1. Introduction
Hydrological conditions are critical drivers of river ecosystems and play an essential role in maintaining energy processes, biological interactions, and physical habitat conditions [
1]. Climate variability and human activities are two critical factors influencing runoff alterations. Runoff formation is closely related to climate change, such as precipitation and potential evapotranspiration. Land use change and construction of large-scale water conservancy projects can change a river’s runoff and its original seasonal and annual distribution by affecting rainfall interception and actual evapotranspiration [
2,
3]. Runoff is severely disturbed in many parts of the world [
4,
5]. Liu et al. [
6] found significant changes in the flow of 24% of the world’s large rivers. Li et al. [
7] summarized long-term runoff records from 22 subbasins of the Yangtze River in China. They concluded that the construction of dams would impact hydrological factors such as water level, flow, and sand content in the downstream basin. The study of climate change and impact of human activities on runoff has become an essential scientific issue due to the importance of avoiding and reducing economic losses caused by frequent floods and severe drought disasters [
8,
9].
Effects of development and construction of large reservoirs on the degree of runoff change and its influencing factors have been a hot research topic at home and abroad. The most widely used method is the range of variability approach (RVA), which analyses the variability of the overall hydrological situation of a river [
10,
11]. This method, proposed by Richter [
12], allows the river to be divided into different periods by human activity, climate change, etc. The extent to which the river’s hydrology changes in a sudden change year can be obtained by exploring hydrological indicators, which has been discussed by many scholars. Eum et al. [
13] suggested that changes in flow magnitude and timing may lead to changes in reservoir operations using the range of variability approach (RVA) to assess each hydrological indicator to investigate the impact of changes in these indicators on the reservoir. Tonina et al. [
14] suggested that changes in the vital hydrological indicators can significantly affect aquatic organisms’ diversity and population dynamics, with implications for habitat quality within a watershed. Pfeiffer et al. [
15] found that changes can influence the water balance in river catchments in precipitation. Human activities such as dam construction, irrigation, and land use changes have significantly altered the hydrological state of rivers.
Changes in meteorological factors such as precipitation, temperature, and radiation are inextricably linked to alterations in the runoff. Many scholars have used hydrometeorological data to fit empirical relationships between the water and heat balances of different watershed types, confirming the validity of the Budyko hypothesis, which is that changes in river runoff are mainly caused by climate change and human activities [
16]. The method is widely used in globally significant river basins [
17,
18]. Dividing the United States into seven study regions, Heidari et al. [
19] concluded that the southern and southwestern United States are likely to experience prolonged droughts in future periods. At the same time, the western part is likely to experience more humid conditions. Xia et al. [
20] quantitatively estimated runoff alterations in the upper reaches of the Han River based on the elastic coefficient method and the hydrological simulation method. They discussed the differences in runoff caused by natural and artificial factors, which showed that human activities have a more significant impact than natural factors. Exploring the assessment of changes in river runoff characteristics and the causes of their alterations is essential to understanding and predicting the impact of changes in river flow regimes on the ecology within river basins.
The Min River is a first-class tributary of the upper reaches of the Yangtze River. In recent years, construction of Min reservoirs has gradually gained attention, forming a cluster of terraced reservoirs containing strings and mixed links [
21,
22]. The construction of a reservoir impacts the climate within the Min River basin, which will inevitably cause changes in runoff and some disturbance to the ecosystem within the basin [
23]. The existing studies on runoff changes and their factors in the Min River basin are limited to the study of trends and abrupt changes, lacking an understanding of the causes of changes in hydrological indicators before and after abrupt changes in runoff and the impact of changes in habitat quality on runoff. Composition, structure, and function of ecosystems, such as rivers and wetlands, are inextricably linked to their hydrological characteristics. Changes in habitat quality and land use can affect hydrological cycle processes of runoff, altering the spatial and temporal distribution of water resources [
24]. Therefore, the study of hydrological changes in the Min River basin is essential to the ecological restoration and sustainable development of the upper Yangtze River basin.
In this paper, based on the previous studies, we use the ecohydrological indicator range of alteration (IHA–RVA) and the hydrological alteration method to evaluate the hydrological situation in the Min River basin and analyze the degree of influence of climate variability and human activities on the runoff in the Min River basin in a more objective way. FAO Penman–Monteith formula was used to calculate the potential evapotranspiration in the Min River basin. According to the long-term water balance formula, drying indicator, and six Budyko hypothesis formulas, the influence of climate change and human activities on the runoff was calculated. This study provides a reference for the Min River basin to cope with climate variability and develop and utilize water resources more rationally.
