## 4 Results and Discussion
### Simulations of Dam Outflow  
After examining the appropriateness of method used, the nonlinear relationship between water level and storage capacity (\(H\)-\(S\) curve) was developed using the GIS Functional Surface toolset based on the DEM data for 5 dams in North Korea including Hwanggang dam, April 5 dam 1, April 5 dam 2, April 5 dam 3, and April 5 dam 4 as shown in Table 3. These relationships were then applied in the model to simulate dam outflow as mentioned in Section 3.3. The estimated results show the dominance of storage capacity of Hwanggang dam compared to other dams. This dam is a multi-purpose dam and is also reported to be the largest dam in North Korea and has strong effects on discharge variation and geomorphological change downstream of the Imjin river [9; 31]. The dam is manually operated based on various rules of the North Korean government, with reference to the actual conditions of the dam. Hence, it is nearly impossible to automatically simulate dam outflow using a hydrologic model. However, using the duration of water release and storage in Hwanggang dam detailed in Section 4.1, the dam operation module in the model could be calibrated by changing threshold water level of the dam in the first timestep of each duration. The effectiveness of the dam operation module is evaluated by comparing observations and simulation using the dam-integrated SURR model at Gunnam station for events in 2009, 2010, 2011, and 2012 (Figure 3). Figure 3 shows that the simulated discharges have high correlation with observations, especially in the events where it is confirmed that the streamflow is affected by the amount of water released from the Northern dams. These results prove that the dam consideration module is able to indicate good fit between observations and simulations if the information of dam operation is available. To evaluate impacts of dam outflow in the Northern part of the Imjin basin on the Southern downstream region, it is necessary to consider inflow and outflow of Hwanggang--the largest dam in the Imjin river basin. The differences between inflow and outflow (so-called controlled discharge Qctr, with Qctr = Qinflow-Qoutflow) for events in 2009, 2010, 2011, and 2012 are shown in Figure 4. The differences between inflow and outflow of Hwanggang dam show the high capability of controlling discharge of the dam. The impacts of the dam become more significant during high rainfall. Referring to the dam affected period defined in Section 4.1, it is clear to see that using the dam operation, the water from the dam is able to change significantly the discharge in downstream basin, with the controlled discharge has a very wide range, from approximately \(-\)7000 to 6000 m\({}^{3}\)/s, where the negative value shows amount of water released and the positive value shows the amount of water stored. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Hwanggang Dam** & \multicolumn{2}{c}{**April 5 th Dam 1**} & \multicolumn{2}{c}{**April 5 th Dam 2**} & \multicolumn{2}{c}{**April 5 th Dam 3**} & \multicolumn{2}{c}{**April 5 th Dam 4**} \\ \hline \multirow{2}{*}{ \begin{tabular}{c} _Water_ \\ _level (m)_ \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _Storage_ \\ _capacity_ \\ _(\(10^{6}\) m\({}^{3}\))_ \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _Water_ \\ _level (m)_ \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _Storage_ \\ _level (m)_ \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _Water_ \\ _level (m)_ \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _Storage_ \\ _capacity_ \\ _(\(10^{6}\) m\({}^{3}\))_ \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} _Water_ \\ _level (m)_ \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _Storage_ \\ _capacity_ \\ _(\(10^{6}\) m\({}^{3}\))_ \\ \end{tabular}  } \\ \hline
73 & 0.0000 & 43 & 0.0037 & 60 & 1.8848 & 99 & 0.0401 & 106 & 0.6831 \\
75 & 0.1422 & 44 & 0.0066 & 61 & 3.4020 & 100 & 0.0738 & 107 & 1.3976 \\
80 & 5.