Reservoir-induced hydrological alterations and ... - Springer Link

Stoch Environ Res Risk Assess (2014) 28:2119–2131 DOI 10.1007/s00477-014-0893-4

ORIGINAL PAPER

Reservoir-induced hydrological alterations and environmental flow variation in the East River, the Pearl River basin, China Qiang Zhang • Mingzhong Xiao • Chun-Ling Liu Vijay P. Singh

•

Published online: 10 May 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract The East River basin is the major source of water supply for megacities in the Pearl River Delta and Hong Kong. Intensifying development of water resources and reservoir-induced hydrological alterations negatively affect ecological hydrological requirements. In this study, hydrological alterations and environmental flow variation are determined. Results indicate that: (1) multi-day maxima have reduced, while multi-day minima have increased, due to hydrological regulations of water reservoirs; (2) hydrological regimes of the East River have also been severely affected by hydropower generation, leading to a greater frequency of high and low pulses of lesser duration, and these effects are increasingly evident from the upper to lower East River basin; (3) owning to the water being released rapidly for hydropower generation or flood protection, the number of hydrologic reversals have increased

Q. Zhang (&) M. Xiao Department of Water Resources and Environment, Sun Yat-sen University, Guangzhou 510275, China e-mail: [email protected] Q. Zhang M. Xiao Key Laboratory of Water Cycle and Water Security in Southern China of Guangdong High Education Institute, Sun Yat-sen University, Guangzhou 510275, China C.-L. Liu Pearl River Hydraulic Research Institute, Pearl River Water Resources Commission, Guangzhou 510611, China V. P. Singh Department of Biological and Agricultural Engineering, Texas A & M University, College Station, TX 77843-2117, USA V. P. Singh Department of Civil and Environmental Engineering, Texas A & M University, College Station, TX 77843-2117, USA

after reservoir operations, also with increasing rise and fall rate; and (4) the alteration of three different types of environmental flow components have been shown in the study, which can be used to support the determination of environmental flow requirements in the East River basin. Keywords Indicators of hydrological alteration (IHA) Range of variability approach (RVA) Environmental flow East River basin

1 Introduction Increasing demands on water resources due to booming socioeconomic development and growing population have resulted in an intensifying, complex conflict between the development of rivers as hydrological and energy sources and their conservation as biologically diverse, integrated ecosystems (Sparks 1992; Postel et al. 1996; Tharme 2003). What’s more, over half of the world’s accessible surface water is already appropriated and will be increased to an astounding 70 % by 2025 (Postel et al. 1996; Postel 1998). However, a vast body of literature supports that a natural flow regime (i.e., the natural flow variability in terms of magnitude, frequency, duration, timing and rate of change) is important to sustain river environment and aquatic ecosystems (Poff et al. 1997; Richter et al. 1997; Arthington et al. 2006). Thus, natural flow regimes are of primary interest in designing environmental flows and therefore essential for water resources management and planning. Faced with the complexity inherent in natural systems, effective ecosystem management of aquatic, wetland and riparian systems requires that existing hydrological regimes be characterized using biologically relevant

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hydrological parameters. With 32 parameters which provide information on some of the most ecologically significant features of surface and groundwater hydrological regimes influencing aquatic, wetland, and riparian ecosystems, Richter et al. (1996) developed Indicators of Hydrological Alteration (IHA) for assessing the degree of hydrological alteration attributable to human disturbance within an ecosystem. To set streamflow-based river ecosystem management targets, Richter et al. (1997) proposed the range of variability approach (RVA) based on the IHA. Since its inception, RVA, developed by Richter et al. (1997), has been applied extensively in the United States (Richter et al. 1997, 1998) and Australia (Arthington et al. 1998). Hence, in this study RVA was employed to provide a comprehensive statistical characterization of ecologically relevant features of flow regime in the East River basin. With the added environmental flow components in the IHA software (Mathews and Richter 2007; The Nature Conservancy 2009), the environmental flow was also analyzed in the study, and this can be used to support the determination of environmental flow needs in the East River basin. The East River basin is a major tributary of the Pearl River basin in South China with a drainage area of 35,340 km2, accounting for about 5.96 % of the Pearl River basin (Fig. 1). Water resources in the East River basin have been highly developed and heavily committed for a variety of uses, such as water supply, hydropower, navigation, irrigation, and suppression of seawater invasion

