Drought mitigation operation of water conservancy projects under severe droughts

Drought is one of the most serious climatic hazards (Borgomeo et al. 2015; Zhang et al. 2022c; Zaniolo et al. 2023). Due to the combined effects of climate change and human activities, water shortage and drought events have become more frequent and severe in recent decades (Jehanzaib et al. 2020; Zhang et al. 2022a; Wang et al. 2023b). Some regions have even suffered from record-breaking extreme droughts, which undoubtedly pose a great threat to both economic and social development and the safety of people's lives (Lopez-Nicolas et al. 2017; Ault 2020; Wang et al. 2023a).

The frequent occurrence of drought events has attracted the attention of scholars. They have carried out extensive research on drought indicators (Vicente-Serrano et al. 2010; Shah & Mishra 2020; Lin et al. 2023b), drought prediction (Fung et al. 2020; Pan et al. 2023; Pande et al. 2023), drought assessment (Karnieli et al. 2010; Veijalainen et al. 2019; Meza et al. 2020), and so on. Also for drought mitigation, scientific scheduling of water conservation projects is regarded as the most essential and effective measure (Tu et al. 2003; Wu et al. 2019; Zhang et al. 2022b). Based on the regulation capacity of the water conservation projects, the mismatch between the natural inflow and the water usage can be improved to maximally meet water demand, and mitigate the adverse impact of droughts. Research shows that the reservoir hedging rule is effective, which rations water supply slightly in the current period and reserves water for future use, thus avoiding severe water shortage events in the future (Karamouz & Araghinejad 2008; Seo et al. 2019; Luo et al. 2023). For instance, Chang et al.(2019) proposed a hedging policy triggered by the seasonal drought prevention limiting water level (DPLWL), and its use in the Yellow River Basin proved the superiority of the proposed hedging rule. Meanwhile, the improved rules perform better during normal drought events and with individual drought years than severe droughts or with successive drought years. Zhang et al. (2022b) divided the static drought-limited water level into dynamic drought warning water level and drought depletion water level. The optimal dynamic controlling scheme of a multi-year regulation reservoir effectively reduced agricultural crop yield loss risk due to drought disasters. All these drought mitigation studies demonstrate the superiority of hedging rule optimization in reducing regional water shortage.

However, these existing studies generally targeted moderate droughts (Shih & ReVelle 1995; Jin & Lee 2019; Raso et al. 2019). Its conflict between water supply and demand can be well resolved by optimizing the hedging rules of the single reservoir or simple reservoir group system (Choi et al. 2020; Lin et al. 2023a). Yet the extreme droughts of concern in this paper impact water supply over a wide range. Thus, mitigation of severe drought involves numerous water sources, users, and water conservation projects (Cunha et al. 2019; Zhang & Shen 2019). The projects include the inter-basin water transfer project, the reservoirs of different scales, and the micro conservation projects. Different projects have different regulation capacity, water users, and different resistance ability for drought, especially for severe droughts. How to rationally utilize all these water conservancy projects with different regulation capacities to satisfy the water demand of different water users is an essential issue (Chae et al. 2022; Kim et al. 2022).

To address the above issue, a joint operation framework of large, medium, small, and micro water conservation projects for drought mitigation is constructed in this study. In this framework, we firstly simplified the system complexity through the critical water source identification and cooperative regulation mode development. Subsequently, an optimization model of drought-limited water levels is established for the critical reservoirs. The hedging rules are optimized to minimize the water shortage indexes for each user, enabling the temporal reallocation of water resources. Finally, multiple emergency regulation scenarios will be formulated for regions whose water demand are still unsatisfied. Through these three steps, severe drought damages will be mitigated during mega-droughts.

Extreme droughts are characterized by prolonged durations and high intensities, and it is necessary to coordinated operate all the water conservation projects together to effectively mitigate the drought damages. Typically, these projects include underground and groundwater projects, such as the regional water diversion projects, and inter-basin water transfer projects. Among these, the regional and inter-basin water transfer projects can reallocate the water resources in spatial, and reservoirs can reallocate the water resources in temporal. The various sizes of reservoirs serve different functions in drought mitigation, as outlined in the following,

• (1) Large and medium-sized reservoirs have strong storage regulation ability and are the backbone reservoirs against mega-droughts. These reservoirs can reserve water during wet years, creating a reserve for use during drought years, thereby playing a critical role in prolonged drought mitigation. Additionally, in the face of intense drought, reservoirs with high capacity have a surplus of available water for supply, while low-capacity reservoirs may face empty storage. In essence, reservoirs with higher regulation capacities exhibit superior drought resistance capabilities, and the optimal operation of these reservoirs is vital for drought mitigation.

