Hydro- biogeochemical responses to environmental change at a granite catchment in Japan estimated from 43-years records

In Japan, rainfall patterns have been changing and extreme storm events are increasing as the effects of climate change. These events will not only cause many natural hazards such as floods, but also some kinds of changes of hydrological responses of the catchments. However, the responses may different depending on the attributes and/or background conditions of each catchment. In this study, we will consider the hydro- biogeochemical responses at a granite catchment in Japan based on 43-years observation records. The observation was conducted in Kiryu Experimental Watershed (KEW), belonging in the Tanakami mountain range. Historically, this area was devastated because of overuse of forest, and soil erosion was severe problem for the downstream. About 120 years ago, erosion control works had started, and the problem was generally cleared around 1980's. In this study, we use the data for precipitation and discharge rate since 1972, sediment transport since 1976, and streamwater chemistry since 1990 obtained from KEW. The number of rainy days is decreasing but days with larger rainfall intensity is increasing in KEW. The annual baseflow ratio is decreasing and annual direct runoff ratio is increasing as the result of the changing pattern of rainfall. The sediment transport was constrained by the erosion control dams, however, it abruptly increased since 2010 because overaged dams were destroyed by recent large precipitations. The effects of rainfall patterns are unclear in the monthly streamwater chemistry. However, the chloride conc. is decreasing and nitrate conc. is increasing for decadal periods. These changes may be the reflection of forest condition; KEW has 60-year-old unmanaged, Japanese cypress forest. It is the typical of Japanese artificial forest, and degradation of the condition may be reflected in the streamwater chemistry, that is, chloride shows the decrease of evapotranspiration and nitrate shows the decrease of nutrient uptake. Our results means that climate change can be detected in hydrological cycle. Moreover, the change, or degradation of infrastructure and/or ecosystem conditions are occurring in accordance with the climate change. Therefore, long-term monitoring is valuable to expect future, unexpected phenomena.