Stoch Environ Res Risk Assess (2014) 28:2119–2131

(Chen et al. 2010). In recent years the East River has been providing water supply to about 80 % of Hong Kong’s annual hydrological demands (Chen et al. 2010; Wong et al. 2010). Therefore, the availability and vulnerability of the East River water resources systems are of great importance for sustainable social and economic development in the Pearl River Delta, one of the economically developed regions in China, and also for sustainable water supply for Hong Kong. Except for over-exploitation of water resources, the hydrological system of the East River is highly regulated by water reservoirs. Using multi-scale entropy analysis, Zhou et al. (2012) found that the construction of water reservoirs (such as the Xinfengjiang and Fengshuba water reservoirs, Fig. 1) greatly increases the degree of complexity of hydrological processes, and this influence is subjected to damping with the increase of distance between water reservoirs and hydrological stations. In addition, based on daily hydrological data covering 1952–2002 from Longchuan, Heyuan and Lingxia stations, Chen et al. (2010) evaluated hydrological alterations along the upper and middle portions of East River with the use of RVA. However, a sufficiently long hydrological record will improve the accuracy of IHA, and more important are ecoenvironmental implications of hydrological alterations along the East River basin. Development of water resources and hydrological regulations of streamflow regime should take it into consideration environmental streamflow requirements of a river basin. Further, updated streamflow data will be helpful for evaluation of hydrological alterations and possible implications. Therefore, the objectives of this study are: (1) to statistically characterize the temporal variability of hydrological regimes in the East River with the use of IHA; (2) to quantify hydrological alterations associated with dam operations by comparing the hydrological regimes from pre- and postimpact time periods in the East River; and (3) to analyze the eco-environmental influences of hydrological alterations along the East River. The study is organized as follows: Introduction of data is presented in Sect. 2; and analysis methods are described in Sect. 3. Section 4 presents and discusses the results of the study, and, finally, in Sect. 5, the conclusions of the study are discussed. Results of this study will be relevant in scientific water resources management for East River basin whose water resources are highly developed.

2 Data

Fig. 1 Locations of water reservoirs and hydrological stations

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Daily streamflow data at four hydrological stations, i.e., Longchuan, Heyuan, Lingxia and Boluo, were collected (Table 1). The quality of dataset is firmly controlled before its release and all the data are free of missing values. During

Stoch Environ Res Risk Assess (2014) 28:2119–2131 Table 1 Information on hydrological stations and daily streamflow series for the preand post-impact periods

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Station

Longitude

Latitude

Longchuan

115°150 E

24°070 N

Heyuan

114°420 E

23°440 N

Lingxia

114°340 E

23°150 N

Boluo

0

114°28 E

0

23°18 N

Table 2 Information on water reservoirs Water reservoir

Construction period

Drainage area (km2)

Storage capacity (billion m3)

Xinfengjiang

1958–1962

5,740

13.98

Fengshuba

1970–1974

5,150

1.94

Baipenzhu

1977–1987

856

1.22

the period of 1958–1974, a number of reservoirs, such as the Xinfengjiang and Fengshuba reservoirs (Table 2) were built in the East River basin for multiple purposes, such as flood control, water supply, and hydropower. The Xinfengjiang reservoir, the largest multi-functional water reservoir in the East River basin with a storage capacity of 13.9 billion m3, began to store water in October 1959. The Fengshuba reservoir, with a storage capacity of 1.9 billion m3 and a drainage area of 5,150 km2, began to store water in October 1973. To evaluate the influence of water reservoirs on hydrological regimes, streamflow series were divided into pre- and post-segments based on the time when water reservoirs began impounding. The hydrological regimes at the Longchuan station are only influenced by Fengshuba reservoir, and then the change point of pre- and post-segment of streamflow series was 1973. As the Heyuan station is located just downstream to the Xinfengjiang reservoir, the pre and post streamflow series of Heyuan station were divided by the year of 1959. For Lingxia and Boluo stations, however, they are located downstream far away from the Xinfengjiang reservoir. Zhang et al. (2012) found that there is a mutation in 1973 at Lingxia and Boluo stations, thus the pre and post streamflow series of the two stations were divided by the year of 1973, the same as at the Longchuan station. The divisions of the streamflow series are also shown in Table 1.

