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HYDROLOGICAL PROCESSESHydrol. Process. 27, 444–452 (2013)Published online 19 March 2012 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.9278

Abrupt behaviours of streamflow and sediment load variationsof the Yangtze River basin, China

Qiang Zhang,1,2,3* Vijay P. Singh,4 Chong-Yu Xu5 and Xiaohong Chen1,2,31 Department of Water Resources and Environment, Sun Yat-sen University, Guangdong, China

2 Key Laboratory of Water Cycle and Water Security in Southern China of Guangdong High Education Institute, Sun Yat-sen University, Guangdong, China3 School of Geography and Planning, and Guangdong Key Laboratory for Urbanization and Geo-simulation, Sun Yat-sen University, Guangzhou, China4 Department of Biological and Agricultural Engineering, and Department of Civil and Environmental Engineering, Texas A & M University, College

Station, TX, USA5 Department of Geosciences and Hydrology, University of Oslo, Blindern, Oslo, Norway

*CEnE-m

Co

Abstract:

Monthly sediment load and streamflow series spanning 1963–2004 from four hydrological stations situation in the main stem ofthe Yangtze River, China, are analysed using scanning t-test and the simple two-phase linear regression scheme. Results indicatesignificant changes in the sediment load and streamflow from the upper reach to the lower reach of the Yangtze River. Relativelyconsistent positive coherency relations can be detected between streamflow and sediment load in the upper reach and negativecoherency in the middle and lower reaches. Interestingly, negative coherency is found mainly for larger time scales. Changes insediment load are the result mainly of human influence; specifically, the construction of water reservoirs may be the major causeof negative coherency. Accentuating the human influence from the upper to the lower reach results in inconsistent correlationsbetween sediment load and streamflow. Decreasing sediment load being observed in recent years has the potential to alter thetopographical properties of the river channel and the consequent development and recession of the Yangtze Delta. Results of thisstudy are of practical significance for river channel management and evaluation of the influence of human activities on thehydrological regimes of large rivers. Copyright © 2012 John Wiley & Sons, Ltd.

KEY WORDS abrupt behaviours; scanning t-test; the simple two-phase linear regression scheme; hydrological regimes; theYangtze River basin

Received 30 October 2011; Accepted 17 February 2012

INTRODUCTION

The hydrological regimes of a river represent anintegrated basin response to climatic inputs, withprecipitation being dominant (Zhang et al., 2001; Zhanget al., 2010). Statistical properties of streamflow varia-tions are critical for the evaluation of regional availabilityand variability of water resources (e.g. George, 2007;Brabets and Walvoord, 2009; Chen et al., 2011; Zhanget al., 2011a). Human activities heavily interfere withhydrological processes, particularly sediment load. Theseactivities, especially the construction of dams and waterreservoirs, are primarily responsible for the reduction interrestrial sediments to coastal areas (James et al., 2006).Walling and Fang (2003) pointed out that reservoirconstruction was probably the most important influenceon land–ocean sediment fluxes. That is why a goal of theInternational Geosphere Biosphere Program and its coreproject, Land Ocean Interaction in the Coastal Zone, hasbeen to survey the terrestrial sediment supply to coastsand analyse perturbations in this flux (Syvitski, 2003).Sediment load and streamflow changes of large rivers

in China have been investigated a great deal. Analyzing

orrespondence to: Qiang Zhang, Department of Water Resources andvironment, Sun Yat-sen University, Guangzhou 510275, China.ail: [email protected]

pyright © 2012 John Wiley & Sons, Ltd.

