Evaluation of water shortage and instream flows of shared rivers in South Korea according to the dam operations in North Korea

The Han River basin consists mainly of three river basins: North Han River, South Han River, and Imjin River basins. Notably, the North Han River and Imjin River basins, excluding the South Han River basin, are essentially shared rivers originating in North Korea. The Korean Peninsula features a mountainous terrain in the east and lower terrain in the west, contributing to the complexity of efficient water resource management and making it difficult. In particular, the effective water resource management for shared rivers is paramount due to the uncertainties posed by climate factors, including frequent extreme droughts and localized torrential rains, in addition to the ongoing political uncertainties between South and North Korea.

In particular, the North Han River and Imjin River basins are essentially shared rivers that originate in North Korea and are connected by the military demarcation line between South and North Korea. This situation makes it exceptionally challenging to manage the water resource in the entire Han River basin. In the case of the North Han River basin, a substantial 1.7 billion tons per year of water used to flow from the upper reaches of the North Han River to South Korea before the construction of the Imnam Dam in North Korea. However, following the Imnam Dam's operation, the inflow decreased, prompting the construction of the Peace Dam as a response to North Korea's water attack plan. Consequently, a total of seven dams are currently in operation within the North Han River basin. In the Imjin River basin, another shared river, the Hwagang Dam, is operated in North Korea, while dams such as the Gunnam Dam and Hantangnag Dam are being constructed and operated for flood control purposes. It is worth noting that the operation of dams in North Korea, which are interconnected with the Han River basin, plays a pivotal role in the efficient management of water resources within the Han River basin.

Many domestic studies have focused on the impact of water resource management and dam operations in South and North Korea. Kim et al. (2002) conducted a review of the current status of water resources in the Imjin River basin and proposed a plan for efficient utilization of water resources through cooperation between South and North Korea. In a study by Lee et al. (2008), problems related to shared rivers in South and North Korea were diagnosed, and suggestions were made to promote a cooperative system concerning water resources in North Korea. Myeong & Kwon (2010) analyzed past precipitation characteristics and forecasts to assess variations in water resources in North Korea. KEI (2009) and Ahn et al. (2011) examined the quantity and quality of water resources in the North Han River basin following the operation of the Imnam Dam in North Korea, highlighting the water shortage situation. Ahn et al. (2011) conducted a comprehensive evaluation of the impact of the Imnam Dam's operation and river diversion in the North Han River basin and its effects on the downstream. They also analyzed the specific situations related to the shared rivers between South and North Korea and the disadvantages resulting from hydrogeographic asymmetry. Sah (2017) reviewed the water dispute between the Imjin River and North Han River basins based on theoretical discussions on integrated water resource management between countries.

In order to ensure the efficient water resource management in the Imjin River and North Han River basins, both of which involve shared rivers, it is essential to analyze linkage management and operation systems of dams and to secure the instream flow for the river ecosystem conservation and agricultural activities. In studies focusing on instream flow, Olsen et al. (2013) reviewed hydrological models used for estimating instream and environmental flows suitable for different rivers in Denmark. They also recommended appropriate instream flow ranges for each river. Kang et al. (2018) examined various concepts related to instream flow and suggested improvement policies aimed at pragmatic water resource management. Their approach emphasized the correlation between instream flow and river water usage. Zhang et al. (2018) conducted an analysis of instream flow shortages resulting from dam operations, significant fluctuations in instream flow due to human activities, and the impact of instream flow on the water environment in the Huanghua River, China. Woo et al. (2019) employed the SWAT (Soil and Water Assessment Tool) to assess changes in water quality and the aquatic ecosystem in basins. This assessment was conducted through additional dam releases aimed at meeting instream flow requirements specified in major multi-purpose dams within the Han River basin.

