Evaluation of hydrological regime alteration and ecological effects in the middle and lower of the Yangtze River, China Open Access

River flow regimes are considered primary drivers of riverine ecosystems and have become a fundamental part of ecological informatics for riverine ecosystems (Arthington et al. 2010). Alterations of natural streamflow regimes modify the distribution and availability of riverine habitat conditions, with potentially adverse consequences for native biota. A natural flow regime is significant in sustaining river environments and aquatic ecosystems and has been widely adopted as a paradigm for conserving and restoring ecological integrity (Gao et al. 2012, 2021). Climate change may increase global atmospheric temperatures and accelerate the water cycle, while human activities alter water resources’ spatial and temporal distribution (Nie et al. 2021). Human activities such as large reservoir construction will change the natural flow pattern of rivers and bring adverse effects on ecology. Large reservoirs disrupt the continuity of the river and inevitably lead to changes in the streamflow regime, sediment regime, and water temperature regime of the river (Gierszewski et al. 2019).

Numerous studies have developed and applied multivariable approaches to quantify the relations between hydrologic alteration and ecological diversity to evaluate the ecological effect of reservoir operations (Wang et al. 2019; Yang et al. 2021). Many indicators have been developed to describe the characteristics of river flow regime changes (Olden & Poff 2003; Poff & Zimmerman 2010). To quantitatively evaluate how human activities impact the hydrologic regimes within an ecosystem, Richter et al. (1996) proposed 33 indicators of hydrologic alteration (IHA) to show intra- and inter-annual variations of flow regimes. In addition, Richter et al. (1997) applied IHA and proposed the range of variability approach (RVA) to set streamflow-based river management targets. These methods have been widely used in analyzing hydrologic variability and aquatic ecosystem integrity. Pfeiffer & Ionita (2017) found high alterations in some hydrological indices and significant changes in river hydrological conditions in Germany's Elbe and Rhine rivers due to climate change factors. Eum et al. (2017) evaluated 12 hydrological indicators representing the intensity and timing of hydrological conditions. The previous study concluded that the increase in precipitation due to climate warming had caused significant alterations in hydrological conditions in the upper Athabasca River Basin. Tian et al. (2019) analyzed the hydrological change of the Wuding River and its potential driving factors, indicating that the hydrological change of the river is mainly affected by land-use change and reservoir construction, and human activities have a greater impact on the change in the hydrological situation. A study of the Yellow River has shown that the construction of large reservoirs has highly altered the hydrological regime in the river's middle reaches (Zhao et al. 2014).

The Yangtze River is the largest river in China and the third-largest globally, with abundant water and rich species resources. It plays a vital role in environmental protection and China's economic development (Jiang et al. 2020). With increasing climate change and human disturbance in watersheds, natural river systems worldwide have been altered to serve various human purposes, including water supply to cities and rural areas, power generation, navigation, and flood control. The construction of large water structure projects will regulate river runoff, changing its runoff and its original seasonal and intra-annual distribution (Han et al. 2017). Furthermore, studies have shown that dam construction is the leading cause of the extinction, threat, or endangerment of nearly one-fifth of the world's 9,000 identifiable freshwater fish species in the last hundred years. (Zhang et al. 2019a, 2019b) The Three Gorges Water Conservancy Project, in particular, has contributed to economic development while also changing the natural hydrological situation of the river (Chen & Wu 2011; Jiao et al. 2019). The impoundment of the Three Gorges Dam (TGD) in 2003 directly affected the hydrological situation of the Yangtze River and the breeding and proliferation of migratory aquatic resources in the river, indirectly affecting the hydrological changes in Dongting Lake and Poyang Lake in the middle and lower reaches of the Yangtze River. To a certain extent, the living environment of biological resources in the ecosystem of the Yangtze River Basin has been changed. The early resources and yields of some local fish (such as FFMC and Chinese sturgeon) have been significantly reduced, which has a long-term ecological effect on the biological resources of the Yangtze River Basin, resulting in a sharp decline in fish resources and a decrease in biodiversity in the Yangtze River system (Li et al. 2021). As shown, assessing ecological hydrology in the middle and lower reaches of the Yangtze River plays a vital role in maintaining ecosystem health and aquatic biodiversity in the Yangtze River Basin.

