Effects of reservoirs on seasonal discharge of Irtysh River measured by Lepage test

Feng HUANG*, Zi-qiang XIA, Li-dan GUO, Fu-cheng YANG

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Hydrology and Water Resources, Hohai University, Nanjing 210098, P. R. China

3. Sichuan Electric Power Design and Consulting Co., Ltd., Chengdu 610016, P. R. China

Effects of reservoirs on seasonal discharge of Irtysh River measured by Lepage test

Feng HUANG*1,2, Zi-qiang XIA1,2, Li-dan GUO1,2, Fu-cheng YANG3

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Hydrology and Water Resources, Hohai University, Nanjing 210098, P. R. China

3. Sichuan Electric Power Design and Consulting Co., Ltd., Chengdu 610016, P. R. China

The Irtysh River is an international river partially joining the territories of China, Kazakhstan, and Russia. Cascade reservoirs have been constructed in the upper reaches of the river and their effects on the seasonal discharge of the middle and lower reaches were analyzed considering the mean and dispersion of the seasonal discharge. The Lepage test, which is a nonparametric, two-sample test for detecting location and dispersion, was used to measure the significance of difference between the pre-dam and post-dam seasonal discharge. The results show that the reservoirs’ effects on the seasonal discharge varied with the season. In the middle reaches of the river, the summer and autumn discharge decreased significantly and their inter-annual variabilities also decreased significantly. The summer and autumn precipitation over the Irtysh River Basin changed little before and after the operation of the reservoir, which indicates that the discharge changes mainly due to water storage of the reservoirs. The reservoirs store water in summer and autumn and store more water in a wet year, which leads to the reduction of the mean and dispersion of the summer and autumn discharge. The winter discharge increased significantly because the reservoirs released water for power generation. The spring discharge changed slightly. In the lower reaches, only the winter discharge increased significantly, and the other seasonal discharge changed slightly. The reservoirs’ effects on the seasonal discharge are more significant in the middle reaches than in the lower reaches.

seasonal discharge; effects of reservoirs; Lepage test; Irtysh River

1 Introduction

One of the main hydrologic impacts of dams is the decrease in streamflow variability due to peak flow damping and minimum flow enhancement, which lead to flow homogenization downstream of dams. The homogenization also occurs with respect to the inter-annual variability of streamflow (Ye et al. 2003; Yang et al. 2004a; Lu and Siew 2006; Lajoie et al. 2007; Poff et al. 2007; Assani et al. 2011; Zhang et al. 2012), where the trend becomes almostuniform downstream of dams. From an ecological standpoint, more uniform streamflow and variability cause homogeneity of aquatic fauna, leading to significant loss of biodiversity and the loss of native species (Moyle and Mount 2007; Guo et al. 2011; Yu et al. 2014).

Three reservoirs were constructed in the upper reaches of the Irtysh River in the 1950s and 1980s. Considering the influences of the reservoirs, the seasonal discharge series were divided into pre-dam and post-dam sub-series, and the significance of the difference between the two sub-series was measured by the Lepage test (Lepage 1971). The Lepage test has been demonstrated to be more statistically powerful than other similar tests, such as the Student’s t test, the chi-square test, and the Wilcoxon-Mann-Whitney test (Hirakawa 1974). This method has few underlying assumptions and has been successfully applied in many hydrological and meteorological studies (Yonetani and McCabe 1994; Chen et al. 2009; Yang et al. 2009; Zhang et al. 2009; Liu et al. 2011).

The hydrological regime of the Irtysh River has changed dramatically over recent decades, probably due to the influence of human activities and climate changes (McClelland et al. 2004). Yang et al. (2004b) found that the streamflow decreases in summer and increases in winter in the upstream reaches of the Irtysh River Basin. Huang et al. (2012) found abrupt decreases in annual runoff and abrupt decreases in variability and concentration degree of intra-annual runoff in the middle reaches of the Irtysh River. Generally, much attention has been paid to the change trend and the intra-annual variability in previous studies. In this study, the seasonal discharge of the Irtysh River was analyzed, with a focus on its inter-annual variability. The results can add to our understanding of the impacts of multi-year regulating reservoirs on the Irtysh River and provide a vital basis for river management.

