Journal of Hydrology 368 (2009) 96–104

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Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol

Multiscale variability of sediment load and streamflow of the lower Yangtze River basin: Possible causes and implications Qiang Zhang a,b,*, Chong-Yu Xu c, V.P. Singh d, Tao Yang e a

State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China Institute of Space and Earth Information Science, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China c Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway d Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX 77843-2117, USA e State Key Laboratory of Hydrology-Water Resources and Hydraulics Engineering, Hohai University, Nanjing 210098, China b

a r t i c l e

i n f o

Article history: Received 9 September 2008 Received in revised form 20 January 2009 Accepted 22 January 2009

This manuscript was handled by G. Syme, Editor-in-Chief Keywords: Sediment load Streamflow variations Scanning t-test Scanning F-test Yangtze River basin

s u m m a r y Long monthly streamflow and sediment load series observed at the Datong station located in the lower Yangtze River basin were analyzed using the scanning t-test, F-test and coherency analysis techniques. The results indicated that: (1) different changing properties of the first and the second moments of the hydrological series on different time scales were observed, reflecting different driving factors influencing the hydrological processes of the lower Yangtze River basin; (2) a generally decreasing trend can be identified after the mid-1980s. Significant abrupt changes in sediment load were analyzed in the sediment load series. However, more complicated changing patterns can be observed in the changes in streamflow. Generally decreasing sediment load and increasing streamflow gave rise to anti-phase relations between sediment load and the streamflow on longer time scales. In-phase relations between sediment load and streamflow on shorter time scales may imply a considerable influence of the hydrological dynamics on sediment transport; and (3) human activities, particularly the construction of water storage reservoirs, exerted a massive influence on sediment load variations. Construction of a large amount of water reservoirs on the tributaries of the Yangtze River and the Gezhouba Dam on the mainstem of the Yangtze River seem to be the main factors responsible for abrupt changes in the sediment load. Construction of the Three Gorges Dam causes a sharp decrease and unstable variability in sediment load variations, which may pose new challenges for the ecological environment conservation and the deltaic management of the Yangtze Delta region. Ó 2009 Elsevier B.V. All rights reserved.

Introduction The reduction in the river-supplied sediment to coastal areas and its consequent impact on the coastal environment has become a global topic in recent years (e.g., Syvitski et al., 2005). This is because the population of the world is increasingly moving toward coastal areas—about 60% (3.6 billion) of the world’s population lives within 60 km (37 miles) of sea coasts (UNESCO, 1998). Human activities and construction of dams and water reservoirs in particular should bear the major responsibility for the reduction in terrestrial sediment to coastal areas. Walling and Fang (2003) pointed out that reservoir construction is probably the most important influence on land–ocean sediment fluxes. Therefore, a goal of the International Geosphere Biosphere Program (IGBP) and its core project,

* Corresponding author. Address: State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China. Tel.: +852 26096639. E-mail address: [email protected] (Q. Zhang). 0022-1694/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2009.01.030

Land–Ocean Interaction in the Coastal Zone (LOICZ), has been to survey the terrestrial sediment supply to the coast and to analyze human perturbation of this flux (Syvitski, 2003). A number of studies have investigated changes in sediment load and streamflow, associated underlying causes and subsequent impacts on the coastal environment. Zhang et al. (2008b) analyzed annual water discharge and sediment load series (1950s–2004) at nine stations in the main channels and main tributaries of the Zhujiang (Pearl River), demonstrating a significantly decreasing sediment load at some stations in the main tributaries and more stations have witnessed significantly decreasing sediment loads since the 1990s. They also indicated that the decreasing sediment load of the Pearl River basin was the result of hydrological regulations of water reservoirs. Similarly, tremendous changes in water discharge and sediment load have also occurred in other rivers in China, such as the Yellow River basin. The changes in streamflow and sediment load of the Yellow River basin can also be beneficial. The mean annual sediment load of the Yellow River in China has declined markedly due to reduced precipitation, increased water

