With the rapid development of hydropower exploitation in China, changes in runoff and sediment transport have become a significant issue that cannot be neglected. In this study, the Han River was selected as a study case, where the runoff variation and changes in sediment load at the Baihe, Huangjiagang, Huangzhuang, and Xiantao stations were analyzed in different time periods. The results indicate that impact of cascade hydropower exploitation on runoff and sediment transport is significantly different even during the same time periods. After reservoir regulation, the decreasing of sediment load is faster than that of runoff. Strong positive correlation between runoff and sediment load exists during different time periods, while reservoir operation leads to different turning points at the Baihe, Huangjiagang, Huangzhuang, and Xiantao stations in the middle and lower Han River. As a key driving factor, runoff variation contributed to sediment transport with different impact index
With the rapid development of the social economy and improvement of modern science and technology, human activities have affected the environment deeply, which has attracted extensive attention. In particular, a large number of water conservancy projects were built in recent years, and water resource that humans must depend on has been under control to a certain degree. However, runoff process and sediment transport in rivers have also changed greatly. How to evaluate the influence of water resource exploitation on surface runoff and sediment yield and predict the tendency of hydrological variation have been a significant issue [
In the past 50 years, the process of runoff and sediment has experienced a great change with comprehensive exploitation and utilization of water resources in China [
The objective of this paper is therefore to investigate the influence of cascade hydropower exploitation on runoff and sediment transport. The Han River basin was selected as a case study. The effects caused by a series of water conservancy facilities, such as the Danjiangkou, Wangfuzhou, and Cuijiaying reservoirs, were analyzed. The dominant factors that affect the changes in runoff and sediment transport were identified and evaluated in order to provide a scientific basis for the comprehensive management in the Han River basin.
The Han River, located in central China (Figure
Locations of the Han River, tributaries, dams, and hydrological stations, whose data were used in this study.
Hydrological data of annual runoff and suspended sediment load of Huangjiagang, Xiangyang, Huangzhuang, and Xiantao stations along mainstream of the Han River were collected from the Changjiang Water Resources Commission, China (CWRC) [
Detailed hydrological data information used in this study.
Stations | Drainage area (km2) | Data length | |
---|---|---|---|
Runoff | Sediment load | ||
Baihe | 59115 | 1955–2012 | 1955–2005 |
Huangjiagang | 95217 | 1955–2013 | 1955–2013 |
Huangzhuang | 147528 | 1955–2013 | 1955–2013 |
Xiantao | 216579 | 1955–2012 | 1955–2006 |
In order to investigate the changing trends of runoff and sediment transport in the Han River basin, five-year moving average method, rank-based Mann-Kendall method, and slope change ratio of cumulative quantity method were used in this study to test the evolutionary trend of runoff and sediment transport in the past decades.
Average values of data series can be calculated from the former and later period values of hydrological sequence
Rank-based Mann-Kendall test is a nonparametric statistic method to detect monotonic trends in series of environmental data, climate data, or hydrological data [
Under the null hypothesis of the sample, the statistic
The total annual data which were considered in this study are usually taken as independent and identically distributed in hydrology. Therefore, this method was used in this study.
Besides the traditional Mann-Kendall test, we also calculated the statistic variables
A double mass curve is a plot of cumulative values of one variable against the accumulation of another quantity during the same time period. It is used to determine the changes between the two datasets. If the two datasets are consistent, the slope of the double mass curve is linear. A break in the slope of the curve would indicate that conditions have changed. The method was introduced in this paper to evaluate the impact of runoff variation on sediment transport in the Han River basin in a quantitative manner. Generally, the changes in runoff directly affect the sediment transport. Since the slope of double mass curves represents the proportionality between the two quantities, the contribution rate of runoff variation to sediment transport can be expressed as the slope of these curves.
In order to determine whether these turning points are caused by runoff variation or sediment changes, the single mass curves are also involved in this study. The changing ratio of the slope of single mass is introduced in this study to discuss the changes before and after the turning points determined by double mass curves.