Therefore, we quantitatively assessed the runoff changes in the middle and lower reaches of the Yangtze River using ecological indicators to reflect the ecological response process of the TGD construction to changes in the ecological and hydrological conditions of the middle and lower reaches of the Yangtze River. Taking 2003 as the turning point, the contribution rate of anthropogenic disturbance and climate change runoff was clarified using the slope change ratio of the accumulative quantity. Moreover, we analyzed the highly correlated indicators with changes in fish resources. The results of this study can provide an essential basis for river governance and ecological restoration.

The Yangtze River is the longest river on the Eurasian continent, traversing a distance of 6,300 km. It originates in the Tanggula Mountains of the Tibetan Plateau, flows eastward through 11 Chinese provinces, and finally reaches the East China Sea. It discharge tends to increase as it proceeds downstream due to the contribution of numerous tributaries and because precipitation increases from the upper drainage basin towards the lower Yangtze. The TGD is located on the upper Yangtze River, and its drainage area is approximately 1 million km². The middle and lower Yangtze River, located downstream of the dam, has a drainage area of 0.8 million km². The Yichang gauge, 38 km downstream of the TGD, controls the river discharge of the upper Yangtze. The Hankou and Datong gauges control the middle and lower reaches discharge, respectively. The middle Yangtze River Basin (from Yichang to Hukou) extends 955 km, accounts for 38% of the total drainage area, and provides 30% of annual runoff. The lower Yangtze River Basin (from Hukou to Datong) extends 338 km, covers 7% of the total drainage area, and generates 20% of the basin's total discharge. The Yichang, Hankou, and Datong stations control the discharge of the upper, middle, and lower reaches, respectively. The distribution of hydrological stations is shown in Figure 1.

In this study, Yichang Station, Hankou Station, and Datong Station, the main streams of the Yangtze River, were selected as flow control stations. The daily flow data from 1960 to 2020 were observed from the ‘Yangtze River Basin Hydrological Yearbook.’ The daily meteorological data of 119 basic stations in the middle and lower reaches of the Yangtze River were selected from the China Meteorological Data Network (http://data.cma.cn/).

The null hypothesis, that there is no trend, is accepted at a significance level of 0.1 if the standardized statistic z is less than 1.64. A positive z indicates an increasing trend, while a negative one represents a decreasing trend.

The cumulative departure curve was used to detect the preliminary turning points. Then, the M–K method and the moving t-test were employed to verify the turning points. The M–K method can test the trend and mutation of long time series. These tests have been inspected as useful techniques to identify the inflection points of streamflow data in previous studies. More details of the calculation procedures used in the three techniques can be obtained from Xue et al. (2021), Du et al. (2019), and Zhang et al. (2016).

The IHA is employed to evaluate the hydrologic alteration between pre- and post-dam periods (Zhou et al. 2020). In this study, zero-flow days were not observed at the three stations during the period; thus, the parameter ‘number of zero-flow day’ was not included. In addition, 32 parameters were categorized into five groups, i.e., the magnitude, timing, frequency, duration, and rate of change (Table 1).

The RVA method is used to quantitatively evaluate the degree of hydrologic alteration by analyzing daily streamflow records in the pre- and post-dam periods. The RVA targets are based on the range of pre-impact data distributions, which have been used as a reference to define the extent of natural flow regime alteration. If most of the IHA values in the post-impact period are within the range of RVA targets, the river hydrologic regime is minimally affected by the reservoirs. Otherwise, the hydrologic regime can be seriously altered, and the ecological system can be severely damaged (Swades & Rajesh 2021).

The cumulative slope of change analysis method principle is that if runoff changes are influenced only by the precipitation factor, the slope of the cumulative curve of precipitation and runoff should change in the same multiplicative ratio over the year (Wang et al. 2019). The sum of all influences on the variable is defined as 1. By calculating the ratio of the cumulative slope change rate of each influencing factor, the influence degree of the influencing factor on the hydrological variable is determined. The slope of the linear relationship between year and cumulative runoff is assumed to be and (in 10⁸ m³/a), and the slope of the linear relationship between year and cumulative precipitation is assumed to be and (in mm/a), before and after a certain inflection point of cumulative runoff change (Wang et al. 2015).

