Research on Responses to Climate Change of Runoff Pattern and Water Temperature for Rivers in a Cold Snowy Region

This study addresses how river runoff patterns and water temperatures respond to climate change, based on estimations of runoff and heat flux using changes in the long-term hydrologic cycle and interactions between the atmosphere and the land surface. Recently, the effects of climate change caused by global warming have become obvious in Hokkaido, Northern Japan, and in other cold, snowy regions. The IPCC Fifth Assessment Report (IPCC AR5) expressed growing certainty that global warming is under way. In the future, rises in river water temperatures and increases in oxygen demand due to the decomposition of organic matter induced by those rises may cause the water environment to deteriorate and may adversely affect inland fisheries and ecosystems. Specifically, in rivers of cold, snowy regions, an impact assessment of climate change on species of salmon and trout adapted to cold water temperatures is important from the viewpoint of water environmental conservation and fishery resources. River water temperatures are thought to be influenced by runoff components, such as the ratio of groundwater to surface water. For example, it is inferred that if the groundwater component accounts for a large share of the groundwater, then river water temperatures will not be affected much by the rise in atmospheric temperatures. Therefore, it is necessary to properly estimate the runoff components by using the characteristics of hydrologic processes in cold, snowy regions, such as snowmelt. First, existing climate change data (MRI-NHRCM 20), such as temperature and precipitation based on the RCP emission scenario, which has a 20-km-mesh scale, were modified to 1-km-mesh data by down-scaling based on spatial interpolation (so-called statistical downscaling). Second, current and future river runoff and water temperatures were estimated by inputting the grid-based downscaling data into a model that quantifies fluxes of heat and water between the atmosphere and land surface (LoHAS) as well as a distributed runoff model (which is based on the tank model). As a result, it was estimated that the river water temperatures may rise by about 6.6 ℃ with the temperature changes in May, and it was estimated that the period during which masu salmon, a salmonid important to Hokkaido, can inhabit rivers may decrease.