Q. Zhang et al. / Journal of Hydrology 368 (2009) 96–104

withdrawal and the implementation of comprehensive soil and water conservation and sediment control programs within the loess region of the Middle Yellow River (Mou, 1991; Milliman, 1997; Walling and Fang, 2003; Wang et al., 2003; Walling, 2006). The Yangtze River (Changjiang, Fig. 1), being the longest river in China and the third longest river in the world, plays a vital role in the economic development and ecological environmental conservation of China. Numerous water reservoirs have been built in the Yangtze River basin. However, most of the reservoirs are located in the tributaries. Based on Xu (2005), up to the end of 1980s, there are 11,931 water reservoirs appeared in the upper Yangtze River basin with a total storage capacity of 2.05  1010 m3. The construction of the Three Gorges Dam started in 1993 and will be ended in 2009 with the total storage capacity of 3.93  1010 m3. The Gezhouba Dam started its construction in 1970 and the operation started in earlier 1980s with the total storage capacity of 1.58  109 m3. The appearance of water reservoirs greatly altered the hydrological processes of the Yangtze River basin (Zhang et al., 2006b, 2008a). The sediment load and water discharge changes directly affect the ecological environment of the river basin and also influence the propagation and recession as well as the wetland ecological environment of the Yangtze Delta. A number of studies have investigated the changes in water discharge and sediment load. Zhang et al. (2006a) analyzed annual maximum streamflow and water level, indicating increasing streamflow in the middle Yangtze River and consistently increasing water level from the upper to the lower reaches of the river. Based on annual streamflow and sediment load data of hydrological stations along the mainstem and main tributaries of the Yangtze River, Zhang et al. (2006b) indicated that water reservoirs exerted more influence on sediment transport than water discharge and this influence was more significant in the tributaries than in the mainstem of the Yangtze River basin. Chen et al. (2001) analyzed hydrological records (covering a 100-year period) from the upper, middle, and lower portions of the Yangtze River.

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They found increasing streamflow from the upper to the lower reaches but identified decreasing sediment load. They also pointed out quite stable sediment load changes at the Datong station. Yang et al. (2006) analyzed the 1951–2004 time series of annual sediment supply and coastal bathymetric data, illustrating a significant decreasing trend in riverine sediment supply since the late 1960s and attributed this decreasing sediment to dam construction. They also indicated that the subsequent result of the decreasing sediment load may be responsible for the recession of the deltaic coast which poses a great challenge to coastal management. It should be noted here that the hydrological data the previous study used were annual sediment load and streamflow data. Therefore, higher resolution of the hydrological data should be analyzed to achieve deeper insight into variations of sediment load and streamflow of the Yangtze River basin, and which is sure to be practically and scientifically significant in sound understanding of influences human activities exert on hydrological processes and in ecological environmental conservation of the Yangtze River delta. Moreover, more than one factor results in alterations of the hydrological processes and these driving factors have the potential to influence changing properties of sediment load and streamflow on different time scales. Thus, it is necessary to understand abrupt changes and associated statistical properties of the hydrological series on diverse time scales with robust techniques. This is the major motivation of this study. Better understanding of the dynamic processes of sediment transport and water discharge needs more detailed datasets, such as monthly data (Walling and Fang, 2003). Therefore, in the current study, we analyzed long monthly sediment load and streamflow data from the Datong station since this station is the last control hydrological station reflecting the flux of sediment load and water discharge from the Yangtze River basin to the East China Sea. Statistical techniques were used with the aim to obtain a better understanding of the variations in streamflow and sediment load

Fig. 1. Location of the Yangtze River basin and the Datong hydrological station. TG Dam: Three Gorges Dam; GZB Dam: Gezhouba Dam. Shaded gray area denotes the Yangtze River basin.

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Fig. 2. Sediment load and runoff series of the Datong station, the Yangtze River basin.

on different time scales. Thus, the objectives of this paper were: (1) to analyze general trends within long sediment load and streamflow series at the Datong station; (2) to investigate streamflow and sediment load variations on different time scales; and (3) to discuss possible causes, particularly the influence of Three Gorges Dam on the dynamic processes of sediment load and streamflow of the lower Yangtze River, and implications for the wetland ecological environment of the deltaic coast. Data

cesses on a longer time scale and human activities such as construction of water reservoirs can promptly cause decreasing sediment transport and the abrupt changes occurred. Abrupt changes occurred to the hydrological series are usually the results of sharp alterations of the way the influencing factors work on the hydrological processes or new driving factors available. Thus, it is necessary to understand abrupt changes of the hydrological series on diverse time scales. In this study, abrupt changes were analyzed using the scanning t-test technique on different time scales. Stable or unstable status of hydrological variations is another important statistical property of the hydrological series since that unstable sediment load and streamflow variations have the potential to seriously affect scouring and filling of river channels and also for the growth and recession of the river delta. Stable or unstable variations of the hydrological series were evaluated by the change in the standard deviation, which was analyzed by the F-test technique. Moreover, phase and anti-phase relations between streamflow and sediment load variations were determined by the coherency analysis method. The scanning t-test, the scanning Ftest and the coherency analysis methods are now introduced. Jiang et al. (2002) grafted the wavelet technique (Kumar and Foufoula-Georgiou, 1994) onto the Student t-test and the F-test (Cramer, 1946) with the aim to develop algorithms for scanning t-test and F-test, respectively. The scanning t-test attempts to detect significant changes in the first moment (subseries mean or average) on different time scales within a long time series; while the scanning F-test attempts to analyze significant changes in subseries variance (the second moment) on various time scales (Jiang et al., 2007). In the scanning t-test, statistic t(n, j) is defined as the difference of the subsample averages between every two adjoining subseries with equal subseries size (n) expressed as:

