Assessment of ecological flow alterations induced by hydraulic engineering projects in the Han River, China

As one of the most fundamental elements in diverse ecosystems, rivers play essential roles and support mankind's survival. A flow regime that reflects the hydrological process has been of critical concern in the integrity and biodiversity of riverine ecosystems (Tonkin et al. 2018). However, warming climates and intensifying anthropogenic activities have caused significant alterations in flow regimes and exerted inevitable impacts on riverine ecosystem security (Li et al. 2020; Deng et al. 2022; Hatamkhani et al. 2022). Therefore, studies regarding the impacts of altered hydrologic regimes on biodiversity have captured considerable attention, intending to improve water resources management and ecological security maintenance (Palmer & Ruhi 2019; Mezger et al. 2021).

The variation in the hydrological regime counts mainly on climate change and anthropogenic activities. In general, instream flow is mainly controlled by precipitation in the natural periods. Climate change, characterized by rising temperatures, lifted sea levels, and shifted precipitation patterns can play a role in the well-being and resilience of the riverine ecosystem (Yang et al. 2021). Moreover, intensifying anthropogenic activities exert pervasive impacts on the ecosystem. Hydraulic engineering represented by reservoirs becomes incredible during human-induced time. The construction and operation of reservoirs alter their nature attributes and translate to copious eco-environmental problems (Yousefi & Moridi 2022). The impacts of dams can be mainly classified into four categories: (1) economic interest can be generated by hydropower generation but it leads to declining downstream flow, and some wetlands downstream even face drying up (Spanoudaki et al. 2022; Yang et al. 2023); (2) the dams separated the river channels and the water body with a low flow rate lacks high self-purification capacity, then water quality deterioration and eutrophication emerge (Zhu et al. 2023); (3) the water stored in the reservoir features in layer temperature and the natural temperature pattern of discharge can be altered and have a negative impact on fish spawning (Benjamin et al. 2020); (4) the upstream sediment is mostly deposited in the reservoir and the clear discharge has a strong scouring ability, often causing the downstream riverbed to be deeply eroded and cut down (Gierszewski et al. 2020). The coarsening of the riverbed affects the survival of insects, which are the bait for certain fish species. Therefore, we need to gain a deeper understanding of their impacts on river ecosystems and biodiversity (Pal & Sarda 2020; Siala et al. 2021).

Ecologists have identified flow regime variations as the principal drivers of key ecological processes in riverine ecosystems (Palmer & Ruhi 2019; Hatamkhani & Moridi 2023). The generally accepted approach for assessing hydrologic regimes focuses on developing comprehensive indicator frameworks to quantify their variability (Gao et al. 2009; Jumani et al. 2020). Richter et al. (1996) proposed Indictors of Hydrologic Alteration (IHA) to evaluate the impacts of anthropogenic activities on hydrologic alteration. IHA contains five categories of flow variability in terms of magnitude, frequency, duration, timing and rate of change (Li et al. 2018). The Range of Variability Approach (RVA) based on IHA employs 75 and 25% percentiles as the threshold of each indicator before disturbance for the natural flow regime. This approach can not only delineate flow regime alteration but also forge correlations between diverse hydrologic indicators and ecological attributes of habitats, including physical and chemical environments. It remains the most used technique for quantitatively evaluating hydrologic alteration and the potential ecological impacts of altered flow regimes. However, the data redundancy existing across IHA indicators with some indicators being highly intercorrelated hampered the water resources management in practice (Clausen & Biggs 2000; Cheng et al. 2019). It presents an obstacle to the effective application of ecologically centered river flow regimes, especially those pertaining to reservoirs. The integrated degree of hydrological variation (D₀) and Dundee Hydrological Regime Alteration Method (DHRAM) emerged for the assessment of hydrologic alteration (Black et al. 2005; Shiau & Wu 2007). These evaluation approaches, however, are incapable of dealing with the specific ecological flow variability. Vogel et al. (2007) suggested ecological flow indicators (eco-surplus and eco-deficit) based on the flow duration curve (FDC) to address this issue. It can demonstrate the instream flow surplus and deficit at different temporal scales (annual or seasonal scales) and shed new light on the examination of hydrologic alteration. However, research in this area remains limited despite the potential benefits.

