Abstract
In order to investigate the impact of global warming on the inland waters of Japan, we examined the rates of rises in water temperature in nine dam reservoirs that had kept long-term records of vertical temperature observations. Water temperatures at the shallowest and deepest layers were selected as being representative for our investigation, and their trend lines were obtained using time-series analysis. Our first expectation was that the deepest water temperatures would be little-affected by meteorological disturbances and might even show clearer temperature rises than those in the shallowest water. However, all the nine reservoir trend lines from 1993 to 2006 in the shallowest layers showed an upward tendency probably attributable to recent rises in air temperature. In some reservoirs, the ratio of water temperature rises exceeded the ratio of rises in air temperature. In contrast, trend lines of the deepest layers during the same period revealed both upward and downward tendencies. Those results suggested that water temperatures in the deepest layers were affected by a reduction in volume due to accretions of sand, enhanced stratification, or diminished vertical mixing, rather than to a rise in temperature due to a concomitant rise in air temperature. To determine the impacts of these effects will be an important issue in future investigation.