Pre-impact time series

Post-impact time series

7,699

1952.4.1–1973.3.31

1974.4.1–2009.3.31

15,750

1951.4.1–1959.3.31

1960.4.1–2009.3.31

20,557

1954.4.1–1973.3.31

1974.4.1–2009.3.31

25,325

1954.4.1–1973.3.31

1974.4.1–2009.3.31

Drainage area (km2)

manageable parts, Richter et al. (1996) developed the IHA. The IHA method is based on 32 biologically relevant hydrological parameters to statistically characterize intraannual hydrological variation (Table 3), and then an analysis of the inter-annual variation will be done on these attributes for comparison of hydrological regimes before versus hydrological regimes after that have been altered by human activities. The hydrological parameters computed by IHA are based on the hydrological year, i.e., from April 1st to March 31st of the next year. Definitions of these 32 parameters (Table 3) have been given by Richter et al. (1996). 3.2 Range of variation approach (RVA) Based on IHA, RVA (Richter et al. 1997) evaluates the potential hydrological alterations. RVA uses the pre-impact natural variation of the IHA parameter values as a reference for defining the extent to which natural flow regimes have been altered. The full range of pre-impact data for each parameter is divided into three different categories. For non-parametric analysis, the boundaries between categories are based on percentile values, which are specified by the user. As an example, the default is to place the category boundaries 17 percentiles from the median. This yields an automatic delineation of three categories of equal size: the lowest category contains all the values less than or equal to the 33th percentile; the middle category contains all the values falling in the range of 34th to 67th percentiles; and the highest category contains all the values greater than the 67th percentile (Table 4). If the expected and actual frequencies of the ‘‘postimpact’’ values of the IHA parameters should fall within each category, a Hydrological Alteration (HA) factor can be calculated for each of the three categories as follows (The Nature Conservancy 2009; Chen et al. 2010): Observed frequency expected frequency expected frequency

3 Methodologies

HA ¼

3.1 Indicators of hydrological alteration (IHA) (Richter et al. 1996)

A positive HA value means that the frequency of values in the category has increased from the pre- to the post-impact period (with a maximum value of infinity), while a negative value means that the frequency of values has decreased (with a minimum value of -1). In this study, RVA was

To decompose the temporal complexity inherent in a streamflow regime into ecologically meaningful and

ð1Þ

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Table 3 Summary of IHA parameters and ecological implications (Richter et al. 1996; The Nature Conservancy 2009) IHA parameter group

Hydrological parameters

Ecosystem influences

Group 1: Magnitude of monthly water conditions

Median value for each calendar month

Habitat availability for aquatic organisms Soil moisture availability for plants Availability of water for terrestrial animals

Group 2: Magnitude and duration of annual extreme water conditions

Annual maxima 1-day means

Balance of competitive, ruderal and stress tolerant organisms

Annual minima 3-day means

Creation of sites for plant colonization

Annual maxima 3-day means

Structuring of aquatic ecosystems by abiotic vs. biotic factors



Structuring of river channel morphology and physical habitat conditions

Annual maxima 7-day means Annual minima 30-day means Annual maxima 30-day means Annual minima 90-day means Annual maxima 90-day means Base flow index: 7-day minimum flow/mean flow for year Group 3: Timing of annual extreme water conditions

Julian date of each annual 1 day maximum

Compatibility with life cycles of organisms

Julian date of each annual 1 day minimum

Predictability/avoidability of stress for organisms Access to special habitats during reproduction or to avoid predation

Group 4: Frequency and duration of high and low pulses

Number of high pulse each year Number of low pulse each year Mean duration of high pulses within each year Mean duration of low pulses within each year

Frequency and magnitude of soil moisture stress for plants Frequency and duration of anaerobic stress for plants Availability of floodplain habitats for aquatic organisms Nutrient and organic matter exchanges between river and floodplain

Group 5: Rate and frequency of water condition changes

Rise rates: mean of all positive differences between consecutive daily values Fall rates: mean of all negative differences magnitude between consecutive daily values Number of hydrologic reversals

conducted with the IHA software from http://www.con serveonline.org/workspaces/iha. 3.3 Environmental flow components Drawing from holistic methodologies developed around the world, the ability to calculate characteristics of five different types of Environment Flow Components (EFCs), including low flows, extreme low flows, high flow pulses, small floods, and large floods, has been added to the IHA software (Mathews and Richter 2007). These five types of flow components represent the full spectrum of flow conditions, and must be maintained in order to sustain riverine ecological integrity. Not only is it essential to maintain adequate flows during low flow periods, but higher flows and floods and also extreme low flow conditions perform