The purpose of this study is to evaluate whether dams situated in the shared rivers of the North Han River and Imjin River basins maintain sufficient instream flow, taking into consideration the operations of the Imnam and Hwangang Dams in North Korea. First, this study will conduct a brief analysis of the water balance of dams in the North Han River and Imjin River basins. Second, this study will assess the impact of the operations of the Imnam Dam and Hwangang Dam in North Korea on the dams in North Han River, both before and after the operation. Finally, building upon the operations of the Imnam Dam and Hwangang Dam, this study will evaluate whether the dams in these two basins have effectively sustained instream flows and propose appropriate countermeasures to ensure adequate instream flows for these dams. By employing these methods, it is possible to formulate a plan to ensure that the dams situated in the North Han River and Imjin River basins, both of which are in the unique situation of being shared rivers, stably secure the instream flow for effective water resource management.

The total area of the Han River basin, including areas located in North Korea, is 34,647 km², accounting for 35% of South Korea's territory. The river itself has a length of 459.3 km. The Han River originates in Taebaek, Gangwon-do and is composed of two main tributaries: The North Han River and the Imjin River. Both of those tributaries are shared rivers that connect with North Korea.

The North Han River boasts several topographical advantages in terms of irrigation policies and stands out as the river with the most active utilization of water resources among South Korean rivers. This distinction is owed to the presence of several dams, including the Cheongpyeong, Uiam, Chuncheon, Soyanggang, and Hwancheon Dams. The North Han River serves as the primary tributary of the Han River, excluding the Imjin River. Its basin covers an area of 10,761.2 km², with a river length of 291.3 km. This accounts for approximately 41.3% of the entire Han River basin (see Table 1). Originating from the north of the DMZ (Demilitarized Zone), the North Han River traverses its basin before merging with the South Han River, ultimately flowing into the Paldang Dam basin (KEI 2015).

The Imjin River, originating in Hamgyeongnam-do, North Korea, is the first tributary of the Han River, beginning at its estuary and forming a boundary with the North Han River basins. The upper reaches of the Imjin River feature a steep riverbed slope, while wide plains are distributed in the middle and lower reaches. However, the lower reaches have a gentle riverbed slope, making them vulnerable to significant flood damage. Furthermore, the Imjin River serves as a vital water supply source for the northern part of the metropolitan area, playing a crucial role in water resource utilization and river management. For detailed specifications of the Imjin River basins, please refer to Table 2.

The Imnam Dam diverts the upper stream of the North Han River to the East Sea, causing considerable disruptions to hydroelectric power generation as well as agricultural and industrial water supply in South Korea. Furthermore, conflicts in flood control within the shared rivers of South and North Korea have resulted in frequent damages caused by unauthorized dam release in North Korea's upper reaches. Since 2001, North Korea has released dam discharges a total of 10 times, providing advance notices only on two occasions. Consequently, flood damages in the Imjin River due to unauthorized dam release have occurred on five occasions. In the context of irrigational conflicts, the Hwanggang Dam, located in Imjin River basins, was completed and began operating in 2008. An assessment was conducted to determine whether the dam operation led to a decrease in streamflow. The finding showed a notable reduction in streamflow from North Korea after 2008, along with a broader variation in flow during the dry season. The analysis revealed that the Hwanggang Dam not only contributed to decreased precipitation but also diminished stream ratios during the dry season, affecting the lower reaches of the Imjin River in South Korea.

One of the goals of this study is to analyze the water balance in the North Han River and Imjin River basins. By conducting this water balance analysis, it is possible to verify whether the existing dams located in the North Han River and Imijin River basins are operating normally.

This study places particular focus on understanding the relationships among ‘total release from the upper dams’, ‘inflow from the corresponding dam’, ‘inflow from the upper dams’, and various factors including the total release, dam inflow, and the locations and geographical characteristics of observation stations for each basin, to evaluate the water balance; the definitions of these factors are established as follows:

• The total release of corresponding dam = hydroelectric release of the corresponding dam + spillway release of the corresponding dam + other releases of the corresponding dam

• The inflow of the corresponding dam = total release of the upstream dam + basin inflow of the upper basin-intake flows of the corresponding dam

• The basin inflow of the upper basin = inflow of the corresponding dam − total release of the upstream dam