sediment load datasets from 1950 to 2005 from fourgauging stations in the main stem of Yangtze River,Wang et al. (2007a,b) found distinct stepwise decreasesin sediment load, which they attributed to both naturaland anthropogenic impacts. Using the 1951–2004 timeseries of annual sediment supply and coastal bathymetricdata, Yang et al. (2006) illustrated a significantdecreasing trend in riverine sediment supply since thelate 1960s and attributed this decreasing sediment to damconstruction. They indicated that the subsequent resultof decreasing sediment load may be responsible forthe recession of deltaic coast, which poses a greatchallenge to coastal management. Zhang et al. (2006)found that water reservoirs exerted more influence onsediment transport than water discharge, and thisinfluence was more significant in the tributaries than inthe main stem of the Yangtze River. Analyzing annualwater discharge and sediment load series (from the 1950sto 2004) from nine stations in the main channel and maintributaries of the Zhujiang (Pearl River), Zhang et al.(2008c) demonstrated a significant decreasing sedimentload at some stations in the main tributaries, and sincethe 1990s, more stations have witnessed significantlydecreasing sediment loads. It should be noted that theinfluence of human activities or natural factors, such asprecipitation change, is subjected to different time scales,for example, climate change usually influences the

445HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER

hydrological processes at longer time scales as comparedwith that of human activities, such as the construction ofwater reservoirs. The previous studies seem to havemostly ignored this important consideration, which is themajor reason for this study (Chen et al., 2001; Lu et al.,2003).The Yangtze River (Changjiang, Figure 1), the longest

river in China and the third longest river in the world,plays a vital role in the economic development andecological environmental conservation within China.Numerous water reservoirs have been built in the YangtzeRiver basin, and most of these reservoirs are located in thetributaries. Xu (2005) stated that up to the end of 1980s,there were 11 931 water reservoirs in the upper YangtzeRiver basin with a total storage capacity of2.05� 1010m3. The construction of the Three GorgesDam started on December 14, 1994, and ended in 2009with a total storage capacity of 39.3� 109m3. Theconstruction of Gezhouba Dam started in May 1970,and its operation started in December 1988, with a totalstorage capacity of 1.58� 109m3. The building of waterreservoirs has greatly altered the hydrological processesin the Yangtze River basin (Zhang et al., 2006). It shouldbe noted here that, when compared with the Three-GorgeDam, the Gezhouba Dam is much smaller in terms ofvolume capacity (1.58� 109m3 of Gezhouba Dam vs39.3� 109m3 for Three Gorges Dam). It has only alimited influence on hydrological processes exceptsediment transport several years afterwards. However, it

92 E 96 E 100 E 104 E

Chongq

Yangtze R. basin

CHINA

1

Huanghe R.

Figure 1. Location of the Yangtze Rive

Copyright © 2012 John Wiley & Sons, Ltd.

may have considerable impacts on ecological environ-ment such as the migratory fishes from the middle andlower reaches of the Yangtze River to the upper reaches.Using monthly data from four stations in the Yangtze

River, the objectives of this study therefore were asfollows: (1) to investigate abrupt changes in sedimentload and streamflow at different time scales, (2) toinvestigate the causes of the abrupt changes, and (3)discuss implications of these changes.

DATA

Monthly sediment load (kg/s) and monthly streamflow(m3/s) from January 1963 to December 2004 (Table I)from four major hydrological stations in the YangtzeRiver were obtained collected from the Changjiang(Yangtze) Water Resource Commission, which firmlycontrols the quality of data before release. Locations ofthe stations are shown in Figure 1. There were no missingdata in the streamflow series, but missing data were foundin the sediment load series in 1979 at the Yichang stationand in 1965, 1966, 1968, 1976 and 1978 at the Datongstation. No missing data can be found in the sedimentload and streamflow series at other stations considered inthis study. The existence of missing data results in adecrease in the sample size available for analysis. Tomake full use of the data without loss of its statisticalproperties, missing values were filled in with multi-annualmean sediment load of the respective individual months.

108 E 112 E 116 E 120 E

32 N

24 N

36 N

28 N

3 4

4

TGD

Minjiang R.

Jialingjiang R.

Wujiang R.

1

2

3

Three Gorges Dam (TGD)

ing

Nanjing

Jinshajiang R.