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Drought stress on plants Entrapment of organisms on islands, floodplains Desiccation stress on low-mobility stream edge (varial zone) organisms

important ecological functions (The Nature Conservancy 2009). Exact definitions of these five types of EFCs and their ecosystem influence can be referred to The Nature Conservancy (2009). In this study, low flows, high flow pulses, and small floods have been calculated for the flood season (from April to September) and non-flood season (from October to March) to analyze how much those flows have been altered, shown in Table 5. The algorithm used to calculate the three types of EFCs are as follows (The Nature Conservancy 2009): (1) Making the initial split between high flows and low flows. All flows greater than the 75th percentile of daily flows are classified as high flows. And all flows less than or equal to the 50th percentile of daily flows are classified as low flow events. When flows are between high flow and low flow thresholds, a high flow

Stoch Environ Res Risk Assess (2014) 28:2119–2131 Table 4 RVA thresholds for four hydrological stations in the East River basin for IHA parameters

IHA parameter groups

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Longchuan

Heyuan

Low

Low

High

Lingxia High

Low

Boluo High

Low

High

Group 1: Monthly magnitude April

2.81

4.06

7.25

17.16

7.29

11.61

7.65

12.05

May

4.00

7.91

14.69

19.26

12.35

20.61

13.90

23.13

June

6.21

11.96

23.61

29.96

19.08

37.52

24.82

51.31

July

3.42

5.78

11.08

18.18

13.63

17.52

15.77

25.94

August

3.22

4.67

8.15

12.21

14.54

18.40

17.99

24.85

September

2.68

3.68

6.94

9.36

11.42

14.31

14.20

18.49

October

1.79

2.93

4.04

6.49

8.02

11.14

8.58

12.18

November

1.64

2.39

3.55

5.53

6.07

9.31

6.76

10.68

December

1.27

2.03

2.75

4.67

5.31

8.13

5.69

8.84

January February

1.26 1.48

2.05 2.02

2.45 3.79

4.82 4.73

3.99 5.08

7.63 7.90

4.71 5.40

8.22 8.20

March

1.28

2.63

4.72

9.69

5.58

7.78

6.15

8.09

Group 2: Magnitude and duration of annual extremes 1-day minimum

0.64

0.91

0.68

1.93

2.53

3.46

2.72

3.79

3-day minimum

0.67

0.96

1.66

2.42

2.57

3.60

2.79

4.00

7-day minimum

0.69

1.03

1.96

3.03

2.95

4.44

2.88

4.76

30-day minimum

1.04

1.27

2.08

3.68

3.91

5.85

4.17

6.22

90-day minimum

1.30

1.84

2.76

4.17

5.13

7.41

5.53

7.99

1-day maximum

43.47

68.10

92.32

139.40

92.65

143.20

124.60

185.60

3-day maximum

35.35

46.29

83.40

108.70

75.62

125.30

113.30

161.80

7-day maximum

23.09

36.77

56.70

84.29

54.26

100.40

82.98

130.80

30-day maximum

12.65

26.91

39.34

50.26

34.47

60.01

45.36

73.54

90-day maximum

8.77

13.23

24.36

34.04

25.16

35.48

33.19

47.54

Base flow index

0.13

0.20

0.11

0.21

0.20

0.27

0.16

0.23

119.10 172.00

87.88 157.30

261.80 174.90

42.00 166.60

91.00 183.60

60.20 171.00

92.60 210.80

Group 3: Timing of annual extremes Date of minimum Date of maximum

77.00 148.30

Group 4: Frequency and duration of high and low plus Low pulse count

4.00

7.74

2.94

4.15

4.00

8.00

3.60

6.00

Low pulse duration

5.00

10.07

7.92

10.68

4.50

9.70

4.20

8.20

12.26

14.00

9.97

15.00

8.60

11.00

8.00

10.00

3.00

3.37

3.00

4.02

3.50

5.00

4.00

7.10

High pulse count High pulse duration

Group 5: Rate and frequency of change in conditions Rise rate

0.37

0.60

1.01

1.40

0.63

0.90

0.88

1.31

Fall rate

-0.28

-0.21

-0.74

-0.45

-0.73

-0.59

-0.85

-0.64

Number of reversals

81.78

95.00

84.97

97.21

89.40

116.40

78.40

96.60

will begin when flow increases by more than 25 % per day and will end when flow decreases by more than 10 % per day. (2) Choosing the number of high flow classes to specify, and in the paper there are two classes, which will be called high flow pulses and small floods. All the high flow events that have a peak flow greater than or equal to 2 year return interval event will be assigned to the small flood class. All the flow events with a peak flow less than this value will be assigned to the high flow pulse class. The return intervals for small floods and flow level thresholds

used to define high flow pulses are based on data from the pre-impact period.

