## 2 Study Area  
The relation between North Korea and South Korea is complicated with extreme conflicts as the two nations were originally a single nation and have been divided since the Korean War in 1953. This sensitive relation requires studies focusing on the transboundary region between the two countries to be more considerate. The Imjin river, passing through North and South Korea, crossing the Demilitarized Zone (DMZ), and joining the Han River downstream of Seoul before emptying into Gyeonggi Bay in Yellow Sea, is the well-known Korean transboundary basin. It is the seventh largest river in Korea with a total basin area of 8139 km\({}^{2}\) and a total length of 273.5 km (Figure 1a).  
The Northern part of the Imjin river basin is predominantly mountainous, so the river gradient is relatively steep, and many streams pass through the valleys. The uppermost part, which heads from Hamgyeongnam-do Maskiryoung Duryu mountains, varies in elevation from 1500-1800 m above sea level. Therefore, the inflow of water increasing dramatically due to the high amount of rainfall may cause flood damage downstream. Especially in recent decades, North Korea has constructed five dams upstream of the Imjin river, including Hwanggang dam, April 5 dam 1, April 5 dam 2, April 5 dam 3, and April 5 dam 4, of which the April 5 dam 1 and April 5 dam 2 are located in a tributary downstream of Hwanggang dam, whereas the April 5 dam 3 and April 5 dam 4 are located upstream  
Figure 1: The Imjin river basin, Korean peninsula: (**a**) Subbasins, river network, and elevation (Digital Elevation Model, DEM); (**b**) Location of 68 rain-gauge stations, 3 meteorological stations, 3 flow stations, and Thiessen polygons.  
of Hwanggang dam [9]. The largest multi-purpose dam, Hwanggang, which is located 42 km north of the DMZ, completed construction and started operating in North Korea in 2007.  
In the Korean Peninsula, flood season normally lasts from the beginning of June through to the end of September, and in this season, high river discharge is naturally caused by a large amount of precipitation over the basin. Annual precipitation of the Imjin river basin is 1100 mm, with the highest hourly rainfall recorded at 120 mm [24]. About 74% of the total annual discharge occurs between June and September, roughly parallel to the monsoonal climate, with its wet summers and dry winters. Due to the high amount of precipitation, the Imjin river basin encounters a number of flood events during this time. After constructing a range of hydraulic structures in North Korea, the downstream is facing not only a natural flood hazard due to high rainfall but also an artificial flood risk from the dam.  
Because of the situation of the study catchment, with two third of area placed in North Korea, the data in the Northern part of the basin are hard to access due to political boundaries for transboundary rivers. Additionally, Paju-si, a military zone located in this region that is used in river-crossing operations, is also influenced directly by flood hazards. The improvement of flood forecasting systems is intrinsically related to military operations. Nevertheless, the area includes 38 subbasins with 68 rain-gauge stations, three meteorological stations, and three flow stations (Figure 1b), in which all the hydrometeorological stations are located in the Southern area and there is no observation of weather or flow obtained in the Northern area due to limited accessible sources. Hydrological modelling for the Imjin river basin has become a specific challenge for South Korean scientists and engineers.