Adaptive calculation of river ecological flow considering the variable lifting volume under changing conditions

The science and practice of ecological flows is an approach to protecting and restoring aquatic biodiversity, ecosystem integrity, and important ecological services by managing freshwater flow regimes (Ćosić-Flajsig et al. 2020). Existing studies show that ecological flows have been compromised or are at risk in most aquatic systems around the world, and the cumulative global impacts are severe (Vörösmarty et al. 2015; Bunn 2016; Zhou et al. 2022). Changes in hydrological conditions, growing demands for freshwater, more flow regulation, fragmentation of aquatic habitats, and less water for the environment overall in coming decades make the urgency for the implementation of environmental flows greater than ever (Arthington et al. 2018; Agashua et al. 2022; Mobilia et al. 2023). Judicious determination and supervision of ecological flow can achieve the balance between urban economic development and ecological protection to support biodiversity, resilient ecosystems, and socially valued ecological services (Acreman et al. 2014; Poff et al. 2016; Chen 2020; Taniguchi-Quan et al. 2022).

At present, ecological flow and its related definitions and connotations have not been completely unified, but under the promotion of relevant international organizations, it is becoming more and more consistent (Wei et al. 2022). The 2007 Brisbane Declaration provided a widely recognized definition of environmental flows that has since been cited in over 30 scholarly books and hundreds of journal publications and reports, testifying to the value of a consolidated, widely accepted statement of the essence and vital purpose of environmental flows (Gleeson & Richter 2018; Kennen et al. 2018). The 2018 Declaration altered the original wording from ‘…quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems…’ to ‘quantity, timing, and quality of freshwater flows and levels necessary to sustain aquatic ecosystems’; the revised definition meets the call to embrace flowing (lotic), standing (lentic), and groundwater-dependent ecosystems (GDEs), as well as aquatic ecosystems that may alternate between these states (Arthington et al. 2018). Nowadays, ecological flow is generally divided into basic and suitable ecological flow. The former is the minimum flow for the basic needs of river ecosystems, and the latter is the flow for constructing the most suitable habitat for rivers (Akter & Tanim 2018; Jia et al. 2020).

There are more than 200 methods (four major categories) related to the assessment of ecological flows, each of which has its advantages and disadvantages (Tharme 2003). The hydrological methods determine the recommended flow regimes of the river through historical or estimated monthly or daily flow data, in a relatively simple way, and are generally used for reference or comparison (Qu et al. 2022). These include the Tennant (Huang et al. 2019; Prakasam et al. 2021a, 2021b), 7Q10, Q90, Lyon, Texas (Yu et al. 2013; Opdyke et al. 2014), range of variability approach (RVA) (Monico et al. 2022), basic flow methodology (BFM) (Alomía Herrera & Carrera Burneo 2017), Tessmann (Pastor et al. 2014), frequency curve method, flow duration curve (FDC) (Maddu et al. 2022) and its improved method, etc. The hydraulics method relates various parameters of the hydraulic geometry of a watercourse channel to discharge, mainly including the wetted perimeter method (Prakasam et al. 2021a, 2021b), R2-CROSS method (Oikonomou et al. 2021), Lotic Invertebrate Index for Flow Evaluation (LIFE) method (Extence et al. 1999), and Adapted Ecological Hydraulic Radius Approach (AEHRA) (Liu et al. 2011). However, such methods require tedious field work and an examination of each cross-sectional area leads to negative results in terms of time and data consumption (Prakasam & Saravanan 2022).

Two other types of approaches that require very large scientific expertise and high costs are the habitat and the holistic methods. A typical representative of the habitat method is the Instream Flow Incremental Methodology (IFIM) (Bovee 1982), and if employed correctly, IFIM allows the values of every legitimate stakeholder to be taken into account. In addition, the River Hydraulic Habitat Simulation (RHYHABSIM) (Kelly et al. 2015) considers the flow requirements of the indicator species over its entire life cycle; Computer-Aided Simulation (CASMIR) model for instream flow requirements in regulated streams uses the FST software to establish flow and biotype correlations; and Physical Habitat Simulation (PHABSIM) (Johnson et al. 2017; Wang et al. 2018) for assessing the natural flow requirement can be made independently of the naturalized flow data with relative flexibility. In general, the habitat approach is site-specific, costly to implement, and mainly applied at micro- and meso-habitat levels focusing on one or a few river reaches. The holistic method focuses on the structure of the complete ecosystem and integrates multidisciplinary expert opinions to consider the correlation between multiple factors, and comprehensively assess the river requirements of the ecosystem in different states (Chen et al. 2014). Examples include Building Block Methodology (BBM) (Yarnell et al. 2022) and Downstream Response to Imposed Flow Transformation (DRIFT) (Wineland et al. 2022).