4 Results and discussions To statistically characterize the temporal variability of hydrological regimes in the East River basin, IHA have been calculated. To quantify hydrological alterations associated with reservoir operations, the box-and-whisker

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Streamflow (cms)

30

Pre

Post

20

Maximum 75 pctile Median 25 pctile Minimum

10 0

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

5

Ratio

4 3 2 1 0

1-day min 3-day min 7-day min 30-day min 90-day min 1-day max 3-day max 7-day max 30-day max 90-day max Base flow

6

Ratio

4

2

0

Date min

Date max

Lo pulse count

Lo pulse L

Hi pulse count

Hi pulse L

Rise rate

Fall rate

Reversals

Fig. 2 Box-and-whisker plots for Longchuan station in the East River basin, and the magnitudes of HA for high, middle and low RVA categories are also indicated by the width of the box from the top down for convenience, where red for positive and green for negative. It should be noted that the boundaries of RVA targets, the 34th and 67th percentiles, for all of the IHA parameters are not shown in the

figure, the ratio in the figure represents the quotient divided by the median of IHA parameters in the pre-impact period, and ‘‘Lo’’ and ‘‘Hi’’ are the abbreviation of ‘‘Low’’ and ‘‘High,’’ respectively, ‘‘pulse L’’ means ‘‘pulse duration’’. These are the same for Figs. 3, 4 and 5, and will not be repeated

plots, showing the median, 25th and 75th percentile values, and minimum and maximum values of IHA, for the preand post-impact time frames are presented. For Longchuan, Heyuan, Lingxia and Boluo stations, the box-and-whisker plots are displayed in Figs. 2, 3, 4 and 5, respectively. For RVA analysis, the range of streamflow variations for the East River were characterized by generating the 32 IHA parameters from the pre-impact period to define the extent of natural flow regimes. The full range of pre-impact data for each parameter was divided into three different categories, the boundaries of RVA targets, the 34th and 67th percentiles, for Longchuan, Heyuan, Lingxia and Boluo stations, as illustrated in Table 4. During the post-impact period, many of the annual values of the 32 IHA parameters fell outside of the RVA targeted range due to the reservoir operation. The HA for high, middle and low RVA categories of the Longchuan, Heyuan, Lingxia and Boluo stations were also calculated (Figs. 2, 3, 4, 5). In order to evaluate the environmental flow variation associated with reservoir operation, the box-

and-whisker plots, showing the median, 25th and 75th percentile values, and minimum and maximum values of three types of EFCs, for the pre- and post-impact period were also constructed. For Longchuan, Heyuan, Lingxia and Boluo stations, the box-and-whisker plots are shown in Figs. 6, 7, 8 and 9, respectively.

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4.1 Magnitude and duration of annual extreme hydrological conditions An evident impact of flood control operations of water reservoirs on hydrological processes is the virtual elimination of high-magnitude flooding. It can be seen from Figs. 2, 3, 4, and 5 that frequencies of multi-day maxima at Longchuan, Heyuan, Lingxia and Boluo stations have decreased. However, the frequencies of multi-day minima have increased from the pre- to the post-impact period. This could be due to the fact that the reservoirs attempt to store water during the flood season for later dry season use for water supply and hydropower generation.

Streamflow (cms)


2125

Pre

60

Post

40 20 0

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.


Dec.

Jan.

Feb.

Mar.

8

Ratio

6 4 2 0


Ratio

6 4 2 0

Date min

Date max

Lo pulse count

Lo pulse L

Hi pulse count

Hi pulse L

Rise rate

Fall rate

Reversals

Fig. 3 Box-and-whisker plots for Heyuan station in the East River basin, and the magnitudes of HA for high, middle and low RVA categories are also indicated by the width of the box from the top down, where red for positive and green for negative

4.2 Timing of annual extreme hydrological conditions With a shift from middle to high and low RVA categories, the timings of annual maxima and minima have also changed (Figs. 2, 3, 4, 5). Due to increased storage capacity of water reservoirs, the frequency of high RVA categories in terms of timing of the annual maxima and minima increased from the pre- to the post-impact periods. However, the frequencies of values in the low RVA categories for the timings of annual maxima and minima have also increased from the pre-impact period, which may be attributed to the reservoirs impounding or releasing water prematurely.

