ESTIMATION OF ERODIBILITY COEFFICIENTS BASED ON GEOPHYSICAL FOREST PROPERTIES IN OGOUCHI DAM WATERSHED

Abstract

 Forests generally provide protection against sediment-related disasters, as they reduce the impact of raindrops that erodes the topsoil during extreme rainfall events. However, under poor and ineffective management, they become susceptible to more erosive raindrops which develop and grow in the tree canopy layer before falling to the ground. In this study, preliminary field investigation were carried out to evaluate the differences in geophysical properties and responses to raindrop-impact-induced erosion among fenced Government-managed forest (TMGf), unfenced Government-managed forest (TMG), and private forest (Pri) in the upstream catchment of Ogouchi Dam. Based on the gathered data, an integrated analysis, which approximates the kinetic energy caused by smaller-sized raindrops directly falling from open sky and the larger-sized raindrops developed in the tree canopy layer, was applied. The total erodibility coefficient C_TEr for each forest group was determined by summing up the standardized values from these two, minus the probable effect of forest floor protection. Finally, the annual sediment yield for TMGf, TMG, and Pri were calculated by associating their respective C_TEr to MUSLE land cover and management factor C_MUSLE in SWAT. The outputs of the simulation reliably demonstrated the difference in sediment quantity between poorly- and effectively-managed forest in Ogouchi Dam watershed.