1. Introduction
Hydrological conditions refer to changes in the hydrological elements of natural water bodies, such as rivers, lakes, and reservoirs; over time and space [
1], which play a crucial role in maintaining the stability of river ecosystems [
2,
3]. In 1996, Richter [
4] proposed hydrological alteration indicators and evaluated the changes in hydrological conditions in the basin using 5 groups of 33 indicators. Currently, the indicator of hydrologic alteration (IHA) is widely used for the assessment of various scenarios, such as changes in hydrological conditions [
5]; ecological environmental impact [
6]; and ecological environment flow estimation [
7]. The hydrological conditions correspond to the ecological effects of the river basin. The range of variability approach (RVA) method can calculate the degree of hydrological alteration and reflect the evolution of hydrological conditions in a basin [
8,
9,
10]. Many scholars have combined the IHA with RVA (IHA-RVA) to analyze hydrological alteration in different river basins. First, by calculating the value of the IHA index and then by calculating the degree of change of each index using the RVA method.
Changes in hydrological conditions due to climate change and human activities pose serious challenges to the stability of watershed ecosystems and resource management [
11,
12]. Related studies have shown that climate change and human activities impact river basins in China in different ways [
13]. River runoff in the basins of the northern regions is more influenced by human activities, whereas in the southern basins, climate change is the main factor affecting runoff variability [
14]. Many researchers have explored the impact of changing environments on runoff from different perspectives. Alrajoula [
15] explored the impact of dam construction on hydrological conditions and analyzed its impact on the watershed ecosystem. Lu [
16] analyzed the water-sand mediation scheme of the Xiaolangdi dam on the hydrological alteration of the lower Yellow River, and Ashraf [
17] evaluated the impact of climate change and river regulation on the flow system under a cold climate. The effects of climate change and human activity on watershed runoff has become a major research hotspot [
18,
19].
The Yongding River basin receives scarce precipitation [
20,
21] and is a typical water-scarce basin in northern China [
22,
23,
24]; thus, the severe water shortages have received considerable attention from researchers [
25,
26,
27]. The agricultural economy is an important economic source in the Yongding River basin, and water for agricultural irrigation is an important factor contributing to water scarcity in the Yongding River [
28,
29]. There are 52 large- and medium-sized irrigations in the upper part of the basin, with an irrigated area of 3795 km
2 [
30,
31]. With economic development, the agricultural water consumption has increased dramatically, resulting in the degradation of the Yongding River ecosystem [
32,
33]. In 2020, the Ministry of Water Resources included the Yongding River in second place in the list of key rivers and lakes assessing ecological flow [
34]. Upstream Xiangshuipu was set as the assessment section to strengthen the ecological protection of the Yongding River and promote the construction of an ecological civilization.
Using the Xiangshuipu section as the study area, this study used the IHA-RVA method to identify the most ecologically relevant hydrological indicators (ERHIs) and to solve the problem of redundant information in 33 IHA indicators. The evolution of the hydrological conditions was determined by calculating the hydrological variability. Based on the WetSpa model, four scenarios were established to simulate the daily runoff in the section from 1962 to 2021. The contribution of climate change, irrigation water withdrawal, and reservoir storage to hydrological alteration were quantitatively calculated, and the main driving factors affecting hydrological alteration were identified. The results of this study provide a reference for ecological management of the Yongding River basin.
3. Methodology
3.1. IHA-RVA Method
- (1)
IHA indicator system
The IHA indicator system is commonly used to represent the characteristics of the change in hydrological conditions and to evaluate the process of change. It is currently the most widely used set of indicators. The IHA indicator system is divided into five groups based on flow magnitude, epoch duration, occurrence period, frequency, and the rate of change; and contains 33 hydrological indicators with corresponding ecosystem impacts (
Table 1). To eliminate or reduce the response of hydrological indicators to interannual climate change, it is recommended that the length of hydrological data should be at least 20 years [
35]. In the IHA index system, the characteristics of river flow change are depicted by several aspects, such as monthly flow and duration; extreme flow and pulse flow occurrence time; frequency and duration; and flow change frequency.
- (2)
Range of variability
To analyze the ecohydrological characteristics of rivers, Richter [
36] proposed the RVA in 1997 and defined RVA thresholds that help determine the degree of change in the IHA. The period of stability was considered as the period before and after the variation, when the hydrological conditions were more stable for a longer period. The degree of hydrological alteration was used to quantitatively assess the degree of change in ERHIs after disturbance using 75% and 25% as the upper and lower limits of the RVA thresholds of ERHIs during stable periods, respectively. The degree of hydrological alteration is defined as follows:
where D
i is the degree of hydrological alteration of the
i-th ERHIs; N
i0 is the number of years in which the
i-th ERHIs remains within the RVA threshold in the unstable period; N
ie is the number of years in which the
i-th ERHIs are expected to fall within the RVA threshold in the unstable period;
r is the proportion of the
i-th ERHIs within the RVA threshold in the stable period (50% in this study); and N
T is the number of years recorded in the affected time series in the unstable period. The criteria for determining the degree of hydrological alteration of the ERHIs are listed in
Table 2.