In the North Han River basins, the water balance analysis began with the Imnam Dam in North Korea, followed by the Peace Dam, Hwacheon Dam, Chuncheon Dam, Soyanggang Dam, Uiam Dam, Cheongpyeong Dam, and Paldang Dam. To assess the fundamental dam operation status, this study obtained dam operation data from WAMIS (www.wamis.go.kr) covering the period from May 1992 to December 2020, recorded on an hourly and daily basis. In the Imjin River basins, due to limitations in obtaining accurate hydrological data regarding inflow and release of the Gunnan Dam and Hantangang Dam, this study utilized observation data from multiple stations: observation data from the Pilseung bridge station, located between the Hwanggang Dam and Gunnam Dam, spanning from January 2002 to December 2020; observation data from the Imjin bridge station, situated downstream of the Gunnam Dam, covering the period from January 2001 to December 2020; observation data from the Sarang bridge station, located downstream of the Hantangang Dam, available from January 2004 to December 2020; observation data from the Biryong bridge station, situated at the junction of downstream rivers of the Gunnam Dam and Hantangang Dam, covering the period from January 2001 to December 2020.

Second, the results of water balance for the Imjin River basins using daily data were analyzed. As a result, the streamflow at the Imjin bridge station averaged 110 m³/s, and the total release of the Gunnam Dam was 98.3 m³/s (as reported by K-water 2016). Consequently, the estimated basin inflow between the Gunnam Dam and Imjin bridge station was 12.4 m³/s. The streamflow at the Biryong bridge was 207.6 m³/s, and the combined streamflow from the Imjin bridge and Sarang bridge stations amounted to 186.5 m³/s. This led to estimated basin inflow at the junction of downstream rivers of the Gunnam Dam and Hantangang Dam of 21.1 m³/s. Additionally, the estimated basin inflow between the Biryong bridge and Tongil bridge was 40.8 m³/s. Despite limitations in data acquisition, upon scrutinizing the available data, it is affirmed that the basin inflows between dams and observation stations have been reasonably calculated and estimated for the water balance analysis in the Imjin River basins.

The analysis method for assessing flow variations in the North Han River aimed to evaluate the impact of the Imnam Dam operation in North Korea on the North Han River basins. Due to limitations in acquiring observed flow data for flow variations, the inflows of the Peace Dam were assumed to be equivalent to the releases of the Imnam Dam in North Korea. Furthermore, the Peace Dam, constructed solely for water control and defense against water attacks, releases 100% of its inflow as is. As a result, the total release from the Peace Dam accounts for the majority of the inflow to the Hwacheon Dam. In essence, to scrutinize the impact of dams situated on the shared rivers of the North Han River due to the Imnam Dam operation, it is imperative to examine the flow variations at the Hwacheon Dam.

Consequently, the inflow and total release of the Hwacheon Dam were examined with a specific focus on the operation of the Peace Dam. The results are outlined in Table 4, which compares monthly areal-averaged precipitation and monthly flow in the Hwacheon Dam basins for the entire period (encompassing both flood and dry seasons) and only the dry season before and after the Imnam Dam operation (in 2003). In Table 4, areal-averaged precipitation, dam inflow, and dam release for the entire period amounted to 3.4 mm, 58.1 m³/s, and 56.6 m³/s, respectively. Notably, when considering the entire period before and after the construction of the Imnam Dam, although the areal-averaged precipitation experienced a slight decrease from 3.5 to 3.4 mm, the monthly inflow to the Hwacheon Dam decreased significantly, dropping from 73.0 to 52.8 m³/s (a decrease of 27.7%). As inflows decreased, the total release also saw a substantial reduction, falling from 75.3 to 45.0 m³/s (a decrease of 40.2%). Additionally, during the dry season, areal-averaged precipitation slightly increased from 0.4 to 0.6 mm after the construction of the Imnam Dam. However, the inflow and total release of the Hwacheon Dam decreased from 15.5 and 30.7 to 9.1 m³/s (a decrease of 41.3%) and 21.8 m³/s (a decrease of 29.0%), respectively. These results confirm that the Imnam Dam operation played a significant role in the decrease in inflow and total release in the North Han River.