A linear relationship was assumed before and after any abrupt changes; the changing ratio of the slope of single mass curves can be expressed as
Similarly, the changing ratio of the slope of cumulative sediment load can be expressed as
Therefore, the extent of runoff variation impact on the sediment transport before and after turning time can be estimated as
When the abrupt changes and linear trends were detected in the time series, the impact of runoff variation on sediment transport before and after abrupt changes can be quantitatively estimated by (
The data used in this study were collected from four gauging stations (Baihe, Huangjiagang, Huangzhuang, and Xiantao) in the Han River basin, which covered the original data of long-term records of runoff and sediment discharge for nearly 60 years (1955–2013). In order to detect the monotonic trend and analyze the changing points of runoff under the exploitation of cascade hydropower, the annual runoff data series at the four gauging stations were used.
Based on the data series of runoff in the Han River basin, the process diagrams of the runoff at the four stations were obtained using the five-year moving mean method. The results are shown in Figure
The runoff variation at the four gauging stations in the Han River basin.
Baihe
Huangjiagang
Huangzhuang
Xiantao
It can be seen from Figure
In addition, the trend and slope of the linear-regression line are nearly the same at the stations (b) Huangjiagang, (c) Huangzhuang, and (d) Xiantao. This may be related to the layout position of the three stations. It can be seen that the maximum runoff occurred in 1964 at Huangjiagang, Huangzhuang, and Xiantao station, while the maximum runoff did not appear at Baihe station in that year. Therefore, it can be inferred that the main runoff is from the middle and lower Han River basin and its tributaries, including Danjiang River, Tangbai River, and Nan River. Moreover, it can be seen from Figure
Based on the method of rank-based Mann-Kendall, the statistics of runoff data series were calculated, and results of abrupt changes are shown in Figure
Baihe
Huangjiagang
Huangzhuang
Xiantao
The up and lower dotted lines are the confidence intervals at the 5% significance level in the Mann-Kendall test. It can be seen from Figure
In addition, the curves of
In the same way of the runoff analysis, the trend and abrupt changes in sediment load were analyzed at the four stations based on the annual data series of sediment.
Figure
Changes in the sediment transport at the four gauging stations in the Han River basin.
Baihe
Huangjiagang
Huangzhuang
Xiantao
The changing processes of the sediment transport above indicate that the changes in sediment transport do not always show well time consistency at all the hydrological stations in the same river basin [
Abrupt changes in sediment transport were detected by the Mann-Kendall test, and the results are shown in Figure
Mann-Kendall test curves of sediment transport at the four stations in the middle and lower Han River.
Baihe
Huangjiagang
Huangzhuang
Xiantao
It can be seen from Figure
It is well known that runoff changes usually have influence on the sediment transport to a certain extent [
Runoff and sediment variation at the four stations in different periods.
Baihe
Huangjiagang
Huangzhuang
Xiantao
Correlations between runoff and sediment at different stations in the middle and lower Han River.
Baihe
Huangjiagang
Huangzhuang
Xiantao
It can be seen from Figure
With the rapid development of economy in the Han River basin, the consumption of domestic and industrial water increased at a fast pace. Therefore, the runoff at those stations began to decrease to a certain degree after 1985 (as shown in Figure
The above analysis suggests that there are great differences in the relationship between runoff and sediment load for different periods. When abrupt changes occur at some turning time, the relationship between runoff and sediment load could have a significant change. Therefore, it is very important to find the turning points. However, due to the theoretical limitation of Mann-Kendall test, the turning points cannot be surely determined by the above analysis. We introduced a double mass curve of annual sediment load and runoff to further study the relation between runoff and sediment load. By plotting the accumulation of runoff and sediment load, the data will plot as a straight line, and the slope of this line will represent the constant of proportionality between the two quantities. A break in slope indicates a change in the constant of proportionality [
Double mass curve of runoff and sediment load at each station along the Han River.
Baihe
Huangjiagang
Huangzhuang
Xiantao
The double mass curve of runoff and sediment load at the Baihe station shows a turning point and the slope of this curve also decreases gradually from 1983, which indicates a decrease in sediment from this year. For the Huangjiagang station, the abrupt change in 1958 has a close relationship with the construction of the Danjiangkou dam. For Huangjiagang, Huangzhuang, and Xiantao stations, the time of the turning points occurring in 1967 for these three stations coincided with the operation of Danjiangkou reservoir in the Han River basin. And all these three stations are located in the downstream of the Danjiangkou reservoir. The locations of these stations and the time coincidence indicate that these turning points occurring in 1967 are caused by the reservoir operation. Moreover, the dual effects of both reservoir operation and increase in water consumption make the two stations (c) and (d) have abrupt changes in relationship between runoff and sediment transport in 1984. Generally, Figure
From Figure
Cumulative curves of runoff in different time periods.
