The D-A-R approach: a method for determining ecological flow based on the component structure of ecological protection objects and dynamic hierarchical adjustment to the annual flow process

In the decades, since the Brisbane Declaration (2007) called upon governments and other decision makers to integrate environmental flows into water management, practitioners have continued to seek ways to expand the implementation of flow restoration or protection (Jeffrey et al., 2018). Although regional and seasonal water scarcity occurs frequently in China (Wang et al., 2014), and the contradiction among domestic, production and ecological water is prominent in some watersheds (Qiao, 2020), the Chinese government still attaches great importance to the determination and implementation of ecological flow of rivers or lakes. In April 2020, the Ministry of Water Resources of China (MWRC) issued the Guiding Opinions on the Determination and Guarantee of Ecological Flow of rivers or lakes (the following abbreviation Opinions), which clearly pointed out that the ecological protection objects and ecological flow thresholds of rivers or lakes should be clearly defined, the ecological flow management of rivers or lakes should be strengthened according to law, and the ecological flow determination and guarantee of rivers or lakes should be effectively done to help the construction of ecological civilization.

At present, although indicators such as the lowest ecological water level and the ecological water quantity of the section are basically in line with the standards in China, the negative impact of the development and utilization of water resources has become increasingly apparent with the successful implementation of hydraulic engineering (Song et al., 2003; Savenije & Zaag, 2008). In the dry season, there are still a few substandard situations, and the guarantee of ecological water in the sensitive period is not optimistic. In some watersheds, the basic ecological water quantity target is too low. In general, it is a work that needs to be comprehensively strengthened to formulate the target amount of ecological water in the sensitive period of rivers or lakes, and how to guarantee it (Wang & Hu, 2022).

Affected by the monsoon climate, water resources in China are unevenly distributed in spatial and temporal, and there are dry and wet years. In dry years, the imbalance between the supply and demand of water resources is particularly prominent. In the period of water shortage, the contradiction among the demand of domestic, production and ecological water is more prominent. Meanwhile, the Chinese government attaches great importance to promote ecological progress, which raises the demand for ecological flow management policies and further exacerbates this contradiction.

Inspired by the theory of ‘Hedging rule’ (Shih & ReVelle, 1994), at the same time, according to the practical experience, which the Chinese government has implemented different amplitude reduction strategies to domestic and production water in recent years, based on the water shortage situation during the urban emergency water supply period, a new ecological water management strategy (the D-A-R approach) is proposed in this article. By combining this with the assessment of water scarcity in the watershed, various waters are synchronously reduced by this approach, and the water shortage in local periods is allocated to the whole year to deal with this contradiction.

The main contributions of this study are as follows: the local governments can use the D-A-R approach to dynamically adjust the ecological flow based on the prediction of water inflow in the current year and the simultaneous reduction of various water demands, so as to alleviate the contradiction among domestic, production and ecological water in the watershed. The rationality of the D-A-R approach has been verified in the Dingnan River watershed, but the D-A-R approach is universal, and managers of other watersheds can use this approach to formulate water scheduling schemes during the water shortage period.

In the 1940s, environmental flow was first proposed and studied by American scholars (Poff & Matthews, 2013), and the research was mainly focused on rivers. Subsequently, the theory of river eco-environmental water demands and other related concepts have been put forward. Lorey proposed the Tennant method in 1976 (Leroy, 1976), which laid the theoretical foundation for the study of ecological flow, and then the American Nature Conservancy (McFarlane, 2014) and European Union Water Framework Directive also put forward the theory of ecological flow and applied it to the practice of protecting the water ecosystem (Acreman & Ferguson, 2010). The current definition of ecological flow is best expressed in the 2007 Brisbane Declaration, which describes environmental flow as ‘the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems’ (Poff & Matthews, 2013).