There are many efforts in the calculation and management of ecological flows by scholars and related authorities around the world (Çadraku 2022; Jekabsone et al. 2022). Xia et al. (2023) applied probability-weighted FDC to calculate the ecological flow of the Ganjiang River south of Poyang Lake in China. Yang et al. (2022) calculated the suitable minimum ecological flow of the study reach after the completion of the hydropower station, based on a 2D shallow water model with high-precision solution methods and graphics processing unit (GPU)-accelerated performance, combined with Tennant, hydraulics method, and habitat suitability models to obtain habitat conditions of the river for fish survival during non-spawning periods and effective habitat areas during the spawning period under different discharges. Pei et al. (2022) determined ecological flow thresholds in the downstream area of Bengbu Sluice in the Huai River Basin of China by using habitat simulation and hydrological approaches for mutual validation. Baruah et al. (2023) employed a two-dimensional eco-hydraulic model and multiple criteria decision analysis (MCDA) coupled framework in the Dikhow River, India, to investigate the favorable habitat condition for the nearly threatened Notopterus chitala fish locally known as Chital fish. To assess ecological flow needs in groundwater-influenced streams, Yarnell et al. (2022) applied the California Environmental Flows Framework (CEFF) in two river systems in California, USA, which offered flexibility to incorporate information on the seasonal and spatial dimensions of groundwater influences in the development of ecological flow targets. Greco et al. (2021) applied the Indicators of Hydrologic Alteration methodology (IHA by TNC) coupled with the valuation of the Index of Hydrological Regime Alteration (IARI by ISPRA) to define the ecological flow in each monitoring of the Agri River in Basilicata (Southern Italy) cross-section to support sustainable water resource management and planning.

Although there are many ecological flow-related studies, most of them focus on the calculation or flow variation, the results are usually deterministic or under scenarios (Lyu et al. 2019; Yan et al. 2021; Elenius & Lindström 2022). It has poor adaptability and operability for normal and emergency management. To continuously improve the river ecology quality, ecological flow should be adjustable, in the case of changing hydrological conditions, regulatory capacity, and ecological services (Liu 2021). Thus, the aims of this paper are as follows: (i) Adopting a new method for determining the ecological flow of the river, that is, adding a variable lifting volume to the ecological baseflow, and feasibility and adaptability analysis was carried out; (ii) Giving solutions to the problems involved in the use of the method, including the ecological baseflow and water demand calculation, weighing the water requirements of different ecological protection objects, priority classification, supply and demand balance analysis, and flow process management; (iii) Giving the optimal combination of ecological flows in each month and providing decision support for water allocation when both the stakeholders (industry, agriculture, ecology, etc.) and ecology are taken into account.

The observed daily flow series used from 1950 to 2018 in this study were obtained from The Yellow River Commission of the Ministry of Water Resources P.R.C. These data were measured by the LJC and WJB hydrological stations. Based on these daily flow sequences from 1950 to 2018, we obtained monthly and annual runoff data.

The proposal of ecological flow originates from the water-related environmental and ecological problems caused by the disorderly development and utilization of river resources by human beings and climate change, which makes ecological baseflow insufficient. The determination and supervision of ecological flows are largely influenced by human cognitive biases and management objectives (Wei et al. 2020). Ecology is more than ecology; livelihood, water for industry, agriculture, etc., all need to be taken into account. The changing supply and demand of water necessitates the adjustment of ecological flows accordingly.

From the concept that ecological baseflow is the maintenance of constant river flow, the calculation methods are summarized, mainly based on hydrological methods (Table 2). To more reasonably obtain the ecological baseflow of seasonal rivers, the improved annual distribution method was selected for the calculation.

The inflow changes dynamically from month to month during the year, and the ecological service objects and their priorities in specific river sections are different. Thus, according to the actual situation of the river upstream and downstream, it is necessary to determine the service categories and their combinations inside and outside the river, other water users (living water intake, agricultural irrigation, and industrial production), divide the priorities, and then carry out the ecological baseflow and various ecological water demands calculation. Some of the calculations required to determine the ecological flow of two key cross-sections are described below.

• (1)

  Prioritization

By investigating the important hydraulic habitat parameters required for fish survival, such as flow velocity, water temperature, water depth, water width, and wetted area, the relationship curves of suitability and flow velocity/water temperature of different fish were constructed (Jiang et al. 2009). On this basis, the flow processes required for fish spawning and migration were determined (Wei 2015).

