Evaluation of instream ecological flows based on hydrological alteration in the Upper Huai River, China

Climate change and human activities serve as the primary drivers for alterations in the flow regime (Frederick & Major 2017; Cavalcante et al. 2019). The full extent of the impact of human activities on the flow regime and ecology remains uncovered, particularly with the escalating industrial, agricultural, and domestic water use, as well as reservoir construction (Wang et al. 2020; Guan et al. 2023). During the natural period, characterized by minimal or no human activity, the flow regime was predominantly influenced by climatic factors like precipitation and evaporation (Patterson et al. 2013; de Freitas 2020). Nevertheless, heightened human activities, including reservoir storage and increased water use for economic and social development, have led to alterations in the river flow regime (Lin et al. 2017; Maskey et al. 2022; Yang et al. 2022). Despite numerous studies identifying the effects of climate change and human activities on river flows, there has been limited discussion regarding their impact on river ecosystems (Gao et al. 2010; Wei et al. 2013; Cheng et al. 2019; Yang et al. 2022). These studies have predominantly concentrated on annual-scale changes in precipitation forecasts, and their practical implementation in the basin remains unknown (Yang et al. 2022).

Indicator analysis approaches are commonly utilized to assess the flow regime in rivers (Poff & Zimmerman 2010). Richter et al. (1996) introduced the indicators of hydrological alteration (IHA), which is based on the consideration of the normal functioning of river ecosystems and the dynamics of streamflow throughout a year. The range of variability approach (RVA), derived from the IHA, compares the flow regime under natural conditions with that after hydrological alteration and gives a threshold value for each indicator (Richter et al. 1997). Vogel et al. (2007) introduced ecological surplus and ecological deficit indicators to evaluate instream ecological flow in a river, based on the flow duration curve (FDC). Subsequently, ecological surplus and ecological deficit were refined to use the 25th and 75th percentiles of the FDC as thresholds, meeting the practical needs of river management. In addition, the overall degree of alteration (⁠⁠) and the Dundee Hydrological Regime Alteration Method (DHRAM) have been developed to assess integrated hydrological alterations (Black et al. 2005; Shiau & Wu 2007). However, relying on a single ecological flow indicator assessment method may overlook specific hydrological features (Zhang et al. 2015). Despite numerous studies assessing instream ecological flow and hydrological alteration in rivers, the corresponding ecological effects have not been thoroughly examined and clearly elucidated (Zhang et al. 2018; Li et al. 2020). Therefore, an approach of evaluating flow regime from the perspective of protecting the river ecosystems is essential.

To quantitatively assess instream ecological flow and its impact on river ecosystems, we employed a multi-indicator assessment framework in this study. The study area selected for this research is the Upper Huai River, characterized by multiple large reservoirs and a dense population. The study aims to (1) analyze instream ecological flow surplus and deficit, (2) assess the degree of hydrological alteration in rivers using IHA and DHRAM, and (3) evaluate the impact of flow alteration on river ecosystems, as indicated by river biodiversity.

The Upper Huai River, situated in eastern China, refers to the region upstream of the Wangjiaba station and encompasses a basin area of 30,075 km² (Figure 1). The western mountainous regions of the basin exhibit higher elevations, generally exceeding 1,000 m above sea level, whereas the central and eastern areas comprise extensive plains with a more gradual topography. The basin experiences an average annual precipitation exceeding 800 mm, with a rainfall distribution favoring the northern and mountainous regions over the southern and plain areas, and a disparity between seasons, with less precipitation in spring and winter and more in summer and autumn. The construction of numerous large reservoirs in the basin for flood storage, irrigation, and power generation has induced alterations in streamflow dynamics to some extent, posing various challenges to the stability of the ecosystem. In addition, the basin has witnessed a rapid surge in population and urbanization, leading to a significant transformation in the land use/cover conditions of the Upper Huai River.

The daily streamflow time series during 1961–2020 at the Wangjiaba station were obtained from The Huaihe River Commission of the Ministry of Water Resources P.R.C. Precipitation data of the 7 precipitation stations during 1961–2020 were obtained from China Meteorological Data Service Centre (http://cdc.cma.gov.cn/).

This study employs the Mann–Kendall test to identify the change point in the streamflow time series at the Wangjiaba station spanning 1961–2020. Subsequently, the streamflow time series is segmented into two distinct periods: pre-alteration (i.e., natural period, characterized by minimal or no human activity) and post-alteration.

Here, ED(ES) represents the ecological deficit and surplus, A₂₅ (A₇₅) denotes the area encircled by a specific FDC and the corresponding 25% (75%) FDC; A₅₀ signifies the area enclosed by a specific FDC and the 50% FDC.

The Dundee Hydrological Regime Assessment Method (DHRAM), another hydrological alteration approach, was proposed by Black et al. (2005). The DHRAM classifies the absolute changes of the mean and coefficient of variation (CV) of the IHA parameters into three impact categories, namely, category 1 representing the lowest degree of change, category 2 representing the medium degree of change, and category 3 representing the highest degree of change. The classification results are shown in Supplementary material (Table S2). For the mean in group 1, if the change in the IHA parameters was less than 19.9%, then the point was recorded as 0; if the change in the IHA parameters ranged from 19.9 to 43.7%, then the point was recorded as 1; if the change in the IHA parameters ranged from 43.7 to 67.5%, then the point was recorded as 2; if the change in the IHA was more than 67.5%, then the point was recorded as 3. The same approach was applied to other groups as well.

Considering the sum of the points of the mean and CV for each group, total point was obtained. The total point allows conversion to a final DHRAM classification of impact severity on a 1–5 class scale (Supplementary material, Table S3). The point value is associated with the integrated degree of hydrological alteration and can help in evaluating the extent of potential damage to aquatic ecosystems. Higher points lead to greater alterations in the flow regime, indicating higher vulnerability of the river ecosystem to damage.