• (2) Small-sized reservoirs have weak storage regulation ability and usually experience the situation of empty and thus no water to be supplied during the droughts. Nevertheless, they can supply more water in the early stage of drought to reduce the water supply pressure on large-size reservoirs and thus increase their water supply capacity during the mega-drought events.

• (3) The micro-projects, such as the water delivery trucks and underground pumping wells, are very flexible. Their agility in responding to localized water shortages contributes significantly to ensuring water security during drought events.

Therefore, this study proposes a framework for the joint operation of large, medium, small, and micro water conservation projects to optimize their respective roles in the drought mitigation. This framework consists of three sub-models, outlined as follows.

First, establish the supply–demand relationship between the water source projects and the water consumers by creating a supply–demand network diagram. On the demand side, determine the regular water demand and the minimum water demand during droughts, along with the priority of different water consumers. On the water supply side, analyze the water supply capacity of various projects and determine their maximum water supply capacity during droughts.

Second, establish a cooperative regulation mode for various hydraulic engineering projects. In this mode, the projects supply water in order of their regulation capacities, from weak to strong. Specifically, water from the inter-basin water diversion project supplies first, followed by the small, medium, and large-scale reservoirs. Additionally, optimize the operation rules, known as the hedging rules, of the critical reservoirs, particularly the large-scale reservoir. Hedging rules, triggered by drought-limited water levels (DLWL), are optimized to allocate water resources in temporal, thereby avoiding extreme water shortages instead of frequent minor shortages.

Third, during the severe drought events, the water demand may not be satisfied in the regions with weak regulation capacity projects. Therefore, an emergency regulation module should be activated to implement emergency water supply measures when significant water shortages persist. These emergency measures include reducing or suspending water demand from users, such as suspending industrial water consumption. Additionally, enhancing decentralized water supply infrastructure, such as the pumping wells and water tankers, should be considered.

The second mode is the critical part of the framework, and thus following sections introduce the optimization of the hedging rules of critical reservoirs.

There might be multiple critical reservoirs in the study region, i.e., a multi-reservoir system, and the aggregation–decomposition method is adopted to aggregate the multiple reservoirs to a virtual reservoir.

• (1) The aggregate model

According to Equation (7), the partition ratio varies over time, and the reservoir with greater water availability supplies more water. This suggests effective utilization of reservoir capacity compensation. However, the reservoir regulation capacity is not considered in the partition ratio, and this might lead to water spillage for the reservoirs with weak regulation capacity. Therefore, Equation (7) is applied to determine the initial value of the partial ratio, and it will be enlarged for the reservoir with spillage, ensuring water utilization efficiency.

When the reservoir water level is higher than the DWWL, the water demand of all water users can be met. When the water level falls below the DWWL but remains above the DPWL, the water supply needs of higher-priority users can be satisfied, while those with lower priority are restricted. When the water level drops below the DPWL, the water supply requirement of all water users should be restricted. These restriction ratios can be optimized, typically set at 0.7 for agriculture and 0.9 for domestic and industrial water supply.

In order to optimize the DLWLs, an optimization model should be established, introduced as follows.

• (1) Objectives

There are typically four types of water users: agriculture, industrial, domestic, and ecological users, and they compete with each other, especially during droughts. Among them, ecological users have the highest water supply priority which is determined by the concept of green development in this region. It is then followed by domestic and industrial users, with agriculture having the lowest priority. Different water supply strategies result in varying drought mitigation effects, including the frequency, severity, and duration of water scarcity. This study adopts a comprehensive indicator, the water shortage index, proposed by the U.S. Army Corps of Engineers (1971), to assess the water shortage risk for each user. This indicator effectively reflects the overall water scarcity risk, encompassing frequency, severity, and duration. Moreover, greater water scarcity leads to larger economic losses, which should be avoided. Therefore, the quadratic of water shortage index is adopted. The objectives of the optimization model are described as follows.

