Research on eco-environmental flow based on the improved holistic method and its application in Wei River Basin Open Access

The Wei River is a typical representative of strong competitive water use in Northern China. With the continuous increase of development and utilization intensity of water resources in the river basin, social and economic water use has occupied the eco-environmental water use for a long time. Facing the severe trend of resource constraints, serious environmental pollution and ecosystem degradation, governments at all levels have always attached great importance to the protection and construction of the ecological environment. Driven by various projects and plans, eco-environmental protection has been quite effective, and eco-environmental water use has increased year by year, especially in the past five years, but the proportion of water use is still less than 5%. The eco-environmental flow is considered as a critical component for supporting sustainable water resources allocation and management. How to determine the reasonable and feasible eco-environmental flow is a key problem.

Many methods have been developed for the calculation of eco-environmental flow, which can be divided into four categories: hydrological methods, hydraulic methods, habitat simulation methods and holistic methods. Among them, hydrological methods use a fixed flow percentage as the required flow process of the river based on a long series of historical runoff data, which are generally used for reference or method comparison (Wang et al. 2002), such as the Tennant method (Tennant 1976; Wu et al. 2022), Texas method (Matthews & Bao 1991), RVA method (Richter et al. 1997; Zhang et al. 2018), 7Q10 method (Eslamian et al. 2010; Wei et al. 2019), dynamic calculation method (Pan et al. 2013), monthly frequency calculation method (Li et al. 2007), and monthly guaranteed frequency method (Wang et al. 2021). The hydrological methods concentrate on single hydrological elements, with low accuracy.

The hydraulic methods express the eco-environmental flow by hydraulic parameters, such as width, depth, flow velocity and wetted perimeter. The representative methods include the wetted perimeter method (Gippel & Stewardson 1998; Cheng et al. 2019) and R2-Cross method (Mosley 1982). Although the methods take hydraulic factors into account, they need a large amount of measured data, which makes the realization difficult.

Compared with the above methods, the habitat simulation methods, such as the IFIM method (Armour & Taylor 1991; Pan et al. 2015), PHABSIM (Williams 1996; Zhang et al. 2021), and CASMIR method (Xu et al. 2016), consider the hydrology, hydraulics, water quality and biological demand, etc. These methods have the advantages of satisfying the demand of representative species, ignoring watershed planning and the whole riverine ecosystem.

The holistic methods emphasize the integrity of the river system, which is considered to be a comprehensive ecosystem and consists of multiple components. This method is not limited by analytical tools, and can determine the flow process required by each component of the system through various methods, and finally determine the comprehensive environmental flow required by the whole river ecosystem. As a multi-disciplinary crossing method, a large amount of basic data is needed. Thus, the method is relatively complex. The representative methods include the BBM method (King & Louw 1998), FSR method (O'Keeffe et al. 2002), and EPAM method (Cottingham et al. 2002).

During the last few decades, scholars’ research on eco-environmental flow has turned to the coupling of multiple methods to overcome the defects of a single method. Zhang et al. (2011) proposed a simple, practical and easy-to-manage comprehensive method by combining the hydraulic wet perimeter method and R2-Cross method with the hydrological Tennant method. Jiang et al. (2009) proposed a new method, namely the flow recovery method which combines a hydrological method and a habitat simulation method. Li et al. (2018) considered different protection targets at different stages, coupled with a variety of calculation methods to determine the river ecological flow. Huang et al. (2019) coupled the matter-element analysis method with the hydrological Tennant method, and carried out an ecological flow process evaluation study based on the Me–Tennant method. Xu et al. (2016) calculated ecological flow by using the eco-hydraulic method according to the flow velocity range of fish migration. Yang & Chen (2006) combined the monthly guarantee rate method with the hydrological index method and proposed a calculation method for dynamic ecological environmental water demand.

As can be seen, the above methods have played an important role in both the theory and practice of eco-environmental flow. However, the above calculation methods are not unified and there is a lack of systematic and universal calculation methods. These methods mostly focus on one or more protection objectives, and the results are usually one or a group of specific values, which is not conducive to the implementation and supervision of eco-environmental flow and is not operable. The eco-environmental flow is a scientific concept, but also a management tool, and the determination of eco-environmental flow should better adapt to development and change.

Actually, a river ecosystem has natural–social dual attributes. The contradiction between socio-economic and eco-environmental water use is irreconcilable, especially in the dry season. The key question is how to balance the interests and determine the eco-environmental flow which is fair and acceptable to all stakeholders. The aim of this study is to introduce an improved holistic method to estimate the eco-environmental flow. The improved holistic method applies stakeholder theory to the development and utilization of water resources. On the basis of determining the ecological water demand of each sub-item, this method considers the inflow of tributaries, the water intake of key sections, the principle of water balance, and related planning and research results, and integrates the opinions of various stakeholders. Finally, the three-level management targets of eco-environmental flow are put forward, which provides new ideas and references for comprehensively and scientifically promoting river and lake eco-environmental flow control.