To adhere to the government's instream flow requirements for the dam operation, the release flows from all dams must consistently surpass the specified instream flow. Anticipating the potential expansion of the environmental flow concept in the future, it is likely that higher flows than the current instream flow standards will be necessary. Failure to meet these standards will result in a heightened consideration of the environmental impact on the river. Therefore, this study analyzed two main issues based on the criterion of whether the current dam operation plans adequately fulfill the instream flow requirements. First, this study investigated whether the operation of each dam within the North Han River basins, as well as coordinated operations between these dams, conformed to the instream flow standards. It also explored deviations that might occur due to factors such as insufficient inflow, specific dam operation purposes, or preserved dam operation plans for releases. Second, this study assessed whether dam operations in North Korea influenced the satisfaction of the instream flow standards. Table 5 illustrates the standard instream flow requirements for each dam in the North Han River basins, as announced by the government. These standards increase downstream, taking into account water intake and river basin area. Since the Peace Dam serves a specific purpose and does not separately announce instream flow standards, the instream flow for the Peace Dam was estimated at 1.12 m³/s using the area ratio between the Hwacheon Dam and Peace Dam.

These findings indicate that not only the Hwacheon Dam but also the majority of the dams located in the North Han River basins failed to meet the instream flow requirements when assessed using hourly, daily, and monthly data. In the analysis results using daily and monthly data, it appeared that the periods of insufficient instream flow were significantly reduced as the data were converted according to the different time units (e.g., from hourly data to daily data). However, these changes in results were attributed to the data conversion process between time units. For accurate instream flow analysis, it is imperative to evaluate dam release based on hourly data. Particularly, the analysis using hourly data, which allows for more precise assessment of instream flow compliance, revealed that most dams consistently fell short of meeting the standard instream flow. This deficiency can be attributed to the focus on hydroelectric power generation in dams designed for such purposes and multi-purpose dams, which are operated for both water supply and hydroelectric power generation. Consequently, it was evident that these dams did not meet the standard instream flow when hourly data were employed. In this context, if the condition of failing to secure the instream flow persists over an extended period, the possibility of adverse effects on the aquatic environment and ecosystem increases.

Table 6 presents the shortage amounts of instream flow and total period of insufficient instream flow for dams in the North Han River basins, analyzed using hourly data. The results show that the total shortage amounts of instream flow were the largest for the Peace Dam, totaling 1,471,113.2 m³, with a total period of instream flow shortage lasting 77,921 h. The Soyanggang Dam had total shortage amounts of instream flow estimated at 1,192,700 m³, occurring over a period of 119,076 h. It is worth noting that while the total shortage amounts of instream flow tended to decrease as one moved downstream from the Peace Dam, located at the top of the basin, to the Paldang Dam, located at the bottom, the absolute values of standard instream flow are larger in lower basins.

As a result, the total shortage amounts were estimated to be larger for the downstream dams compared to the upstream dams. In other words, the analysis revealed that the shortage period of instream flow decreased as one moved downstream, while the shortage amounts increased. When calculating the annual shortage amounts of instream flow for each dam, the results showed that the shortage amounts of instream flow were as follows: The Peace Dam had an estimated shortage at 37.2 million tons, Hwacheon Dam, 145.8 million tons, Chuncheon Dam, 163.5 million tons, Uiam Dam, 595.4 million tons, Soyanggang Dam, 315.9 million tons, Cheongpyeong Dam, 299 million tons, and Paldang Dam, a substantial 1,667 million tons. It was observed that the annual shortage amounts of instream flow for each dam were proportional to the average shortage amounts of instream flow.

Furthermore, this study investigated whether the operation of the Imnam Dam in North Korea affected the operation plans of dams located in the North Han River basins and the satisfaction of the standard instream flow thorough dam release. Using data from 2003, when the Imnam Dam operation commenced, the inflow, total release, spillway flow, release for the hydroelectric power generation, total shortage amounts, and period of instream flow were calculated and compared by dividing into both before and after the dam operation (refer to Table 7). In Table 7, this study compared the inflow fluctuations for each dam before and after the Imnam Dam operation in North Korea. The inflow changes for the dams were as follows: Hwacheon Dam decreased by −40%, Chuncheon Dam increased by +20%, Uiam Dam decreased by −30%, Cheongpyeong Dam decreased by −40%, and Paldang Dam decreased by −20%. The increase in inflow at the Chuncheon Dam was attributed to slightly increased precipitation in its dam basins since 2003. Due to the Imnam Dam operation, the inflow of each dam decreased, leading to corresponding reduction in the total release of dams located in the North Han River basins. Although each dam had a different operation plan, the dams in the North Han River basins were managed by reducing both the total release amounts for hydroelectric power generation and spillway release amounts.