Ecological flow research started in the 1970s in China. Although it started later than in other countries, the research progressed faster. Scholars have successively proposed some methods: Flow Restoration Method (Gordon et al., 2004), Monthly Guarantee Rate Method, Improved Wetted Perimeter Method, Improved 7Q10 Method, Hydro-ecological Corresponse Method and so on (Sang, 2021), which are mostly ecological flow research methods improved with the relevant experience from other countries (Mark & Loren, 1982; Lamb, 1989). In addition, some scholars divided ecological flow into components based on different ecological protection objectives (Peng, 2020), including the maintenance of basic river functions, the integrity of river habitats, the protection of fish habitats, pollution control, landscape maintenance and so on, and then selected the corresponding ecological flow accounting methods based on different ecological protection objectives (He, 2021).

In this article, the idea of dividing the ecological flow into components is used, and the ecological flow demand of each component of the ecosystem is considered, so as to determine the annual process of ecological flow more reasonably.

Water scarcity assessment can effectively alleviate the harm of water scarcity to the economy and society. At present, many scholars have evaluated the degree of water scarcity. Jia (2023) used water quantity–quality indicators to define water scarcity and quantify the extent of regional water scarcity. Veldkamp et al. (2016) used the water crowding index and population density factors to assess the global water scarcity risk and divided three types of water scarcity levels: moderate, severe and absolute. Qian et al. (2021) used the clustering algorithm RLCA and Fisher discriminant analysis to build a water resources scarcity risk assessment model; used indicators such as water demand, water supply and population to divide the four water resources scarcity risk levels of lower, low, medium and high and evaluated the water resources scarcity risk levels of all counties and districts in Tianjin in 2020. The above studies have carried out the assessment of water resources scarcity degree at different spatio-temporal scales based on different evaluation indicators, which provide the necessary scientific basis for the proposed water adjustment strategy in this article.

Therefore, this study mainly investigated the water demand (domestic, production and ecological water demand) and the available water supply (incoming water and engineering water supply) in the study area, calculated the regional water scarcity and divided three types of water scarcity levels: light, medium and severe to evaluate the water scarcity level, and proposed reasonable and feasible water adjustment strategies.

When faced with water scarcity, different local governments will adopt different water policies to deal with this situation. Ahsan et al. (2022) studied the measures taken by Bangladeshi households to deal with water scarcity and found that most households would take six measures to deal with the pressure of water scarcity, namely, reducing vegetable production, reducing livestock production, paying more to access water, increasing time for water collection, preserving water and using reserves to collect water. Stakhiv et al. studied the evolution of drought management policies in the United States and found that at the beginning of the 20th century, increasing water supply capacity was the most effective drought management option. Now, the policy of reducing various water demands is the main direction of the government to deal with drought and has received strong support (Stakhiv et al., 2016). Matikinca et al. studied the water-saving policies adopted by the government in Cape Town, South Africa, in the face of severe drought, and found that mandatory restrictions on urban water use to a certain extent are more effective in reducing water demand than raising water prices, and water restrictions were an effective measure to deal with water scarcity (Matikinca et al., 2020). Similarly, Tortajada et al. studied the implementation policies of water management departments in five Spanish cities in the face of drought and found that each region has adopted pricing and non-pricing measures. In the case of drought, non-pricing measures (water restrictions) have a greater impact on water decision-making (Tortajada et al., 2019). According to the literature retrieval, the research on water restrictions focuses on domestic and production water, while the research on water reduction for the annual process of ecological flow is very scarce.

The ‘Hedging rule’ is a method in water resources management. By considering the possibility of water scarcity in the future, water supply is limited in advance within a certain range to reduce the loss of total water scarcity, so as to obtain greater total water supply benefits.

Drawing on the experience of the ‘Hedging rule’, the ‘synchronous reduction strategy’ is proposed to strengthen water allocation to water users with potential future water scarcity by reducing the prophase water consumption of all water users to degrade the probability of suffering more serious water scarcity events in the later stage.

In order to verify the rationality of the D-A-R approach, the Dingnan River watershed was taken as a research case. Then, the study area, data sources, research steps and specific research methods of the D-A-R were introduced in this section.

The flow monitoring data (1976–2019) and flow velocity monitoring data (2017–2021) of Shengqian (II) hydrometric station were provided by Hydrology Bureau of Jiangxi province. The water-use data were derived from the Xunwu County Water resources bulletin (2018–2020), the Anyuan County Water resources bulletin (2018–2020) and the Dingnan County Water resources bulletin (2018–2020).