Figure 2 shows the annual mean streamflow variation at the Yichang, Hankou, and Datong hydrological stations from 1960 to 2020. Yichang Station, Hankou Station, and Datong Station have the same fluctuations in inter-annual flow changes, with maximum values in 1954 and 1988, and minimum values in 2006 and 2016. Years with extreme streamflow fluctuated greatly, and overall decreased trend. The M–K averages of annual streamflow at the three stations are −1.7, −0.5, and −0.45, respectively as shown in Table 2. This shows that the Yichang Station streamflow downward trend is the most obvious, Hankou Station and Datong Station streamflow change is not obvious. The storage of water for power generation at the Three Gorges in 2003 was one of the major causes of the significant drop in streamflow at Yichang Station. Upstream inflows account for 47.98% of the annual streamflow from the Datong station, suggesting that over 50% of the incoming water comes from intra-territorial replenishment. The lower reaches of the Yangtze rely on riverine supplementation for streamflow, with annual streamflow increasing from top to bottom along the river at each hydrological station. The annual runoff of the Yangtze River has no obvious change trend.

The M–K test method revealed that the annual average flow rate of Yichang Station and Hankou Station both changed significantly in 2003, and Hankou Station changed abruptly in 2004. In order to verify the mutation point, as shown in Table 3, a sliding t-test and cumulative distance level method was used to test the annual average discharge of three hydrological stations in the middle and lower reaches of the Yangtze River. Combining the impact of human activities and climate change, China's largest water conservancy project, the TGD water conservancy project, began to store water in 2003, and TGD's savings and regulation energy exceeded 22 billion cubic meters. This has brought unprecedented negative impacts to the middle and lower reaches of the Yangtze River, such as the degradation of wetlands and estuary deltas and the loss of biodiversity. To sum up, this study selects 2003 as the year of sudden change in runoff changes in the middle and lower reaches of the Yangtze River based on the results of the three inspection methods and the actual situation of large-scale water conservancy project construction in the basin.

The research quantitatively analyzes the degree of change of 32 hydrological indicators in five groups and discusses the impact of reservoir construction on rivers by changing 32 hydrological indicators. The daily runoff data were divided into Pre-alteration (1960–2002) and Post-alteration (2003–2020). Table 4 shows the size of the 32 IHA in Yichang Station, Hankou Station and Datong Station in the period and period, as well as the change degree of 32 IHA.

The first group of IHA is the monthly average streamflow of each hydrological station. Figures 3 and 4 summarize the magnitude of median monthly water conditions before and after the TGD impoundment, as obtained by IHA. The magnitude of the monthly median flow is more than the pre-impact values from December to March, particularly in March. However, from July to September, the monthly median flow decreased after 2003, particularly in July at Hankou and Datong stations. Yichang had the most significant decrease in October. The Yichang Station experienced moderate hydrological alteration of 53%. Hankou underwent a large change with a hydrological change of 69%. Datong had a low alteration with a hydrological alteration of 23%. These changes were directly related to the operation mode of the TGD, which affects the middle and lower reaches of the Yangtze River. According to the rules regulating the TGD, the increase in runoff was caused by water release in dry seasons, and the decrease in runoff was caused by the impoundment of the TGD in wet seasons.

The second set of IHA is the magnitude of the annual extreme streamflow at each hydrological station. Figures 5 and 6 show the changes in the annual extremum flow of each hydrological station. It can be seen from the figure that the average annual minimum flow and maximum flow have been reduced to varying degrees. In contrast, the medians of the 1-, 3-, 7-, 30-, and 90-day maximum flows decreased (Table 4). The annual minimum flow varies greatly, while the annual maximum flow varies less. These results indicate that Yichang and Hankou were influenced more than Datong. This finding mainly resulted from the fact that Datong is far from the TGD and the strengthened forcing of Poyang Lake, Dongting Lake, and other tributaries on the river, which made the flow variations flatter and had buffering effects on the river flow variations. For example, the annual 1- and 7-day minimum flows at Yichang and Hankou were more significant than those during the pre-impact period and fell less frequently within the RVA target range, with high alterations. The high alterations of the extreme flows after the TGD impoundment directly influenced the stability of the river ecosystem, the river landscape, and the construction of natural habitats. In addition, the maximum annual flows decreased, which affected the nutrient transport between the channel and the detention area and damaged the distribution of plant communities.