tðn; jÞ ¼ ðxj2  xj1 Þ  n1=2  ðs2j2 þ s2j1 Þ1=2

ð1Þ

3

In this study, we used the monthly streamflow data (m /s) and monthly sediment load data (kg/s) observed at the Datong station (Fig. 1). The time series of streamflow and sediment load data cover the period from January 1951 to December 2006 and January 1963 to December 2006, respectively (Fig. 2). All hydrological data were provided by the Changjiang (Yangtze) Water Resource Commission (CWRC). The quality of the hydrological data was firmly controlled before its release. The sediment load data used in this paper refer to suspended sediment. Sediment concentration data were collected on a monthly basis using equal width increment sampling and sediment yields were determined using the relationship between runoff and sediment concentration (Zhang et al., 2006b). There are no missing data in the streamflow series, and missing data can be found within the sediment load series in 1965, 1966, 1968, 1976, and 1978. The existence of missing data results in a decrease in the sample size available for the analysis. To make full use of the data we have, and also to keep the statistical properties of the entire series, we replaced the missing data with the multi-annual mean sediment load of the respective individual months. We believe that this processing procedure can largely keep the basic statistical properties of the series and has little influences on the general trends and also on our analysis results. Methodology In this study, we used the difference of the hydrological series from its mean value to show the relative changes of streamflow or sediment load from its mean in different periods. A negative difference indicates a decrease and vice versa. We note that there is more than one factor influencing the variations of streamflow and sediment load series and the way the driving factors alter the hydrological processes is different in terms of time scales. Some factors like climate changes can alter the hydrological pro-

where

xj1 ¼

j1 1 X xðiÞ; n i¼jn

s2j2 ¼

jþn1 1 X ðxðiÞ  xj2 Þ2 ; n  1 i¼j

xj2 ¼

jþn1 1 X xðiÞ; n i¼j

s2j1 ¼

j1 1 X ðxðiÞ  xj1 Þ2 ; n  1 i¼jn

in which subsample size n may vary as n = 2, 3, . . . , 1.0, the abrupt change is significant at the 95% confidence level. tr(n, j) < 1.0 denotes a significant decrease and tr(n, j) > 1.0 a significant increase. The scanning F-test technique defines significant changes in subseries variances. Statistic Fr(n, j) is defined as:

8 2 2 > < ðSj1 =Sj2 Þ=F a ; for Sj2 < Sj1 ; for Sj2 ¼ Sj1 or Sj1 ¼ 0; F r ðn; jÞ ¼ 0; > : 2 2 ðSj2 =Sj1 Þ=F a ; for Sj2 > Sj1 ;

Sj2 ¼ 0; ð3Þ

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where the subsample standard deviations Sj1 and Sj2 are calculated in the same way as in Eq. (1). Fa is a threshold value based on the effective degree of freedom after the correction of dependence and in a normalized distribution for the time series. The effective degree of freedom for the correction of the dependence was estimated as (Hammersley, 1946):

When statistic trc ðn; jÞ > 1:0 with both jtru ðn; jÞj; jtrv ðn; jÞj > 1:0, the two series have abrupt changes in the same direction; while if t rc ðn; jÞ < 1:0, the two series have abrupt changes in opposite directions (Jiang et al., 2002). The coherency of abrupt changes between monthly streamflow series and monthly sediment load series can be taken as an indication of the interaction between these two series on decadal and basin scales.

Two time intervals with obvious positive differences of sediment load can be identified as: 1963–1970 and 1975–1985. After the late 1980s, sediment load turned to decrease. After about 2000, this decrease tendency became sharp, represented by larger changing grades, i.e., more dense contours of sediment load difference. Comparatively, the sediment load of January, February, November, and December experienced mild variations, and this was particularly true for sediment changes in January and February. Fig. 3 shows that the sediment load of January and February fluctuates around the mean value. The sediment load of November and December showed a positive difference before the mid-1970s and turned to a negative difference after the mid-1970s. The increase and decrease can be observed before 1970 and after 2000, respectively. Comparatively, streamflow changes are characterized by complex changing properties (Fig. 4). In terms of streamflow changes in January, February, and March, negative differences can be detected before the mid-1980s. After 1985, streamflow turned to a positive difference. Streamflow in April, May, and June had a negative difference before the late 1980s. Positive differences can be detected during late the 1980s and 2002. After about 2002, streamflow changes turned to the negative difference. With respect to streamflow changes in July–December, three time intervals with positive streamflow differences can be identified: before 1970, 1980–1985, and 1990s–2002. Correspondingly, three time intervals with negative streamflow differences can be found: 1970– 1980, 1985–1990s, and 2002–2006. Generally, decreasing streamflow was observed in March–December after 2002.

