The Effect of Weather on the River Temperature and the Water Quality of the off-channel Reservoir

The reservoirs are major water facilities in Taiwan. The water quality of reservoirs will affect the quality of public water supply. Due to land overexploitation and human activities are deteriorating the water quality of the reservoirs. Problems of the eutrophication and the reservoir sedimentation are worse than before, which the water resource management has to face. In this research, we have investigated the effects of inflowing water temperature in addition to the water quality of Keelung river on a subtropical off-channel reservoir, Hsin-Shan Reservoir. By using the water quality model CE-QUAL-W2 as a tool and we did simulation of the algal concentration as well as concentration of nutrients with the estimation of the inflow position by the buoyant jet model to get the estimates as close as the real situation. Furthermore, we established the methods to estimate the river temperature and other environmental factors affected the eutrophication through the meteorological data. Finally, the method to control and prevent the algae bloom in Hsin-Shan Reservoir will be discussed. In this study, in order to simulate the river temperature, respectively, we established the model to predict the river water temperature by using weather information. The results show that the average relative error is about 5% between the predicted river water temperature and the real water temperature. The monthly average relative error is about 3 to 7%. In general, the simulation of river water temperature has reasonable accuracy. With estimated river water temperature, the simulation results indicate that the nutrients were completely mixed in the reservoir at winter, and was unevenly influenced by the thermal stratification and the final position of the inflow in summer. The model can accurately simulate temporal and spatial variation of the concentration of algae. Besides, we found that algae concentration in reservoir has highly correlation with the amount of nutrient supply to epilimnion. The nutrient supply to the surface water and algae growth would be reduced if the inflow temperature is low enough to avoid breaking into the thermocline to the surface on account of the buoyancy of the inflow water. With high accuracy of the estimates of the inflowing water temperature, CE-QUAL-W2 model with the consideration of the buoyant jet mechanism are able to simulate the water quality of Hsin-Shan Reservoir very well. The method can not only alert to the occurrence of algal bloom but also predict the impact of the climate change in the future. Therefore, the authority of Hsin-Shan Reservoir should closely watch the temperature of river water and choose appropriate operational approach to preserve the water quality in the reservoir when it is continuously climbing up within a short period of time. Keywords:CE-QUAL-W2、river temperature prediction、buyant jet mechanism、eutrophication、off-channel reservoir