A Practical Method for Ecological Flow Calculation to Support Integrated Ecological Functions of the Lower Yellow River, China

Abstract

The lower Yellow River is characterized by low water discharge and a high sediment load, resulting in a fragile aquatic ecosystem. It is important to develop a reasonable method of ecological flow calculation that can be applied to the water-scarce rivers like the Yellow River. In this paper, we selected the Huayuankou hydrological station in the lower Yellow River as our study site and assessed the ecological flow using several methodologies including the monthly frequency calculation method, the sediment transportation method, the habitat simulation method, and the improved annual distribution method. Based on the seasonal applicability of the four methods across months of the year, we established an ecological flow calculation method that considers the integrated ecological functions of the lower Yellow River. In this method, ecological flow in the lower Yellow River during the dry season (November to March) can be determined by using the improved annual distribution method, ecological flow in the fish spawning period (April to June) can be calculated using the habitat simulation method, and the ecological flow during the flood season (July to October) can be calculated using the sediment transportation method. The optimal ecological flow regime for the Huayuankou section was determined using the established method. The ecological flow regimes derived in our study ranged from 310 m³/s to 1532 m³/s. However, we also observed that the ecological flow has a relatively low assurance rate during the flood season in the lower Yellow River, with the assurance rate not exceeding 63%. This highlights the fact that more attention should be given in reservoir regulations to facilitating sediment transport downstream.

1. Introduction

The Yellow River is the second-longest river in China. It serves as the principal water supply source and ecological barrier for the northwestern and northern regions of China [1,2]. The river is responsible for supplying water to 12% of the national population and 15% of cultivated land in China, thereby highlighting a pronounced issue of water scarcity. From the 1970s to the late 1990s, the synergistic effects of climate change and water resource exploitation led to frequent occurrences of flow cessation. This period was characterized by severe downstream channel siltation, riverbed elevation, contraction of the main channel, and degradation of delta wetlands [3]. These developments collectively imposed a series of impacts on the sustainable utilization of water resources and the ecosystems of the Yellow River basin.

In 1999, the Yellow River Conservancy Commission initiated a unified basin-wide water resources regulation, which effectively curbed the problem of mainstream flow cessation. However, in recent years, dramatic economic growth and social development have precipitated a continuous increase in water consumption within the basin. Currently, the utilization rate of surface water has escalated to 86% in the Yellow River basin, substantially surpassing the basin’s water resource carrying capacity. The source region of the Yellow River has experienced accelerated ice and snow melt, while the middle and lower reaches have witnessed significant declines in both runoff and sediment transport. These changes have exacerbated the imbalance between water supply and demand, posing severe threats to the health of the riverine ecological systems [4]. Therefore, it is necessary to reasonably determine the ecological flow indicators in order to provide a basis for the water resources regulation.

Ecological flow is the water discharge necessary to sustain the fundamental functions of riverine ecosystems. It generally refers to the quantity, timing, and quality of water flows required to support the survival of both aquatic and riparian organisms [5]. To date, over 200 methodologies have been developed for calculating ecological flow, which can be broadly divided into four primary categories: hydraulic method, hydrological method, habitat simulation method, and holistic method [6]. The hydraulic methods determine the ecological flow of a river based on hydraulic parameters such as width, depth, flow velocity, and wetted perimeter. These methods have been most commonly applied to assess the minimum instream flow, but they are not suitable for high-sediment rivers with frequent changes in riverbed morphology [7]. The hydrological approach recommends ecological flow values based on either the percentage of average flow rate or a specific exceedance probability derived from flow frequency curves. The most representative methods include the 7Q10 method, Tennant method, and Texas methods. These methods focus on the physical characteristics of the river, but do not take into account its biological characteristics [8]. Habitat simulation methods represent an advanced development of the hydraulic method, utilizing the hydraulic conditions required by aquatic organisms to establish ecological flow [9]. However, habitat simulation methods primarily focus on conservation of specific riverine species, often neglecting the entire river ecosystem. As a result, the ecological flows may not always align fully with the management requirements of the river system. In the 1990s, scholars proposed holistic methods, which address the limitations of habitat simulation approaches that focus on one or two species. These methods emphasize that rivers function as complex ecological systems, necessitating that we consider the interactions among various biotic and abiotic components [10].