Furthermore, due to hydropower generation wherein water is stored in the reservoir until sufficient water head is attained to generate power efficiently and then rapidly released through the dam turbines, the frequencies of high and low pulses of lesser durations have increased from the pre-impact period to the post-impact period. It can be seen from Figs. 2 and 3 that the frequencies of low pulse counts in the high RVA categories have increased for the Longchuan and Heyuan stations; this may be attributed to the two stations located downstream close to the Fengshuba and Xinfengjiang reservoirs. 4.4 Rate and frequency of change in hydrological conditions

4.3 Frequency and duration of high and low pulses The pulse behavior of the East River has been severely affected. As the reservoirs store flood season flow for later dry season use for water supply, the streamflow has increased in dry seasons, and the frequencies of low pulse durations in the low RVA categories have increased, while they have decreased in the middle and high RVA categories. Meanwhile the frequencies of low pulse counts in the low RVA categories have also increased for the Longchuan, Heyuan, Lingxia and Boluo stations (Figs. 2, 3, 4, 5).

The effect of hydropower generation on the hydrological regimes is not only to trigger higher frequencies of high and low pulses of shorter durations but also to increase the number of hydrograph rises and falls, and rise and fall rates (Figs. 2, 3, 4, 5). However, as reservoirs attempt to store water to reduce peak flows during flood episodes, the variability of rise rates has been reduced from the preimpact period, as the box-and-whisker plots of rise rates are more centralized than the pre-impact period (Figs. 2, 3, 4, 5). Furthermore, it can be seen from Figs. 3 and 5 that the

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80

Streamflow (cms)

Pre

Post

60 40


20 0

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

4

Ratio

3 2 1 0


5

Ratio

4 3 2 1 0

Date min

Date max

Lo pulse count

Lo pulse L

Hi pulse count

Hi pulse L

Rise rate

Fall rate

Reversals

Fig. 4 Box-and-whisker plots for Lingxia station in the East River basin, and the magnitudes of HA for high, middle and low RVA categories are also indicated by the width of the box from the top down, where red for positive and green for negative

frequencies of rise rates for the Heyuan and Boluo stations have decreased in the high RVA categories; this may be due to the fact that the streamflows at the Heyuan and Boluo stations are regulated strongly, as the Heyuan station is located downstream close to the Xinfengjiang reservoir with the largest storage capacity and Boluo station is located at the lower East River basin. 4.5 Magnitudes of monthly hydrological conditions The magnitudes of monthly hydrological conditions along the East River have also been altered by the construction of reservoirs. Since reservoirs tend to store water during flood seasons for later dry season use for water supply and hydropower generation, the frequency of monthly streamflow in non-flood seasons in the high RVA categories has obviously increased. Because reservoirs tend to reduce peak flows in flood seasons for flood control, Figs. 2, 3, 4 and 5 indicate that the largest monthly streamflow is mainly concentrated in June during the pre-impact period, and this streamflow has been reduced as a result of flood control with decreased frequency in the high RVA categories after the pre-impact period. In addition, the decreased frequency of monthly

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streamflow can be observed in April and May in the high RVA categories after the pre-impact period at the Heyuan station. Furthermore, the box-and-whisker plots for Longchuan, Lingxia and Boluo stations show that the variability of monthly streamflow in each calendar month has increased after the pre-impact period except in June and September (Figs. 2, 4, 5). As the two centers of high streamflow in the flood season during the pre-impact period are in June and September, the reservoirs mainly store water to reduce peak flows in these two months and it is reasonable that the variability of monthly streamflow in these two months would decrease during the pre-impact period. 4.6 Low flows As the amount of aquatic habitat available for most of the year is determined, the seasonally-varying low flow levels in a river impose a fundamental constraint on a river’s aquatic communities (The Nature Conservancy 2009). It can be seen from Figs. 6, 7, 8, and 9 that the median values of low flows during each calendar month have increased from the pre- to the post-impact period at Longchuan, Heyuan, Lingxia and Boluo stations and the variability of


2127

Pre

Streamflow (cms)

100

Post

50

0

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.


Dec.

Jan.

Feb.

Mar.

