## 1 Introduction  
An estuary refers to a point where the final outlet of a stream converges with the marine environment, and is a water body that combines fresh water and seawater [1]. This leads to the creation of a productive ecosystem [2], and acts as a natural runoff filter to improve water quality [3]. In addition, by artificially preventing marine circulation through seawall construction and creating an estuarine reservoir-closed water environment, estuaries supply water from diverse sources, and some areas use estuarine water resources as agricultural water [4, 5]. Estauarine reservoirs are the final destination of upstream pollution sources when considering the geographical features of being situated in estuaries, and most estuarine reservoirs suffer from water quality pollution because of limited seawater circulation [6, 7, 8]. In particular, in the Ganwol estuarine reservoir, our study site, because of reclamation works, deposit atomization has resulted from tide speed decline [9], inflow of livestock wastewater and domestic wastewater as a point source pollution, and inflow of agricultural land outflow as a nonpoint source pollution [10]. This has led to the resupply of organic nutrients within the reservoir because of the resuspension of fine sediments and an increase in the phosphorous inner load [11]. The water quality of estuarine reservoirs is crucial for biodiversity, sustainability [12], and the production of high-quality, eco-friendly agricultural products on reclaimed lands [13]. Therefore, to improve the eco-friendly characteristics of estuarine reservoirs and reclaimed lands, it is important to develop water quality management measures based on a comprehensive understanding of the upstream estuarine reservoirs, inflowing pollutants, and estuarine reservoir water body features. When analyzing estuarine reservoir watersheds and water environment changes based on water quality pollution management measures, applying actual methods and conducting various assessments are useful for improving accuracy and reducing the uncertainty of data. However, there are limitations in terms of finance and time. Additionally, there is a lack of data on the flow and water quality of all streams flowing into estuarine reservoirs or on their water levels and water quality changes. Therefore, modeling is generally used to examine various scenarios. As estuarine reservoirs encompass watersheds and reservoirs, it is necessary to develop three-dimensional watershed models, hydrological models, and water bodies within estuarine reservoir models. To create a linkage model, a watershed model is the boundary condition for the hydrodynamic and water quality model of a three-dimensional numerical model, and internal and external linkages are necessary [14]. Several studies have examined the linkage between watershed and reservoir models using the hydrological simulation program-Fortran (HSPF) model [15] and the environmental fluid dynamics code (EFDC) model [16], which has been used to simulate the watershed runoff and reservoir water quality of a river estuary [14] or a river-reservoir combined system [17]. The HSPF-EFDC linkage model has also been used to enhance the prediction accuracy of water quality in rivers using the ensemble Kalman filter (EnKF) technique [18; 19]. Although the HSPF-EFDC linkage model has the advantage of being able to easily import the HSPF input file through the linkage file, this study evaluates the water quality improvement scenarios by linking the soil and water assessment tool (SWAT) [20] and EFDC models. The SWAT model has been widely used in studies on the upstream nonpoint source pollution of agricultural systems [21; 22; 23; 24; 25; 26]. In particular, [25] reviewed several studies and concluded the following: the SWAT model applied in various countries performs well in nonpoint source pollution simulation. The sediment simulation capability is also important in the simulation of nonpoint source pollution.