Results

Scanning t- and F-test of streamflow variations

Difference analysis of sediment load and streamflow changes

We present the scanning t-test analysis for the long streamflow series to define when the change points occurred in the lower Yangtze River basin (Fig. 5). The streamflow series from the Datong station displayed an abrupt decrease in 1956 (on a time scale of 45 months), 1977 (on a time scale of 54 months), 1984 (on a time scale of 45 months), and 2001 (on a time scale of 64 months). Among these detected abrupt changes, only abrupt changes in 1956 and 1984 were significant at the 95% confidence level. Similarly, Fig. 5 also indicates an abrupt increase in 1963 (on a time scale of 91 months), 1972 (on a time scale of 45 months), 1980 (on a time scale of 32 months), and 1988 (on a time scale of 38 months). The significance test indicated that only the abrupt changes in 1980 and 1988 were significant at the 95% confidence level. Generally, different abrupt changes can be observed in the streamflow series from the Datong station which varies with different time scales. On a time scale longer than 180 months, only

Ef ðnÞ ¼ f ðnÞ 

" k X

#1 r 2 ðsÞ

;

rðkÞ ! 0;

ð4Þ

s¼0

where f(n) is the degree of freedom listed in the F table. A local minimum in Fr(n, j) < 1.0 denotes a significant change towards a smaller variance, i.e., the record becomes much steadier; whereas a local maximum in Fr(n, j) > 1.0 indicates a significant change towards a larger variance, i.e., the record becomes much unsteadier (Jiang et al., 2007). Finally, the coherency of abrupt changes between two series u and v was defined as

t rc ðn; jÞ ¼ sign½tru ðn; jÞt rv ðn; jÞfjtru ðn; jÞt rv ðn; jÞjg1=2

ð5Þ

12 11 10 9 8 7 6 5 4 3 2 1

11 9 7 5 3 1 -1 -3 -5 -7 -9 -11 -13 -15 1965

1970

1975

1980

1985

1990

1995

2000

Sediment load (kg/s)

Months

To evaluate the changes in sediment load in comparison with the mean for individual months, we computed the difference of sediment load for each individual month. For the sake of easier understanding of sediment load and streamflow changes of each month and also for the concise presentation of our results, we demonstrated the results in Fig. 3. This figure was made with Surfer software package. We compared all the interpolation methods available in Surfer software package and found that local polynomial method is proper because it maximally illustrate the changes of hydrological series as they are. Fig. 3 illustrates different changing properties of sediment load with respect to different months. Similar changing properties can be identified for sediment load changes in March–October: a positive difference was detected before the late 1980s and a negative difference after the late 1980s.

2005

Years Fig. 3. Secular variations of monthly sediment load difference at the Datong station. Solid lines indicate positive difference and dashed lines denote minus difference of the sediment load.

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12

8 6 4 2 1965

1970

1975

1980

1985 Years

1990

1995

2000

2005

Streamflow (m3/s)

Months

10

5000 4000 3000 2000 1000 0 -1000 -2000 -3000 -4000 -5000 -6000 -7000 -8000 -9000

Fig. 4. Secular variations of the monthly streamflow difference at the Datong station. Solid lines indicate positive difference and dashed lines denote minus difference of the streamflow series.

512

0.8

Time scale (months)

362

0.6

256

0.4

181

0.2

128

0 -0.2

91

-0.4

64

-0.6

45

-0.8

32

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Years

-1

Fig. 5. Contours of the normalized scanning t-test standardized by the ‘‘Table-Look-up” critical value t0.05 for the standardized streamflow series at the Datong station. Contour interval is 0.2 and the zero-contour is hidden. Solid lines denote positive values and dashed lines negative values.

one significant increase was found in 1980. However, on a time scale shorter than 180 months, four abrupt decreases and five abrupt increases were identified without considering whether the abrupt increase or decrease was significant at the 95% confidence level or not; wherein only abrupt changes in 1956, 1980, 1984, and 1988 were significant at the 95% confidence level. Abrupt changes in streamflow in terms of the subseries mean characterized the variation of mean streamflow. The standard deviation describes whether streamflow changes are stable or unstable. The stability of streamflow changes may exert a considerable influence on the conservation of the wetland ecological environment. Thus we analyzed the variation of standard deviation of the streamflow series on different time scales using the scanning F-test technique. The scanning F-test of the streamflow series (Fig. 6) was calculated on the same time scales as in Fig. 5. It is seen from Fig. 6 that many frequent variations of standard deviation can be observed on short time scales. On time scales longer than 91 months, however, two positive (increases in subseries variances) and three negative (decreases in subseries variances) changes were detected, with local maxima and minima in the contours (Fig. 6). Only the positive change in 1991 was significant at the 95% confidence level. On time scales shorter than 91 months, six negative and four positive significant changes, except the positive changes in 1963 which was not significant at the 95% confidence level, were detected with local minima and maxima in the contours. In order to characterize the stable or unstable features of each episode partitioned by abrupt changes of streamflow mean, we combined the results of t- and F-test, as shown in Fig. 7. It can be