3.2. WetSpa Model
The WetSpa model, proposed in 1996 [
37], is a distributed hydrological model based on physical mechanisms for simulating water–air transport and energy exchange among the soil, vegetation, and atmosphere at the daily watershed scale (
Figure 2). The model considers the processes of precipitation, interception, snowmelt, depression, infiltration, evapotranspiration, surface runoff, and subsurface runoff, and uses a multi-layer model to represent the water and energy balance of each grid cell. The grid cell size was set to 1 × 1 km, and 11 flow production parameters and 4 sink parameters were involved in determining the flow rate model. The flow production process is calculated using the grid as a unit, and the flow confluence process is calculated according to the upstream and downstream relationships one-by-one sub-basin.
The WetSpa model is vertically divided into four layers: the vegetation canopy, surface layer, soil layer, and the groundwater aquifer. After precipitation has evaporated in the first layer, the model determines the surface yield by analyzing the land use, soil type, slope, rainfall intensity, and soil water content of the grid cells. Surface runoff is formed after meeting the amount of ground fill. The infiltrated part will form soil water, which will continue to move laterally to form a soil midstream as the soil water content increases, or it will infiltrate downward to form groundwater. After the infiltrated amount of water partially exceeds the storage capacity of the underground aquifer, it will flow out in the form of underground runoff. Surface runoff, underground runoff, and mid-loam flow together make up the total runoff on each grid.
Daily runoff data from the Xiangshuipu section from 2017 to 2021 were used to calibrate and verify the model parameters. The simulated time step is 1 day. We considered 2017 as the warm-up period, 2018–2019 as the calibration period, and 2020–2021 as the verification period. As the Xiangshuipu section is adjacent to the Xiangshuipu Reservoir, the storage of the reservoir has a significant influence on the simulation results of the section runoff. Therefore, the inflow of the Xiangshuipu Reservoir was calibrated and simulated. As the inflow is difficult to measure directly in the reservoir, it is usually calculated according to the observed outflow and reservoir storage using the water–balance method. However, the inconsistent observation of outflow requires imputation data, which will cause the calculated inflow to have abnormal values. But the timing and value of peaks are not impactful to the later analysis. From the simulation results (
Figure 3), the relative biases (RB) of the calibration and validation period are 0.15 and 0.08, respectively. The result shows that these ‘errors’ are in an acceptable range and the simulated runoff is close to the actual runoff. This indicates that the WetSpa model is applicable to the simulation of runoff processes in the Yongding River basin and can be used for subsequent research analysis. RB is calculated as follows:
where
is the simulated value of t-time flow;
is the observed value of t-time; and RB is the relative biases of the simulated value and the measured value. The closer the RB value is to 0, the smaller the difference between the simulated flow and the measured flow. When |RB|< 0.2, the simulation results are considered acceptable.
3.3. Driving Factor Analysis Model
As shown in
Figure 4, the water withdrawal activity between the critical section and the upstream section can be generalized as one interval water withdrawal unit. The flow rate (Q) can be expressed as follows:
where
I is the water arriving from upstream during the period; ∆S is the reservoir storage volume; ∆W is the inter-district water withdrawal; and
L is the evaporation and seepage losses from the river.
Equation (4) and
Figure 4 show that the changes in the ecohydrological conditions of the basin are mainly influenced by three factors: climate change, water withdrawal, and reservoir storage. To quantitatively determine the degree of influence of the three factors, the multi-series contribution split method was used. The related formulae are as follows:
where E
i,1 refers to the current series (considering reservoir and withdrawal effects); E
i,2 refers to the original series (not considering reservoir and withdrawal effects); E
i,3 refers to the series only considering water withdrawal effects; and E
i,4 refers to the natural series (before variation of climate conditions).
The above four series are all calculated during the stable period. To obtain the natural series, firstly, the climate conditions during the stable period need to be revised to the level of the natural period (namely, no variation occurred). Then, the WetSpa model is driven by the revised climate conditions to obtain the daily runoff process under natural conditions.
m is the number of ERHIs indicators. Vi,1, Vi,2, and Vi,3 are the rate of change of the i-th ERHI compared to the natural series. The positive and negative of Vi,s can be seen as the relationship between Vi,s in which the change of the indicator is positively or negatively correlated. βc, βw, βr are the contributions of climate change, interval water withdrawal, and reservoir storage in relation to the change in ERHI indicator values, respectively. To avoid a small Vi,1 value, which excessively contributes to rate calculation results, when the absolute value of Vi,1 was <3%, no statistical analysis was performed.