Ecological flow in high-sediment rivers has received significant research attention. De Jalón et al. [11] suggested that sediment dynamics need to be considered when specifying instream ecological flow. Sun et al. [12] examined and quantified the volume of inflowing water needed to meet multiple demands in the Yellow River Estuary. The research study determined that the minimum, medium, and high levels of annual environmental flows are 134.22 × 10⁸, 162.73 × 10⁸, and 274.9 × 10⁸ m³, respectively, in the Yellow River Estuary, which represent 23.7, 28.7, and 48.5% of the natural river discharge. Zhang et al. [13] analyzed the multi-year trends of annual and seasonal changes in water discharge in the upper Yellow River and indicated that the construction and operation of reservoirs are the primary factors driving changes in ecological flow regimes. The research argues that the effects of cascade reservoir operations on ecological flow regimes are even more significant than those from individual reservoirs, posing serious risks to downstream riverine ecosystems. Zhang et al. [14] proposed a quantitative method for determining the flow rate changes in reservoir ecological scheduling. The approach utilized the daily flow rate and daily flow-rate increment to characterize the flow process. Kemper et al. [15] found that the development and utilization of a watershed-scale sediment–ecological connectivity framework highlights the value of sediment as a critical ecological resource to be managed jointly with flow to ensure the maintenance of vital riverine ecosystems.

Current research on ecological flow mainly focuses on hydrological methods. The Yellow River’s high sediment load, inherent water scarcity, and fragile aquatic ecosystem contribute to variations in the prioritization of its ecological functions over time, resulting in differing ecological flow requirements within the river. However, each ecological flow calculation method has its own limitation, making it challenging to use a single approach to adequately meet the water demand for the whole ecological functions of a sediment- laden river such as the Yellow River. Therefore, it is essential to study the applicability of ecological flow methods across different periods of a heavy-sediment river. This will not only meet multiple ecological objectives for the sustainable management of the Yellow River but also facilitate the establishment of a comprehensive ecological flow framework tailored to sediment-laden rivers.

This study focuses on the Huayuankou hydrological station in the lower Yellow River (Figure 1), where multi-year runoff and sediment data have been collected. We employ various eco-hydrological methods to estimate the ecological flow regimes at the Huayuankou station. A comparative analysis is conducted to assess the applicability of these methods. We also propose ecological flow regimes designed to meet diverse ecological protection objectives, thereby providing the basis for optimizing reservoir regulations in the Yellow River basin.

2. Materials and Methods

2.1. Study Area and Data Collection

The lower Yellow River (Figure 1) extends for a total length of 786 km and encompasses a watershed area of 23,000 km², with an average riverbed gradient of 0.12%. This river stretch is characterized by low water flow and high sediment loads [16]. Additionally, the lower Yellow River is one of the most severely affected segments in terms of drought and flood disasters, exacerbating the conflict between water supply and demand. The wide and shallow river channel offers important habitats for both avian and aquatic species, facilitating their survival and reproduction [17].

Huayuankou station (Figure 1) is an important hydrological station in the lower reaches of the Yellow River, located in Zhengzhou, Henan Province. It controls a watershed area that comprises 97% of the Yellow River basin. The frequent channel migration in this section makes the channel morphology extremely sensitive to instream flow and sediment conditions, while also posing negative impact on the riverine ecosystem. We collected the daily flow data and monthly sediment transport data from the Huayuankou hydrological station for the period spanning 1980 to 2021. We also collected measurement data on erosion and deposition of the lower reaches of the Yellow River for the same period.

2.2. Methods for Calculating Ecological Flow

2.2.1. Monthly Frequency Method

The monthly frequency method involves analyzing the frequency curve of the monthly average flow based on a lengthy series of flow data. The suitable ecological flow is determined by taking the monthly runoff corresponding to a 50% probability [18,19]. In this method, it is necessary to select the period with minimal human activity interference for the analysis of ecological flow. In the early 1960s, the impoundment of the Sanmenxia Reservoir led to a significant reduction in the minimum monthly flow in the downstream of the Yellow River. Following the implementation of reservoir regulation in the 1970s, the water scarcity during dry periods was alleviated, approaching the natural conditions observed in the 1950s and 1960s [20]. The impoundment of the Xiaolangdi Reservoir in 1999 further significantly affected the runoff regime of the lower Yellow River, resulting in notable variations throughout the year. Consequently, we utilized the monthly runoff data from 1980 to 1999, prior to the impoundment of the Xiaolangdi Reservoir, to analyze the ecological flow regime of the river.