First, the ecological flow is measured according to the water demand during the sensitive period of the protected objects. The types of special ecological protection objects (SEPOs) and their sensitive period are determined, the flow requirements in the sensitive period are focused on, so as to determine the annual flow process of SEPO. The annual process of ecological flow based on the component structure is obtained by taking the maximum value of flow required by SEPO and ecological basic flow each month.

Second, the degree of water scarcity in the watershed is assessed. The available water supply under different water inflow frequencies (considering engineering measures such as reservoir storage and extraterritorial water diversions), as well as the corresponding domestic, production and ecological water demand are analyzed, and then the monthly water scarcity is calculated to assess the degree of water shortage in the watershed.

Third, combined with the assessment of the water scarcity degree in the watershed, the domestic, production and ecological waters are reduced synchronously under three scenarios of ‘general type, saving type and constrained type’, in which the reduction range of water consumption of all users has been clarified during the water shortage period.

The Opinions required local governments to specify the ecological protection objects of rivers or lakes and divided them into two categories. One is the basic ecological protection objects such as the basic form, basic habitats and basic self-purification ability of rivers or lakes. The other is important ecologically sensitive areas, aquatic biodiversity, sediment transport, the ability of estuary to resist salt tide and other SEPOs. Based on desktop data review and field investigation, the ecological flow of the Dingnan River is divided into two components based on different ecological protection objects in this article. One is to satisfy the basic ecological protection objects including the basic form of rivers or lakes, basic habitats and basic self-purification capacity of the ecological basic flow. The other is the ecological flow that satisfies the SEPO.

The Dongjiang Ecological Flow Implementation Plan recommended eight hydrological methods for ecological basic flow calculation. The results and adaptability of these ecological basic flow calculation methods are quite different. For the Dingnan River, we have carried out the optimization method and adopted the monthly flow variation method to calculate ecological basic flow (Cao et al., 2022; Table 1).

Based on the comprehensive investigation and scientific literature review, it is determined that the main SEPO of the Dingnan River, including four domestic fish (Mylopharyngodon piceus, Ctenopharyngodon idella, Hypophthalmichthys molitrix and Aristichthys nobilis), Opsariichthys bidens, Oreochromis mossambicus and Acrossocheilus fasciatus.

The sensitive period is the spawning and juvenile stage of fish, including migration, spawning and juvenile growth. Through a large number of scientific literature reviews, there is still a lack of research on the sensitive period of O. bidens, O. mossambicus and A. fasciatus, and the sensitive period of four domestic fish is mainly considered in this article. The differences in watershed, climate and experimental facilities are considered in the above scientific literature. The spawning period of SEPO is from April to July, and the juvenile growth period is from May to September.

In the non-sensitive period (month beyond the sensitive period), the growth of fish is mainly considered, and all the above fish are the SEPOs that need to be studied.

Relevant research results show that whether in the sensitive or non-sensitive period, the basic hydrological conditions of fish are in accordance with the appropriate flow velocity to determine the threshold [v_l,v_u]. Therefore, the maximum value of the lower threshold value of all fish in the same period can be selected as the ecological flow velocity in this period (Zhang et al., 2017).

Then, according to the relationship between velocity and flow of river, the ecological flow of two periods is calculated, which constitutes the annual process of ecological flow. In this article, the measured flow results of Shengqian (II) station (2017–2021) are adopted to construct the relationship formula between flow velocity and flow rate _.

Water demand includes production water demand, domestic water demand and ecological environment water demand. Various water demands on a monthly scale are calculated in this article.

The water demand outside the river channel can be calculated according to field investigation or desktop data (2018–2020).

During the period of water scarcity, the main ways to alleviate the contradiction between ecological water and other water demand such as domestic and production include engineering and non-engineering measures. Specifically, the available water supply in the water scarcity month can be increased by scheduling water storage projects such as reservoirs or by engineering measures such as water diversion from outside the region. The total water demand can also be reduced through various emergency management measures for synchronous reduction of water. However, the degree of water scarcity that engineering measures can solve is limited, and the combination of engineering and non-engineering measures can solve the problem of water scarcity more effectively.

