Hydro-biogeochemical functions of a riparian wetland in a forested catchment

Riparian wetland, one of a landscape in a forested catchment, has a distinctive hydro-biogeochemical functions from the unsaturated hillslope because it is continuously saturated. Mountain rivers in Japan generally have steep channel and there are many check dams to prevent from the severe soil erosion resulted from the overuse of forest. Sediments are deposited and riparian wetlands are formed in the area upslope from the dam. In this study, we observed the flow dynamics and chemical characteristics of the surface- and groundwater in such a wetland. Depending on sediment depositional patterns, soil material is sandy in upstream and lower layer of downstream of the wetland and clayey in upper layer of downstream. The results of piezometric observation showed the spatially heterogeneous flowpath of the groundwater; the flow concentrated to the lower layer of downstream, and the hyporheic exchange flow was hardly observed at the upper layer of downstream. The seasonal variability of the groundwater chemistry showed that the wetland was reduced more in summer. It was highly reduced in the clayey soil through the year. On the other hand, the reduced condition was moderated during winter in the sandy soil. These variations are result from the hydrological processes. The groundwater flow through the sandy soil easily supplies the oxygen. The degrees of effects of these hydrochemical processes on the streamwater depended on the precipitation amount. As the surface water less contacted to the wetland in a pluvial year, the effects weakened. These results suggest that the spatio-temporal heterogeneity of the hydrological processes are reflected on the biogeochemical characteristics in the forested catchment.