## Materials and Methods
### Dam Operation Module  
A module called AutoROM was developed as a part of the SURR rainfall runoff model to simulate dam outflow based on the concept of the automatic reservoir operation method, which is a common method widely used to operate dams/reservoirs. In this method, it is assumed that the reservoir stores the total amount of inflows until the pool reaches the specific target level (or threshold water level) and then releases the total amount of inflows after reaching that level. The nonlinear relationship between water level and storage (H-S curve) is the key aspect of the method used to simulate dam outflow in this study. To develop this relationship for a transboundary region, where the data are generally scarce, such as Northern dams of the Imjin river basin, the GIS functional surface toolset was applied using features and various surface types. To apply this toolset, it is necessary to obtain the digital elevation model (DEM) data. Moreover, a decrease in accuracy of estimation due to the increase of DEM scale has been reported [29]. Hence, high resolution DEM data are required for the best estimation. In order to examine the appropriateness and accuracy of the GIS Functional Surface toolset, the parameters of the Andong dam basin in South Korea were estimated and evaluated by comparison of data provided by the Korea National Committee on Large Dams (KNCOLD). The watershed area, total storage capacity, and the effective storage volume of Andong dam were calculated using the GIS Functional Surface toolset then compared with the provided parameters. The estimated results are 1574.74 km\({}^{2}\), 1.19 km\({}^{3}\), and 1.09 km\({}^{3}\), and the provided parameters are 1584.00 km\({}^{2}\), 1.25 km\({}^{3}\), and 1.00 km\({}^{3}\) for watershed area, total storage capacity, and the effective storage volume, respectively. The difference between estimated and provided parameters are \(-\)9.73 km\({}^{2}\) (\(-\)0.61%), \(-\)0.06 km\({}^{3}\) (\(-\)4.8%), and \(+\)0.09 km\({}^{3}\) (\(+\)9%) for watershed area, total storage capacity, and the effective storage volume, respectively. The results indicate that the method of estimating the low capacity of the dam using the GIS functional surface toolset is appropriate. Due to various specific reasons related to the binational transboundary region, it is difficult to collect all of the data required to apply the AutoROM method in the Northern part of the Imjin river basin. Hence, a number of assumptions are used to solve this problem, including: (1) All of the spillway structures considered in this study are free-orifice type; (2) the initial water level used in the model setup is equal to spillway height; (3) initial dam storage used in model setup is equal to storage corresponding to spillway height; and (4) if the dam level is below the spillway, only discharge for hydro power is considered. Based on these assumptions, the computation of dam outflow was developed using the theory of hydraulics of spillways with limited information of dams. From the nonlinear H-S curve estimated using the GIS toolset, water storage or water level at a certain level can be roughly linearly interpolated if either of the two variables is defined. The dam outflow was computed based on the change of storage and water level due to the inflow in case of dam water level over the specific target level.