The most serious problems of the Wei River contain four aspects: (1) Due to the decrease in water resources, ecological water is stolen by industrial and domestic water consumption, which leads to seasonal blockage of the water channel and habitat destruction. (2) Sediment accumulation is one of the most serious problems in the downstream of the Wei River, which leads to a large amount of sediment entering the river and adds great pressure to flood discharge. (3) Most sections of the Wei River are polluted heavily, with water quality worse than level V according to the Environmental Quality Standard for Surface Water and the Environmental Bulletin of Shaanxi Province. (4) The water quality of the wetland landscape in Shaanxi Section of the Wei River is deteriorating, the aquatic ecological function is degenerating, and the habitat of plants, animals and microorganisms is being destroyed.

According to the planning targets and characteristics of the Wei River, the eco-environmental flow is divided into four parts: ecological basic flow, water demand for sediment transport, environment flow, as well as water demand for water landscapes. Among them, ecological basic flow is the minimum flow to maintain the basic morphology and ecological function of rivers. Due to serious soil and water loss in the downstream of the Wei River, water demand for sediment transport must be guaranteed in order to keep the shape of the riverbed and make flood discharge unblocked. Water pollution directly leads to the destruction of the eco-environment function. It is necessary to maintain a certain amount of water in the river to achieve the water quality target determined by the water function zoning. According to the comprehensive treatment planning, three new water landscapes are planned along the main stream of the Wei River, with a total area of 2,200,000 m².

In this study, five hydrological stations, namely Linjiacun, Weijiabu, Xianyang, Lintong and Huaxian, were selected as representative control sections, corresponding to river sections 2, 5, 8, 11 and 13, respectively. The hydrology, sediment and water function zoning data of Shaanxi Province from 1950 to 2018 were used to calculate the eco-environmental flow of each section.

The riverine ecosystem has natural and social properties. The following questions should be paid more attention when stressing the whole ecosystem.

• (1)

  Under the condition of limited water resources, the competition of water use among various users is fierce; it is difficult to meet the water demand of all users at the same time. It is necessary to discuss through stakeholder consultation, so as to realize the balance of interests. The main reason why eco-environmental flow is difficult to implement is that the existing users have their own spokespeople, and they are trying to protect their own interests. These interests are related to water supply security, food security, flood control security, energy security, etc., involving a wide range of departments. The ecosystem cannot participate in the discussion of stakeholders because there is no spokesperson. Therefore, government departments should, as representatives of the public benefits of the ecosystem, hold roundtable meetings, discuss and determine together, and shoulder the responsibility of protecting the ecosystem.

• (2)

  It is necessary to implement a hierarchical management system for eco-environmental flow. At present, the concept of adaptive management emphasizes two points: one is to adopt the latest technology, and the other is to obtain the best course of action. Due to the uncertainty of water resources systems, even if all the sub-items of eco-environmental flow are determined, it may not be able to get enough water to the right place at the right time. Therefore, it is necessary to establish hierarchical management targets of eco-environmental flow in order to coordinate the relationship between eco-environmental water use and socio-economic water use. In view of this, a new improved holistic method is proposed in this paper.

The improved holistic method, on the premise of identifying the eco-environmental protection targets and the eco-environmental function regionalization, adopts a variety of methods to calculate the flow process for the composition of the riverine ecosystem, such as ecological basic flow for fish existence, water demand for sediment transport, environmental flow, and water demand for water landscapes. In addition, the eco-environmental flow for key sections should take water balance into account, such as the inflow of tributaries and water intake from key sections. Then the eco-environmental flow process is finally determined after a roundtable discussion among all stakeholders. The eco-environmental flow for the Wei River is determined by four sub-items consisting of the ecological basic flow, water demand for sediment transport, environment flow and water demand for water landscapes. The calculation method of each sub-item is as follows:

(1) Ecological basic flow for fish existence

The product of flow velocity and cross-sectional area can be used as the ecological basic flow. This paper proposes the concept of low-limit ecological basic flow and extremely low-limit ecological basic flow. The former can satisfy fish existence for the vast majority. The latter, considering the annual and seasonal hydrologic variation, can only guarantee fish existence for parts of the river, and allow the shrinking of living space for fish in dry seasons.

(2) Water demand for sediment transport

(3) Environmental flow

In order to reflect the requirements of water environment protection for flow in dry seasons, this study adopted the design flow of the water function zone as the environmental flow, and the comprehensive flow of the ecological environment should not be lower than the design flow of the water function zone. The calculation of this flow is based on the measured flow data of the existing hydrological station, and the minimum monthly flow of each year is sorted from the largest to the smallest. The frequency calculation results are plotted as a logarithmic frequency curve, which is matched with the p-III curve. In this paper, the minimum monthly flow under 90% guarantee rate is used as the design flow of the water function zone.