In the case of the Hwacheon Dam and Paldang Dam, there was a strong correlation between the decrease in total release and the reduction in release for hydroelectric power generation. Conversely, the reduction in total release of other dams was highly correlated with the decrease of spillway release. The increased ratios of the total shortage amount and periods of instream flow storage, resulting from decrease in dam inflow and total release due to the Imnam Dam operation, were calculated as follows: 330 and 330% for Hwacheon Dam, 280 and 280% for Chuncheon Dam, 300 and 310% for Uiam Dam, 330 and 330% for Cheongpyeong Dam, and −20 and 110% for Paldang Dam. As such, the gravity of these issues is significant as the trend of increasing shortage amounts and periods is likely to persist in the future.

As analyzed above, the current dam operation plans do not effectively meet the standard instream flow. Particularly, considering the significant impact of the Imnam Dam operations in North Korea on the dams in the North Han River basins and satisfactory instream flow, there is an urgent need to develop improvement strategies to address these issues.

This study also analyzed variations of streamflow within the Imjin River basins to assess the impact of Hwanggang Dam operation in North Korea. As it was not possible to directly obtain inflow and release data from the Hwanggang Dam in North Korea, data from the Imjin bridge station located between downstream of the Hwanggang Dam and upstream of the Gunnam Dam were utilized. Given that the flow fluctuation at the Imjin bridge station significantly affected Gunnam Dam's inflow, it was simultaneously assumed that the streamflow at the Imjin bridge station represented both the release from the Hwanggang Dam and the inflow to the Gunnam Dam. Furthermore, this study conducted an analysis of flow fluctuations at the Biryong bridge station, located in the lower Imjin River basins, to assess the impacts of the Hwanggang Dam operation on the Gunnam Dam and the Hantangang Dam operation on the Imjin River basins. The data, monthly streamflow, and water level data from the Imjin bridge station (from January 2001 to December 2020) and data from the Biryong bridge station (from January 2003 to December 2020) were analyzed. Table 8 presents monthly areal-average precipitation and monthly streamflow data for the Imjin bridge and Biryong bridge stations in the Imjin River basins during the total and dry periods, encompassing the entire period (including flood and dry seasons) before and after the Hwanggang Dam operation.

According to the analysis results from the Imjin bridge station presented in Table 8, the average precipitation and streamflow during the total period were 115.5 mm and 113.7 m³/s, respectively. There was no significant difference in precipitation before (115.5 mm) and after (113.9 mm) the commencement of the Hwanggang Dam operation. However, the monthly streamflow decreased significantly from 138.7 m³/s before the dam operation to 98.3 m³/s after the dam operation, representing a substantial decrease of −29%. During the dry season (October to May of the following year), there was little change in precipitation, but the streamflow decreased by −53% following the Hwanggang Dam operation. As a result, the analysis indicates that the Hwangang Dam operation in North Korea was the primary cause of the decreased streamflow in the Imjin River basins. Furthermore, the results obtained from the data at the Biryong bridge station also yielded very similar findings to those from the Imjin bridge station. This suggested that the operation of the Hwanggang Dam has a significant impact on reducing the inflow and release of the Gunnam Dam. It also implied that even if the Hantangang Dam were to be constructed and operated, it would not provide substantial assistance in securing the streamflow of the Imjin River basins.

This study reviewed whether the operation of each dam and the linkage operation between dams in the Imjin River basins secured the instream flow. Additionally, it examined whether dam operations in North Korea impacted instream flow satisfaction in the Imjin River basins.

