Impact assessment of water-level management on water quality in an estuary reservoir using a watershed-reservoir linkage model

In Korea, estuary reservoirs are one of the main water resources for agriculture, generally used for water supply, while ensuring an appropriate water level. The recent inflow of pollutants from watersheds is aggravating water pollution problems in these reservoirs; to address this issue, the government has implemented effective measures, including the control of water level and basin-management measures. Effective water-level management of estuary reservoirs can reduce the environmental impact on freshwater and coastal fishing. However, the current water-management measures mainly focus on securing water and ensuring the feasibility of sea dike construction. Therefore, in this study, we evaluated the effect of drainage gate operation on the water quality of the Ganwol reservoir. We established a watershed-estuary linkage model, the Hydrological Simulation Program-Fortran, Environmental Fluid Dynamics Code, and Water Quality Analysis Simulation Program (HSPF-EFDC-WASP) linkage simulation system, to comprehensively consider the watershed and water-body characteristics and study the water quality changes for differently managed water-level scenarios. The effect of water level on the water quality of the estuary reservoir was evaluated by considering differently managed water-level scenarios in the linkage model. The results indicated that the difference in the water quality for each water-level scenario was larger at the lower point (GW1) of the estuary reservoir than at the upper point (GW3). At GW3, the water quality tended to improve with the increasing water level. GW1 was significantly affected by the operation of the drainage gate; at GW1, the lower the management level was, the higher the water-quality improvement effect, owing to the improvement in the flow in the estuary caused by the increase in the discharge frequency. The study results can be used as basic data to prepare an optimal water resource management plan for estuary reservoirs by estimating the water-level impact on their future hydrology and water quality. •A watershed-estuary linkage model was developed to analyze the water quality changes of an estuary reservoir.•Differently managed water level conditions were considered in the linkage model.•Water-level management of estuary reservoirs can reduce environmental impact.•It is crucial to induce water circulation through discharge while keeping the managed water level low.