In South Korea, rainfall deficits lasting for three years (2014–2017), have decreased the reservoir storage level. The Boryeong reservoir, located in the Geum River western sea basin, recorded the lowest reservoir storage. The Boryeong water transfer project has supplied water resources to the east side of the basin. This is an emergency water supply project operated when there is a water supply shortage in the reservoir. It is essential to use appropriate settings for the operation scheme. In this study, a reservoir simulation model was built by applying a water supply adjustment standard, which reduces the water supply when reservoir storage is decreased to a specific level. Four operation cases (no operation, continuous operation, and two temporary operations) were used to analyze the effects of the project. The temporary operations activate when the reservoir storage decreases. The volumetric reliability of the water supply is enhanced by 1.8–2.2% in the temporary operation cases. This study used cost-benefit analysis to analyze the two temporary operation cases. Operating the transfer project when reservoir storage goes below a certain level is more efficient than other cases.
Global climate change has caused frequent abnormal weather phenomena that increase rainfall and temperature variability (Bates et al. 2008). Operating water resource facilities becomes difficult when there are more frequent droughts and floods (Trenberth 2008). Droughts cause a widely spatial and temporal range of damage and directly affect people’s lives through the water supply, economy, and environment (Knutson et al. 1998; Feyen and Dankers 2009). However, since drought causes damage over a long period of time and cannot be relieved by temporary rainfall, it is difficult to predict the damage scale or create a plan for reducing the damage.
The average annual rainfall in South Korea is 1281 mm, which is approximately 1.6 times greater than the global annual rainfall. About 70% of the total annual rainfall is concentrated in the flood season, which is historically challenging for managing water resources. Since the 2000s, long term rainfall deficits have occurred frequently, and the reservoir inflows have decreased, leading to insufficient water supply. As drought damage increased, the South Korean government constructed the Four Major Rivers Restoration Project (FMRRP) from 2008 to 2013 to secure additional water resources. Multifunctional weirs and new dams were constructed in four major rivers: Han, Nakdong, Geum, and Youngsan. In 2015, the Korean government established the Water Supply Adjustment Standard (WSAS) in preparation for reservoir water shortage, which reduces the reservoir supply when storage decreases (Ministry of Land and Infrastructure and Transport (MOLIT) 2015).
Even after the FMRRP was completed, the lack of rainfall (2015–2017) caused severe drought damage to the four major rivers and other river basins. The annual average rainfall values for 2015, 2016, and 2017 from the Boryeong weather station at the Geum River western sea basin, corresponded to 63%, 64%, and 69% of the average annual rainfall of 1244 mm. The rainfall for 2015–2017 flood seasons was only 26%, 36%, and 69% of the long-term average value of flood season rainfall of 842 mm, leading to a summer drought. Restrictive water rationing was enforced for four months from November 2015 to slow down the decreasing rate at which the reservoir was filling, which had lasted from 2014 and to prepare for the worst situation of water supply interruption. However, the storage’s rate did not recover to more than 50% before 2017.
In September 2015, the MOLIT, which is responsible for managing water resources in South Korea, concluded that if the reduction in Boryeong reservoir’s storage lasted until 2016, then a significant water shortage would occur in the recipient area. The government decided to construct the Boryeong water transfer project (BWTP) to supply water from the FMRRP to the Boryeong reservoir. The BWTP is an inter-basin water transfer project that is intended to take water from downstream of the Baekje weir in the Geum River and supply it to the Bangyo stream, upstream of the reservoir.
Inter-basin water transfer takes water from basins in which water is abundant and supplies it to areas where there is insufficient water. For example, northeastern California has abundant water resources, whereas southern California lacks adequate water resources. To resolve this water resource imbalance, the California state government implemented the State Water Project in the 1960s (Israel and Lund 1995). The Colorado River Aqueduct (1941) was built to transfer water from Lake Havasu to Lake Mathews (Zetland 2011). These water transfer projects became popular as climate change exacerbated.