Yayanjiang R.

Danjiangkouwater reservoirHanjiang R.

Poyang Lake

Taihu Lake

Hydrological station

Pingshan st.

Hankou st.

Dongting Lake

GZBGZBYichang

Nanjing

ShanghaiShanghai

Wuhan

Yichang st.Gezhouba Dam(GZB)

223

Datong st.

r basin and the hydrological stations

Hydrol. Process. 27, 444–452 (2013)

Table I. Sediment load and streamflow data at four hydrologicalstations of the Yangtze River basin

Stationname

Drainage area(km2)

Streamflowseries

Sediment loadseries

Pingshan 485 099 1963–2004 1963–2004Yichang 1 005 501 1963–2004 1963–2004Hankou 1 488 036 1963–2004 1963–2004Datong 1 705 383 1963–2004 1963–2004

446 Q. ZHANG ET AL.

METHODOLOGY

Zhang et al. (2009) revised a method developed by Lundand Reeves (2002) for detecting abrupt hydrologicalchanges, and the revised method was applied in thisstudy. The original method (Solow, 1987; Easterling andPeterson, 1995; Lund and Reeves, 2002) can be written as

Xt ¼ m1 þ a1t1 þ etm2 þ a2t2 þ et

�(1)

where Xt is the dependant variable representing a hydro-meteorological series; m1 and m2 are the mean values ofthe two sub-series, respectively, divided by c, theassumed change point; a1 and a2 are the regressivecoefficients of the two sub-series, respectively; t1 and t2are the time interval of the two sub-series, respectively,being defined as: 1≤ t1≤ c and c< t2≤ n, respectively,where c is the possible change point to be tested; n is thetotal length of the two sub-series; and et is mean zeroindependent random error with a constant variance.Relations between streamflow and sediment load were

analysed by using a coherency technique, which entailstwo steps: scanning t-test and coherency analysis basedon the scanning t-test. In the scanning t-test (Jiang et al.,2002), statistic t(n, j) is defined as the difference of thesubsample averages between every two adjoining sub-series of equal subseries size (n) expressed as

t n; jð Þ ¼ �xj2 � �xj1� �� n1=2 � s2j2 þ s2j1

� ��1=2(2)

where

�xj1¼1n

Xj�1

i¼j�n

x ið Þ;�xj2¼1n

Xjþn�1

i¼j

x ið Þ; s2j1¼1

n�1

Xj�1

i¼j�n

x ið Þ��xj1� �2

;

s2j2 ¼1

n� 1

Xjþn�1

i¼j

x ið Þ � �xj2� �2

;

in which n is the subsample size varying as n= 2, 3,. . . ,< N/2 or may be selected at suitable intervals; andj = n+ 1, n+ 2 ,. . . , N–n+ 1 is the reference time point. Itshould be noted that hydrological series are usually auto-correlated. Thus, the Table-Look-Up Test (von Storchand Zwiers, 1999) was adopted to modify the significancecriterion of statistic t(n, j) based on lag-1 autocorrelationcoefficients of the pooled subsample and the subsamplesize n. Criterion t0.05 for the correction of the dependencewas employed to determine significant changes in time


scales longer than 30 years. For shorter subsample sizes,the critical values are overly restrictive. Because thesignificance level varies with n and j, the test statistic wasnormalized, to make values comparable, as

tr n; jð Þ ¼ t n; jð Þ=t0:05 (3)

when tr(n,j)> 1.0, the abrupt change is significant at the95% confidence level. tr(n,j)<�1.0 denotes a significantdecrease, and tr(n,j)> 1.0 a significant increase. Finally,the coherency of abrupt changes between two series u andv was defined as

trcðn; jÞ ¼ sign½truðn; jÞtrvðn; jÞ�fjtruðn; jÞtrvðn; jÞjg1=2 (4)

When statistic trc(n, j)> 1.0 with both j tru(n, j) j ;jtrv(n, j) j> 1.0, the two series have abrupt changes in thesame direction, whereas if trc(n, j)<� 1.0, the two serieshave abrupt changes in opposite directions (Jiang et al.,2002). The coherency of abrupt changes between monthlystreamflow series and monthly sediment load series can beregarded as an indication of the interaction between thesetwo series on decadal and basin scales.