The construction of cascade reservoirs and water transfer projects in the Han River poses a great challenge to the river ecosystem and the surrounding ecological environment (Li et al. 2020; Yan et al. 2022; Zhu et al. 2023). The previous studies mainly focused on the overall trend of runoff under the dual impacts of climate change and anthropogenic activities, showing a deficiency in the hydrologic alteration with associated ecological risks due to reservoir disturbance. The lack of deep exploration of seasonal variations also inhibits potential practical implementations. Furthermore, the Han River undergoes several key eco-environmental issues related to hydrologic alteration and the water resources system becomes more complex. Higher frequency of algal bloom outbreaks and less biodiversity urge to conduct research in this field (Xia et al. 2020; Xin et al. 2020). Given the mentioned research gaps and requirements, more comprehensive and thorough studies on how hydrologic alteration affects ecosystems need investigation.

Therefore, the main objective of this study is (1) to employ eco-surplus and eco-deficit to systematically evaluate instream ecological flow alteration and analyze driving factors at multiple time scales; (2) to utilize IHA framework to determine D₀ and DHRAM for assessing the degree of alteration and the level of risk; (3) to assess and investigate the ecological response to ecohydrological variability resulting from reservoir operation. We expect that this study will provide useful information on the evolutionary features of river ecohydrological circumstances, as well as the effects of climate change and anthropogenic activities on the basin at different time scales.

The red and blue dashed line in Figure 1 represents 25th–75th quantiles of FDC, respectively. The black solid line refers to the annual or seasonal FDC of a given year. The annual or seasonal FDC above the 75% quantile is defined as eco-surplus, suggesting that the hydrological element exceeds the demand value. The annual or seasonal FDC below the 25% quantile is defined as eco-deficit, suggesting that the hydrological element is lower than the demand in the river ecosystem.

The IHA approach is used for quantifying the degree of hydrologic alteration and their influence on river ecosystems since these indicators represent major hydrological features with ecological relevance. These indicators are classified into five groups (see Supplementary material, Table S2): the monthly mean flow, annual extreme flow magnitude and duration, annual extreme flow occurrence time, number and duration of flow pulses, rate, and frequency of flow changes. However, IHA can only calculate the alteration degree of each indicator between the natural condition and disturbed condition, the health degree of the riverine ecosystem posterior to the change is neglected. The RVA method was used to quantify the acceptable threshold of indicators in the IHA for quantitative assessment of the alteration degree. The 25 and 75% percentiles are usually defined as the lower and upper thresholds with the consideration of ecosystem restoration.

The DHRAM assumes that the structural damage causing danger to the ecosystem is proportional to the cumulative hydrologic alteration and connects IHA indicators to ecological repercussions based on risk. The degree of change in the mean and variance of each category is used to obtain an integrated score. The DHRAM assessment is divided into 5 levels corresponding to different ecological risk degrees: level 1 denotes no risk to the ecological system and level 5 denotes serious hazard to the ecosystem. The greater the value, the higher the degree of variability in the flow regime and the higher the risk of environmental harm. The specific procedures for attaining DHRAM can be referred to Black et al. (2005).

In this study, daily streamflow data series of two major hydrological stations (i.e., Huangjiagang and Huangzhuang stations) in the Han River are collected from the Hydrological Bureau of Yangtze River Water Resources Commission. The daily meteorological data series (i.e., precipitation) are obtained from the China National Meteorological Administration, with the missing data filled with the correlation analysis of the nearby meteorological stations and surface-interpolated using the Thiessen polygon method.