The effectiveness of ε-NSGAII in solving multiobjective optimization problems has been demonstrated in various studies (Kasprzyk et al. 2012, 2013, 2017). Regarded as a top-performing search tool among the state-of-the-art multiobjective evolutionary algorithms (Reed et al. 2013), ε-NSGAII ensures both convergence and diversity of the Pareto approximate set by employing an ε-dominance archive and adaptive population sizing. Hence, it is adopted to optimize DLWL. For details regarding parameter settings of this algorithm, please refer to previous research (Zhang et al. 2017). To eliminate the effect of population generation uncertainty, we conduct 10 random-seed runs. Each run employs ε-NSGAII for 500,000 evaluations. The final Pareto approximate set is obtained by combining the best individuals across all runs with a nondominated sort.

The water scarcity situation is particularly severe in Chuxiong Prefecture, located in the central part of Yunnan Province, covering a total area of 28,438 km². The climate in this region is subtropical monsoon, with the flood season lasting from May to October, and the dry season lasting from November to April. The Chuxionglucheng District, serving as the primary water receiving district in Chuxiong, is taken a case study. The water resources project group in Chuxiong Prefecture includes one large-scale reservoir (QingShanZui Reservoir), three medium-scale reservoirs (Jiulongdian, Zhongshiba, and Xijinghe Reservoirs), over 100 small-scale reservoirs, and the Central Yunnan Water Transfer Project. The projected annual water demand in Chuxionglucheng for the year 2,040 is 319.52 million m³, with industrial and agricultural sectors constituting 69% of this demand. The water demand process is depicted in Figure 4(c) and 4(d).

For comparison, a region with weak regulation hydraulic engineering projects, Nanhualongchuan District, is selected to validate the performance of the proposed framework. The water engineering projects include inter-basin diversion, Laochanghe Reservoir, Maobanqiao Reservoir, and Xijinghe mid-size reservoirs, along with small-scale reservoirs and local water diversion and lifting projects. The total active reservoir storage of three medium reservoirs is 30.91 million m³. The water demand is 71.93 million m³.

As demonstrated in Section 3, there are numerous hydraulic projects in the Chuxionglucheng District. Coordinating the regulation of the water conservancy projects to optimize their regulatory functions and efficiently utilize water resources poses a significant challenge. To address this challenge, we firstly evaluate the regulatory capacity of each project, measured by the ratio between the active storage and average annual runoff. Table 1 illustrates that Qingshanzui Reservoir is the key water source during droughts in Chuxionglucheng, since it is an over-year and large-size reservoir.

Second, the significance of a reservoir is greater when it has a larger water supply volume and supplies users with higher priority. In the Chuxionglucheng District, Qingshanzui and Jiulongdian reservoirs play a crucial role, as they supply 90% of the region's water demand to users with high priority. Specifically, Jiulongdian Reservoir supplies domestic users, while QingShanZui Reservoir mainly serves industrial users.

Considering the above three factors, it is concluded that QingShanZui and Jiulongdian Reservoirs are the critical water sources for drought mitigation in the Chuxionglucheng District.

Furthermore, in conjunction with the regulatory performance of each reservoir, the prioritized sequence principle for the use of water sources ranges from weak to strong regulatory capabilities. Subsequently, a coordinated control scheme for the hydraulic engineering group in the study area was established: for domestic and industrial water supply, the preferred order of water resources is Dianzhong Diversion Water, followed by small-scale reservoirs, and then medium-sized and large-scale reservoirs. For agricultural irrigation use, priority is given to water diversion from the river, small-scale reservoirs, and Dianzhong Diversion Water, with medium and large-scale reservoirs as the next option.

According to the coordinated operation principle proposed in the last section, which prioritizes utilizing water resources from low-capacity reservoirs, the remaining water demand from the entire Chuxionglucheng region should be met by the Qingshanzui Reservoir and Jiulongdian Reservoir. The water supply from the two reservoirs should meet the following criteria to ensure the water security of Chuxionglucheng.

• (1) Water shortage ratio

• • a. Agricultural water supply: The shortage ratio should not exceed 0.54.

  • b. Industrial and domestic water supply: The shortage ratio should not exceed 0.11.

• (2) Reliability

• • a. Agricultural water supply: The reliability should not be less than 0.75.

  • b. Industrial and domestic water supply: The reliability should not be less than 0.95.

In the figure, the arrow directions indicate the preferred objectives, with the ideal optimal solution located at the left corner of the graph. It is observed that there exists a non-linear competitive relationship between the shortage indexes of public and agriculture. That is to say, improving the water supply benefits for public water uses will inevitably reduce that for agriculture.