The study also examined and compared daily total release, streamflow, and instream flow for each station, including the Gunnam Dam, Hantangang Dam, Imjin bridge station, Sarang bridge, and Biryong bridge, to analyze whether the instream flow requirement in the Imjin River basins was being met. Specifically, this study focused on determining whether the streamflow at each station, corresponding to the total releases from the dams, consistently met the instream flow requirements. Table 9 presents the standard instream flow data for each station in the Imjin River basins.

In Figure 7(a), within daily data time series at the Imjin bridge station, there was a noticeable reduction in streamflow from 2014 to 2015, coinciding with a period of low nationwide precipitation. This period exhibited significantly lower streamflow compared to other years. Figure 7(b) illustrates the shortage in instream flow that fell below the standard instream flow (7.32 m³/s) at the Imjin bridge station, located downstream of the Gunnam Dam. Notably, there was a concentration of instream flow shortages after 2011. During the years 2014 to 2015, the insufficiency in instream flow was the most pronounced, aligning with the period of low precipitation. For the analysis of the daily data from the Biryong bridge station, Figure 7(c) presents the time series of daily streamflow at the Biryong bridge station along with the seasonal characteristics of the Korean Peninsula. Additionally, Figure 7(c) displays the daily shortage amounts of instream flow that failed to meet the standard instream flow (11.84 m³/s) of Sarang bridge stations, located downstream of the Hatangang Dam. Similar to the findings for the Imjin bridge station, there was a concentration of rapid instream flow shortages in the years 2014 and 2015, which were characterized by lower precipitation levels.

Table 10 displays the official standard instream flow, total and average instream flow shortage amounts, and the number of days within instream flow shortage (shortage days). This analysis was based on daily data from streamflow and water level observation stations in the Imjin River basins. For the total shortage amounts of instream flow, Imjin bridge station, Sarang bridge station, and Biryong bridge station recorded 2,612.2, 1,132.7, and 450.2 m³/s, respectively. As observed above, the decrease in the instream flow shortage amounts downstream was attributed to the larger proportion of the basin inflow. On days when instream flow was insufficient, Sarang bridge and Biryong bridge stations reported 402 and 169 days of shortages, respectively. The annual shortage amounts of instream flow for each dam were calculated, resulting in estimates of 4.38 million ton/year for the Imjin bridge station, 3.7 million tons/year for the Sarang bridge station, and 3.5 million ton/year for the Biryong bridge station. It was noted that these annual shortage amounts of instream flow for each dam were proportional to the average shortage amounts.

This study aimed to determine whether the satisfaction of instream flow and operation plans of dams in Imjin River basins were influenced by the operation of the Hwanggang dam in North Korea. To assess this, we calculated and compared the shortage amounts and periods of instream flow at each streamflow observation station before and after the Hwanggang dam operation, using the operation of the Imnam Dam in 2008 as a reference point. The Sarang bridge station, located downstream of the Hantangang Dam, was included in the analysis for comparison purposes, as it was not directly affected by the operation of the Hwanggang Dam.

Table 11 presents data on the streamflow, instream flow, and instream flow shortage amounts and periods for each dam before and after the Hwanggang Dam operation. When comparing the daily streamflow fluctuations before and after the Hwanggang Dam operation, the Imjin bridge station experienced a decrease of −36.9% (from 147.4 to 93.0 m³/s), while the Biryong bridge station experienced a decrease of −35.3% (from 269.1 to 174.0 m³/s). Despite the Imjin bridge station meeting the standard instream flow requirements before the Hwanggang Dam operation, there was a significant increase in instream flow shortage amounts and periods afterward. The total shortage amounts and periods of instream flow were calculated as 2,621.2 m³/s and 784 days, respectively, indicating a substantial intensification of shortages. At the Biryong bridge station, it was determined that the total instream flow shortage amounts and periods increased significantly after the Hwanggang Dam operation by 14,156% (from 3.1 to 447.1 m³/s) and 16,800% (from 1 to 168 days), respectively. The substantial increase in both the total shortage amounts and periods at these two stations highlighted the significant impact of the Hwanggang Dam operation in North Korea on the dams located in the Imjin River basins. Failure to secure instream flow downstream of the Hwanggang Dam is expected to have a severe impact on the river ecosystem and water quality. Of particular concern is continuous and repetitive occurrence of instream flow shortages, as depicted in Figures 7 and 8.