(4) Water demand for water landscapes

(5) Three-level management targets for eco-environmental flow

The key of the improved holistic method is to coordinate the relationship among the eco-environmental system, economic system, and social system and guarantee the implementation of eco-environmental flow. The three-level management targets for eco-environmental flow are proposed to guarantee the implementation of eco-environmental flow. The first management target (extremely low-limit eco-environmental flow) and the secondary management target (low-limit eco-environmental flow) include different sub-items of eco-environmental flow. The former includes extremely low-limit ecological basic flow, water demand for water landscapes, and inflow of tributaries, while the latter includes low-limit ecological basic flow, water demand for water landscapes, environmental flow, inflow of tributaries, and water intake from key sections. Both are determined by a roundtable discussion. The third management target (suitable eco-environmental flow), which is similar to the natural ecological status, is determined by the minimum monthly average flow. The three-level management targets for eco-environmental flow for key sections can be finally determined by roundtable discussion among all stakeholders.

All the calculation results are presented in Tables 2,³⁴⁵⁶–7. Tables 2,³⁴–5 present the results for each sub-item of eco-environmental flow. Tables 6 and 7 show the three-level management targets and eco-environmental flow processes.

(1) Ecological basic flow for fish existence

By habitat investigation on the mainstream of the Wei River (Wang et al. 2013), fish could live in most parts of the Wei River, with the maximum river width 60 m and river depth 0.5 m, while the fish could only live in parts of the Wei River, with the river width unchanged and river depth 0.3 m. Thus, the ecological basic flow with 0.3 m and 0.5 m river depth, which represent the extremely low-limit ecological basic flow and low-limit ecological basic flow, respectively, could be obtained as shown in Table 2.

(2) Water demand for sediment transport

Sediment deposition in the downstream of the Wei River leads to a sharp decrease in the flood discharge capacity of the main channel, and the channel oscillation intensifies the further deterioration of the downstream river regime. Based on the hydrological and sediment data of Xianyang Station, Zhangjiashan Station, Lintong Station, Zhuangtou Station and Huaxian Station, the monthly water demand for sediment transport of Lintong Section and Huaxian Section in the lower reaches of the Wei River in wet years (under 25% water inflow frequency), normal years (under 50% water inflow frequency) and dry years (under 75% water inflow frequency) were calculated, as shown in Table 3. It can be seen that the water demand for sediment transport was mainly concentrated in the flood seasons (July to October). From the wet year to the dry year, the water demand for sediment transport of Lintong Section in flood seasons was 63.99 × 10⁸m³, 43.22 × 10⁸ m³ and 28.54 × 10⁸ m³, respectively, which accounted for 81.32%, 57.00% and 93.02% of the total annual water demand for sediment transport, respectively; the water demand for sediment transport of Huaxian Section in flood seasons was 72.26 × 10⁸m³, 44.89 × 10⁸ m³ and 28.41 × 10⁸ m³, respectively, which accounted for 86.26%, 59.10% and 71.04% of the total annual water demand for sediment transport, respectively. With the decrease of natural inflow, the annual water demand for sediment transport decreased gradually. That is because the incoming sediment amount in the Wei River mainly concentrates in flood seasons, during which more water and sediment are transported (Song et al. 2005). In addition, for the same typical year, the water demand for sediment transport from Lintong Section to Huaxian Section showed an increasing trend.

(3) Environmental flow

In this paper, the minimum monthly flow under 90% guarantee rate was used as the design flow of the water function zone. According to the corresponding relationships between key sections and water function zones, the environmental flow for each key section was calculated as shown in Table 4.

(4) Water demand for water landscapes

The water exchange cycle method was used to calculate the water demand for water landscapes. The water depth was 2–2.5 m. The changing water cycle was about 60 days to better ensure water quality and landscape effect. Combined with the actual situation and planning objectives of each water landscape, the water demand for each water landscape is shown in Table 5.

In addition to the above water demand for sub-items, the eco-environmental flow for key sections should also consider the inflow of tributaries and the water intake of key sections. There is a large number of tributaries in the south and north banks of the Wei River. According to the principle of proximity, among the five key sections, the Laoyu River on the south bank of the Wei River is 2.55 km upstream from the Xianyang Section. Thus, the eco-environmental flow of Xianyang Section should also consider the inflow of the tributary; the value was 0.63 m³/s (which was equivalent to 10% of the natural runoff of the tributary). In addition, there is a pumping irrigation project between Lintong Section and Huaxian Section, that is, the Wei River water was extracted from Jiaokou Town for irrigation, and the annual water diversion flow was 8.00 m³/s. When determining the eco-environmental flow of key sections, it should be taken into account.