4

Ratio

3 2 1 0


Ratio

10

5

0

Date min

Date max

Lo pulse count

Lo pulse L

Hi pulse count

Hi pulse L

Rise rate

Fall rate

Reversals

Low flows

Fig. 5 Box-and-whisker plots for Boluo station in the East River basin, and the magnitudes of HA for high, middle and low RVA categories are also indicated by the width of the box from the top down, where red for positive and green for negative

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Flood season

Non-flood season

High-flow pulses Small floods

Peak Dur

Time Freq Rise Fall

Peak Dur

Time Rise Fall

Pre Post

Peak Dur Time Freq Rise Fall

Fig. 6 The box-and-whisker plots of three types of EFCs for the preand post-impact period at Longchuan in the East River basin. The high flow pulses and small floods have been calculated separately for flood season and non-flood season while the small floods can be

neglected in the non-flood season. And the y-axes for the high flow pulses and small floods are just used to analyze the variation of the same parameters in the pre- and post-impact periods, and these are same for Figs. 7, 8, and 9

low flows has increased from the pre-impact period in the non-flood season for all the stations. However, in the flood season, the variability of low flows has been reduced from

the pre-impact period in May and June at Longchuan; in May, July and September at Lingxia; and in May, July, August, and September at Boluo. Owing to the downstream

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Low flows

2128

Apr

May

Jun

Jul

Sep

Oct

Nov

Dec

Jan

Feb

Small floods

Flood season Peak Dur

Time Freq Rise Fall

Peak Dur

Time Rise Fall


season and non-flood season while the small floods can be neglected in the non-flood season

Low flows

Fig. 7 The box-and-whisker plots of three types of EFCs for the preand post-impact period at Heyuan in the East River basin. The high flow pulses and small floods have been calculated separately for flood

Mar

Pre Post

Non-flood season

High-flow pulses

Aug

May

Jun

Jul

Flood season

High-flow pulses

Peak Dur

Time Freq Rise Fall

Aug

Sep

Oct

Nov

Small floods

Peak Dur

Dec

Time Rise Fall

Jan

Feb

Mar

Pre Post

Non-flood season

Apr


Fig. 8 The box-and-whisker plots of three types of EFCs for the preand post-impact period at Lingxia in the East River basin. The high flow pulses and small floods have been calculated separately for flood


location close to the Fengshuba and Xinfengjiang reservoirs, the low flows in the flood season at Longchuan and Heyuan stations experience more variability than at Lingxia and Boluo stations. However, as the pre-impact period at Heyuan is not long enough, the results of low flows at Heyuan may be not credible enough.

4.7 High flow pulses

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High flow pulses provide important and necessary disruptions in low flows. They deliver a nourishing subsidy of organic material or other food to support the aquatic food web, and also provide fish and other mobile creatures with

2129

Low flows


May

Jun

Jul

Sep

Oct

Nov

Small floods

Flood season

High-flow pulses

Aug

Peak Dur

Time Freq Rise Fall

Peak Dur

Dec

Non-flood season

Apr

Time Rise Fall

Jan

Feb

Mar

Pre Post


Fig. 9 The box-and-whisker plots of three types of EFCs for the preand post-impact period at Boluo in the East River basin. The high flow pulses and small floods have been calculated separately for flood


increased access to up- and downstream areas (The Nature Conservancy 2009). It can be seen from Figs. 6, 7, 8, and 9 that the variability of peak, duration and time of high flow pulses have increased from the pre-impact period in the flood season for all the stations, meaning that the operations of reservoirs have increased the variability of the high flow pulses in the flood season. The median values of times of high flow pulses have also increased from the pre-impact period in the flood season for all the stations, which may be attributed to the reservoirs attempting to store water to reduce peak flows during flood episodes. The variability of frequency, rise rate and fall rate of high flow pulses in the flood season have not changed from the pre-impact period while the median values of frequency and rise rate have decreased and the median values of fall rates have increased (Figs. 6, 7, 8, 9). Meanwhile, it can be seen from Fig. 9 that the maximum value of rise rate in the post-impact period is much larger than the value in the preimpact period at Boluo station which is different from the patterns at the other three stations. This may be attributed to the fact that reservoirs on the upstream release water rapidly to vacate the reservoir capacity for flood protection or hydropower generation while the rise rates of Boluo station are relatively low in the natural state as it is located on the downstream of East River basin. In the non-flood season, the alterations of high flow pulses from the pre-impact period to the post-impact period are different from those in the flood season. It can be seen from Figs. 6, 7, 8, 9 that the frequencies of high flow pulses