Many studies focused on the topic of inter-basin water transfer (De Andrade et al. 2011). Yevjevich (2001) reviewed the geomorphological and hydrologic elements for planning inter-basin transfer. Cox (1999) studied the social, economic, and environmental effects of inter-basin transfer. De Andrade et al. (2011) compared the water transfer projects of the U.S., Australia, and other countries. Ballestero (2004) mathematically simulated the amount of transferred water, and water prices to solve the social conflicts between the donor and recipient basin. Dosi and Moretto (1994) analyzed a water transfer project considering uncertainty to determine the size of water detention tanks to store the water transferred through the project.
Recent studies focused on operation methods and effects of the water transfer project. Studies in China used optimization techniques to analyze the quantities and resultant economic effects of water transfer projects to relieve the water resource imbalance between the southern and northern regions (Zeng et al. 2014). Other studies analyzed water transfer operations using optimization techniques, such as Bilevel Programming and Improved Particle Swarm Optimization in China (Guo et al. 2012; Zeng et al. 2014), and fuzzy Multi Criteria Decision Making was applied to determine water transfer operation plans (Xuesen et al. 2009). Xi et al. (2010) established reservoir linkage operation plans for transfer facilities and analyzed the economic effects of reservoir simulation. Peng et al. (2015) analyzed the flood control capacity of reservoirs connected with water transfer by using rainfall forecasts and water supply data.
Based on the literature review, many studies determined the projects’ quantities and operational plans using optimization techniques. The inter-basin transfer projects were operated to supply a constant amount of water for all or part of the year. The BWTP is an emergency water supply project that is temporarily implemented when the Boryeong reservoir storage decreases due to drought. This study carried out various operational cases for the BWTP and reservoir simulation that analyzed effect on water supply and the Boryeong reservoir’s yield. The historical inflow data (1998–2017) were used in the simulation model. The WSAS (MOLIT 2015) was applied to the model’s construction to reduce the determined quantity of water when reservoir storage decreased. The BWTP’s operational effects were analyzed using volumetric reliability, the duration of water supply reduction, and the spillway release data.
Boryeong Reservoir and Water Transfer Project
The Boryeong multipurpose reservoir located in the Geum River western sea basin, been in operation since 1998. The reservoir’s total storage capacity is 116.9 MCM (million cubic meter), and the effective storage capacity is 108.7 MCM. It is planned that the reservoir will supply 90.5 MCM for domestic and industrial use, 4.9 MCM for agricultural use, and 11.4 MCM for instream water annually. Water from the reservoir is supplied through water intake conduits. In total, 95% of the planned domestic and industrial water is supplied to the Boryeong multiregional water supply system through the first outlet. The remaining 5% of the domestic and industrial water and 100% of the instream water are supplied through the second outlet. Agricultural water is supplied from April to September through the third outlet (Fig. 1).
Droughts have decreased the reservoir inflow in South Korea ever since the early 2000s. The domestic water supply has been disrupted in the middle and southern parts of Korea (Fig. 2a). The Korean government carried out the Four Major Rivers Restoration Project to secure water resources by dredging rivers and building multifunctional weirs. A total of 16 weirs have been built and operated in the four major rivers since 2013. Three weirs were built in the Geum River including Baekje weir (Fig. 2b). The Baekje weir in the downstream of the Geum River is 7 m high and 31 m long, has a storage capacity of 23.6 MCM and releases at least 17 m3/s for power generation and river maintenance throughout the year.
This study used the hydrologic data from the Baekje weir (2013 to 2017) to determine the sufficient quantity of water to the Boryeong reservoir. The result showed that an inflow of at least 29.7 m3/s could be secured even in non-flood seasons via the operation of the multipurpose reservoir located upstream. The weir has been releasing all inflows while maintaining a storage rate of more than 90%. The weir can release 115,000 m3/day to the Boryeong reservoir even during a severe drought.