RESULTS AND DISCUSSIONS

Changes in sediment load and streamflow at Pingshanstation

Pingshan station is located upstream to the GezhoubaDam and Three Gorges Dam (Figure 1). Streamflow hasbeen increasing at a time scale of> 128months. Thedecrease and increase can be found to be intermittent atdifferent time scales (Figure 2). Regions dominated bysignificant change points also are found to distributesporadically within the time scales versus time space.Comparatively, patterns of distribution of change pointsof sediment load are in approximate agreement with thoseof streamflow. Increasing tendency is found mainly atlonger time scales, and the abrupt changes in sedimentload at shorter time scales, such as < 64months, arerelatively complicated. For sediment load and streamflowchanges, the long-term tendency is dominated by theincreasing trend, and these results are good line withthose by Zhang et al. (2006). Earlier study suggested thatthe river suspended sediments are mainly from the upperYangtze River Catchment (Pan, 1999); the sedimentsfrom Jinshajiang River alone accounts for about 39.4% ofthat in Yichang station (Xu, 2005). Over-exploitation inthe upper Yangtze Catchment led to an increasing trend ofsediment load in the Pingshan station (Pan, 1999).Figure 3 shows abrupt changes in streamflow (upper

panel of Figure 3) and sediment load (lower panel ofFigure 3). Different changing properties of streamflowcan be identified during different time intervals. Specif-ically, before 1965, streamflow was characterized to beincreasing. Decreasing streamflow can be observedduring 1965 and the early 1970s. The 1970s and 1980s

Hydrol. Process. 27, 444–452 (2013)

1965 1970 1975 1980 1985 1990 1995 2000

1965 1970 1975 1980 1985 1990 1995 2000

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256Pingshan: streamflow

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256Pingshan: sediment load

Figure 2. Change points on different time scales of streamflow andsediment load variations of the Pingshan station. Dashed lines showdecreasing trend after the change point, and solid lines indicate increasingtrend after the change point. Thick solid and dashed lines denotesignificant change points. The meanings of the line styles are the same forsubsequent figures. The x-axis denotes time scales with unit of months;first, we should identify the change points in the plot, that is, the regionscircled by thick dashed or solid contours, and then read the time scalesfrom x-axis and then the time when the change point occurred from the y-axis. In so doing, the time scales and the time when the change pointoccurred can be read from the plot. The same procedure will be followed

in the subsequent figures

1960 1970 1980 1990 2000

−2

−1

0

1

2

3

Sta

ndar

dize

d st

ream

flow

(m3 /

s)

1960 1970 1980 1990 2000−2

0

2

4

6

Sta

ndar

dize

d se

dim

ent l

oad

(kg/

s)

Figure 3. Linear trends of time intervals separated by change points forthe streamflow and sediment load series at the Pingshan station

1965 1970 1975 1980 1985 1990 1995 2000

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256

Figure 4. Coherency between sediment load and streamflow series at thePingshan station

1965 1970 1975 1980 1985 1990 1995 2000

1965 1970 1975 1980 1985 1990 1995 2000

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256 Yichang: streamflow

Tim

e sc

ales

(m

onth

s)

Yichang: sediment load

8

16

32

64

128

256

Figure 5. Change points on different time scales of streamflow andsediment load variations of the Yichang station. Dashed lines showdecreasing trend after the change point, and solid lines indicate increasingtrend after the change point. Thick solid and dashed lines denotesignificant change points. The meanings of the line styles are the same for

subsequent figures


are featured by increasing streamflow. Decreasingstreamflow is found after 2000. Abrupt changes insediment load shown in the lower panel of Figure 3 aresimilar to those of streamflow changes, implying that thetransport of sediment load is subjected mainly to thehydrodynamic characteristics of the river channel (Luet al., 2003). Coherency analysis of sediment load and


streamflow (Figure 4) also indicates positive relationsbetween sediment load and streamflow by positivecoherency at various time scales.