seen that episodes of high streamflow are usually characterized by larger standard deviations, meaning unstable streamflow variations; and episodes of low streamflow are usually characterized by smaller standard deviations, implying stable streamflow variability. Moreover, a larger standard deviation can also be identified in the transition period from episodes of low to high mean streamflow (Fig. 7). After 2003, the standard deviation of streamflow becomes small (upper panel of Fig. 7) and the negative difference of streamflow also occurred after 2003. The general trend of the standardized streamflow (seasonal variations have been removed by SSFj,i = (SFj,i  SFj,mean)/SFj,std, where SSF is the standardized streamflow series; SF is the raw streamflow series; j denotes the month, i.e., January, February, and so on; i denotes the length of the series; SFj,mean denotes the mean streamflow of month j; SFj,std means the standard deviation of month j) indicated slightly increasing streamflow, but the increase was not significant at the 95% confidence level. However the standard deviation obtained by the scanning F-test was decreasing. Therefore, the monthly streamflow at the Datong station was slightly increasing and became stable over time. Specifically, two time intervals were identified based on Fig. 7 characterized by increasing monthly streamflow, i.e., from mid-1950s to early 1970s and from mid-1980s to 2006. Scanning t- and F-test of the sediment load variations The results of scanning t-test of the sediment load series from the Datong station are shown in Fig. 8. Visual comparison between

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512

Time scale (months)

362 256 181 128 91 64 45 32

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Years

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 -1.4

Fig. 6. Contours of the scanning F-test on standardized streamflow series at the Datong station at 95% confidence level. Contour interval is 0.2 with the zero-contour lines hidden. Solid lines denote positive values and dashed lines negative values.

Streamflow (m3/s)

Standard deviation

Figs. 5 and 8 reveals distinctly different patterns of scanning t-test results of the sediment load series when compared with those of the streamflow series. Based on the local maxima and minima of the t-test values, on a time scale longer than 91 months two negative (decreasing sediment load) centers were identified (Fig. 8). Significant abrupt changes were detected in 1976 and 1985. The

1.6 1.4 1.2 1 0.8 4 2 0 −2

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Years Fig. 7. Episode averages and standard deviations of the standardized streamflow series at the Datong station. The thin gray solid line of the lower panel denotes standardized streamflow series.

sediment load during 1976–1985 was 5.7% less than that before 1976; and the sediment load after 1985 was more than 32.6% less than that during 1976–1985. On time scales shorter than 91 months, the sediment load series was partitioned into six negative (decreasing sediment load) and two positive (increases in sediment load) centers based on the local maxima and minima of the t-test statistic values (Fig. 8). These six negative abrupt changes centered in 1969 (on a time scale of 64 months), 1976 (on a time scale of 45 months), 1985 (on a time scale of 54 months), 1992 (on the time scale of 64 months), 1998 (on a time scale of 38 months), and 2003 (on a time scale of 45 months). However, the abrupt changes in 1976, 1998, and 2003 were not significant at the 95% confidence level. The sediment load after 2003 was 59.8% less than that before 2003. Two significant positive abrupt changes centered in 1980 (on a time scale of 45 months) and 1988 (on a time scale of 38 months). The scanning F-test results of standardized sediment load series are shown in Fig. 9. For comparison, the scanning F-test of the sediment load was computed on the same time scale as that of the ttest (Figs. 8 and 9). Different properties of contours of the F-test can be identified in Fig. 9 in comparison with those of Fig. 8. Many frequent variations can be observed on shorter time scales. On time scales longer than 91 months, one positive (increase in subseries variance) and two negative (decreasing subseries variances) significant changes were detected by identifying the local maxima and minima in the contours (Fig. 9). On time scales shorter than 91 months, four negative and two positive significant changes were

512

Time scale (months)

362 256 181 128 91 64 45 32

1965

1970

1975

1980

1985

1990

1995

2000

2005

1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 -1.4

Years Fig. 8. Contours of the normalized scanning t-test standardized by the ‘‘Table-Look-up” critical value t0.05 for the standardized sediment load series at the Datong station. Contour interval is 0.2 and the zero-contour is hidden. Solid lines denote positive values and dashed lines negative values.