2.2.2. Sediment Transportation Method

The sediment transportation water requirement refers to the flow discharge necessary to effectively transport sediment through a given river segment, ensuring that it reaches designated points without exceeding allowable sedimentation levels [21]. According to statistical analysis of hydrological data, approximately 85% of the annual sediment transport in the lower Yellow River occurs during the flood season. Thus, it is essential to focus on the sediment transportation flow requirement during this period. The sediment transportation flow requirement during the flood season is influenced by factors such as the sediment flux, water discharge, and river channel sedimentation. These factors exhibit a strong statistical correlation [22], as represented by the following formula:

$$W=22Ws-42.3(Ys+Cs)+86.8$$

(1)

for the water requirement during the flood season (m³), where $Ws$ denotes the monthly sediment flux during flood season (t), as was calculated by using the measured data of flow discharge and sediment concentration; $Ys$ indicates the allowable sedimentation in the river channel (t); and $Cs$ refers to the amount of riverbed scouring during non-flood periods (t).

When calculating the water requirement for sediment transportation during the flood season, it is also essential to consider the riverbed scouring during non-flood periods. Shen et al. [22] established a quantitative relationship among the runoff, sediment load, and riverbed scouring in the lower Yellow River, based on data during non-flood periods. The corresponding calculation formula is as follows [20]:

$$Ws-Cs=\left\{\begin{array}{cc}0.002W{m}^{2.13}& Wm\le 50\\ 0.03Wm-0.67& Wm>50\end{array}\right.$$

(2)

where $Wm$ is the monthly river runoff during non-flood periods.

The allowable sedimentation $Ys$ in the river channel is determined by the flow discharge and sediment flux. A nonlinear statistical relationship has been established between the allowable sedimentation and sediment flux at the Huayuankou cross-section. The formula can be shown as

$$Ys=0.4268Ws-1.4124$$

(3)

Due to the prioritization of flood control and power generation during the operation of the Xiaolangdi Reservoir, there has been a notable reduction in peak flow rates during the flood season, coupled with an increase in flow discharge during non-flood periods. This has significantly altered the original regimes of flow discharge and sediment transport within the downstream river reach. The sediment fluxes show a decrease of 82% in the flood season and 81% throughout the year (Figure 2). So, it is suitable to utilize sediment and flow data after operation of the Xiaolangdi Reservoir for the study of the sediment transportation water requirement.

Consequently, we conducted a frequency analysis of the sediment flux during the flood season at the Huayuankou station from 2002 to 2021, thereby generating a theoretical frequency curve for sediment flux during this period. Typical years were then selected based on sediment flux corresponding to a design guarantee rate approaching 50%. Then, we used Formulas (1)–(3) to calculate the monthly water requirements for sediment transport during the flood season spanning from July to October.

2.2.3. Habitat Simulation Method

The habitat simulation method involves the construction of a habitat model to simulate the relationship between flow rates and the habitat distribution, thereby investigating ecological flows that fulfill the needs of aquatic organisms [23]. Fish are essential bioindicators of the ecological health of river and lake systems, with the overall suitability of their habitats serving as a direct reflection of the impacts that hydrological regimes exert on riverine ecosystems. For the purpose of habitat simulation in this study, we selected the Yellow River carp (Cyprinus carpio) as a target fish species. This species is an important economic fish within the Yellow River, with its spawning period occurring from April to June and a wintering period spanning from November to March. Prior research has indicated that the Huayuankou river section serves as a crucial spawning ground for this species [24]. Wang et al. [25] developed a habitat simulation model for the Yellow River carp, investigating the relationship between fish habitat suitability and river flow discharge. The study suggested the ecological flow that fulfilled the spawning and overwintering requirements of the Yellow River carp in the Huayuankou river section. We adopted these research results in our study.