Boryeong Water Transfer Project
The BWTP was constructed to reduce water shortage damage to the recipient basin of the Boryeong reservoir. This project transfers water from the downstream of the Geum River to the Boryeong reservoir (Fig. 2c). The project is approximately 21 km long and is made of 1 m diameter cast iron pipes. The BWTP built water intake stations, pumping stations, and water purification facilities to minimize the topographic and ecological problems. Operating these supplementary facilities incurs additional costs for the energy requirement and water treatment. Operating the project when the reservoir storage drops is more efficient than running it continuously.
Reservoir Simulation Model
Reservoir operation analyses are conducted using optimization and simulation. A simulation is used to conduct a reservoir operation analysis (Yeh 1985; Wurbs 1993; Simonovic 2000; Ranjithan 2005; Fayaed et al. 2013), as was used in this study to analyze the operation of the BWTP. RiverWare, Water Evaluation and Planning System (WEAP), and the Hydrologic Engineering Center’s reservoir simulation (HEC-ResSim) are commonly used. The HEC-ResSim model has an advantage for reservoir simulation in that it reflects the circumstances of a complex water system, and it is possible to simulate release according to the condition of the reservoir, such as with an if-then rule. This study used HEC-ResSim 3.1 (USACE 2013) to simulate the complicated operation of the Boryeong reservoir and the BWTP.
Water Supply Priority and Adjustment Standard
Multipurpose reservoirs provide water, control floods, and generate hydropower. Water supply includes domestic and industrial, agricultural, and instream purposes. When reservoirs cannot satisfy the demand for water during drought, the reservoir should release less water, based on the water supply priorities that are established for the various water, uses as stated above.
The two prominent characteristics of droughts are the facts that they can last for a long period and it is difficult to predict when they will end. Supplying the same amount of water when the reservoir storage level drops in the drought season can be strenuous. The Water Supply Adjustment Standard (WSAS) is intended to reduce the reservoir water supply when the reservoir storage drops below a certain level, which has been applied to 15 multipurpose reservoirs since 2015. This standard is set in four levels (concern, caution, alert, and emergency) based on certain reservoir storage that can supply the contracted domestic and industrial, agricultural, and instream water users during the severe drought (Fig. 3). Once the reservoir storage goes below the concern level, the domestic and industrial sectors get only their contracted amount of water supply. At the caution level, the reservoir ceases the instream water supply. During the alert level, the reservoir stops supplying agricultural water demand. At the emergency level, the reservoir provides 90% of the contracted amount to the domestic and industrial sectors. These four levels were applied to the reservoir simulation model.
This study built a model using historical daily inflow data for the reservoir from 1998 to 2017. The reservoir inflow decreased from 2012, and the total annual inflow decreased and went below the annual planned supply (106.6 MCM) from 2014 to 2017 due to severe drought.
Operation Standard of the BWTP
The BWTP is operated only when the reservoir storage decreases due to drought. If the project is operated too rarely, it may be difficult to achieve an emergency water supply. If the project is operated too frequently, the operation cost increases, and the total reservoir inflow rapidly leads to increasing spillway release. Determining project’s operation period is crucial for the BWTP to operate efficiently.
The Four BWTP operation cases are (1) no operation, (2) continuous operation, (3) temporary operations I, and (4) temporary operations II. In case 1, the BWTP is not operated at all during the simulation period. This case was used to analyze the effect of the BWTP operation compared with cases 2, 3, and 4. In case 2, the BWTP is operated continuously during the simulation period. This case was used to analyze the water supply capacity of the reservoir when the project is operated maximally. In cases 3 and 4, the BWTP is operated temporarily when the reservoir storage decreases. The references were set using the concern, caution, alert, and emergency levels of the water supply adjustment standard.
After drought season, the reservoir needs some recovery time before it can operate normally. The termination condition was set to be higher than the beginning condition of temporary project operation. The simulation results show that when the termination condition was set one level higher, the reservoir storage exceeded the certain reference reservoir storage via temporary rainfall. However, the daily inflow decreased again the next day, and the reservoir storage went below a certain level. This led to significant variability of operation, and the termination condition of temporary operation was set to be two levels higher than the beginning condition for temporary operation. Case 3 starts at the alert level and ends at the concern level. Case 4 starts at the emergency level and ends at the caution level (Fig. 4).