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512

Time scale (months)

362 256 181 128 91 64 45 32 1965

1970

1975

1980

1985

1990

1995

2000

2005

1.8 1.4 1 0.6 0.2 -0.2 -0.6 -1 -1.4 -1.8 -2.2 -2.6

Years Fig. 9. Contours of the scanning F-test on standardized sediment load series at the Datong station at the 95% confidence level. Contour interval is 0.2 with the zero-contour lines.

1.68

1.09

Just as shown in Fig. 7, we combined the results of F-test with those of t-test to characterize the changes in sediment load within the episodes partitioned by the t-test as stable or unstable features (Fig. 10). It can be seen that episodes with larger sediment load mean values usually corresponded to episodes with a larger standard deviation (unstable) and vice versa. This result is similar to that of streamflow variations. However, two episodes were exceptions: 1975–1980 and 2003–2006. These two episodes were characterized by low sediment load mean values but higher standard deviations; in other words, these two episodes were characterized by decreasing but unstable sediment load changes (Fig. 10). Coherency relations between sediment load and streamflow series

0.49

Sediment load (kg/s)

Standard deviation

observed, centering in 1971 (on a time scale of 45 months), 1985 (on a time scale of 54 months), 1991 (on a time scale of 76 months), 1993 (on a time scale of 32 months), 1997 (on a time scale of 32 months), and 2002 (on a time scale of 38 months) (Fig. 9). The change years in the second moment (F-test) were different from those in the first moment (t-test).

3 1 −1 1963 1968 1973 1978 1983 1988 1993 1998 2003 Years

Fig. 10. Episode averages and standard deviations of the standardized sediment load series at the Datong station. The thin gray solid line of the lower panel denotes standardized sediment load series.

What was discussed above characterized the abrupt changes and stable or unstable properties of the sediment load and the streamflow variations by using the scanning t- and F-test techniques. We also conducted coherency analysis of abrupt changes of these two hydrological series (Fig. 11). It can be observed that the coherency of significant changes in the streamflow series with those in the sediment load series showed one negative (anti-phase) center around 1988 on time scales of 128–181 months. This result indicated that on a longer time scale, sediment load and streamflow variations were in anti-phase relations. On time scales shorter than 91 months, six positive (in-phase) coherency centers were detected around the mid-1970s, the late 1980s, the early 1980s, the mid-1980s, the late 1990s, and 2000–2005. Therefore, on shorter

512 1

Time scale (months)

362

0.8

256

0.6 181

0.4

128

0.2 0

91

-0.2

64

-0.4

45

-0.6

32

1965

1970

1975

1980

1985 Years

1990

1995

2000

2005

Fig. 11. Coherency between sediment load and streamflow series at the Datong station.

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time scales, sediment load variations mostly coincided with those of streamflow, showing influences of hydrological dynamics on the transport of suspended sediment in river channels. However, from the viewpoint of a longer time scale, anti-phase relations between sediment load and streamflow indicated other driving factors besides hydrological dynamics. Discussions and conclusions The Datong station is the last control station of the Yangtze River basin. The sediment load and streamflow variations measured at the Datong station are taken as input of the terrestrial flux from the Yangtze River basin into the East China Sea. In this study, we analyzed long monthly sediment load and streamflow series by using scanning t-test and F-test techniques and coherency analysis with the aim to understand variations in sediment load and streamflow on different time scales. The following conclusions can be drawn from this study. The sediment load turns to decrease after the 1980s and this decreasing tendency becomes sharp after 2000, which is mainly reflected by dense contour lines of the sediment load difference. Seasonal variations of sediment load show moderate variations in January, February, November, and December. This is particularly true for sediment changes in January and February. Larger variability can be identified in the changes of the sediment load in June, July, August and September. Comparatively, streamflow series exhibits complex changes. The negative difference of streamflow was detected before the mid-1980s and positive difference after 1985 for January, February, and March. Streamflow in April, May, and June shows a negative difference before the late 1980s, followed by a positive difference during the late 1980s and 2002. After about 2002, streamflow changes turn to the negative difference. Generally, decreasing streamflow can be observed in March–December after 2002. Streamflow shows an increasing trend and sediment load a decreasing trend, although these trends are not significant at the 95% confidence level. This may explain the anti-phase relationship between the sediment load and streamflow. The results of scanning t-test indicate that significant abrupt changes in sediment load occurred in 1976 and 1985. However, the significant abrupt change in 1985 was detected at almost all time scales, showing that this abrupt change in 1985 is the principle abrupt change in sediment load at the Datong station. Studies (Zhang et al. 2008a) have shown that streamflow variations in the Yangtze River basin are mainly the result of climate change, and precipitation changes in particular. The transport of sediment load is heavily influenced by human activities, such as construction of dams, land use, forestation/deforestation, and so forth. Construction of the Gezhouba Dam started in 1970 and the operation started in the early 1980s with a total storage of 1.58  109 m3, which exerts a tremendous influence on the sediment load transportation (Chen and Huang, 1991). Furthermore, up to the end of the 1980s, there were 1880 water reservoirs constructed in the Jinshajiang River basin with a total storage of 2.813  109 m3 (Xu, 2005). The construction of these water reservoirs on the mainstem and tributaries of the Yangtze River trapped large amounts of sediments and gave rise to significant abrupt changes in sediment load in the mid-1980s. The streamflow at the Datong station is increasing, although this increasing trend is not significant at the 95% confidence level. This result is in agreement with that based on annual streamflow analysis (Zhang et al., 2006b). Streamflow changes in the lower Yangtze River are mainly attributed to the spatial and temporal distribution of precipitation (Zhang et al., 2005). The results of coherency analysis, showing anti-phase relationships between sediment load and streamflow on longer time scales, confirm the general trend

