CHANGES IN RIVER WATER TEMPERATURE WITH RAINFALL

Abstract

River water temperature has been studied from various view points such as biological or ecological. Even in hydrology, river water tern perature has been treated only as a factor of water quality, and there are few investigations about changes in river water temperature with precipitation-runoff process besides snow-melting. In this paper, changes in river water temperature during a rainfall-runoff event were studied in the small vegetated watershed which is located about 20 km northeast of Mt. Fuji and has a drainage area of 0.12km2. Under a heavy rainfall of typhoon on Aug. 19-22, 1981, river water temperature was measured together with air temperature, soil tempera-ture, groundwater temperature, groundwater level, discharge, rainfall and surface runoff. As a result, it was observed that river water temperature rose suddenly about 0.2_??_0.5°C when surface runoff occurred and hydrograph reached its peak. On the other hand, the peak of groundwater level lagged behind rises of river water temperature. Groundwater tempera-ture and soil temperature kept constant. There were no remarkable changes in air tempera-ture during that time. Therefore, the cause of this phenomenon was attributed to a direct runoff accompanying thermal transfer. Two simulation models of changes in river water temperature with rainfall were present-ed. One model takes heat balance and mixing of water body into consideration, and the other model is based on a tank model. The former model represented that if direct runoff component occupies about 10% of total runoff, river water temperature rises about 0.2_??_0.5°C. Hydrograph separation by using specific conductance showed that direct runoff was nearly equal to 10% of total runoff at the peak of hydrograph. The latter model also proved that rises of river water temperature occur due to direct runoff.