2 Study area and data

The Irtysh River (Fig. 1), the largest tributary of the Ob River, is 4 248 km long and drains an area of approximately 160 × 104km2. It originates in the Altai Mountains in the Sinkiang Uyghur Autonomous Region, in China, flows west through Zaysan Lake and northwest across Eastern Kazakhstan, and finally joins the Ob River in Khanty-Mansiysk City, in Russia. The range of the mainstream basin in Kazakhstan and China is 59.6 × 104km2, of which 5.6 × 104km2is the headwater area (Hrkal et al. 2006). The Irtysh River has two main tributaries: the Ishim River and the Tobol River, which are both on the left side. The Ishim River is 2 450 km long and drains an area of 14.4 × 104km2. The Tobol River is 1 591 km long and drains an area of 42.6 × 104km2.

The Irtysh River Catchment has various natural conditions. The basin can be divided into two completely different parts in terms of morphology: the headwater area and a flat country area. The headwater area lies in the Altai Mountains, where local rocks are crystalline Paleozoic complex metamorphic rocks with some magmatic intrusions. The flat country area of the river catchment is part of the southern tip of the Siberian platform. Different types ofclimate can be observed in the headwater area. This region is an important water source for the whole catchment, and long-term annual total precipitation varies between 1 500 and 2 000 mm. The region has more evenly distributed precipitation than the Siberian platform. The flat country area, which is in the middle reaches of the Irtysh River Catchment, shows characteristics of a continental climate with extreme temperature values in the winter and summer. Generally, spring is from April to May, which is the snowmelt period, summer is from June to August, autumn is from September to October, and winter is from November to March of the next year.

Large cascade reservoirs (Table 1) have been constructed in the upper reaches of the Irtysh River (Fig. 1). The Ust-Kamenogorsk Reservoir is the re-regulating reservoir of the Bukhtarma Reservoir, whose effective volume is only 1.7 × 108m3, and, thus, its influence on the discharge regime is relatively smaller compared with the Bukhtarma Reservoir. Therefore, the study period was segmented by the year 1960, when the Bukhtarma Reservoir was constructed. The Shul’binsk Reservoir was constructed in 1989. Thus, the post-dam discharge regime has mainly been influenced by the Bukhtarma Reservoir before 1989 and by cascade reservoirs after 1989.

Table 1 Reservoirs in upper reaches of Irtysh River

Fig. 1 Irtysh River Basin

Two hydrometric stations with long-term observed data, one in a middle reach and the other in a lower reach of the Irtysh River, were selected (Fig. 1). The Omsk Hydrometric Station is located at 55.0°N and 73.4°E and drains an area of about 76.9 × 104km2. The multi-year mean runoff of the Omsk Hydrometric Station is about 277 × 108m3. The Tobolsk Hydrometric Station is located at 58.2°N and 68.2°E and drains an area of about 150 × 104km2. The multi-year mean runoff of the Tobolsk Hydrometric Station is about 676 × 108m3. Monthly discharge data of the two hydrometric stations from 1936 to 1999 were collected. To investigate the reservoirs’ effects on the seasonal discharge, the discharge series were divided into two sub-series: the pre-dam series (1936 to 1960) and the post-dam series (1961 to 1999).

The Irtysh River has different water sources in different seasons. It is mainly supplemented by precipitation in summer and autumn. Twelve meteorological stations (Fig. 1) with long-term observed precipitation data were selected (Table 2). The monthly precipitation data from summer and autumn were collected and analyzed with the Lepage test.