• (3)

  Water demand for landscape

The first step in calculating the lifting volume is to determine the ecological baseflow, the categories of ecological services, and water demand priority. It will be used as input conditions for the next step. Among them, the boundary of the basic interval is fixed, which is equal to the ecological baseflow; this is also the lower limit of the interval of the competitive interval. The upper limit of the interval is determined according to different conditions, and the flexible and variable part in the middle is the variable lifting volume. On this basis, the ecological flow is comprehensively determined.

The core of the competitive interval is the determination of the feasible lifting volume, which is determined by two components. The first is the residual water volume, which is the remaining water after deducting the ecological baseflow; the second is the compressible water volume, which refers to the volume that can be reduced by elastic water in agriculture and industry. The determination of the competition interval has important guiding significance for the normalized river management of ecological flow.

• (1)

  Ecological baseflow calculation

The ecological services of the middle section of the Weihe River during the dry period include aquatic products (fish), watershed landscape, and biological habitat, with overload rates of 35.9, 16.1, and 12.3%, respectively. Since the ecological baseflow is the minimum flow to ensure the continuous flow of the river, it has the highest priority and must be guaranteed monthly; the fish spawning period (April–June) has higher flow requirements and should have a higher priority than landscape water demand; the flood season (July–October) has more incoming water and the evaporation in winter (December–February) is small, so the landscape water demand can be guaranteed without additional increase, and it has a certain periodicity, which can be guaranteed at monthly intervals.

• (3)

  Water demand for fish migration

One of the representative fishes in the middle and upper reaches of the Weihe River is carp, whose spawning time is generally from April to June, with a duration of 1–3 days. According to the relationship curves between suitability and water depth/flow velocity, the optimal water depth/flow velocity of carp is 1–1.5 m and 0.2–0.6 m/s. The water levels covering most of the side beaches of LJC and WJB transect are 610.4 and 496 m, respectively. When the water depth and suitable flow velocity are 1 m and 0.3 m/s, respectively, combined with the cross-sectional morphology, the two typical cross-section flow required for fish spawning and the migration period is 20.2 and 16 m³/s, respectively.

• (4)

  Water demand for landscape

The ecological landscape projects in LJC and WJB cross-sections are Jinwei Lake and a stepped water storage project. The water surface area of the two is 1.4 and 1.6 million m², respectively. The landscape volume is 350 and 400 m³, respectively. The water exchange cycle is 60 days, and the average water depth is 2.5 m. The water demand of the landscape of LJC and WJB cross-sections was calculated by using the water exchange cycle method are 0.7 and 0.8 m³/s, respectively.

• (5)

  Water demand for agricultural irrigation

In addition to various ecological services, the reach from LJC to WJB mainly undertakes the irrigation task of Baojixia Irrigation District (Lin & Li 2010; Lin et al. 2013). Among them, the Loess Plateau Irrigation District is diverted from the LJC reservoir, and the Lower Plateau Irrigation District is diverted from the WJB diversion. Based on the investigation of crop types, irrigation quotas, and water consumption periods in the irrigation area, the irrigation water demands of crops in different typical years were determined using the quotas index method, and the calculation results are shown in Table 3.

The ecological flow is not greatly affected by the diversion of water from LJC cross-sections in wet years, but insufficient after the diversion of water in normal years, and worse especially in dry or withered years. Therefore, reducing agricultural water consumption and tapping water-saving potential in irrigation districts will greatly contribute to the guarantee of ecological flow.

• (6)

  Ecological flow calculation

Step 1: Determine whether the remaining water can guarantee the irrigation of two specific cross-sections by assuming that the ecological baseflow is fully satisfied each month.

Step 2: Determine whether the ecological baseflow can satisfy the ecological requirements of fish and landscape in two specific cross-sections.

Step 3: Determine whether the remaining water can satisfy the lifting amount; if it can be fully or partially satisfied, calculate the volume, the lower limit of the ecological flow is the ecological baseflow, and the upper limit is the lifting volume; if it cannot be satisfied, it will not be increased, and the upper and lower limits of ecological flow are the same, which is ecological baseflow.

The ecological flow calculation results of the LJC and WJB cross-section according to the above steps are shown in Table 4.

