Difference of Ecosystem and Hydrological control on Long-term water quality between adjacent subcatchments in a forested catchment in central Japan

We have been monitoring the water quality in the Kiryu Experimental Watershed (KEW; 5.99 ha), Japan, and four subcatchments for more than 20 years. The climate of KEW is warm temperate. The artificially planted Japanese Cypress around 1960 covers whole of the KEW. The geology of KEW is weathered granite. The concentrations of SiO2 and Na, which are mainly supplied from weathering processes, in streamwater were different between the catchments, and the difference was controlled by hydrological conditions in each catchment; in the catchment where shallow groundwater contribution to the streamflow is large, the concentrations were lower. Conversely, in the catchment where deep, bedrock groundwater contribution is large, these concentrations were higher. The K+ concentration which cycles between soil and plants showed clear seasonal variations and the differences between the catchments were small. Considering the long-term trend of streamwater chemistry, the electric conductivity (EC), Cl-, and SO42- concentrations have been gradually decreasing during recent 10 years in all catchments. On the other hand, the NO3- concentration has been commonly increasing in recent 5 years in all catchment. The difference of concentrations between the catchments were depending on the difference of the redox condition caused by soil sediment. One of the subcatchments experienced the disturbance in early 90's and subsequent increase of NO3- concentration, and the residence tine distribution shows the effects of the disturbance is still remaining. Recently, the disturbance is expanding in another subcatchment, and one of the reason of the disturbance is soil erosion. Moreover, the streamwater NO3- concentrations are also gradually increasing even in undisturbed catchments. These facts imply that this 50-year-old unmanaged artificial forest may start degrading, and the biogeochemical cycle may start changing. The long-term dynamics of streamwater chemistry is a good diagnosis tool of the ecosystem. Therefore, it is important to continue the monitoring and to consider the mechanisms. We will discuss about the mechanisms and the differences between adjacent subcatchments considering the water quality of the precipitation and the groundwater as well as the hydrological parameters at the coming meeting.