Climate changes and human development activities lead to changes in water resources in watersheds [1
], and the changes cause hydrological and ecological changes in rivers, resulting in a decline in water quality and difficulty in obtaining stable water resources. As water resources decrease, various problems can occur in terms of quantity and quality, so it is important to secure additional water resources of good quality. In these circumstances, a dam plays significant roles in water-utilization, flood control, and drought relief systems, such as disaster prevention. However, it is difficult to secure good quality of water for water-utilization due to environmental problems and limitations on the availability and the capacity expansion of dams. About half of South Korea’s population is depending on drinking water sources from the Paldang Lake located in South Korea that consists of flows from the South Han River, the North Han River, and the Gyeongancheon. The North Han River is composed of about 43.4% of the inflows into the Paldang Lake [4
], which is the Transboundary River between South Korea and North Korea, and can be divided into the upstream and downstream basins by the Demilitarized Zone (DMZ). The Kumgangsan Dam is located in the upstream basin of North Han River (in North Korea), and five other dams (i.e., Peace dam, Hwacheon Dam, Chuncheon Dam, Euiam Dam, Cheongpyung Dam) are located in the downstream basin (in South Korea). The Kumgangsan Dam was constructed by North Korea in 2003 for the purpose of hydroelectric power generation. Such transboundary rivers (e.g., North Han River) have been considered as a major issue for several reasons, such as water supply and pollution, in international disputes in the 21st century [5
]. Water resource disputes for Transboundary Rivers are frequent and critical issues among countries because their total water demand is greater than the water available for sharing. Various studies have been carried out to prevent the disputes based on various water allocation approaches, such as bankruptcy solution [7
], Nash bargaining solution [8
], combining bankruptcy solution with asymmetric Nash bargaining solution [9
], and Asymmetric Nash–Harsanyi Leader–Follower game model [10
]. However, Korea is the only divided nation in the world that experiences difficulty in sharing the information of water resources in the dam located in North Korea. Therefore, since it is difficult to apply the water allocation approaches in Korea, the quantitative analysis of reservoir/discharge volume of the Kumgangsan dam and the effect analysis of reduced flow rate on the downstream of the dam are needed to maintain the water resources in South Korea.
Human development activities have a great impact on the hydrology and hydrogeology in Transboundary rivers, such as decreasing the downstream water amount [11
]. In particular, dams are a significant cause of decreased downstream water resources. Since the construction of the Kumgangsan dam, there have been many issues related to the decreased flow and power generation in South Korea. In 2003, North Korea built the dam in upstream North River without consultation with South Korea. After the completion of Kumgangsan Dam, the amount of inflow into the North Han River decreased by 1.6 billion m3
per year (decreased up to 61%, 64.1%, and 80.2% during midrange flow, dry conditions, and low flow conditions, respectively) in the normal operation of the dam, and the amount of power generated by the three hydroelectric power plants in South Korea (Hwacheon Dam, Chuncheon Dam, Uiam Dam) decreased by 141.2 GWh (about 22%) [13
]. Also, the Kumgangsan Dam has caused hydrological changes, artificially altering the water on the east coast. For instance, a power plant in North Korea has been using the water from the Kumgangsan Dam to produce electricity. The Euiam Lake, in which the discharge of the Chuncheon Dam flows (located in downstream of Kumgangsan Dam watershed), is an important source of drinking water that directly affects the water quantity and quality in the Paldang Lake. The water quality in the Euiam Lake has been affected by water pollution, such as green algae blooms and eutrophication raising nutrient (nitrogen and phosphorous, etc.) concentrations, because the pollutant dilution effect has been decreased by the reduced water amount after the Kumgangsan Dam construction [14
]. Thus, water quality improvement, available water, and power generation at hydropower plants are dependent on the discharge of the Kumgangsan Dam. In addition, there was a change in the water environment in the Euiam Lake and the Chuncheon Lake; there was an unusual proliferation of Bryozoans because of the decreased water amount following the construction of Kumgangsan Dam [15
]. This is a big issue in South Korea, causing water shortages, water quality deterioration, and serious economic losses, especially during dry season. The amount of inflow into the downstream of North Han River during dry and drought seasons was remarkably decreased after the Kumgangsan Dam construction.