Table 2 Meteorological stations of Irtysh River Basin

3 Methods

The Lepage test is a nonparametric, two-sample test for detecting location and dispersion, and for measuring the significant difference between two samples, even if the distributions of parent populations are unknown. The Lepage test is a sum of the squares of the standardized Wilcoxon’s and Ansari-Bradley’s statistics:

where W and A are Wilcoxon’s and Ansari-Bradley’s statistics, respectively, E is the mean value, and V is variance value. HK is the index of the Lepage test. It follows the chi-square distribution with two degrees of freedom. If HK exceeds 5.99 (4.21, 9.21), then the difference between the two samples corresponds to a significant confidence level of 95% (90%, 99%). The Lepage test is calculated as follows: Let x= (x1, x2,...xn1) and y= (y1, y2,...yn2) be two independent sam ples with sizes n1and n2, respectively. XY= (x1,x2,...xn1,y1,y2,...yn2) is the combined sample of x and y. Assume that ui=1 if the ith smallest data in XY belongs to x and ui=0 if it belongs to y.

The statistics in Eq. (1) can be derived based on the following equations:

If n1+n2is even,E(A) and V (A) are estimated as

Otherwise,

Statistical features of the segments divided by the change point are represented by the mean and the coefficient of variation (Cv). Furthermore, the empirical frequency curves (EFCs) of the segments are drawn to display the changes of the mean and the dispersion.

The EFC is a simple and effective method of summarizing the distribution of discharge for a given catchment. The shape of the EFC is determined by the precipitation pattern, size, and physiographic characteristics of a catchment. The shape of the EFC can also be influenced by water resources development and land use. The EFC is widely used to measure the discharge regime as it provides an easy way of displaying the complete range of discharge. It can also be used to assess the changes in the discharge regime affected by human activities and climate changes, by considering discharge changes in percentiles. Each value of discharge (Q) has a corresponding exceedance probabilityp, and an EFC is simply a plot of Qp, which is the pth quantile or percentile of Q versus exceedance probability p, with p defined as

The quantile Qpis a function of observed discharge, and since this function depends upon observations, it is often termed the empirical quantile function.

4 Results and discussion

4.1 Lepage test results of seasonal discharge change

The seasonal discharge series were divided into two sub-series by the year 1960, when the Bukhtarma Reservoir began to operate. The Lepage test was used to measure whether the post-dam seasonal discharge changed significantly. The results are shown in Table 3.

For the Omsk Hydrometric Station, the discharge changed more significantly in summerthan in other seasons. Both the mean and the standard deviation of the summer discharge series decreased after the operation of the reservoir. The mean of the summer discharge decreased from 1 730 m3/s to 1 041 m3/s. The autumn discharge also changed significantly and the pattern was similar with that of the summer discharge. However, the mean of the winter discharge increased by 31%, from 368 m3/s to 483 m3/s, after the operation of the reservoir. The spring discharge changed slightly after the operation of the reservoir.

The Omsk Hydrometric Station is located in a middle reach of the Irtysh River, where the annual runoff is nearly equal to the effective volume of the Bukhtarma Reservoir, so its seasonal discharge was dramatically changed by the operation of the reservoir. The Tobolsk Hydrometric Station is located in a lower reach, where the water sources are more complex, so the seasonal discharge was not significantly influenced by the operation of the reservoir. The Lepage test shows that only the winter discharge significantly increased, by 25%, from 717 m3/s to 893 m3/s. The Irtysh River is mainly supplemented by the groundwater in the winter, so the water released from the reservoirs increases the winter discharge directly and effectively.