Table 7 shows the eco-environmental flow processes for each month of the key sections. As can be seen, from January to April and July to December, the low-limit eco-environmental flow should be guaranteed in normal years and drought years, and the extremely low-limit eco-environmental flow should be satisfied in severe drought years. For the other months, only the low-limit eco-environmental flow needs to be met. In addition, the eco-environmental flow process for Lintong and Huaxian sections should also take the eco-environmental flow for sediment transport into account; the water demand for sediment transport was mainly concentrated in the flood seasons (July to October).

In recent years, climate change and human activities have led to the overall decrease of annual precipitation and runoff in the Wei River Basin. The guarantee degree of eco-environmental flow is an important basis for river and lake health evaluation. Therefore, this study compared the calculation results of eco-environmental flow with the measured runoff in the past ten years. The ratio of the number of days in which the eco-environmental flow can be met to the total number of days in the recent ten years is the guarantee degree of eco-environmental flow. The guarantee degree of eco-environmental flow of the key sections could be obtained, as shown in Table 8.

It can be seen that Lintong Section and Huaxian Section achieved the three-level management targets of eco-environmental flow, and the guarantee degree of eco-environmental flow at all levels was more than 90%. The guarantee degree of extremely low-limit eco-environmental flow and low-limit eco-environmental flow of Xianyang Section was above 90%, the guarantee degree of suitable eco-environmental flow was lowest, which was 75.3% in non-flood seasons and 78.4% in the whole year, and the average damaged days were 60 days and 79 days, respectively. The guarantee degree of extremely low-limit eco-environmental flow of Weijiabu Section was 81.6% in non-flood seasons and 85.2% in the whole year, which was close to 90%, while the guarantee degree of low-limit eco-environmental flow and suitable eco-environmental flow was below 90%, being 32.6% in non-flood seasons and 45.2% in the whole year, and the average damaged days were 163 days and 200 days, respectively. Among all the sections, the guarantee degree of eco-environment of Linjiacun Section was the lowest, and the guarantee degree of eco-environment flow at all levels was far less than 90%, which is mainly due to irregular and large-scale water diversion in Baojixia Irrigation District.

Therefore, for Linjiacun, Weijiabu and Xianyang Sections, in order to achieve three-level management targets of eco-environmental flow, it is necessary to carry out reservoir ecological operation, adjust the operation rules of key reservoirs, intensify efforts to prevent and control water pollution, and improve the utilization rate of reclaimed water. In addition, it is also necessary to accelerate the implementation progress of inter-basin water diversion projects as far as possible to realize the coordinated development of economic society and ecological environment in the Wei River Basin.

The water cycle of social-economic systems has changed the quantity and quality of water resources in natural systems, which has caused a series of ecological and environmental problems. In this study, stakeholder theory was applied to the development and utilization of water resources, and a new method of stakeholder participation in determining river eco-environmental flow was proposed. The processes of eco-environmental flow in Shaanxi Section of the Wei River were studied. The main conclusions are as follows.

The improved holistic method adopts a variety of methods and models to calculate the sub-item of water demand. Considering the principles of tributary inflow, water intake of key sections and water balance, as well as the opinions of various stakeholders, makes the calculation results of eco-environmental flow more scientific and reasonable.

The three-level management targets of eco-environmental flow, including extremely low-limit eco-environmental flow, low-limit eco-environmental flow and suitable eco-environmental flow, are operable and dynamic, and can adapt to demand changes in practical application and provide a new idea and reference for comprehensively and scientifically promoting reservoir ecological regulation.

The development and utilization of water resources is done under the participation of many stakeholders. As a representative of the public interest of the ecosystem, the government participates in the discussion of stakeholders, protects eco-environmental water use, and better coordinates the relationship between eco-environmental water use and socio-economic water use.

Conceptualization, N. Wei; Methodology, N. Wei and K. M. Lu; Data analysis, K. M. Lu and F. Yang; Writing – original draft preparation, N. Wei and K. M. Lu; Writing – review and editing, N. Wei and J. C. Xie; Supervision, J. C. Xie. All authors have read and agreed to the published version of the manuscript.

This research was supported by the National Natural Science Foundation of China (Grant Nos. 51709222 and 51979221), Natural and Science Basic Research Program of Shaanxi Province (Grant Nos. 2017JQ5076 and 2019JLZ-15) and Water Conservancy Science and Technology Planning Project of Shaanxi Province (Grant No. 2020slkj-1). The authors are grateful to the editors and the anonymous reviewers for their insightful comments and suggestions.

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

The authors declare there is no conflict.