2.2.4. Improved Annual Distribution Method

The annual distribution method calculates the ecological flow for each month for the river by determining the ratio of the multi-year average annual runoff to the minimum monthly average runoff [26]. The ecological flow calculation using this method reflects the annual variability of river discharge. However, for sediment-laden rivers in northern China, the seasonal fluctuations in flow discharge are particularly pronounced. Consequently, the use of a uniform ratio does not adequately capture the critical characteristics of runoff variability in these rivers. We divided the months of the year into three distinct periods: the flood season (July to October), the dry season (November to March), and the fish spawning period (April to June). To calculate the ecological flow for each period, we utilized the monthly runoff at a 90% assurance level as a substitute for the minimum monthly average runoff. The calculation process is detailed as follows (Figure 3):

(1)

We calculated average monthly runoff $\overline{q_i}$ and the flow rates corresponding to a 90% assurance level for each month $q_{90\%,i}$ from 1980 to 1999.

(2)

We calculated the multi-year average runoff ${\overline{Q}}_k$ and the flow rates corresponding to a 90% assurance level $Q_{90\%,k}$ for both the flood season and dry season, and then we calculated the ratio ${\eta }_k$ for each period. The formulas for these calculations are outlined as follows:

$${\overline{Q}}_k={\displaystyle \sum _{i=1}^n\overline{q_i}}$$

(4)

$$Q_{90\%,k}={\displaystyle \sum _{i=1}^n{q}_{90\%,i}}$$

(5)

where n represents the number of months included within each respective period.

(3)

We calculated the ecological flow for each month throughout the year by using the average monthly runoff $\overline{q_i}$:

$$Q_i={\eta }_k{\overline{q}}_i$$

(6)

3. Results and Discussion

3.1. Comparison of Ecological Flow Results

Due to the particularity of a high sediment concentration and a complex ecological water demand in sediment-laden flows, the instream ecological flow should be ensured so as to meet the specific ecological functions of the river in different periods. The ecological flow methods in this paper include the monthly frequency method, sediment transportation method, habitat simulation method, and improved annual distribution method, which are widely used in calculating the instream ecological water demand. Each method has its own applicability. The monthly frequency method and improved annual distribution method rely mainly on the historical hydrological data. The advantage of these methods is that they can account for the seasonal variation in runoff. But they may fail to directly satisfy the flow requirements of aquatic organisms. The sediment transportation method only focusses on the minimum water use for sediment transportation, which is crucial for flood control. The habitat simulation method explores the ecological flow that satisfies the spawning and survival of a particular species or population by simulating the relationship between flow rates and habitat suitability. This paper investigates the applicability of these methods for calculating the ecological flow in the Yellow River, which is regarded as a typical river with a heavy sediment concentration.

The results obtained from each method were compared within each respective period. In the dry season period, the ecological flow results obtained through the monthly frequency method consistently exceeded those from other calculation approaches (Figure 4). Conversely, the habitat simulation method yielded the lowest results, exhibiting minimal variations across the months. This discrepancy can primarily be attributed to the habitat simulation method’s reliance on an eco-hydrodynamic model, which simulates the relationship between river flow and the suitability of the fish habitat. This approach thus calculates the flow regimes necessary to maintain the ecological functions of fish habitats.

In this study, we selected the Yellow River carp as the target species, analyzing the suitable ecological flow during its spawning and wintering seasons. Notably, the wintering season for Yellow River carp typically spans November to March, coinciding precisely with the dry season period. Water depth is the primary factor influencing the habitat suitability of Yellow River carp during their wintering period, while the impacts of river flow and water velocity are relatively minimal. The relationship between the habitat-weighted usable area (WUA) and river flow indicates that at a flow rate of 300 m³·s⁻¹, the WUA for Yellow River carp in the Huayuankou river section is maximized [25]. This suggests that maintaining flow levels around this threshold during the wintering season is critical for ensuring optimal habitat conditions for the species.