4. Results and Analysis
4.1. Mutagenicity Analysis
Using the M–K test method, it can be seen that the intersection of the runoff and precipitation statistics of the two curves of UF and UB is at the 0.05 significance level, which indicates that the hydrological situation of the Min River basin may have changed abruptly in 1993 and 1997 (
Figure 2a,d). From the results of the sliding
t-test method, it can be concluded that the intra-annual distribution uniformity of the runoff volume passes the 0.05 significance level and the intra-annual distribution uniformity of the precipitation volume does not pass the 0.05 significance level. It can be seen that the annual runoff volume showed abrupt alterations in 1968 and 1993, and the years of abrupt alterations in annual precipitation were 1990 and 1993 (
Figure 2b,e). The cumulative distance level method shows that the annual runoff had the maximum value in 1993 and 2002, and the annual precipitation had the maximum value in 1993 (
Figure 2c,f). Combining the above three methods, 1993 was chosen as the year of the sudden alteration in the annual runoff and the annual precipitation in this study. The daily runoff data from the Gaochang hydrological station and the daily precipitation data from the Min River basin were divided into base period T
a (1960–1993) and alteration period T
b (1994–2019).
4.2. IHA Hydrological Indicator Alteration Degree Analysis
As climate variability and human activities are important factors affecting runoff alterations, considering only daily flow alterations can only describe the changing state of the Min River basin at the macroscopic level, and to further analyze the causes of the runoff alterations in a more detailed way, recording and analysis of the climate variability data are essential. Precipitation has a direct impact on runoff alterations in climate variability. Considering daily precipitation data, they can reflect not only the rise and fall of rivers directly, but also the alterations of aquatic organisms and riparian habitats.
Table 3 shows the degree of change of 32 hydrological indicators in five groups before and after abrupt changes in the runoff and precipitation. The influence of reservoir construction on rivers is discussed by changing 32 hydrological indicators.
Average flow and precipitation of each month: As can be seen from
Table 3, the average monthly flow at the Gaochang hydrological station increased to different degrees after an abrupt alteration, while the average monthly precipitation in the Min River basin increased and decreased, but the increase and decrease were more moderate; the alteration of the average monthly flow was high in May and medium and low in other months; the alteration of the average monthly precipitation was high in July. After 1993, the number and storage capacity of reservoirs increased significantly, which increased the runoff after the alteration and caused specific disturbance to the reproduction of aquatic organisms.
Annual extreme flow and precipitation: The annual mean extreme minimum flow at the Min River basin’s Gaochang hydrological station generally increased after an abrupt alteration, while the extreme values showed different degrees of decrease, of which the minimum seven-day flow was highly altered, and the rest were altered to a moderate or small degree. The reservoir construction and operation after 1993 affected the original flow polarity alteration process of the river. The annual extreme precipitation alteration range is more drastic, which influences the flow alteration. Such extreme daily alterations in the precipitation are frequent and unpredictable, and extreme alterations in the precipitation can lead to alterations in the natural river hydrological processes, causing disturbances to species that are not adapted to frequent alterations in the water flow.
Timing of the annual extreme flow and precipitation occurrence: After the sudden hydrology alteration at the Gaochang hydrological station, the time of occurrence of the annual maximum flow and the annual minimum flow was changed at a low degree, indicating that the alteration of hydrological indicators in this group had a little alteration on biological abundance. The annual precipitation alterations were low, and climate variability had no significant effect on the time of occurrence of the annual extreme flow.
Frequency and duration of high and low flow (precipitation): After the alteration, the number of flow pulses changed moderately, and the pulse duration changed to a small degree. The precipitation indicators are all low-level changes, and the impact is negligible. The high and low diachronic changes of runoff and precipitation affect the structure and function of the river ecosystem, change the soil water content on both sides of the river, and make the riparian vegetation disappear [
42].
The rate of alteration and frequency alteration of flow and precipitation. After the abrupt alteration, the rate of flow increases, and the number of reversals have different degrees of increase, and the rate of flow decrease reaches 100% as high change, which makes the allowable range of flow alteration decrease. The annual precipitation decline rate of 78% is highly altered, and the annual precipitation reversal number of 38% reaches moderate change, indicating that climate variability has a certain impact on the flow alteration rate and frequency alteration. The rate and frequency of flow alterations affect the pattern of river rise and fall, thus affecting the survival and reproduction of aquatic organisms. The ecosystem has a limited ability to withstand external alterations, and a reduced range of flow alterations can affect the growth of organisms on both sides of the river, while too frequent flows can destabilize flora and fauna.
Overall degree of alteration of flow and precipitation: The change degree of 32 hydrological indicators of the runoff and precipitation is drawn as a radar map (
Figure 3), which can more intuitively show the change of the hydrological situation of the Min River before and after the alteration. It can be seen from the figure that there were four high changes in the hydrological alteration degree indicator in the Min River basin, namely the average flow in May, 7-day minimum, fall rate, and reversals, 15 moderate changes; the rest are low changes. Among the precipitation alteration indicators in the Min River basin, except for the average precipitation in July, the base index, and the decline rate, which were highly altered, the rest were all altered to a moderate and small degree.