The third group of IHA is the occurrence time of extreme annual streamflow at each hydrological station. Figure 7(a)–7(c) shows the time changes of the annual minimum value at Yichang, Hankou and Datong stations. The Julian dates of each annual 1-day minimum post-impact flow at the Yichang, Hankou, and Datong stations were earlier than those of the pre-impact period. In contrast, the Julian dates of 1-day maximum post-impact flows were later than those of the pre-impact period. Most of these alterations were moderate and low (Table 4). The difference values of the 1-day minimum flow between the two periods were relatively high, and the different values of the 1-day maximum flow between the two periods were small. Therefore, the median Julian date of the 1-day minimum has been strongly affected since the TGD impoundment. These two parameters may be tied to reproduction or mortality events for various species, thereby influencing population dynamics.

The fourth group of IHA is the frequency and duration of high and low streamflow at each hydrological station. Figure 7(d)–7(f) shows the annual low pulse durations at Yichang, Hankou and Datong stations. The frequency and duration of high and low pulses decreased, and the low pulse count has been significant changed with an alteration of 90%. The low pulse count, high pulse count, and high pulse duration were slightly altered at Hankou and Datong with the low or moderate alteration. In contrast, the low pulse duration decreased significantly from nearly 55 days per event to less than 20 days per event at Hankou. The low pulse duration decreased significantly from nearly 58 days per event to 28 days per event at Datong. The median values of the duration of low pulses decreased in the post-impact period; almost all of them were outside the RVA target range with alterations of 68%. In summary, the construction of the TGD reduced the low pulse count and duration, which could prevent drought. However, the diversity of aquatic organisms in the floodplain will gradually decrease because the nutrients and organic materials exchanged between the river and floodplain are closely tied to the duration of high and low pulses (Shields & Knight 2013).

The fifth group of IHA is the rate and frequency of changes in each hydrological station. Figure 7(g)–7(i) shows the annual number of reversals at Yichang, Hankou and Datong stations. Both the rate of increase and number of reversals were strongly affected at Yichang, particularly the number of reversals for which no values were within the RVA target and alteration reached 100%. The alteration of the parameters in this group at Hankou and Datong was generally low, and all of them were categorized as low. The rate and frequency of water condition changes at Yichang were the most significant. It is shown that the TGD had the greatest influence on the discharge at Yichang during peak and frequency modulation. However, flow reversal is closely related to stress, significantly impacting low-mobility stream-edge organisms. Even small changes can destroy the natural hydrological cycle and disrupt the living environment of aquatic organisms.

The 32 hydrologic alteration values at the Yichang, Hankou, and Datong stations on the Yangtze River (Figure 8) were analyzed to investigate the order of IHA caused by the TGD using a non-parametric statistical method. Yichang Station has an average flow in March and April, annual minimum flow, and the number of reversals reached eight hydrological indicators of 100% change; Average monthly flow, minimum 1-day flow, minimum 3-day flow, minimum 7-day flow, and baseflow index at Hankou Station were 100% changed; Datong Station Only Base Flow Index Change 100%. Yichang and Hankou had the highest proportion of IHA index height change, reaching 44 and 37%, respectively, while Datong had only 16% of IHA index height change. The proportion of moderate changes in Hankou was 18%, Yichang 22%, and Datong 25%. The proportion of low change in Datong reached 59%, followed by Hankou at 44%, and Yichang at 34%.

The results indicated that the hydrologic regimes at Yichang and Hankou were dominated by high alteration and that Datong was dominated by low alteration. In particular, the degree of IHA at Yichang (the nearest to the TGD) and Hankou, where the Hanjiang River and Dongting Lake flow into the Yangtze River, was approximately twice as large as that at Datong. This difference may be related to the particular setting of the surface water system in the middle-lower Yangtze River, where the river interacts directly with the two largest freshwater lakes (i.e., Dongting and Poyang Lakes) play buffering roles to varying degrees for the Yangtze River.

Table 5 lists the integrated hydrological alteration indicators. It can be seen that all groups of IHA at the Yichang Station were subject to large degrees of alteration, reaching moderate and high levels. Most groups of IHA were subject to large degrees of alteration at the Hankou Station (except the fourth group), and the first and second groups reached the high alteration level. Only the third group of IHA reached high alteration level at Datong. In contrast, the second and fourth groups of IHA reached moderate alteration, and the rest of the groups of IHA reached low alteration. In general, the TGD significantly influenced Yichang and Hankou, but the impact on Datong was not significant.

The hydrological conditions of most rivers have been altered by climate change and human activities, causing some impact on the quality of river habitats and stressing essential species in the basin. The slope change ratio of cumulative quantity method is used to explore the causes of runoff change and quantify the impact of climate change and human activity on runoff (Figure 9).