have increased from the pre-impact period with the median values of frequencies of high flow pulses in the post-impact period being larger than those in the pre-impact period and it is more at Longchuan and Heyuan stations. This may be attributed to the fact that reservoirs store flood season flow for later non-flood season use for water supply and the two stations are located downstream close to the Fengshuba and Xingfengjiang reservoirs. This is the same for the peak and time of high flow pulses with the median values of the peak and time of high flow pulses having decreased from the pre-impact period in the non-flood season while increased in the flood season. 4.8 Small floods During floods, fish and other mobile organisms are able to move upstream, downstream, and out into floodplains or flooded wetlands to access additional habitats, such as secondary channels, backwaters, sloughs, and shallow flooded areas. These usually inaccessible areas can provide substantial food resources (The Nature Conservancy 2009). It can be seen from Figs. 6, 7, 8, and 9 that durations of small floods have increased from the pre-impact period, also with increased variability. These are consistent with the results of high flow pulses in the flood season. Meanwhile, as the reservoirs on the upstream release water rapidly to vacate the reservoir capacity for flood protection or for hydropower generation, the rise rate of small floods has also increased from the pre-impact period at all the

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Table 5 Summary of environmental flow component parameters calculated in this study and their ecosystem influences (The Nature Conservancy 2009) Type

Hydrologic parameters

Ecosystem influences

1. Monthly low flows

Median values of low flows during each calendar month

Provide adequate habitat for aquatic organisms Maintain suitable water temperatures, dissolved oxygen, and water chemistry Maintain water table levels in floodplain, soil moisture for plants Provide drinking water for terrestrial animals Keep fish and amphibian eggs suspended Enable fish to move to feeding and spawning areas

2. High flow pulses

Frequency of high flow pulses during each season

Shape physical character of river channel, including pools, riffles Determine size of streambed substrates (sand, gravel, cobble)

Median values of high flow pulse event:

Prevent riparian vegetation from encroaching into channel

Duration (days) Peak flow (maximum flow during event) Timing (Julian date of peak flow)

Restore normal water quality conditions after prolonged low flows, flushing away waste products and pollutants Aerate eggs in spawning gravels, prevent siltation Maintain suitable salinity conditions in estuaries

Rise and fall rates 3. Small floods

Median values of small flood event:

Provide migration and spawning cues for fish

Duration (days) Peak flow (maximum flow during event)

Trigger new phase in life cycle Enable fish to spawn in floodplain, provide nursery area for juvenile fish

Timing (Julian date of peak flow)

Recharge floodplain water table

Rise and fall rates

Control distribution and abundance of plants on floodplain

Provide new feeding opportunities for fish, waterfowl

Deposit nutrients on floodplain

stations except Heyuan station. As the pre-impact period at Heyuan station is not long enough, the results of small floods at Heyuan station may be not credible enough.

2.

5 Conclusions and brief discussions The East River basin plays a crucial role in terms of water supply for megacities in the Pearl River Delta and Hong Kong. However, human overexploitation of fresh water resources and significant hydrological alteration (Chen et al. 2010) as a result of the hydrological regulations of water reservoirs exert tremendous impacts on the ecological water requirements and the maintenance of local fluvial ecological systems. In this study, hydrological alterations are analyzed, based on the RVA technique and environmental flow needs and impacts of water reservoirs on the environmental flow changes are determined. The following conclusions are drawn from this study: 1.

Hydrological alterations are evident due to the construction of water reservoirs in the East River basin. The multiday maxima have reduced from the pre-impact period due to the elimination of high-magnitude flooding, and the multi-day minima have increased from the pre-impact period as the reservoirs attempts to capture flood season flow for later dry season use for water supply.

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3.

The hydrological regimes of the East River have been severely affected by hydropower generation. The effect of hydropower generation on the hydrological regime leads to both increased frequencies of high and low pulses of shorter durations and increased numbers of hydrograph rises and falls, and rise and fall rates. Being located downstream close to Fengshuba and Xinfengjiang reservoirs, the pulse behavior of hydrological flows at Longchuan and Heyuan stations has been greatly altered. Thus, the degree of hydrological alterations is in close relation with distance between hydrological stations and water reservoirs. Environmental flow setting is inherently an interdisciplinary process, integrating information about hydrology, morphology, biology, and other aspects of a water body. The EFCs calculated by the IHA software show how much those flows have been altered in the East River basin, which can be used by ecologists, water managers, and other stakeholders to decide on an acceptable amount of flow alteration for different times of the year and for different years.

Acknowledgments This work was financially supported by The National Natural Science Foundation of China (Grant No. 41071020), Program for New Century Excellent Talents in University (NCET), and is fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No.

Stoch Environ Res Risk Assess (2014) 28:2119–2131 CUHK441313). Our cordial gratitudes should be extended the editor, Prof. Dr. George Christakos, and two anonymous reviewers for their professional comments and revision suggestions which are greatly helpful for further improvement of this manuscript.

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