Sediment load and streamflow changes at Yichang station

The upper panel of Figure 5 shows abrupt changes instreamflow at the Yichang station. Figure 6 displaysabrupt behaviours of streamflow and sediment load usinglinear regressive technique. Increasing tendency can befound at time scales of> 128months. Changing char-acteristics of streamflow at time scales of< 128monthsare complicated. Significant change points are identifiedin about 1975, the mid-1980s and the early 1990s.Comparison between the upper panel of Figure 5 and theupper panel of Figure 2 indicates similarity in streamflowchanges at the Yichang and Pingshan stations. Thedifferences, if any, between streamflow changes at theYichang and the Pingshan stations should be due to thestreamflow from the tributaries between the Pingshan and

Hydrol. Process. 27, 444–452 (2013)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005−3

−2

−1

0

1

2

3

4

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005−2

0

2

4

6

Sta

ndar

dize

d se

dim

ent l

oad

(kg/

s)S

tand

ardi

zed

stre

amflo

w (

m3 /

s)

Figure 6. Linear trends of time intervals separated by change points forthe streamflow and sediment load series at the Yichang station

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256

1965 1970 1975 1980 1985 1990 1995 2000

Figure 7. Coherency between sediment load and streamflow series at theYichang station

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256 Hankou: streamflow

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256 Hankou: sediment load

1965 1970 1975 1980 1985 1990 1995 2000

1965 1970 1975 1980 1985 1990 1995 2000

Figure 8. Change points on different time scales of streamflow and sedimentload variations of the Hankou station. Dashed lines show decreasing trendafter the change point, and solid lines indicate increasing trend after the changepoint. Thick solid and dashed lines denote significant change points. The

meanings of the line styles are the same for subsequent figures

448 Q. ZHANG ET AL.

the Yichang stations, such as the Wujiang River(Figure 1). Besides, the storage capacity of the riverchannel between these two stations should be one of thefactors causing differences in streamflow changes at thePingshan and Yichang stations.Changes in sediment load are subjected to different

patterns of changes when compared with those at thePingshan station. Several time intervals were identified tobe dominated by different changing properties ofsediment load. Specifically, significant change pointswere detected in the early 1970s, the early 1980s and thelate 1990s. The construction of the Gezhouba Dam startedin 1970, and its operation started in the early 1980s with atotal storage of 1.58� 109m3, which has been exerting atremendous influence on sediment transport (Chen andHuang, 1991). Furthermore, up to the end of the 1980s,there were 1880 water reservoirs constructed in theJinshajiang River basin with a total storage of2.813� 109m3 (Xu, 2005). These water reservoirs onthe mainstream and tributaries of the Yangtze River havebeen trapping large amounts of sediment and have givenrise to significant abrupt changes in the sediment load inthe early 1980s and the mid-1980s (Yang et al., 2006).The effects of water reservoirs, that is, the Gezhouba andthe Three Gorges Dam water reservoirs, are reflected byabrupt changes in streamflow.Since the start of the construction of the Three Gorges