The contribution of human activities to runoff at Yichang, Hankou, and Datong stations was 73.69, 67.98, and 56.84%. The contribution of climate change to runoff was 26.31, 32.02, and 43.16% (Table 6). The contribution of human activities to runoff in the basin gradually decreased from upstream to downstream, and changes in runoff in the middle and lower reaches of the Yangtze River basin are less affected by climate change than human activities. Runoff from the mainstream channel downstream of the TGD comes mainly from the inflow of flow from the upper Yangtze River Basin, precipitation from downstream, tributaries such as the Han River along its course, and the confluence of flow from Dongting Lake and Poyang Lake. The adjustment and storage function of the hydrological regime has alleviated the impact of the Three Gorges water storage to a certain extent. The closer the lower reaches of the Yangtze are to the mouth of the river, the lower the impact of human activity factors on runoff changes and the more significant the impact of climate change factors on runoff changes.

Four famous major carp (FFMC) species, black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idellus), silver carp (Hypophthalmichthys molitrix), and bighead carp (Aristichthys nobilis), the migratory fish on the Yangtze River, are also the main commercial fish in the Yangtze River Basin, playing an essential role in China's freshwater fishery economy. Figure 10 shows the fry runoff of the FFMC species in the middle and lower reaches of the Yangtze River. Dams often result in hydrologic ‘fragmentation’ or disconnection of the fluvial system, decoupling the affected river reach and its biotic systems from its floodplain. Natural flow plays a profound role in the lives of fish, with critical life events linked to the hydrological regime (e.g., spawning behavior, eggs hatching). Flow alteration changes the magnitude and frequency of high and low flow, often reducing the variability necessary to fish spawning behavior. From Table 4, it can be seen that reservoir regulation resulted in the reduction of flow variability and the alteration of the timing of increasing flows. The breeding season of the FFMC in the Yangtze River is from May to July every year. The triggering stimulus for spawning is an increase in discharge. Spawning occurs when the flow increases, but it stops immediately as soon as the flow starts to recede.

The 32 IHA calculated from the daily flow data of Yichang Station from 1997 to 2019 were selected as environmental impact factors, and the fry runoff of FFMC species (FFMC) in the Jianli section was selected as ecological factors. According to the runoff data of FFMC species fry runoff downstream of the Three Gorges Reservoir from 1997 to 2019, the hydrological indexes of each year are obtained based on the daily flow of the Yichang station 1997–2019. The grey correlation analysis method is used to obtain the correlation between the hydrological indexes and the runoff of fish larvae. The results showed that the average flow rate in June, July, August, October, November, and December, the average flow rate on the 1st, 3rd, 7th, 30th, and 19th days, and the average flood rate had a great influence on the runoff of fish seedlings, and the correlation degree was above 0.9. Among them, the change in average flow in October has the greatest impact on the change in fish sap runoff (Table 7).

This research quantitatively analyzes the changes in eco-hydrology in the middle and lower reaches of the Yangtze River from the inter-annual flow variation and the degree of hydrological variation. The study found that the runoff changes at Yichang, Hankou, and Datong stations in the middle and lower reaches of the Yangtze River were 65, 58, and 46%, respectively. These findings are supported by Peng et al. (2020). Reservoir operations typically reduce peak flows in rivers and reduce downstream flows due to impoundment. Therefore, the change of eco-hydrological indicators in the middle and lower reaches of the Yangtze River has brought adverse effects on the regional ecological restoration (Jiang et al. 2014; Xu et al. 2018).

The drivers of changes in hydrological regimes in the middle and lower reaches of the Yangtze River can be attributed to human activities and climate change. The impact of human activities on hydrological elements is mainly due to the construction of water conservancy projects and changes in land-use (Moniruzzaman et al. 2020; Yang et al. 2022). Since the 1960s, more than 40,000 water conservancy projects have been built on the main and tributaries of the Yangtze River, affecting the continuity of the river and the natural hydrological process. After the operation of the Three Gorges Water Conservancy Project, the percentage of runoff in the middle and lower reaches of the Yangtze River during the flood season in the total annual runoff decreased from 72 to 66%. The percentage of runoff in the dry season in the total annual runoff increased from 11 to 15% (Zhang et al. 2016, 2017). Climate change mainly affects the process of hydrological elements through changes in factors such as temperature, precipitation, and evaporation (Yuan et al. 2016).

