To study and model the interrill erosion process in an unmanaged Japanese cypress (Chamaecyparis obtusa) plantation, soil splash detachment rates were estimated based on the quantification of throughfall raindrop indices. Throughfall drops and soil splash detachment were simultaneously observed in the field, and observed data were compared with estimates produced by previous models. Observations took place over five months in 2005, during six observation periods. Raindrop indices of kinetic energy (KE), momentum (M), and momentum multiplied by the drop diameter (MD) were calculated from drop diameters and velocities. The

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Abstract: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, 137 Cs and 210 Pb 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 137 Cs and 210 Pb ex activities associated with the forest floor materials were considerably higher than those of the stream bank and truck trail. The 137 Cs and 210 Pb 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 137 Cs and 210 Pb 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 Hinoki 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 from 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 as a major source of suspended sediment in unmanaged Japanese cypress plantation watersheds.

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