Influence of different management systems on surface protection and erodibility of a temperate mixed forest

Abstract

Unmanaged artificial forests face severe rates of erosion which could also impose serious threats to their adjacent environment. The Soil and Water Assessment Tool (SWAT) is a widely used physically-based model which estimates sediment yield using the empirical Modified Universal Soil Loss Equation (MUSLE). Although SWAT has been proven to obtain satisfactory results in forested catchments, the lack of representation of surface protection in the MUSLE factors still needs to be addressed. Acknowledging this gap, the present study applied a unique method of evaluating the surface protection and erosion susceptibility of four forest groups (poorly-managed private forest, moderately-managed unfenced Government-managed forest, well-managed fenced Government-managed forest, and natural forest) in the upstream catchment of Ogouchi Reservoir in Japan (262.9 km2) which is dominated by steep slopes (35°–45°). While the individual susceptibility to open and throughfall raindrops can be expressed by stand density, surface protection was found to be additionally influenced by slope and management system. With this, a multiple linear regression equation for surface protection (Pr) was derived from the influencing forest properties. Integrating the combined impacts of open and throughfall raindrops with surface protection, the values of total erodibility coefficient (CTEr) were computed which will then be corresponded to the MUSLE management practice factor (MUSLE P) input in SWAT. Poorly-managed private forests were found to be the most susceptible to raindrop splash erosion due to their exposed surface, while the well-maintained fenced Government-managed forests possess the most ideal condition for erosion control. The results of modeling CTEr at various management scenarios confirmed that the erosion susceptibility of poorly-managed Japanese cypress forest could be about 40% greater than the case of well-managed conifer plantation, a ratio that is close to and within the range of the values earlier reported.