During dry and drought seasons, the majority of streamflow is contributed by base flow that slowly flows underground. It is important to understand the characteristics of the base flow for sustainable water management against climate changes because it has a large impact on the water quality in forest watersheds during dry periods [16
]. Also, the base flow is directly associated with streamflow during the winter season and groundwater recharge [17
], and groundwater quality influenced surface water quality [18
]. Furthermore, the base flow has been a key factor for ecosystem services during summer [19
], and groundwater plays a major role in water demand; not only in quality and quantity, but it also alleviates drought [20
]. The decreased water amount (base flow as well as total streamflow (direct runoff + base flow)) after the Kumgangsan Dam construction has a negative impact on the downstream watersheds and dams; for instance, there is now a shortage of agricultural water and hydroelectric power generation has decreased. Thus, the analysis of hydrological and base flow changes according to the decreased discharge after the dam construction is needed to manage the water resources in the downstream watersheds.
Thus, the objectives of this study are to analyze the hydrological effects of the Kumgangsan Dam constructed in the Transboundary River on the downstream watershed using hydrological modeling under dry conditions and to evaluate the base flow changes seasonally and annually due to dam construction.
The upper stream in the North Han River basin has the best water quality in the whole country and has abundant water [13
], contributing not only high-quality water resources to the metropolitan area, but also clean energy production via hydroelectric power generation. The discharge of the Kumgangsan Dam located at the uppermost stream has a wide effect on the base flow as well as the discharge of downstream dams, especially during dry conditions. Currently, the water resource disputes for Transboundary Rivers are a critical issue internationally. The dam construction on the Transboundary River is a very sensitive issue for securing the water resources in the downstream area. In particular, the study area (North Han River) includes multiple dams and it is difficult to obtain water resource management data from North Korea. This study interpreted the change of the inflows into the downstream dams spatially and temporally by using various spatial–temporal data. In addition, we proposed an approach to estimating the inflows and discharge of the dams quantitatively through hydrological modeling when there is insufficient data about the upstream watershed on a transboundary river. This study also focused on the changes of baseflow due to the dam construction for a transboundary river. As shown in the results of this study, the annual inflow estimated from the hydrological model does not show any significant difference from the actual water volume of Kumgangsan Dam; therefore, it is possible to quantitatively analyze the change in flow during the dry season by considering various scenarios. This study showed that the inflow of the downstream dam was significantly influenced by the discharge of the upstream dam; especially, the decreased flow after the construction of the Kumgangsan Dam considerably influenced the base flow changes. The decreased downstream base flow could lead to a decrease in hydroelectric power generation in the downstream dams as well as the water quantity secured in the downstream watersheds. Reduced flow also adversely affects the downstream water quality [13
], and efforts at the government level are needed to solve these issues. Increasing the discharge from the Kumgangsan Dam is necessary to secure the water resources (baseflow) in the North Han River, but it is difficult to control the Kumgangsan Dam operations in a short time due to the peculiarity between South and North Korea. Alternatives to securing baseflow include securing the reservoir capacity of existing dams and forest thinning in the downstream watersheds. The securing of the base flow is very important not only in terms of water quality improvement in the downstream watersheds, but also for securing hydropower generation. Hence, quantitative interpretation of water resources according to the dam construction on a transboundary river is essential to manage and maintain water resources (e.g., streamflow, discharge, base flow, etc.), especially during dry seasons. Also, the results of this study can be used as the basic data for efficient water management planning during dry seasons through the dry condition scenario and the evaluation of the baseflow rate during the dry season.