Equation (5) was used to calculate the change degree of hydrological indicators in five groups and the whole. It can be seen from
Table 4 that the overall change of the runoff is 45% (moderate change). The overall change in the precipitation is 37% (moderate). The change of the precipitation in the Min River is small, indicating that human activities such as reservoir construction have a more significant impact on the runoff.
4.3. Analysis of the Alterations in Runoff Conditions
In the selected hydrological series of the Min River basin, the annual runoff volume alterations has some oscillations and is unstable, with a slight upward trend in the T
a stage and a significant downward trend in the T
b stage, and the annual runoff volume has a decreasing trend in general; according to the trend test analysis, the test statistic is −2.76, which passes the 95% significance level test; the maximum value of the annual runoff volume was 1004.54 × 108 m
3 (1990); the minimum value was 635.17 × 108 m
3 (2006); compared with the T
a period, the annual runoff in the T
b period decreased by 10.46%. The yearly precipitation fell in both the T
a and T
b periods, and according to the trend test statistic of −1.52, the trend was not significant, and the decrease was not evident in many years; the maximum value of annual precipitation is 1286.78 mm (1990), and the minimum value is 808.17 mm (2006); compared with the T
a period, the yearly rainfall in the T
b period decreased by 6.33%. The annual evapotranspiration fluctuated violently, and the T
a phase showed an apparent decreasing trend. After the breakthrough, the T
b phase increased by 4.9% compared with the T
a phase, showing a general decreasing trend (
Figure 4). The multiyear precipitation in the Min River basin is on a downward trend, but the overall decline is not significant. We know that the annual runoff of the Min River also shows a decreasing trend, which indicates that the alteration of precipitation has a certain impact on the annual runoff.
The calculated results of the runoff situation alterations in
Table 5 show the degree of influence of climate variability and human activities on annual dimensions. The elasticity coefficient of runoff to precipitation is 0.83; the contribution of climate variability and human activities to the runoff alteration is 30.20% and 69.80%, respectively.
4.4. Impact of Land Use Change on Runoff Changes in the Watershed
In recent years, the rapid development of population and economy in the Yangtze River basin has led to a change in vegetation cover. The vegetation cover had improved since 1989 when the state started to take various soil and water conservation engineering measures in the Yangtze River basin.
Figure 5 shows the land use changes in the Min River basin in different periods (1980, 1990, 2000, 2010, 2020).
The land cover change in the Min River basin from 1980 to 2020 shows fluctuating changes: the grassland area coverage increased from 42.15% to 42.58% (105 km
2) (
Table 6); the construction land area changed more between 1980 and 2020 (0.23%), which indicates that with the continuous industrialization and urbanization, the construction land area increased gradually with time; the decreasing and then increasing trend in barren land area and the decrease in the forest, cropland, and wetland area indicate that human activities have had an impact on the habitats in the watershed. In recent years, human activities in the Min River basin have increased significantly, leading to a rapid increase in the proportion of land used for construction and a decrease in wetland areas.
The evolution of land types and land use patterns in the Min River basin are analyzed in
Table 7. The watershed area decreased by 9 km
2 in 1980–2020, converting mainly to grassland and barren land, indicating the severity of soil erosion. Since the implementation of soil and water conservation measures in 1980, the forested area showed a trend of growth followed by a decrease during this period. With the development of hydropower projects and the dramatic increase in human activities, the total forested area continued to decrease (230 km
2). Grassland and construction land increased by 100 km
2 and 321 km
2, respectively, with the most dramatic increase in the construction area during 1980–2020.
6. Conclusions
According to the Mann–Kendall method, the cumulative distance level method, and the sliding t-test method combined with the reservoir construction, the abrupt year of runoff and precipitation was 1993. The annual runoff, precipitation, and evapotranspiration show a downward trend.
Using the RVA method to conduct a comprehensive analysis of 32 hydrological indicators and 32 precipitation indicators at the Min River basin’s hydrological stations, the overall hydrological change at the Gaochang hydrological station was 45% (moderate change). Considering the influence of precipitation on the alteration of runoff, the overall change of rain in the Min River basin was 37% (moderate change). At the same time, human activities such as water conservancy-related construction have brought resources and convenience to people and also caused alterations to the hydrological situation in the basin, thus causing damage to the ecological function of the rivers and alterations to the habitat of aquatic organisms.
The contribution rates of climate variability and human activities to the Min River runoff were 30.20% and 69.80%, respectively. Human activities were the dominant factor influencing the Min River basin’s runoff situation. Climate factors have a smaller influence on runoff alterations.
The Min River basin has seen an increase of 321 km2 in built-up area and a corresponding decrease in the wetland, forest, and cropland area (in terms of land use) from 1980 to 2020, and land use changes in the Min River basin have become a factor leading to runoff changes in the area that cannot be ignored.