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of sediment load and streamflow. However, in-phase relations between sediment load and streamflow seem to indicate the influence of hydrological dynamics on the transport of suspended sediment in the river channel (Milliman and Syvitski, 1992). Dynamic transportation-sedimentation processes of sediment load in the river channel also make the relations between abrupt changes of sediment load and construction of water reservoirs more complicated. Even so, our results still clearly show massive influences of water reservoirs on sediment load variations in the lower Yangtze River basin. The scanning t-test results show a change point in 2003, but this abrupt change is not significant at the 95% confidence level. The scanning F-test shows that sediment changes become unstable after 2003. These changing properties of the first and the second moments may reflect the impact of the Three Gorges Dam on the sediment transport in the lower Yangtze River. The period of 2003–2006 was decided as the post-TGD (Three Gorges Dam) by Chen et al. (2008). Annual sediment load changes also indicate decreasing sediment load after 2003 (Chen et al., 2008). The mean sediment load during 2003–2006 is 59.8% less than that before 2003, showing a tremendous influence of the Three Gorges Dam on sediment load changes in the lower Yangtze River. Sediment load difference analysis indicates that the decrease of sediment load occurred mainly during May–October. This may be due to the operation scheme of the Three Gorges Dam (YRSRI, 1993; Chen et al., 2008). The Three Gorges Dam usually stores water in dry seasons and releases extra water during wet seasons, particularly when high flow events occur. Moreover, wet seasons are usually the periods when production and transport of sediment load occur in the Yangtze River basin. Therefore, human activities, particularly the construction of water reservoirs, give rise to significant decreasing trend of the sediment load transport in the lower Yangtze River. Moreover, the sediment load variations become unstable. These changing properties of the sediment load may imply serious challenges for ecological environment conservation and deltaic management. This paper addressed an important scientific problem about influences of hydrological regulations of the water reservoirs and other influencing factors on hydrological processes such as sediment load and streamflow. Moreover, the importance of this study also lies in: (1) exploring sediment load and streamflow variations on different time scales in that different influencing factors usually alter the hydrological processes on diverse time scales; (2) understanding stable/unstable properties of hydrological processes on different time scales, and which is practically important in sound management practice of ecological environment conservation and the development of the Yangtze Delta; (3) application of updated hydrological data and which is helpful for deeper insight into influences of water reservoirs and other factors on sediment load transportation and streamflow variations in a timely way; and (4) analogously, this study shows another way to study possible influences of water reservoirs on hydrological processes besides wavelet technique as suggested by White et al. (2005). Acknowledgments The research was financially supported by National Natural Science Foundation of China (Grant Nos. 40701015 and 40730635), the innovative project from Nanjing Institute of Geography and Limnology, CAS (Grant Nos. CXNIGLAS200814; 08SL141001; and 08YCZ11007), and by Program of Introducing Talents of Discipline to Universities - the 111 Project of Hohai University. Thanks should be extended to the Changjiang (Yangtze) Water Resources Commission (CWRC) for providing the hydrological data. We also appreciate the comments and suggestions from two anonymous