Baihe
Huangjiagang
Huangzhuang
Xiantao
Cumulative curves of sediment load in different time periods.
Baihe
Huangjiagang
Huangzhuang
Xiantao
The regression curves are fitted well before and after turning time at the four stations. The slopes of accumulation curves of runoff
Slope changes of runoff over time for different periods.
Stations | Time periods | | | |
---|---|---|---|---|
| | |||
Baihe | | 253.81 | — | — |
| 191.76 | −24.45 | — | |
| ||||
Huangjiagang | | 411.92 | — | — |
| 419.6 | 1.86 | — | |
| 341.12 | — | −18.7 | |
| ||||
Huangzhuang | | 447.77 | — | — |
| 411.07 | −8.20 | — | |
| 388.2 | — | −5.56 | |
| ||||
Xiantao | | 516.94 | — | — |
| 426.7 | −17.46 | — | |
| 398.48 | — | −6.61 |
Notes:
As the runoff variation, also sediment load at the four stations shows extremely obvious changes before and after the turning points. Except for the natural time period
Slope changes of sediment load over time for different periods.
Stations | Time periods | | | |
---|---|---|---|---|
| | |||
Baihe | | 0.4632 | — | — |
| 0.202 | −56.39 | — | |
| ||||
Huangjiagang | | 1.5383 | — | — |
| 0.7595 | −50.63 | — | |
| 0.0047 | — | −99.38 | |
| ||||
Huangzhuang | | 1.2132 | — | — |
| 0.3229 | −73.38 | — | |
| 0.1082 | — | −66.49 | |
| ||||
Xiantao | | 0.7836 | — | — |
| 0.3294 | −57.96 | — | |
| 0.1798 | — | −45.42 |
Notes:
Impacts of runoff variation on sediment transport in different periods.
Stations | Time periods | |
---|---|---|
Baihe | | — |
| 43.35 | |
| ||
Huangjiagang | | — |
| −3.68 | |
| 18.82 | |
| ||
Huangzhuang | | — |
| 11.17 | |
| 8.37 | |
| ||
Xiantao | | — |
| 30.12 | |
| 14.56 |
Notes:
With the rapid cascade hydropower exploitation in the Han River basin, runoff and sediment transport displayed great changes. In this study, we quantitatively evaluated the trends of hydrological process and identified the abrupt turning points of runoff and sediment load in the middle and lower Han River during the past 60 years. Some interesting and important conclusions were obtained as follows: Operation of cascade reservoirs has significantly influenced the hydrological process, and runoff and sediment load decreased in different degrees at the Baihe, Huangjiagang, Huangzhuang, and Xiantao stations. Compared with the sediment load, the change in runoff is not so large and shows time consistency. The decreasing trend of sediment transport is very significant at the four stations because of reservoir operation during these periods. There are different turning points of runoff and sediment load at the Baihe, Huangjiagang, Huangzhuang, and Xiantao stations along the river. Abrupt changes occur at the four stations in 1983, (1858, 1967), (1967, 1984), and (1967, 1984), respectively. Although great changes in runoff and sediment load occur during different time periods, correlation still exists between runoff and sediment load before and after turning points. Runoff is always a driving factor for the changes in sediment transport, even under the operation of cascade reservoirs. The impact of runoff variation on sediment transport can be determined quantitatively based on the statistic
The authors declare that they have no competing interests.
This paper was financially supported by the CRSRI Open Research Program (Grant nos. CKWV2014205/KY, CKWV2015202/KY); the National Natural Science Foundation of China (51509273, 51679094, and 51409015); the Natural Science Foundation of Hubei Province, China (Grant no. 2015CFC872); Open Foundation of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering (2015491111); the Open Research Fund of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin (China Institute of Water Resources and Hydropower Research), Grant no. IWHR-SKL-201607; the Technology Demonstration Projects of Ministry of Water Resources (SF2016-10); the Foundation of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin (2016TS07); the Research Development Projects on Application Technology of Heilongjiang Province (GZ16B011); and “the Fundamental Research Funds for the Central Universities”, South-Central University for Nationalities (Grant nos. CZQ15007, CZW15072, and CZW15126).