Our study confirmed that the contribution rate of climate change to the runoff change of Yichang Station, Hankou Station, and Datong Station in the middle and lower reaches of the Yangtze River was 26.31, 32.02, and 43.16%, respectively. These findings are similar to the conclusions of Cheng et al. (2019) that the precipitation and evapotranspiration intensity in the middle and lower reaches of the Yangtze River has not changed much in the past few decades. This indicates that the contribution rate of climate change to changes in hydrological processes is limited, and runoff changes are more sensitive to human activities. In recent years, a series of soil and water conservation measures have been implemented in the Yangtze River Basin, resulting in significant changes in land use. From 1980 to 2005, the grassland area increased by 14,000 km², the forest land decreased by 25,000 km², and the construction use increased by 20,000 km². On this basis, managers can use the sensitivity of runoff to human activities to make appropriate artificial adjustments. Adjustment is needed to the rising duration and falling speed of flood peaks in time to reduce the impact on ecological hydrology (Chen et al. 2020; Li et al. 2020).

Appropriate water flow conditions play a vital role in maintaining the healthy development of aquatic ecosystems. As an essential part of aquatic ecosystems, fish resources are significantly affected by water flow conditions. The construction of water conservancy projects has led to a decrease in the connectivity of rivers and lakes, changes in natural hydrological laws, and changes in water temperature and water quality conditions (Wang et al. 2010). FFMC is the main economic fish in China, and it is a typical semi-migratory fish in rivers and lakes. However, the construction of the Gezhou Dam and the TGD has gravely affected the breeding activities of these migratory fishes and affected the middle and lower reaches of the Yangtze River directly.

The study analyzed the correlation of the effects of changes in eco-hydrological indicators in the middle and lower reaches of the Yangtze River on FFMC. Environmental factors affecting the total amount of economic fish caught in the lower reaches of the dam are mainly hydrological indicators related to high flow, especially the annual maximum flow value of short duration (Li et al. 2013; Xu et al. 2015). After the impoundment of the Three Gorges Reservoir, these high flow values were reduced to varying degrees, and the hydrological changes of the corresponding indicators were mainly medium and low. The decrease in these high flow values is one of the main reasons for the significant decrease in the total catch of economic fish in the lower reaches of the dam (Yang et al. 2020). Therefore, in order to better protect the habitat environment of economic fish and improve the number of fish resources, during the operation of the Three Gorges Reservoir, on the basis of ensuring the safety of flood control, free discharge of large flow before the main flood season or artificial flood peak in spring should be carried out to appropriately increase the high flow value.

This study provides a quantitative assessment of the eco-hydrological processes in the middle and lower reaches of the Yangtze River and an analysis of the impact of the processes on fish stocks in the middle and lower reaches of the Yangtze. From 1960 to 2020, the average annual flow at the Yichang, Hankou, and Datong hydrographic stations showed a decreasing trend. Because of the construction of the TGD Water Conservancy Hub, the runoff in the middle and lower reaches of the Yangtze River changed abruptly in 2003. We found that the eco-hydrological regime in the middle and lower reaches of the Yangtze River was highly changed at Yichang Station and moderately changed at Hankou Station and Datong Station. Of the eco-hydrological indicators, the rate of change and extreme flows are more influenced by the TGD. The study found that changes in runoff had some negative impacts on river ecosystems, particularly on the species and abundance of fish stocks. In contrast, the timing of maximum flows and high/low pulses (except for Yichang) were less influential. The study also quantitatively distinguished the contribution of anthropogenic and climatic factors (precipitation and evapotranspiration) to the impact of runoff changes in the middle and lower reaches of the Yangtze River and found that anthropogenic factors dominated.

The change in runoff from the construction of water projects is a complex issue, and the study of the extent of change in hydrological conditions and its drivers is a long-term topic. Our study can be used to assess eco-hydrological changes in other watersheds. We used a wide range of ecological flow indicators to assess their applicability. There findings may provide managers with practical insights into the ecological restoration planning framework in the lower and middle reaches of the Yangtze River.

This work was supported by the National Nature Science Foundation of China (Grant No. 51779094); Water Conservancy Science and Technology Project of Guizhou Province (KT202008); The Wisdom Introduction Project of Henan Province (GH2019032).

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

The authors declare there is no conflict.