dynamics of water and suspended sediment at the watershed scale, we conducted the hydrological observation and fingerprinting source of suspended sediment using natural and fallout radionuclides as tracers in the Mu River (1270 km2) and the Saru River (1350 km2), Hokkaido, northern Japan. Furthermore, the distributed hydrological SWAT model was applied to evaluate temporal and spatial variation in water and suspended sediment dynamics in the watersheds. Combination of hydrological observation and fingerprinting technique using natural radionuclides enables to estimate the suspended sediment yield for each lithological source area within the watersheds, suggesting that the variation in suspended sediment yield was related to the potential factors of erosion represented by steep slope, landslide and bare surface of slope failure. Cesium-137 activity associated with suspended sediment was almost under detection limit, suggesting that the suspended sediment was likely to originate from subsurface material by stream channel. These results indicated that the suspended sediment was provided mainly from subsurface material to stream channel by mass movement and/or surface erosion on bare surface of slop failure. Meanwhile, the SWAT model could simulate the runoff discharge at each outlet of the rivers, but could not show any good fitting in suspended sediment concentration. This disagreement may be attributable to the erosion model, MUSLE, incorporated into the SWAT model, which can evaluate only the surface erosion on hillslope but the mass movement and/or erosion on bare slope by stream channel. To assess and predict the water and suspended sediment dynamics in the Mu River and the Saru River watersheds, further improvement should be needed for the SWAT model.