The period from April to June each year marks the spawning period for fish species, including the Asian carps and Yellow River carp, in the lower reaches of the Yellow River. During this period, ecological flow results obtained from the improved annual distribution method and the monthly frequency analysis method exhibit a trend of initially decreasing followed by increasing flow (Figure 4). This pattern can be attributed to the Yellow River’s role as the primary irrigation water source for the region. In the lower reaches, the rainfall in the irrigation areas predominantly occurs during the flood seasons, while the spring irrigation period (March to May) contributes only about 16% of the annual rainfall. Consequently, the increased demand for agricultural water during this period inevitably results in a reduction in river flow, impacting the water flow necessary to support aquatic habitats and spawning activities. According to the relationship between the WUA of spawning habitat and river flow for Yellow River carp at the Huayuankou river reach, the WUA for adult fish is maximized when the flow rate ranges from 600 m³/s to 750 m³/s, peaking around 700 m³/s [25]. Thus, the recommended ecological flow rate in the Huayuankou section during the spawning period is critical for ensuring adequate habitat availability and supporting the reproductive success of the fish species.

In the lower reach of the Yellow River, rainfall is primarily concentrated during the flood season, which consequently results in maximum flows occurring during this period. The ecological flow calculation results at the Huayuankou reach (Figure 4) indicate that the maximum values calculated using the monthly frequency method, the sediment transportation method, and the improved annual distribution method all occur in August, reflecting the changes in runoff between the flood and dry seasons. The results from the monthly frequency method are significantly higher than those from the other two methods. This discrepancy can be attributed to the substantial climatic variability in the lower Yellow River region, where extreme weather events have become more frequent, leading to an increase in flood occurrences. During the flood season, the monthly runoff series exhibits a high degree of variability. Historical hydrological data reveal that the skewness coefficients and variation coefficients of the monthly runoff are close to 1 during the flood season [27]. The monthly frequency method relies on a 50% assurance level for the monthly runoff to estimate the suitable ecological flow. As a result, the calculations from this method tend to be biased upward due to the influence of extreme flooding events.

3.2. Optimal Ecological Flow Regimes at Huayuankou Station

We determined the optimal ecological flow regimes required to maintain the river’s ecological functions by comparing the results of the four ecological flow methods and considering the ecological functions of the lower Yellow River. In this paper, the optimal ecological flow should satisfy the seasonal water demand for river functions in the flood season, the dry season, and the whole year under different flow and sediment conditions in a sediment-laden river. The findings indicate that during the dry seasons, the habitat simulation method only considers the hydrological conditions necessary for the overwintering of fish species, neglecting the riverine ecosystem, including the riparian zone and wetland. This has led to an underestimation of the ecological flow required for the river.

The results from the monthly frequency method and the improved annual distribution method show a consistent trend in annual variations (Figure 3). However, the runoff and sediment load in the lower Yellow River are largely influenced by precipitation. The monthly frequency method uses a 50% assurance level for monthly flow as the basis for calculating ecological flows. While this standard can meet the hydrological requirements of the riverine ecosystem, it is often too demanding for the Yellow River basin. In extreme dry years, reservoir regulation according to this standard could exacerbate the conflicts between supply and demand of water resources in the lower Yellow River. In contrast, the improved annual distribution method can effectively satisfy the water resources required by river ecological functions while maximizing the efficiency of water resource utilization. Therefore, we advocate for using the results derived from the improved annual distribution method as the ecological flow during the dry season period.

The spawning period represents the most critical phase in the lifecycle of fish species, characterized by special habitat requirements. During the period of April to June, when fish engage in spawning activities, the primary ecological function of the lower Yellow River is to ensure the hydrological conditions for fish reproduction. The habitat simulation method integrates hydrological factors with the reproductive needs of fish through an evaluation index system for habitat suitability, thereby accurately capturing the flow regimes essential for fish spawning activities. We consider that habitat simulation method is the most reliable approach for calculating the ecological flows during the fish spawning period in the lower Yellow River. The habitat simulation method derives the ecological flow for aquatic fauna conservation by modeling the relationship between flow discharge and habitat suitability. However, this approach predominantly emphasizes fish habitat protection while proving insufficient consideration of macroscopic invertebrates and botanical species. Future study should focus on maintaining the integrity of the riverine ecosystem.

Sediment transportation is an important ecological function that should be satisfied in the lower Yellow River. Given that sediment flux during the flood season accounts for the majority of the annual load, ecological flows during this period must prioritize the requirement for sediment transportation [28]. Both the monthly frequency method and the improved annual distribution method calculate ecological flow based on hydrological indicators; however, these approaches often overlook the river’s sediment transportation functionality, which can lead to either riverbed deposition or excessive erosion. Therefore, we recommend using the sediment transport water requirement method to determine the ecological flow for the lower Yellow River during the flood season. Existing studies have demonstrated that there is a correlation relationship between riverbed variation and flow rates as well as sediment concentrations during the flood season in the lower Yellow River. Nonetheless, due to the limitations of the measured data, the empirical parameters for sediment transportation rates have primarily been established based on observation data between the 1950s and the 1990s [29]. Future research should focus on uncertainty analysis combined with sediment transport equations.