This study focused on the North Han River basins and investigated the ways to address the water shortages through shared rivers. The Imjin River basins were also considered to follow similar approaches as those applied to the North Han River basins.

Table 12 displays monthly instream flow shortage amounts using daily data from the Peace Dam to Paldang Dam in the North Han River basins. It also shows the monthly instream flow shortages for the three upstream dams from the Peace Dam to Chuncheon Dam, which had the highest shortage ratios. Specifically, this study emphasized the importance of considering these three dams in the upper basins when implementing practical solutions aimed at securing flow. As mentioned earlier, the operation of the Imnam Dam in North Korea has significantly reduced the streamflow into the North Han River basins in South Korea, posing a major challenge to maintaining continuous instream flow.

To address shortage amounts of instream flow in the North Han River basins, a proposal can be made to leverage shared rivers, particularly involving the Imnam Dam in North Korea. Under this proposal, the Imnam Dam releases the required instream flow volumes for North Han River basins, and South Korea devises a corresponding compensation plan. To initiate this process, the first step is to calculate the water value necessary to meet the instream flow shortages in the North Han River basins. Based on these calculations, a reasonable compensation plan can be formulated for the required water supply. Numerous studies have already been conducted to assess the water value of instream flow (Halaburka et al. 2013; European Union 2015; Ortiz-Partida et al. 2016).

This study utilized the results of a study performed by Ahn & Kim (2016) on the river ecosystem services, which estimated the values on both the demand and supply sides. On the demand side, this study applied the EVIS (Environmental Valuation Information System) to estimate the beneficiary's WTP (Willingness To Pay), specifically for water resources supplied from the dams in North Korea. On the demand side, this study further divided it into water supply value, water quality purification value, and water disaster prevention value. In Table 13, for the Han River basins, average values were estimated for water supply, water quality purification, and water disaster prevention, amounting to KRW 261.1 billion/year, KRW 213.8 billion/year, and KRW 126.3 billion/year, respectively (Ahn & Kim 2016). Since the primary focus of this study was to secure the instream flow, it was exclusively considered the average cost of the water supply value in Table 13 (bold values in Table 13). To estimate the cost of ensuring an adequate instream flow, this study had made the assumptions that the average cost for water supply corresponds to the annual expenditure required to maintain the standard instream flow of 63.5 m³/s at the Han River bridge, which serves as the final point of the instream flow within the Han River basins.

After estimating the value of water, it was determined that the cost of securing the shortage amounts of instream flow would be approximately KRW 130 billion per 1 million tons annually. This study considered three plans to reserve the shortfall of instream flow: Plan 1: Securing instream flow shortage for all dams in the North Han River basins; Plan 2: Securing the instream flow shortage for three upstream dams; and Plan 3: Securing the instream flow shortage for the Hwacheon Dam in accordance with the Imnam Dam operation. Table 14 represents the estimates based on Plan 1, where the annual shortage amounts of instream flow were calculated at 1,412.8 million tons/year, with an associated cost estimate of KRW 184.2 billion/year. As analyzed earlier, while direct countermeasures for compensation exist, it is more reasonable to pursue cooperative projects related to shared rivers between South and North Korea or infrastructure development in North Korea. These situations require extensive discussions and the attainment of public consensus.

To tackle water resource issues such as instream flow shortage and impacts of dam operations in North Korea within the shared North Han River and Imjin River basins, this study investigated challenges within the dam operation and management system responsible for ensuring instream flow in these shared river areas. It also reviewed various plans to address these challenges. The detailed analysis results are as follows:

• (1) Impact analysis of the North Han River and Imjin River basins due to the dam operations in North Korea.

