A Coupled Modeling Approach (SWAT And CE-QUAL-W2) To Simulate The Characteristics Of Nitrogen Runoff Loss In Shanmei Reservoir Watershed

Water quality safety of the reservoir with the function of drinking water supply is a key issue concerning people’s lives and social development. The water quality in the reservoir is not only affected by non-point source pollution under the conditions of land use and agricultural management practices from the upland watershed, but also influenced with the flow field and processes of movement, transformation and degradation of pollutant. Therefore, it is necessary to couple the watershed and reservoir as an integrated system, and to reveal the influence of non-point source pollution on the water quality of reservoir from the watershed scale. Shanmei Reservoir is an important drinking water source of Quanzhou City but with great issue of non-point source pollution from the upland watershed. High total nitrogen concentrations have deteriorated the water body and accelerated the eutrophication trend, causing a serious impact on the water quality with regard to irrigation and water supply and on the sustainable development of social economy. Therefore, it is meaningful to evaluate the characteristics of non-point source nitrogen loss and their impact on the nitrogen distribution in the reservoir, so as to make effective nitrogen reduction strategy from the watershed and optimize the reservoir operation to improve the water quality of the reservoir.The watershed above the dam of Shanmei Reservoir was selected as the study area. Based on watershed distributed hydrological model SWAT and two-dimensional laterally averaged hydrodynamic and water quality model CE-QUAL-W2, a coupled watershed-reservoir modeling system was developed. Applying the coupled model, the characteristics of temporal and spatial distribution of nitrogen loss from the watershed were analyzed, the distribution of water temperature, dissolved oxygen and nitrate was simulated, the effects of reservoir operation on the nitrogen retention was evaluated, the relationship between the net primary productivity of water and eutrophication is primarily discussed, and the influence of fertilization application on the characteristics of nitrogen loss from the watershed and nitrogen concentration distribution within the reservoir was quantitatively evaluated.The results showed that,1) the coupled model can be used to successfully assess the daily process of hydrodynamic and water quality in complex watershed comprised of upland watershed and downstream reservoir; 2) there were significant spatial differences of nitrogen loss in Shanmei Reservoir watershed, TN load entering into the reservoir mainly came from Taoxi subbasin; the spatial distribution of nitrogen loss was always influenced dramatically by land use types at annual, monthly and daily scales, while the effect of precipitation and runoff on the nitrogen loss varied with time scales; 3) the amount of nitrogen loss from the watershed in wet year was greater than in normal year and dry year; the TN load during the wet season accounted for 80% of the annual total, higher values of TN concentration occurred in dry season rather than in wet season; daily process of TN load shared similar temporal patterns with precipitation and runoff, but showed obvious hysteresis properties; TN concentration process basically showed the opposite trend with precipitation process; 4) the distribution of nitrate concentration in the Shanmei Reservoir showed vertical stratified characteristics with seasonal differences, which was affected by the vertical distribution of water temperature, dissolved oxygen and seasonal distribution characters of nitrogen concentration in the input river of the reservoir; 5) the nitrogen retention in the reservoir was significantly influenced by inflow runoff, nitrogen load and reservoir regulation; 6) net primary productivity of the Shanmei Reservoir had obvious temporal and spatial characteristics, and could reflect the nutritional level of the reservoir in a certain extent; 7) selecting proper amount of fertilizer, fertilizing time, and fertilizer variety could reduce the nitrogen load and decrease nitrogen concentration effectively.