Modeling the impact of land-use and climate change on water quality of a deep dam reservoir

Te-Chi Reservoir is an multipurpose reservoir, which supplies drinking water for a population of ~2,800,000 and generate hydroelectric power in the Taichung city, Taiwan. In the past 10 years, this reservoir experienced several events of algal blooms and extreme drought. According to the historical water quality data, the frenauency of the trophic state for the reservoir has increased in the recent years. The N/P ratios of the reservoir are generally greater than 15, indicating that the limited nutrient of eutrophication is phosphorous. In this study, we developed an integrated model to predict the water quality of the reservoir using an input of 10-year observational data. A hydrological stream flow model (i.e., SWAT) was integrated with the simple phosphorous (P) input-output models (i.e., the Vollenweider model) to simulate the change of the trophic state and the concentration of P in the reservoir. We first investigated the hydrological variability impact on the P load in past three year when the extreme weather (drought) happened.The results showed that the concentraion of total phosphorous (TP) was significantly influenced by the inflow of the river to the reservoir and the precipitation (rainfall). The simulated concentrations of TP in dry seasons were typically higher than that in the wet seasons. During the drought, the internal loading, such as resuspension, played a significant source of P for the reservoir. We also investigated the sources and loads of key water pollutants, especially nitrogen and phosphorous, from the spatial aspect in the watershed of the reservoir. The results indicated that the TP loads of each sub-catchment area ranged from 2.76 to 4.12 kg/h. Furthermore, in order to understant the feasibility of establishment of riparian buffer strip, the effect of the land use change on water quality was simulated. This study demonstrated the application of the integrated SWAT-Vollenweider model for a reservior to identify the drivers of pollutants for managing its watershed to mitigate the potential of eutrophication.