The lifting volume varies in different typical years. In wet years, it can be zero; in normal and dry years, it can be sufficient or increased to some extent, while in special withered years, river flows are extremely low or even disconnected in individual months, and there is a risk of further deterioration of the water ecology. Competition for water between livelihoods, agriculture, industry, and ecology becomes more intense. Frequently, there is no extra flow available for ecological improvement. The main concern for reaches in this study is to coordinate the competitive relationship between irrigation and ecology by dividing the period into 10/day scales within the month, combining the days of water demand between irrigation and ecological requirements, and determining the priority and lifting volume. The lifting volume is not necessary if there are no additional flows.

As the priority of ecological demand of fish spawning and migration is higher than the landscape, and the required flow can satisfy the landscape demand at the same time, we further analyzed the lifting volume in different typical years. The surplus flow in April and June is greater than the shortage in normal years; thus, it can be raised to 20.2 m³/s. However, in dry years, the surplus flow in April is less than the shortage, and the opposite in May; thus a partial lift is available in April and a full lift to 20.2 m³/s in May. There is no surplus flow in June of dry years and in withered years, so the lifting volume is zero.

For the WJB cross-section, the ecological baseflow in October and December of normal years, July and August of dry years, and January to March of withered years could not satisfy the landscape water demand. Except for May of normal years, the ecological baseflow from April to June of the other years could not satisfy the ecological demand of fish migration. Among them, the deficient flows in April and June in normal years were 12.5 and 7.5 m³/s, respectively; 8.5, 10.3, and 14.7 m³/s in April to June in dry years, respectively; and 15.2, 15.0, and 11.9 m³/s in withered years, respectively. As can be seen from Figure 5, the surplus water from April to June in normal years and from April to May in dry years is greater than the shortage, thus it can be raised to 16.0 m³/s; the surplus water in June in dry years is less than the shortage, and there is no surplus water in May to June, thus the lifting volume is zero. In addition, Figure 5 shows the two cross-sections only have extra water for lifting volume in the high flow seasons (April–July) in a year, and the volume that can be used for ecological restoration is also relatively sufficient, which is consistent with the seasonal variation of runoff.

Ecological flow is for dry periods, whose value varies with spatial and temporal distribution, river resources conditions, ecological problems, hydraulic engineering, etc. (Liu et al. 2022). Some functional ecological requirements such as self-purification and sediment transport should be guaranteed in terms of flow volume and its process. The ecological flow calculation method proposed in this paper solves the difficulty of quantifying ecological flow, simplifies the complex problem, avoids the management confusion caused by numerous ecological flow algorithms and varied ecological objects, and gives an interval, which is more adaptable than a value or a set of values, and can be better dovetailed with process-oriented management. Because the method is conceptually clear and the calculation process is explicit, it can be generalized and applied to any cross-section of rivers. It should be noted that when this method is used in conjunction with different qualitative or quantitative methods, it is necessary to fully consider the current stage of regulation and control means and technology, scheduling management level, and gradually improve it from usable to adaptable.

Aiming at the problems of difficult quantitative calculation and management of river ecological flow and poor adaptability of results, an ecological flow calculation method based on the basic concept is proposed. Taking two typical cross-sections of the Weihe River in China as research objects, considering the inflow of different hydrological years, the ecological baseflow, fish migrations, landscape, irrigation, and water demand priorities of each month, it is found that the ecological flow of LJC intervals in normal, dry, withered years calculated by VIAM are (3.4, 22.4), (3.3, 20.2), and (0.7, 7.8) and the WJB cross-sections are (0.6, 20.5), (0.5, 16), and (0.3, 16), respectively, the unit is m³/s. The calculation process takes into account the influence of irrigation on the lifting volume, especially the distribution of the remaining water after deducting the ecological baseflow and irrigation water demand in the irrigation area from the inflow; it can be combined with reservoir scheduling and water distribution. The method allows for both overall planning and attention to the assurance requirements of extreme hydrological years. When there is an emergency in the river, it can also respond quickly to changes and help in decision-making.

In summary, ecological baseflow can ensure continuous flow, and the lifting volume represents the ability of ecological restoration, which is the way to guarantee ecological flow. Its dynamic change characteristics can be described by intervals. The calculation is combined qualitatively and quantitatively, and the specific problems are analyzed to realize the adaptive calculation of river ecological flow. At the same time, the various types of modeling methods are classified and a method library is established, combined with the research of multi-stakeholder interval coordination mechanism, which can make this work become more scientific, practical, and flexible, and make an effective contribution to the ecological health of the river.

The authors are indebted to the reviewers and editors for their valuable comments and suggestions.

This work was supported by the China Postdoctoral Science Foundation (2022M722561).

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

The authors declare there is no conflict.