We recommend using the improved annual distribution method to calculate the ecological flow regimes for the dry season period in the lower Yellow River, as this period features lower river flow that primarily keeps the river ecosystem from destruction. For the fish spawning period, we consider the habitat simulation method, which effectively meets the habitat requirements essential for the reproduction of fish species. Additionally, we propose utilizing the sediment transportation method to determine the ecological flow during the flood season, thereby preserving the overall morphology of the riverbed in the downstream reach of the Yellow River (Figure 5). Considering that a primary goal of reservoir regulation in the Yellow River is to prevent the main channel from experiencing flow interruptions, we have established a minimum ecological flow threshold to ensure continuous flow by using the monthly minimum ecological runoff method [30]. Analysis of the results indicates that the optimal ecological flow regimes derived in this study meet the objective of ensuring continuous flow (Figure 5). The optimal ecological flow varies significantly between the flood season and the dry season within the year. The value of ecological flow in each month is less than the multi-year average values. The optimal ecological flow in this paper can satisfy monthly ecological functions in a sediment-laden river, but it may give insufficient consideration to the water demand of human activity, which is worth conducting a more systematic study in the future.

3.3. Analysis of Ecological Flow Assurance

Based on the annual runoff series for the period of 2000 to 2021 at Huayuankou station, we selected a representative wet year (p = 25%), normal year (p = 50%), and dry year (p = 75%). We then calculated the statistical indices for the mean and standard deviation (STD) of the ecological flow assurance rates (Figure 6).

In wet years, the ecological flow assurance rates during the dry season and the fish spawning period approach 100%, whereas the assurance rates during the flood season were relatively lower (mean = 63%, STD = 29%). In normal years, both the flood season and the fish spawning period exhibit comparatively low ecological flow assurance rates (mean < 60%, STD < 38%). In dry years, the ecological flow assurance rate during the fish spawning period is 100%, while the assurance rate during the flood season falls below 1% (Mean = 4%, STD = 3%).

These results clearly indicate that ensuring adequate river flow for sediment transportation is a fundamental prerequisite for maintaining the ecological integrity of the lower Yellow River. Sediment transport is one of the main concerns in a sediment-laden river. Ecological flow should be ensured so as to transport the sediment downstream. Otherwise, sediment deposition during the flood season can elevate the riverbed, intensify the risk of flooding, and pose a significant threat to the riverine ecosystem. It is important to establish a reasonable reservoir regulations strategy for the Xiaolangdi Reservoir and the upstream reservoirs of the Yellow River. During the flood seasons, the reservoir regulations should be optimized according to the sediment transportation water requirement to create a flow process that is conducive to ensuring sediment scouring downstream, thereby promoting the healthy development of the river ecosystem.

4. Conclusions

The study of ecological flow in a sediment-laden river is of great importance for ensuring the ecological health of the river. In this study, we estimated the applicability of the monthly frequency method, sediment transportation method, habitat simulation method, and the improved annual distribution method in calculating ecological flow at the Huayuankou section of the lower Yellow River. Aiming to maintain the ecological integrity of the river, we established an ecological flow calculation method, which allowed us to derive the optimal ecological flow regime for a sediment-laden river. This ecological flow regime can not only meet the primary conservation goal of ensuring continuous stream flow but also satisfy the flow management objectives during various temporal periods (dry season, fish spawning period, and flood season), thus providing a scientific basis for water resource management in a sediment-laden river like the Yellow River.

According to the results of the ecological flow presented in this paper, the river flow during the dry season and fish spawning period is sufficient, adequately satisfying the ecological flow requirements at the Huayuankou section. However, the ecological flow assurance during the flood season is notably low, primarily due to a lack of adequate runoff for sediment erosion. Therefore, there is an essential need to intensify practical efforts related to water and sediment regulation during the flood season within the framework of water resource management in the Yellow River basin.