The period division might induce sensitivity in the eco-flow metrics calculation. Our study focused on the temporal pattern to investigate the impacts of dams on the hydrological regime and potential ecological environment, rather than the exact values. Here, we set the flow series during 1954–1973 as the baseline period since there is no operation of cascade reservoirs. In 2014, the South-North water transfer project started supplying water to north China, which brought new changes to the ecological environment in the downstream Han River. Therefore, we further attained two periods, i.e., impacted period I (1974–2014) and impacted period II (2015–2020), to consider the impact of cascade reservoir regulation and water transfer on the downstream ecological environment.

On an annual scale, the variation of eco-surplus and eco-deficit at the Huangjiagang and Huangzhuang stations is in line with that of precipitation. In general, the precipitation-streamflow relationship signified that heavier precipitation caused higher streamflow so the streamflow area above the 75% FDC could be enlarged with increasing eco-surplus. The increasing annual FDC component under the 25% FDC in dry years triggers the increase in eco-deficit. However, eco-surplus and eco-deficit were the same in a higher frequency even with less precipitation, which is a result of the low-flow components regulated by the reservoir. It should also be noted that a significant decrease in eco-surplus is observed in both stations with positive precipitation anomalies. Meanwhile, the eco-deficit became more severe when precipitation was not abundant. This situation is in close relation to the South-North Water Transfer Project implemented in the Danjiangkou Reservoir. Large quantities of water resources supplied to the north cause the decreasing flow downstream. The relevant studies also demonstrate that the operation of the South-North Water Transfer Project could leave an impact on the security of water resources and ecology conservation. In comparison with annual eco-flow indicators, the variation of seasonal ones was subjected to larger variability and less stability. A similarity could be identified between eco-flow indicators and precipitation anomalies in the baseline period. However, it was not apparent in the impacted period under the influence of human intervention. The spring eco-surplus resembled the precipitation anomalies in both stations. The difference between the spring eco-deficit and precipitation became more evident in the impacted periods. For summer, the eco-surplus showed an obvious declining trend. In the impacted periods, the eco-surplus of the two stations nearly kept at 0 whatever precipitation changed. Meanwhile, greater eco-deficit could be identified for both stations. The obvious eco-deficit could be observed even in a wet year, which exhibited the high-flow component regulation under the significant impact of the large reservoirs represented by the Ankang and Danjiangkou reservoirs. The lower variation of autumn eco-surplus was identified and the eco-deficit tended to be 0 in the second phase, which was triggered by the reduced high-flow component and the elevated low-flow component. In winter, the eco-deficit at the Huangjiagang and Huangzhuang stations approached 0 in the impacted period, which indicated that the area below 25% FDC curve decreased under the reservoir regulation in dry seasons.

Figure 6 also shows that the duration of high pulses tended to be increasing while the numbers exhibited comparatively moderate changes. Meanwhile, an increase in the count of low pulses could be detected while the duration showed a decreasing trend. The decreasing precipitation and water transfer combined for the increase in the count of low pulses. The mean rise rate and fall rate decreased by 73.01 and 57.94% in the impacted period II at Huangjiagang station, respectively. The frequency as well as peak-load control of power grids also contributed to the increasing numbers of hydrologic reversals.

Equation (3) and DHRAM are employed for the integrated hydrologic alteration evaluation. Based on the 32 indicators in five groups of IHA, DHRAM can provide the quantitative evaluation of the alteration degrees. The assessment results are listed in Table 2. From the perspectives of D₀, both stations in both periods experienced a changing degree of over 48%. The Huangzhuang station exhibits the largest alteration degree, reaching 68.97% in the impact period II, and the alteration degree is 65.35% at the Huangjiagang station. Huangzhuang station faced a high level of alteration in impacted period II. In general, the higher hydrologic alteration degree can be found in the impacted period II relative to that in the impacted period I, which manifests that the implementation of water diversion projects and cascade hydropower engineering took a toll on ecohydrological regimes. The greatly altered regimes could exert adverse effects on the fluvial eco-environment and should arouse extensive attention. DHRAM shows that the total points at the Huangjiagang station are 9 and 10 in two periods, while those of the Huangzhuang station are 10 and 13, respectively. The hydrologic alteration degrees at the Huangjiagang station were both level 3, in two impacted periods, while those at the Huangzhuang station were level 3 and level 4 respectively. The results manifest that both stations are facing moderate ecological risk in impact period I, while the Huangzhuang station is under a high ecological risk degree in the impact period II. The DHRAM evaluation method illustrates similar evaluation results when compared to those by the D₀ evaluations, i.e., the higher hydrological alteration degree can be found in the impact period II.