Dam in 1994 and its completion in late 2008 with a totalstorage capacity of 3.93� 1010m3, the period of 2003–2006 was decided as the post-TGD (Three Gorges Dam)by Chen et al. (2008). Changes in annual sediment loadindicate a decrease in the load since 2003 (Chen et al.,2008). The late 1990s were identified as the significantchange point, and the sediment load has obviously beendecreasing since this change point, which should be the


result of the trapping effect of the Three Gorges Dam.Streamflow was not consistently decreasing or increasingbefore the early 1990s and has exhibited moderatevariations since the 1990s. Changes in sediment loadwere different from those of the streamflow, beingdominated by a general decreasing trend. A suddendecrease was observed after the early 1980s and ~2000.Results of coherency analysis (Figure 7) indicatednegative relations between streamflow and sediment loadafter the early 1980s on a time scale of> 64months,showing tremendous impacts of the trapping effect of theGezhouba Dam on the sediment transport. No negativerelations were found during other time intervals ondifferent time scales, particularly after the late 1990s,which should be caused by concordant changes in thesediment load and streamflow, that is, decreasing tendency,although the magnitudes of changes were different.

Sediment load and streamflow changes at Hankou station

Figure 8 shows abrupt changes in sediment load andstreamflow at the Hankou station (see Figure 1 for its

Hydrol. Process. 27, 444–452 (2013)

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256

1965 1970 1975 1980 1985 1990 1995 2000

Figure 10. Coherency between sediment load and streamflow series at theHankou station


location). In general, similar changes can be found instreamflow when compared with those at the Yichangstation in terms of the time when the change points occur.Changes in streamflow at the Pingshan station aredifferent from those at the Yichang and Hankou stations.These results clearly show different influencing factorsfor streamflow changes at the foregoing three hydro-logical stations, which should be attributed to the unevenspatial distribution of precipitation changes (Zhang et al.,2008a). These results also indicate that streamflow in theYangtze River is influenced by climate change, such asprecipitation, but not by anthropogenic factors, such asthe construction of water reservoirs. Differences inpatterns of streamflow changes at the Hankou station,when compared with those at the Yichang station, alsoshould be caused by streamflow from the Hanjiang Riverand hydraulic regulation of the river channels between theYichang and Hankou stations.It can be observed from the lower panel of Figure 8

that the sediment load is dominated by a decreasingtendency except for a short time interval, that is, 1985 toearly 1990s, which is characterized by increasedsediment load. Figure 9 intuitively clarifies the changesin sediment load and streamflow. Streamflow follows arelatively complicated changing pattern, decreasing afterthe early 1990s, which should be caused by thedecreasing precipitation in the upper Yangtze Riverbasin (Zhang et al., 2008a). Changes in sediment load aredominated by a decrease, and this decrease is abruptlyevident after 2000, which should be attributed to theconstruction of the Three Gorges Dam because of theabrupt change stemming from the construction of theThree Gorges Dam, as reported in earlier studies (e.g.Zhang et al., 2008b).

−2

−1

0

1

2

3

−3

−2

−1

0

1

2

3

4

Sta

ndar

dize

d se

dim

ent l

oad

(kg/

s)

19651960 1970 1975 1980 1985 1990 1995 2000 2005

19651960 1970 1975 1980 1985 1990 1995 2000 2005

Sta

ndar

dize

d st

ream

flow

(m

3 /s)

Figure 9. Linear trends of time intervals separated by change points forthe streamflow and sediment load series at the Hankou station


Coherency analysis (Figure 10) shows that on a longertime scale, such as> 64months, streamflow has anegative relation with sediment load since the mid-1970s. This should be the result of the construction ofGezhouba Dam, which decreased the sediment load in thelower river reach and caused inconsistent relationsbetween sediment load and streamflow on longer timescales. On shorter time scales of< 64months, however,positive coherency was still observed. Therefore, sedi-ment transport load also is heavily influenced byhydraulics on shorter time scales.