In terms of monthly flow, all parameters experienced low or moderate alterations, with March flow undergoing the greatest change. Compared to the pre-alteration period (1961–1987), the number of years that fall within the RVA threshold for March flow decreased significantly during the post-alteration period (1988–2020) (Figure 6(a)). For annual extreme flow, the 7-day maximum flow exhibits the most notable alteration. In 1968, the 7-day maximum flow well surpassed the high boundary of RVA, indicating extreme flooding. Although there have been no large flood processes exceeding 6,000 m³/s since 1987, most years did not fall within the RVA threshold (Figure 6(b)). Regarding the timing of annual extreme flow, the date of the annual minimum flow shifted considerably earlier, from early June to mid-March (the dates for the pre- and post-alteration periods are 155.5 and 78, respectively, Table 1). The hydrological alteration degree of the date of the annual minimum flow is moderate, with a value of −35%, and this parameter rarely falls within the RVA threshold after alteration (Figure 6(c)). The parameter of high-flow duration is crucial to the health of riverine water ecosystems. Certain duration of high-flow processes can promote fish spawning and provide spawning sites and nutrients for fish (Guan et al. 2021). The number of times this parameter falls within the RVA threshold increased slightly after 1987 (Figure 6(d)). Moreover, most indicators have lower CV, indicating a more concentrated distribution of flows throughout the year. These results suggest that the hydrological processes of the river stabilize under the regulation of upstream reservoir operation, leading to a homogenization of streamflow.

The impact points and total points of DHRAM and are illustrated in Table 2. The DHRAM level at the Wangjiaba station is 3, with a total of 7 points, signifying a moderate ecological risk. Furthermore, the overall alteration degree at the Wangjiaba station is 32%, indicating low hydrological alteration. The analysis shows that the region has experienced changes in its ecological hydrological regime and faces a moderate ecological risk. To mitigate adverse effects on river hydrology and ecological damage, implementing various ecological protection measures is essential to restore the natural eco-hydrological regime.

The SI exhibits a closely aligned trend with the ecological surplus and an opposing trend to the ecological deficit, as depicted in Figures 5 and 7. SI peaked at 20 in 1963, underwent a gradual decline, experienced a slight increase in 1976, and saw a steep decline after 1987. Likewise, ecological flows exhibited a surplus in 1963 followed by multiple deficits after 1987. After 2004, river biodiversity exhibited poor conditions, with SI dropping below 10, and ecological flows consistently in deficit. The analyses uncovered a substantial influence of instream ecological flow alterations on SI, where excess ecological flow benefits river biodiversity, while deficient ecological flow has a detrimental effect. Notably, the decline in the fitting curve is markedly higher after 1987 than in the preceding period. River biodiversity has markedly decreased due to human activities in the region, as reported by Tang et al. (2021). The escalating urbanization in the region post-1980s led to the discharge of a substantial volume of pollutants into the river, negatively impacting river biodiversity, as highlighted by Shen et al. (2021).

Both climate change and human activities significantly impact river streamflow. Precipitation is the primary factor influencing the ecological flow of rivers in the Huai River basin, followed by human activities (Zhang et al. 2022). The correlation between precipitation and ecological flow indicators are evaluated and shown in Table 3. During 1961–1987, there is a strong correlation between precipitation and ecological surplus and deficit, with correlation coefficients of 0.75 and 0.73. Following the influence of human activities, the correlation between precipitation and ecological surplus and deficit decreases, reflected in correlation coefficients of 0.63 and 0.58. This implies a reduced impact of precipitation on ecological flow indicators, indicating the involvement of factors beyond precipitation. The study unveils a profound impact of human activities on the river environment, especially concerning fish species (Inogwabini & Lingopa 2013; Ko et al. 2017). The 2015 ecological survey of The Huaihe River Commission of the Ministry of Water Resources P.R.C. reveals a 40.8% decrease in the number of fish species in the upper Huai River compared to the 1980s, indicating a serious and irreversible decline in species diversity. Furthermore, accelerated urbanization and reservoir operations have led to significant reductions in the population size and species of some rare and endemic fish in the Upper Huai River, as documented by Ai et al. (2022).

This study utilizes multiple assessment indicators based on hydrological alteration for a quantitative evaluation of instream ecological flow in the Upper Huai River and their corresponding ecological impact. Based on the precipitation and streamflow time series from 1961 to 2020, several conclusions can be drawn:

(1) The annual streamflow time series experienced an alteration in 1987 at a significance level of 0.05. Subsequent to the alteration, there is a significant decrease in the occurrences of ecological surplus and a significant increase in the occurrences of ecological deficit. In the period 1961–1987, precipitation primarily influences ecological flow, while from 1988 to 2020, human activity emerges as a key factor affecting ecological flow regimes. (2) In the period 1988–2020, several indicators no longer fell within the RVA threshold, potentially elucidating the decline in ecological surplus and the rise in ecological deficit in the basin. The overall alteration degree at the Wangjiaba station is 32%, and the DHRAM level is 3, indicating low hydrological alteration and moderate ecological risk in the region. (3) During 1988–2020, the correlation between precipitation and ecological surplus and deficit weakens, reflected in correlation coefficients of 0.63 and 0.58. The SI exhibits an almost identical trend to ecological surplus and an opposite trend to ecological deficit.

This research is supported by the National Natural Science Foundation of China Youth Fund Project (5210903) and Fourteenth Five-Year National Key Research and Development Project (2021YFC3200203). Special thanks are owed to editors and anonymous reviewers whose comments helped to improve the paper.

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

The authors declare there is no conflict.