5. Discussion
5.1. The Evolution of Hydrological Conditions
By screening the IHA indicators, the analysis of indicator coverage and redundancy showed that the six ERHIs were acceptable. A variation diagnosis of the ERHIs was performed, and the variation period of the Xiangshuipu section was determined to be from 1982 to 1988. The fall and rise rates had a moderate and low degree of change, respectively; and overall, were considered to be relatively low changes. To some extent, this reflects that the flow in the Yang River changed during the study period, but it maintained a relatively stable state in the ecosystem. The maximum and minimum 1 day flows had a moderate and low degree of change, respectively; however, both decreased after variation, indicating that the habitat environment of aquatic organisms was damaged, and the ecological flow required further assurance. The July flows show a low degree of change, indicating that during the flood season, the Xiangshuipu Reservoir can be used to reduce outflow. The dates of maximum flow were low, indicating that changes in hydrological conditions had little impact on fish migration. The overall hydrological conditions of the Xiangshuipu section showed little change, indicating that the water environment of the Yang River was stable during the study period.
5.2. The Impact of Human Activities on the Hydrological Conditions
Changes in the hydrological conditions are mainly influenced by human activities, among which water withdrawal is the most influential. From the different ERHIs, the maximum 1 day and July flows were most influenced by water withdrawal. Both these indicators were in the crop-growing period when agricultural water use was higher. China implemented its land reform policy in 1978. This policy was implemented in the Yang River in the early 1980s and led to a significant increase in agricultural water use. Xiangshuipu is located in Zhangjiakou City, where agricultural production is an important economic source for the local area. According to the statistical bulletin of Zhangjiakou City from 2015 to 2021, the agricultural water use in Zhangjiakou City in all years accounted for more than 50% of the total water use in that year (
Figure 9), with an overall upward trend in agricultural water use. In addition, the Xiangshuipu section is adjacent to the Yang River II Irrigation (a large irrigation) and four other medium-sized irrigations. Accelerated agriculturalization and rapid economic development during this period also led to increased water withdrawal from the irrigation.
Although the influence of reservoir storage on the overall hydrological conditions was small, the minimum 1 day flows were mostly influenced by the effect of reservoir storage.
Figure 10 shows the course curves of the outflow and minimum 1 day flows from the Xiangshuipu Reservoir during the stabilization period. The rise and fall trends of the minimum 1 day flows coincide with the outflow of the reservoir. This indicates that during the dry period, the storage effect of the upstream reservoir has a significant influence on the runoff process of the downstream section, which can reduce water shortage problems, such as basin outflow during the dry period.
5.3. The impact of Climate Change on Hydrological Conditions
Climate change has a negative impact on the evolution of hydrological conditions, with the greatest impact on the maximum 1 day and July flows. The maximum 1 day flows occur mostly from June to August; therefore, the average rainfall and temperature from June to August of each year during the stability period were analyzed. As shown in
Figure 11, although temperature showed an insignificant downward trend (R
2 = 0.06); the downward trend in precipitation was significant (R
2 = 0.68). Precipitation is an important source of runoff, and a decrease in precipitation leads to a decrease in river runoff. The analysis of
Figure 7b,e show that both the maximum 1 day and July flows show an increasing trend during the stabilization period. Under the combined action of different meteorological factors, climate change negatively affected the evolution of hydrological conditions.
5.4. Limitations of this Study
In this study, when we applied the WetSpa model to simulate runoff data, only the effects of reservoir storage and irrigation water withdrawal were considered, such that the simulation results were within a reasonable error range and had scientific validity. However, actual runoff processes are also influenced by various human activities. In addition, we calculated the contribution rate of climate change as a whole while analyzing the trends of change of some climate factors. Therefore, subsequent studies should consider multiple influencing factors and separate climate factors to consider the contribution levels of the different factors, such as rainfall and temperature, to further improve simulation accuracy.
6. Conclusions
Considering that climate change and human activities continue to have an increasingly serious impact on runoff, this study combined the IHA-RVA method with the WetSpa model to calculate the degree of hydrological alteration in the Xiangshuipu section of the Yongding River basin to reveal the changing characteristics of hydrological conditions. Then, by considering climate change, reservoir storage, and irrigation water withdrawal, we simulated the daily runoff process under four scenarios based on the WetSpa model. We also quantitatively identified the key driving factors influencing the hydrological alteration by comparing different hydrological conditions in different runoff periods. The main conclusions are as follows: the IHA indicator was used to identify ERHIs and was based on periods of ecohydrological variability; and we also found that variability occurred during 1982–1988; among the ERHIs, except for the maximum 1 day flows and fall rate, which had a moderate degree of change, all other ERHIs had a low degree of change. Finally, the overall hydrological alteration in the Xiangshuipu was low, with relatively stable changes in hydrological conditions. Overall, human activity is the main factor affecting hydrological conditions, and the influence of climate change is relatively small. The contributions of the three factors, in descending order, were irrigation water withdrawal > climate change > reservoir storage. The contribution of climate change was negative. Irrigation water withdrawal is the most important reason for hydrological change. Therefore, irrigation techniques should be improved in the future to reduce water use in agriculture. Relevant departments should issue relevant policies to achieve the purpose of protecting the water resources of the Yongding River.