This study concentrated on dams and observation stations within the North Han River and Imjin River basins, analyzing the effects of streamflow fluctuation resulting from operations of the Imnam Dam and Hwanggang Dam. Over the entire period, a significant decrease (27.7%) was observed in the average monthly inflow, with the total release of the Hwacheon Dam decreasing by 40.2%. During the dry season, despite an increase in areal-average precipitation following the Imnam Dam operation, the inflow and release of the Hwacheon Dam decreased by 41.3 and 29%, respectively. Furthermore, in the Imjin River basins, the analysis revealed a 29% reduction in streamflow due to the operation of the Hwanggang Dam, with a more substantial decrease of 53% during the dry season.

• (2) Instream flow analysis in the North Han River and Imjin River basins

To assess dam operations in the North Han River basins against the standard instream flow criteria, this study investigated whether all dams within the North Han River basins released streamflow to maintain instream flow. The results showed that the Paldang Dam, Uiam Dam, and Soyanggang Dam had instream flow shortages of 1,667 million tons/year, 595.4 million tons/year, and 315.9 million tons/year, respectively. The total shortage period of instream flow exhibited a tendency to decrease from the Peace Dam to Paldang Dam. However, despite the escalating standard instream flow requirements downstream, the total shortage amounts of instream flow remained significant. Considering the linkage operation of dams in the North Han River basins, most dams showed higher average shortage ratios during the dry season compared to the flood season. Notably, due to the releases from the Soyanggang Dam and the basin inflows from larger basin areas, Soyanggang Dam, Cheongpyeong Dam, and Uiam Dam had relatively low average shortage ratios.

In the Imjin River basins, this study conducted a review to assess whether the operation of each dam and the linkage operation between dams met the standard instream flow requirements. The analysis revealed instream flow shortages at several key points: Imjin bridge, Sarang bridge, and Biryong bridge stations experienced shortages of 4.38, 3.7, and 3.5 million tons/year, respectively. Furthermore, when this study analyzed the impact of the Hwanggang Dam operation in North Korea on instream flow satisfaction, based on data from the Biryong bridge station, it was found that the shortage amounts of instream flow increased by a staggering 14,156% following the Hwanggang Dam operation.

• (3) Solution for water shortages in shared river basins

To ensure instream flow, this study proposed two plans: one involving the utilization of existing dams in the North Han River basins and another considering the connection of the dams in North Korea, taking into account the shared rivers. First, this study reviewed a plan involving dam linkage operation between the Peace Dam and Hwacheon Dam. When considering only the Hwacheon Dam, raising its flood water level from EL. 165.3 m to EL. 178.0 m (777.40 million tons of storage) would enable the Hwacheon Dam to secure an additional 260.7 million tons of instream flow. A broader plan involving dam linkage operation between the Peace Dam and Hwacheon Dam allows for the reservation of necessary instream flow. Furthermore, considering shared rivers, the Imnam Dam in North Korea released streamflow to compensate for instream flow shortages in the North Han River basins, with South Korea developing corresponding compensation plans with the PES. On the demand side, based on the North Han River basins, the total shortage amounted to 1,412.8 million tons per year, with a water supply value estimated at KRW 184.2 billion. These results revealed a disparity between demand and supply, necessitating efforts to reasonably bridge the gap.

The current conflict between South and North Korea over the North Han River and Imjin River basins has been caused by North Korea unilaterally altering the river basins in the upper reaches to generate hydroelectric power. This had a significant impact on the use of water resources in South Korea, downstream. Moreover, it is suboptimal in terms of the economic efficiency of the whole basins in South and North Korea. In particular, since South Korea will incur losses constantly, maintaining the current situation is not reasonable. Therefore, efforts to find practical solutions are necessary.

Professor Lee designed and developed this study and guided data collection and analysis; he also led the write-up of findings. Professor Jang conducted the quantitative data analysis, prepared the results, tables, and charts, and contributed to the write-up.

This study was supported by Daejin University and Korea Environment Industry & Technology Institute (KEITI) through Water Management Program for Drought, funded by Korea Ministry of Environment (MOE) (RS-2023-0023194).

All relevant data are available from an online repository or repositories (www.wamis.go.kr, www.kma.go.kr).

The authors declare there is no conflict.