Assessment of Turbidity In Reservoirs In Japan Induced By Flood Events in Response to Future Climate Change

High and long-term persistence of turbidity especially after a large flood event has been a burning issue in the area of river water environment. High turbidity has sometimes caused water outage and had adverse effects on society. Considering the fact that reservoirs are responsible for more than half of drinking water in Japan, it is of great importance to investigate the turbidity in reservoirs to ensure water supply. Furthermore, climate change may affect the occurrence of higher turbidity in reservoirs since behavior of turbidity is sensitive to the condition of thermal stratification in a reservoir as well as precipitation. Although there have been a number of studies investigating the effect of climate change on turbidity in reservoirs by means of numerical models, most of them examined only one or a small number of reservoirs. However, it is necessary to target reservoirs located across Japan since effects of climate change differ in space. Hence, we applied a vertical 1-dimensional hydraulic model for future prediction of turbidity in more than 30 reservoirs in Japan. We set a reference period and a future period of the simulations as 1981-2000 and 2081-2100 respectively. RCP8.5 scenario has been adopted to generate meteorological conditions. To calculate suspended sediment concentration in inflow, we developed regional sediment rating curves by taking into account differences of sediment yields for each watershed. Regarding future scenarios, Scenario1 takes into account changes of temperature and solar radiation, whereas Scenario2 considers changes of inflow on top of Scenario1, taking into account changes in annual precipitation. As a result, we revealed that profiles of the water temperature and suspended sediment after a flood event might change due to climate change. Different reservoirs showed different responses to climate change. For instance, change rates of duration of annual high turbidity outflow from reservoirs with respect to baseline showed significant variability. Plus, results implied that reservoirs with smaller turn over rate might be more sensitive to climatological change than other reservoirs.