Estimation of suspended sediment sources using 137Cs and 210Pbex in unmanaged Japanese cypress plantation watersheds in southern Japan

To analyse suspended sediment sources in unmanaged Japanese cypress plantation watersheds, field measurements and fingerprinting of the suspended sediment was conducted in the Shimanto River basin in Southern Japan. For sediment fingerprinting, Cs-137 and Pb-210(ex) were detected by means of gamma-ray spectrometry in the surface soil of the forest floor, stream bank and truck trail and mobilized sediment by interrill erosion. The Cs-137 and Pb-210(ex) activities forest floor materials were considerably higher thin those of the stream bank and truck trail. The Cs-137 and Pb-210(ex) activities associated with the suspended sediment were found to vary with the sampling period. Evidently, the suspended sediment can comprise materials generated from the forest floor by interrill erosion and those from the truck trail and/or stream bank. The multivariate sediment-mixing model using Cs-137 and Pb-210(ex) showed that the contribution of the forest floor varied periodically, ranging from 23-56% in the Hinoki 156 subwatershed and from 18-85% in the Hinoki 155 subwatershed. The difference in the average contribution of the forest floor between Hlinoki 156 (46%) and Hinoki 155 (69%) may relate to the presence of truck trail networks in the watershed. The truck trail network can play roles of sediment source and pathway for sediment front forest floor to stream channel due to the concentrated overland flow on the truck trail during heavy rainfall events. These results indicate that the forest floor should be recognized major source of suspended sediment in unmanaged Japanese cypress plantation watersheds. Copyright (C) 2008 John Wiley & Son, Ltd.