Figure 7(b) illustrates the average partition coefficients of water supply for different years. As illustrated in the figure, on the whole, the water supply partition ratio of Qingshanzui Reservoir is more than three times that of Jiulongdian Reservoir. This can be attributed to the fact that the Qingshanzui Reservoir is a large-scale reservoir with strong storage capacity and much water to be supplied. Furthermore, the water supply partition ratio of Qingshanzui Reservoir is notably high during the flood season. This is because Qingshanzui Reservoir locates in the mainstream of the Longchuan River, where water availability is abundant during this period.

The results indicate that the integrated operation of all conservation engineering projects and the optimal drought mitigation scheme can effectively alleviate the adverse impact of drought.

As can be seen from the optimal scenario, since July 2011, there has been little water in the reservoir, resulting in no water available for industrial use. This indicates a severe water shortage event. Therefore, to prevent a concentrated and severe drought, the water resources should be evenly distributed over time, meaning that the water demand should be restricted when a light drought occurs. Meanwhile, there are high shortages in both domestic and industrial sectors, which both rely on Laochanghe Reservoir. Therefore, a scenario for adjusting the water supply structure is proposed, shown as scenario 2. Finally, during a drought, ecological water demand can also be restricted to reserve water for domestic and industrial use, which is scenario 3. The details of the three emergency scenarios are as follows.

Emergency scenario 1: Restricting water demand. The industrial water demand is restricted to 90%, and agriculture water supply is restricted to 50% in Laochanghe Reservoir.

Emergency scenario 2: Adjusting the water supply structure. Laochanghe Reservoir no longer supplies agricultural water use, and restricts the industry to 90% after satisfying the water demand of domestic.

Emergency scenario 3: based on scenario 2, restrict the ecological flow to 10% of the multi-year average streamflow.

As depicted in Figure 9, the periods of water scarcity for industrial and domestic use are postponed, and the duration of water scarcity is significantly reduced. Comparing the dotted curves (Scenario 1) and the solid curves, it is evident that the water shortages in 2011 is improved, but they are not alleviated in 2012–2013. When suspending the agricultural water supply in the Laochanghe Reservoir (Scenario 2), the periods of water shortages for domestic and industry can be further decreased. The occurrences of water shortage for domestic and industrial users are reduced from 27 and 19 times to 25 and 18 times, respectively. Nevertheless, the maximum shortage ratios for industry and agriculture are still the same as the original scenario. This can be improved by imposing restrictions on ecological supply. In scenario 3, the maximum water shortage is reduced to 53%, occurring only once. These results indicate that, by curtailing ecological and agricultural water use, it is possible to significantly mitigate both the severity and duration of water scarcity for domestic and industrial, thereby ameliorating the adverse impacts of drought.

However, the water shortage ratio for domestic 53% is still high. To guarantee the basic water demand, it is necessary to initiate unconventional emergency water sources. This can be achieved by increasing groundwater extraction, augmenting the water diversion volume, and implementing mobile water delivery vehicles to remote regions. Simultaneously, consider the appropriate utilization of the reservoir storage below the dead storage capacity, and so on to prevent severe water shortages.

Severe droughts are occurring more frequently under changing environment. Severe droughts require the coordination of all water engineering projects to mitigate the adverse effects of drought. To this end, this study proposes a framework for the joint operation of multiple water conservation projects tailored heavy and extreme drought. The framework comprises three sub-models: a coordinate operation model for the water conservation projects, an optimal dispatch model for drought mitigation of the critical reservoirs, and an emergency operation model for extreme droughts. The results of the case study conducted in Chuxiong Prefecture indicate that the proposed framework effectively alleviates the water shortages during heavy drought and extreme drought. Specifically, the maximum water shortage ratio decreased from 59 to 45% for agriculture users and from 52 to 8% for industry users.

The proposed drought operation framework provides a novel water supply strategy to fully utilize limited water resources. However, it is developed only based on historical drought processes, and incorporating real-time monitoring and forecasting information is further needed for better optimization of water resources allocation. In addition, the proposed framework only restricted water supply to users in emergency scenarios of the study area, while it can be applied to more drought-stricken areas in future studies, and increase more water supply by activating emergency water sources, such as appropriately utilizing the dead storage capacity.

This research is supported by the National Key Research and Development Program of China (No. 2021YFC3000205) and the Open Research Fund of Hubei Provincial Key Laboratory of Intelligent Yangtze River and Hydropower Science (ZH2102000102).

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.