It can be seen from Figure 7 that the total seasonal eco-surplus increased from 1954 to 1980, followed by a steady decline trend. However, the total seasonal eco-deficit showed a relatively increased trend in these years. The gaps between the total seasonal eco-surplus and eco-deficit became more and more narrow. Particularly, the total seasonal eco-deficit surpassed eco-surplus in recent years.

To investigate the role of influencing ecological flow in the Han River, the correlations between seasonal ecological indicators and seasonal precipitation are analyzed in Table 3.

In the baseline period and the impacted period, the seasonal eco-surplus always showed a positive correlation with the seasonal precipitation (with the largest correlation of 0.82 at the Huangjiagang station in the baseline period) and the negative correlation could be identified between eco-deficit and precipitation (with the largest correlation of −0.78 at the Huangzhuang station in the baseline period). Comparatively, the correlations exhibited a distinct decline in the impacted period, indicating that the direct impact of precipitation had been weakened after the construction of dams.

The intensifying anthropogenic activities have caused great hydrologic alteration and exercised inevitable influence on the riverine ecosystem. This study employed eco-flow indicators (eco-deficit and eco-surplus), SI, IHA, D0 and DHRAM to evaluate ecological flow in multiple time scales while considering ecological responses to evolutionary characteristics of flow regime. The findings can offer valuable implications for water resources management in the Han River. They can facilitate the strictest water resources management in this basin and promote the safety of the South-North water transfer project.

This study found that the fluvial biodiversity closely related to the flow regime showed a declining trend due to reservoir disturbance. For example, the IHA indicators in Group 4 have experienced a high alteration; they can change the frequency and magnitude of soil moisture stress for plants, the availability of floodplain habitats for aquatic organisms and organic matter exchanges between river and floodplain (Cui et al. 2020). Another example is that the monthly flows are related to the habitat availability for aquatic organisms. They can influence water temperature, oxygen levels, and photosynthesis in the water column (Luo et al. 2023). The monthly flows are beyond the thresholds after reservoir operation and the significant change imposed a negative impact on the migration and spawning of fish. The issue of eutrophication in the Han River has attracted widespread attention. Reservoir operation shifted the pattern of the discharge and water velocity, which are reported as the key factors to restrict water bloom in the Han River (Xiao et al. 2024). They all urge the governors to take response measures to prevent deterioration of the ecological environment. The hydrologic alteration should be considered in reservoir operation. Some studies have integrated IHA parameters into the objectives in optimal reservoirs to make the released flow maintain a more natural flow regime (Li et al. 2018; Yan et al. 2021). The governors can set strict water resources management rules such as reducing the hydrologic alteration or optimizing reservoir operation to control key factors to control key factors influencing environmental flow. In recent years, Danjiangkou Reservoir carried out emergency ecological scheduling to control the overgrowth of Elodea Nuttallii or to suppress algal blooms (Xin et al. 2020). Although they can play a role in preventing or limiting the impacts of emergencies, longstanding problems such as the shifting pattern of month flow or extreme water conditions still need further investigation. Governors should balance the hydropower and water supply benefits, as well as the ecological environmental conservation.