Table 3 Statistical features of seasonal discharge and Lepage test results

4.2 EFC analysis

The EFCs of the pre-dam and post-dam seasonal discharge are shown in Fig. 2 and Fig. 3, which describe the changes of the mean and the dispersion of the seasonal discharge. For the pre-dam and post-dam sub-series, the EFCs of the seasonal discharge that changed slightly are nearly coincident. For the Omsk Hydrometric Station, the post-dam EFC of the summer discharge is remarkably lower than the pre-dam EFC, indicating a dramatic decrease in the mean of the summer discharge. Furthermore, the dispersion of the summer discharge also decreased. The summer discharge ranged from 698 m3/s to 2 923 m3/s during the period from 1936 to 1960 and from 586 m3/s to 1 676 m3/s during the period from 1961 to 1999. The dispersion of the autumn discharge also obviously decreased. The autumn discharge ranged from 375 m3/s to 1 760 m3/s during the period from 1936 to 1960 and from 472 m3/s to 910 m3/sduring the period from 1961 to 1999. For the winter discharge at the Omsk and Tobolsk hydrometric stations, the post-dam EFCs are high above the pre-dam EFCs, indicating a significant increase in the mean of the winter discharge.

Fig. 2 EFC changes of seasonal discharge at Omsk Hydrometric Station

Fig. 3 EFC changes of seasonal discharge at Tobolsk Hydrometric Station

For the purposes of flood control, irrigation, power generation, and some other reasons, the reservoirs store water in summer and autumn and release water in winter, which reduces the intra-annual variability of discharge. Furthermore, the reservoirs store more water in wet years to maximize the power generation and reduce the inter-annual variabilities of the summer and autumn discharge.

Feng HUANG et al. Water Science and Engineering, Oct. 2014, Vol. 7, No. 4, 363-372 369

4.3 Lepage test results of summer and autumn precipitation changes

Precipitation is the main water sources of the Irtysh River in summer and autumn. If the summer and autumn precipitation changed little before and after the operation of the reservoir, this can indicate that the discharge changed mainly due to the operation of the reservoir. The summer and autumn precipitation was analyzed with the Lepage test. The results are shown in Table 4. At the meteorological station 36665, the autumn precipitation increased significantly. In general, the summer and autumn precipitation over the Irtysh River Basin changed only slightly and would not have led to the seasonal discharge reduction. Therefore, the reduction in the summer and autumn discharge may be mainly attributed to the operation of the reservoir. It should be noted that some other reasons, e.g., increased agricultural and industrial water use, may also have contributed to the discharge reduction. However, their influences are much smaller compared with those of large reservoirs.

Table 4 Statistical features of summer and autumn precipitation and Lepage test results

5 Conclusions

The Lepage test was used to evaluate the effects of reservoirs on the seasonal discharge of the Irtysh River. The summer and autumn precipitation was analyzed to separate the reservoirs’ effects from the natural influences. The main conclusions are as follows:

(1) For the Omsk Hydrometric Station, located in a middle reach, the summer and autumn discharge decreased significantly and their inter-annual variabilities also decreased significantly. The winter discharge significantly increased and the spring discharge changed slightly.

(2) For the Tobolsk Hydrometric Station located in a lower reach, only the winter discharge increased significantly due to the operation of the reservoir, and the discharge in other seasons changed slightly.

(3) The summer and autumn precipitation changed little after the beginning of operation of the reservoir. This phenomenon indicates that the discharge reduction is mainly attributable to the operation of the reservoir.

(4) The effects of the reservoirs on the seasonal discharge varied with the season. The reservoirs’ effects are greater on the seasonal discharge in the middle reaches than in the lower reaches. The water storage of the reservoirs significantly reduced the mean and dispersion of the summer and autumn discharge in the middle reaches, but only slightly affected in the lower reaches. The reservoirs’ effects on the winter and spring discharge of the middle reaches are coincident with those of the lower reaches. The reservoirs increase the winter discharge but only slightly change the spring discharge.

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(Edited by Fang WANG)

This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2013/B13020312), the Ministry of Water Resources’ Special Funds for Scientific Research on Public Causes, for the People’s Republic of China (Grant No. 201001052), and the Innovative Project of Scientific Research for Postgraduates in Ordinary Universities of Jiangsu Province (Grant No. CXZZ11_0433).

*Corresponding author (e-mail: hfeng0216@163.com)

Received Sep. 17, 2013; accepted Mar. 6, 2014