Related research has demonstrated the resilience of aquatic ecosystems (Shinderman, 2015), which has the ability to defend, adapt and transform when disturbed (Li et al., 2022). Therefore, ecological flow can be moderately reduced during the water shortage period.

In addition, the Chinese government has formulated Urban Water Supply Emergency and Standby Water Source Engineering Technical Standards (CJJT282-2019) in 2019 to cope with sudden water scarcity events in cities, and according to this standard, the water for domestic and production is reduced accordingly. In recent years, cities such as Nanjing (Xia et al., 2021) and Shanwei (Sun & Chen, 2021) have successively adjusted their water use according to the document and achieved remarkable results.

Therefore, using the resilience of the ecosystem to adapt to the water scarcity environment, the ecological flow can be appropriately reduced while domestic and production water is reduced, which will not cause serious damage to the ecosystem during the water shortage period. At the same time, the saved water can help humans allocate water resources more reasonably in spatial and temporal distribution.

Different reduction policies are adopted according to different water scarcity degrees (Table 2), and different percentages of compression water (PCW) are applied to different water users according to the principles of equity and efficiency. In terms of the specific PCW, the priority order of ‘ecological, domestic and production’ is reflected. The PCW used from low to high is ecological water, domestic water, production water, road sprinkling and greening water. According to ecological water, the PCW of water used for SEPO is less than that of ecological basic flow, and in terms of production water, the PCW of agricultural water is higher than that of industry water during the severe water shortage period. The volume of water shall be reduced based on the principle of no abandoned water, and the PCW can be adjusted appropriately according to the actual situation in the wet season.

Thus, the annual flow process based on the component structure is constructed.

The monthly flow monitoring data of Shengqian (II) Hydrological Station from 1976 to 2019 are taken and restored, and the P-III theoretical probability distribution model of each month is constructed by using the restored data. Under the premise that the project guarantee rate reaches 80%, based on the model, the probability of occurrence of ecological flow based on the component structure and monthly ecological flow after reduction is calculated, respectively, and their guarantee degree each month is obtained.

According to the above methods, the ecological basic flow and annual flow process for SEPO were calculated first, and then, the annual process of ecological flow was constructed based on the component structure. Next, the water scarcity degree in the Dingnan River watershed was assessed, and the corresponding ecological flow process was obtained according to the emergency adjustment strategy.

The annual ecological basic flow process of the Dingnan River is calculated by using the monthly flow variation method, and the results are shown in Table 3. The value of ecological basic flow reached the maximum in June, increased from January to June, decreased after July and reached its minimum value in December.

Through scientific literature search, the flow velocity demand of four domestic fish in the sensitive period (Garcia et al., 2013, 2017; Dang, 2017; Jiang et al., 2019; Yang, 2019) and the juvenile growth period (Bai et al., 2013) are obtained. Details are presented in Table 4.

The migration and spawning process of the four domestic fish are comprehensively considered, and the requirements of fish roe floating and embryonic development are considered, and 0.4 m/s is the appropriate flow velocity threshold required by the four domestic fish in the sensitive period.

Similarly, through a scientific literature review, the flow velocity requirements of four domestic fish and O. bidens, O. mossambicus and A. fasciatus during the non-sensitive period (Ji, 2006; Tang, 2015; Tan et al., 2017; Li, 2019; Zhong, 2020) are obtained, as shown in Table 5.

The suitable flow velocity threshold for the non-sensitive period of SEPO is determined as 0.3 m/s based on the suitable flow velocity conditions of seven species of fish comprehensively.

The annual flow process of the Dingnan River based on SEPO can be obtained (Table 6) on the functional relationship in Tables 4 and 5 and Figure 4.

According to the field investigation and the water resources bulletins of Xunwu, Anyuan and Dingnan Counties (2018–2020), various types of water during the year are calculated, as shown in Table 8.

The available natural water quantity (natural incoming water minus abandoned water), engineering water supply and water scarcity are calculated under different water inflow frequencies, which accord to the reduction calculation results of the measured water data for many years, the annual process of water demand and the regulation capacity of water resources in the watershed. The water scarcity rate for the whole year is calculated, as shown in Table 9.