On the other hand, this study has some limitations and needs more improvements in future research. First, the lack of comprehensive data pertaining to the long-term variations in the quantity of biological species and the composition of the community in the Han River precludes the direct computation of SI. Hence, conducting a preliminary assessment of the potential effects on river biodiversity after reservoir construction through Equation (4) is acceptable. Further investigations may profitably focus on cross-validating biodiversity indices with actual biological communities and species data. Next, the sensitivity induced by the period division or the length of the record can also be explored in the further step. In this study, we adopted the maximum length of accessible data and mainly divided the study period into two pieces according to the operation of the Danjiangkou reservoir and the south-north water transfer project. However, the flow regime in the natural period was used as the reference and IHA values change with the length of the data records in the impacted period. The application of IHA also requires at least 15 years for different periods according to previous studies (Kennard et al. 2010; Fantin-Cruz et al. 2015). It also influences the correlation analysis between seasonal ecological indicators and seasonal precipitation. Therefore, the sensitivity induced by data length and division can also be explored in the further step. Moreover, quantifying hydrologic alteration and potential ecological responses under future climate warming and complex human activities is also a challenge for protecting the ecological environment. This exploration can integrate climate models, hydrological simulation, and the influence of reservoir operation. The findings will offer valuable guidance for initiatives aimed at ecological preservation and water resource management.

Han River, one of the research hotspots in China, is subject to climate change and human activities with altered hydrologic regimes. To explore the ecological response under the influence of hydrologic alteration, daily streamflow data of Huangjiagang and Huangzhuang stations and daily precipitation data covering 1954–2020 were collected and eco-surplus and eco-deficit at multiple scales, multiple hydrological indicators, and the fluvial biodiversity index were examined. The main conclusions are as follows:

• (1)

  Ecological flow was mainly controlled by precipitation in the baseline period and dams significantly altered instream ecological flow in the Han River basin. The annual magnitude and frequency of high (low) flow are decreased (increased) under the impacts of reservoir regulation. The seasonal effect of variation of eco-flow indicators subject to FDC is different. The decline of high flow contributed to the increase in eco-deficit in summer. Spring exhibited a decrease in eco-surplus and eco-deficit due to high flow and low flow, respectively.

• (2)

  Flow regimes in the Han River have been altered greatly. An integrated hydrologic alteration was over 48% degrees in both impacted periods and was under moderate ecological risk in the impact period I (ecological hazard class at level 3), while the Huangzhuang station faced high ecological risk with 68.97% degree in the impact period II (ecological hazard class at level 4).

• (3)

  The variation of the SI is closely related to instream ecological flow which is influenced by precipitation and dams. The SI showed a decreasing trend in recent decades although eco-surplus exceeded eco-deficit with a narrow gap. Dams and water transfer projects have extremely changed seasonal contributions of eco-surplus and eco-deficit that caused the decline of fluvial biodiversity.

• (4)

  Good correlation relationships existed between the eco-flow indicators (eco-surplus and eco-deficit) and IHA indicators, implying that they can capture the critical ecological implications identified by IHA indicators, making them a valuable tool for evaluating ecohydrological systems. Overall, the use of eco-surplus and eco-deficit is a practical and efficient approach for assessing river systems.

The findings can enhance our comprehension of the ecological responses prompted by hydrologic alterations. Particularly, it reveals that flow is presently subjected to the influences of reservoir construction and operation while it is primarily influenced by precipitation during natural periods. The insights may provide references for water resources management and ecological security maintenance. However, some limitations in this study are still worth emphasizing. Future research may profitably focus on cross-validating biodiversity indices with proper biological data. The sensitivity induced by the period division or the length of the record can also be explored in the further step. Quantifying hydrologic alteration and potential ecological responses under future climate warming and complex human activities is also a challenge for protecting the ecological environment. This exploration can integrate climate models, hydrological simulation, and the influence of reservoir operation. The findings will offer valuable guidance for initiatives aimed at ecological preservation and water resource management.

This study is financially supported by the National Natural Science Foundation of China (No. U20A20317).

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

The authors declare there is no conflict.