Boryeong Reservoir Storage
The reservoir storage has significant effects on reservoir operation. Changes in the reservoir storage over time were compared for different cases. The reservoir can supply sufficient water and maintain high storage capacity during the entire period, except for during the initial stage of operation in case 2 (Fig. 5b). When the BWTP was operated temporarily (cases 3 and 4), the storage’s decreasing speed was lower than for case 1 because the BWTP was operated when the storage was decreasing. However, when the reservoir storage increased, the storage recovery speed became faster than in case 1 (Fig. 5a, c, d). In case 3, the average reservoir storage was maintained higher than in case 4 because operation began earlier and ended later. The period between 2014 and 2017, saw the most severe drought since the reservoir was operated, and sometimes, there was no water to supply when the BWTP was not operating (case 1). For cases 3 and 4, water was supplied while reservoir storage was maintained at an emergency level for a severe drought.
Boryeong Water Transfer Project Operation Results
An operational plan is needed to minimize BWTP operational costs while maximizing the reservoir’s water supply. Table 1 shows that the BWTP supplied 839.4 MCM of water to the reservoir during the entire period in case 2. Since BWTP operation begins at the alert level and ends at the concern level in case 3, the number of operational days and the size of water supply were 1.3 times larger than in case 4. Since case 1 is model not to operate BWTP, there are not result about BWTP operation.
Volumetric reliability is the ratio of observed water supply to simulated water supply during the design period (Hashimoto et al. 1982). As shown in Table 1, case 2, the volumetric reliability for domestic and industrial, agricultural, and instream water was estimated about 100.0% since water could be supplied at all period except for during the initial period of operation. In case 3, the volumetric reliability for domestic and industrial, agricultural, and instream water was increased by 2.2%, 6.4%, and 8.3% more than in case 1. In case 4, the volumetric reliability for domestic and industrial water was increased by 1.8% compared with case 1.
The Number of Days Reached by the Water Supply Adjustment Standard
Time-based reliability is the probability that no failure will occurs within a fixed period, which is often called the planning period (Hashimoto et al. 1982). The result of the reservoir simulation, however, shows the reservoir level does not reach the low-water level at all period since the WSAS (MOLIT 2015) was applied for the reservoir simulation in this study. For this reason, time-based reliability could not be calculated, and the number of days that reached the water supply adjustment standard was calculated instead to evaluate the temporal reservoir performance.
In case 1, the number of days on which the concern, caution, alert, and emergency levels were reached was calculated to be 289 days, 346 days, 248 days, and 1031 days (Table 1). In case 2, the number of days on which any of the levels of the WSAS were reached was much fewer compared with case 1. The emergency level that should be reduced by 10% of the contracted domestic and industrial water supply was never reached. In case 3, the number of days on which the concern and the caution levels were reached increased by 182 days and 365 days and the number of days on which the alert and the emergency levels were reached decreased by 78 days and 898 days compared with case 1. In case 4, the number of days on which the concern, the caution, and the alert levels were reached increased by 62 days, 379 days, and 75 days and the number of days on which the emergency level was reached decreased by 838 days compared with case 1.
Spillway Operation Result
Although the transferred flow to the Boryeong reservoir through the BWTP increases the water supply, when the reservoir water level exceeds the normal high water level, the water is released through the spillway. It is ideal to increase the water supply and minimize the spillway release. Spillway release in case 2 increased by approximately 1.9 times compared with case 1 (Table 1). In cases 3 and 4, when the BWTP was temporarily operated, the spillway releases were increased by approximately 11.5 MCM and 7.4 MCM, and the number of spillway release days were increased by 7 and 6 days compared with case 1.
When the BWTP was operated for a longer period, the result showed higher volumetric reliability and costs. Four reservoir operation cases were compared in terms of operation costs and water supply benefits (Table 2). The BWTP operation costs were divided into labor, electricity, and water purification costs, and the total cost to operate the project was $0.021 million/day (KDI 2016). The benefits of $0.344/m3 and $0.002/m3 accrued via the water supply and power generation.
