The Effects of Weather and Inflow Water Temperature on the Eutrophic Condition of Shin-Shan Reservoir

Shin-Shan Reservoir located at Anle Dist., Keelung City in Taiwan is an off-channel reservoir, providing water for people living in Keelung City and surrounding area of New Taipei City. In recent years, Shin-Shan Reservoir had heavy algal blooms in summer when the water body was stratified. In this study, we adopted CE-QUAL-W2 model and integrated the buoyant jet mechanism into it to simulate the water quality of Shin-Shan Reservoir. The relationships between inflow rate, temperature and the degree of eutrophication were investigated, as well as those between the variation of eutrophication and the characteristics of inflow. The model was calibrated and verified with the field data in 2014 and 2010, respectively. The results show that the predicted concentrations of phosphorus, nitrate, ammonium and chlorophyll a are close to real data. However, the total phosphorus concentration was underestimated by the model. Model simulations reveal that temperature of inflow water affects the final position of inflow which becomes a buoyant jet in a stratified water body. When the inflow jet is dispersed near the water surface, the nutrients will be loaded in the epilimnion or metalimnion, leading to severe eutrophication. CE-QUAL-W2 model with the buoyant jet mechanism is able to simulate the water quality of Shin-Shan Reservoir better than the model without consideration of the behavior of the buoyant plume. Furthermore, river temperature affects the final position of inflow. If the river water is cooler than the water of the thermocline, the nutrients will not be loaded into surface water, and there will be less growth of algae. On the contrary, if it is warmer, the algae will bloom heavily. Therefore, the authority of Shin-Shan Reservoir should closely watch the temperature of river water when it is climbing up quicker than that of the water in the reservoir, and choose appropriate operational approach to preserve the water quality in the reservoir.