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reviewers and the editor, Prof. Geoff Syme. Their comments greatly helped to improve the quality of this paper. References Chen, S.R., Huang, G.H., 1991. Water level changes of Yichang station after operation of Gezhouba Dam. Yangtze River 1, 30–37 (in Chinese). Chen, Z.Y., Li, J.F., Shen, H.T., Wang, Z.H., 2001. Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41 (2-3), 77–91. Chen, X.Q., Yan, Y.X., Fu, R.S., Dou, X.P., Zhang, E.F., 2008. Sediment transport from the Yangtze River, China, into the sea over the post-Three Gorge Dam period: a discussion. Quaternary International 186, 55–64. Cramer, H., 1946. Mathematical Method of Statistics. Princeton University Press, Princeton, NJ, USA. Hammersley, J.M., 1946. Discussion of papers. Journal of the Royal Statistical Society 8, 91. Jiang, J.M., Mendelssohn, R., Schwing, F., Fraedrich, K., 2002. Coherency detection of multiscale significant changes in historic Nile flood levels. Geophysical Research Letters 29 (8), 112-1–112-4. Jiang, J.M., Gu, X.Q., Ju, J.H., 2007. Significant changes in subseries means and variances in an 8000-year precipitation reconstruction from tree rings in the southwestern USA. Annales Geophysicae 25, 1–12. Kumar, P., Foufoula-Georgiou, E., 1994. Wavelet analysis in geophysics: an introduction. In: Foufoula-Georgiou, E., Kumar, P. (Eds.), Wavelets in Geophysics. Academic Press, San Diego, pp. 1–43. Milliman, J.D., 1997. Blessed dams or damned dams. Nature 386, 325–326. Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. Journal of Geology 100, 525–544. Mou, J., 1991. The impact of environmental change and conservation measures on erosion and sediment loads in the Yellow River basin. In: Peters, N.E., Walling, D.E. (Eds.), Sediment and Stream Water Quality in a Changing Environment. Proc. Vienna Symposium, August, 1991. IAHS Press, Wallingford, UK, pp. 47–52 (IAHS Publication No. 203). Syvitski, J.P.M., 2003. Supply and flux of sediment along hydrological pathways: research for the 21st century. Global and Planetary Change 39 (1–2), 1–11. Syvitski, J.P.M., Vörösmarty, C.J., Kettner, A.J., Green, P., 2005. Impact of humans on the flux of terrestrial sediment to the global coastal ocean. Science 308, 376– 380.

UNESCO, 1998. Coasts and small islands home for two-thirds of world population. 1998 International Year of the Ocean Website. . Von Storch, H., Zwiers, F., 1999. Statistical Analysis in Climate Research. Cambridge University Press, Cambridge. p. 116. Walling, D.E., 2006. Human impact on land–ocean sediment transfer by the world’s rivers. Geomorphology 79, 192–216. Walling, D.E., Fang, D., 2003. Recent trends in the suspended sediment loads of the world’s rivers. Global and Planetary Change 39, 111–126. Wang, Z.Y., Hu, S.X., Wu, X.J., Shao, Y.S., 2003. Delta processes and management strategies in China. International Journal of River Basin Management 1, 173– 184. White, A.M., Schmidt, J.C., Topping, J.D., 2005. Application of wavelet analysis for monitoring the hydrologic effects of dam operation: Glen Canyon Dam and the Colorado River at Lees Ferry, Arizona. River Research and Application 21 (5), 551–565. Xu, J.X., 2005. Variation in grain size of suspended load in upper Changjiang River and its tributaries by human activities. Journal of Sediment Research 3, 8–16 (in Chinese). Yang, S.L., Li, M., Dai, S.B., Liu, Z., Zhang, J., Ding, P.X., 2006. Drastic decrease in sediment supply from the Yangtze River and its challenge to coastal wetland management. Geophysical Research Letters 33. doi:10.1029/2005GL025507. YRSRI (Yangtze River Science Research Institute), 1993. Research on Key Techniques of Sedimentation and Navigation Relevant to the Three-Gorge Project. Wuhan Industrial University Press, Wuhan, China (in Chinese). Zhang, Q., Jiang, T., Gemmer, M., Becker, S., 2005. Precipitation, temperature and discharge analysis from 1951 to 2002 in the Yangtze catchment, China. Hydrological Sciences Journal 50 (1), 65–80. Zhang, Q., Liu, C.L., Xu, C.-Y., Xu, Y.P., Jiang, T., 2006a. Observed trends of annual maximum water level and streamflow during past 130 years in the Yangtze River basin, China. Journal of Hydrology 324, 255–265. Zhang, Q., Xu, C.-Y., Becker, S., Jiang, T., 2006b. Sediment and runoff changes in the Yangtze River basin during past 50 years. Journal of Hydrology 331, 511– 523. Zhang, Q., Chen, G.Y., Su, B.D., Disse, M., Jiang, T., Xu, C.-Y., 2008a. Periodicity of sediment load and runoff in the Yangtze Riverbasin and possible impacts of climatic changes and human activities. Hydrological Sciences Journal 53 (2), 457–465. Zhang, S.R., Lu, X.X., Higgitt, D.L., Chen, C.T., Han, J., Sun, H., 2008b. Recent changes of water discharge and sediment load in the Zhujiang (Pearl River) basin, China. Global and Planetary Change 60, 365–380.