In case 2, the total benefit over 20 years was $737.5 million with sufficient water supply and power generation, but the total operation cost of the BWTP was $150.8 million and the net benefit was thus calculated to be $586.7 million, which is the smallest among all cases. In cases 3 and 4, the net benefits over 20 years were $682.4 million and $680.7 million, respectively. In case 1, the average net benefit was $28.4 million during the drought years (2014–2017), which increased to $29.4 million in case 3 and $29.0 million in case 4. In 2017, when the net benefit was the smallest over the 20 years, the net benefits in cases 3 and 4 were calculated as $25.3 million and $24.0 million. Regarding benefits, case 3 is the more efficient BWTP operation plan compared to case 4.
In case 1, the volumetric reliability for the domestic and industrial water supply was 93%, and 1031 was the number of days reached on the emergency level that should reduce 10% of the contracted domestic and industrial water supply. That means that When WSAS was only applied, domestic and industrial water satisfied 86% of the operation period. In case 2, the reservoir supplied water over the entire period except for the initial period. However, it seemed to be inefficient because the operation costs drastically increased, and the spillway release almost doubled compared to case 1. In cases 3 and 4, the volumetric reliability of domestic and industrial water supply increased by 2.2% and 1.8%, respectively, and the number of days reached on the emergency level decreased by 898 days and 838 days compared to case 1.
A cost-benefit analysis was used to evaluate the operational costs of the BWTP and the benefits of the Boryeong reservoir water supply. In case 1, net benefit was $676.6 million. The net benefits in case 2 decreased by $90 million compared with case 1 because the transfer project was operated when the reservoir storage was sufficient. In cases 3 and 4, the net benefits were increased by $5.8 million and $4.1 million, respectively. The BWTP operation according to temporary operation I (alert-concern) (Case 3), was more efficient than operating the project according to temporary operation II (emergency-caution) (Case 4).
Temporarily operating the BWTP when the reservoir storage decreases is more efficient than continuous operation. The water supply in case 3 was 7.2 MCM larger than case 4 over 20 years. In case 3, the operational days of the BWTP were 183 longer than in case 4. In the cost-benefit analysis, net benefit in case 3 was $1.7 million higher than in case 4. Therefore, case 3 is more efficient than case 4 considering both the Boryeong reservoir water supply and the BWTP operation.
This study analyzed the effects of BWTP operation through reservoir simulation. The reservoir simulation model was constructed for four cases according to the BWTP’s operation conditions from 1998 to 2017. Reservoir storage change, volumetric reliability, the number of days reached on a water supply adjustment standard, spillway release, and a cost-benefit analysis were used as evaluation methods to compare and analyze the operational effects of the BWTP.
The BWTP is temporarily operated for emergency water when the reservoir storage decreases during severe drought. It is important to set project operation conditions that maximize the water supply while minimizing the number of days of the transfer project operation. The water transfer project operation conditions were set based on reservoir storage. Operating the project in advance to reduce the reservoir storage decrease (case 3) is better in terms of the relief it offers and the cost-benefit. It is inefficient to operate the project too much, regardless of demand, due to excessive increases in operating costs and spills.
As temporal and spatial variability of rainfall due to climate change increases, imbalance in water resources can occur in adjacent basins. The inter-basin water transfer in this paper can be used when there are differences in water resources between the two basins. Appropriately operating the inter-basin water transfer can reduce further damage caused by drought and mitigate the spill. This paper can be helpful in evaluating the usability of transfer projects for emergency water supply in the event of severe drought.
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This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Advanced Water Management Research Program, funded by Korea Ministry of Environment (MOE). (83078).
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Choi, Y., Ahn, J., Ji, J. et al. Effects of Inter-Basin Water Transfer Project Operation for Emergency Water Supply. Water Resour Manage 34, 2535–2548 (2020). https://doi.org/10.1007/s11269-020-02574-9
- Inter-basin transfer
- Reservoir operation
- Emergency water supply
- Water shortage