At present, there is a shortage of water resources regulation projects in the river watershed, and the guarantee rate of water supply can only reach 50% after the application of hydraulic engineering measures. The local governments should strengthen the construction of reservoirs and other projects to improve the capacity of water resource regulation. The guarantee rate of water supply can be increased to 80% when the effective storage capacity of the reservoir is increased to 3.3 × 10⁷m³.

When the water inflow frequency is higher than 80%, it is necessary to give full play to the role of engineering and start non-engineering measures to implement varying degrees of reduction of all kinds of water demand. The level of water scarcity at 90, 95 and 98% of the water inflow frequency is defined as light, medium and severe, respectively, in this article.

According to Tables 8 and 9, the reduced water volume of domestic, production and outside the river channel ecological water under three reduction policies of ‘general type, saving type and constrained type’ can be calculated (Table 10), and the total amount of water consumption is equivalent to that of light, medium and severe water scarcity in the three cases, respectively, so as to alleviate the contradiction among domestic, production and ecological water in the watershed.

At the same time, according to formula (12) and the PCW of water for SEPO and ecological basic flow water, the annual process of ecological flow in the river under three reduction policies can be obtained (Table 11). Furthermore, the total annual water amount of ecological flow inside the river channel is obtained (Table 11).

Table 11 shows that the annual process of ecological flow of the Dingnan River is urgently adjusted under the three scenarios of ‘general type, saving type and constrained type’, and their annual water compression ratios are 93, 85 and 75%, respectively.

As shown in Section 3.5, the ecological flow guarantee degree will be significantly improved if engineering and non-engineering measures are considered comprehensively. Under the premise that the engineering guarantee degree reached 80%, the water reduction policy can increase the guarantee degree of ecological flow from 80 to 90, 95 and 98%, respectively, thus effectively resolving the contradiction between supply and demand imbalance during the dry season.

When only the water reduction policy is adopted, the improvement of the guarantee degree of ecological flow is limited (Table 12). In March, July, August and September, the guarantee degree of ecological flow is 100% before the water reduction, and the guarantee degree of ecological flow in other months is increased compared with that before the water reduction. Among them, the average increase in the constrained type was the most significant, reaching 7.6 and 10.4% less than the increase under both engineering and non-engineering measures.

It is worth study on the management policy of water resources which can reduce production, domestic and ecological water to different degrees based on water inflow frequency prediction and watershed water demand gaps. In view of the fact that a policy of production and domestic water reduction during the urban emergency water supply period has already been implemented by the Ministry of Housing and Urban Rural Development (MOHURD) which has played a positive role in ensuring urban emergency water supply. Based on the reduction of production and domestic water in the watershed, the resilience of the ecosystem is considered and the ecological flow is further reduced in this article, which can more effectively resolve the contradiction between supply and demand. It provides a theoretical basis and practical case for managers of other watersheds to formulate water scheduling schemes during the water shortage period.

Ecological flow can be proactively and dynamically adjusted by the D-A-R approach based on water inflow frequency prediction, the D-A-R approach has the potential to be successfully implemented on a larger scale and it can provide a reference for water resources management in other watersheds.

In the dry season, the guarantee of ecological water is not optimistic. At the same time, the contradiction among the demand for domestic, production and ecological water is prominent. By the D-A-R approach based on the ‘Hedging rule’ and the ‘synchronous reduction strategy’, the annual process of ecological flow can be reasonably determined, and the water consumption allocation of water users with potential future water scarcity is strengthened to degrade the probability of suffering more serious water scarcity events in the later stage. In the Dingnan River watershed, the local governments can use the D-A-R approach to make proactive and dynamic adjustment of ecological flow to alleviate the contradiction among domestic, production and ecological water during the water shortage period. In the future, we can popularize the new approach to other southern water-rich watersheds with ‘hydrological similarity’, which can help local managers to formulate new water scheduling schemes.

This research was funded by the Science and technology project of Jiangxi Provincial Water Resources Department, China (202123YBKT20). We acknowledge Taotao Cao, Xingqing Tong and Yuze Wu for their help with the digitalization process.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.