Sediment load and streamflow changes at Datong station

Changes in streamflow were evidently similar to thoseat the Yichang and Hankou stations in terms of abruptchanges (Figure 11). No large tributaries and no massivestreamflow input were available in the lower YangtzeRiver basin, which should contribute to the similarity ofstreamflow changes at the Yichang, Hankou and Datong

Tim

e sc

ales

(m

onth

s) Datong: streamflow

8

16

32

64

128

256

8

16

32

64

128

256 Datong: sediment load

Tim

e sc

ales

(m

onth

s)

Datong: sediment load

1965 1970 1975 1980 1985 1990 1995 2000

1965 1970 1975 1980 1985 1990 1995 2000

Figure 11. Change points on different time scales of streamflow andsediment load variations of the Datong station. Dashed lines showdecreasing trend after the change point, and solid lines indicate increasingtrend after the change point. Thick solid and dashed lines denotesignificant change points. The meanings of the line styles are the same for

subsequent figures

Hydrol. Process. 27, 444–452 (2013)

450 Q. ZHANG ET AL.

stations. In addition, this result further confirms theconclusion that changes in streamflow are mainly theresult of precipitation changes. Sediment load hasgenerally been decreasing particularly since 2000(Figures 12 and 13). The mid-lower basin is a kind of asediment sink (Chen, 1996). Increasing evident increaseof the sediment load at the middle and the lower YangtzeRiver basin when compared with the upper Yangtze Riverbasin is partly because of an important sink thatsignificantly decreases the sediment discharge from theYangtze River into the sea (Chen et al., 2005). It isestimated that the annual sand extraction amounted toabout 40 million tons in the early 1980s and increased toabout 80 million tons in the late 1990s. In this case,sediment load changes are the integrated results of climatechanges and human activities. However, the decrease insediment load at the Datong station in recent decades hasbeen moderate when compared with that at the Yichangand Hankou stations, which should be caused by theerosion of riverbed. The scouring process caused by thedecreased sediment load from the upper and the middle

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

−2

−1

0

1

2

3

4

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005−2

0

2

4

6

Sta

ndar

dize

d se

dim

ent l

oad

(kg/

s)S

tand

ardi

zed

stre

amflo

w (

m3 /

s)

Figure 12. Linear trends of time intervals separated by change points forthe streamflow and sediment load series at the Datong station

Tim

e sc

ales

(m

onth

s)

8

16

32

64

128

256

1965 1970 1975 1980 1985 1990 1995 2000

Figure 13. Coherency between sediment load and streamflow series at theDatong station


Yangtze River caused a moderate decrease, although thedecreasing tendency was still evident. The time intervalafter 2000 witnessed a significant decreasing sedimentload. The decreasing sediment since 2000 can beattributed to the trapping effect of the Three GorgesDam. Streamflow at the Hankou station had beengenerally increasing, but it has been decreasing since2000. In the lower Yangtze River basin, precipitation hasbeen increasing, and it is particularly true for precipitationmaxima and precipitation intensity (Zhang et al., 2008a,2011b). Spatial patterns of precipitation regimes over theYangtze River basin result in spatial distribution ofchanges in streamflow.

CONCLUSIONS

Analysis of monthly streamflow and sediment load datafrom four hydrological stations along the mainstem of theYangtze River shows abrupt changes. The followingconclusions are drawn from this analysis:

1. Climate change and human activities induce changes instreamflow and sediment load at different time scales.Generally, the changes of hydrological regime influ-enced by climate factors, on a long-term basis, wascomparable to the changes by anthropogenic factors.However, hydrological changes caused by humanactivities, such as the construction of water reservoirs,can reflect the anthropogenic influence on a longer timescale. Trapping by water reservoirs can decreasesediment load, and the decrease in sediment load canoccur for a considerably long time. In this sense, theinfluence of water reservoirs on the hydrologicalregime, particularly the sediment load and subsequenteffects, can be far reaching. Besides, the abruptbehaviours of sediment load and streamflow arepresented in time versus temporal scales showingabrupt changes on different time scales, this is thenovel points when compared with research on similartopics within the Yangtze River basin.

2. Coherency analysis shows that sediment transport inthe upper Yangtze River heavily depends on the riverhydraulics, as reflected by the positive coherencybetween sediment load and streamflow at the Pingshanstation. Changes in sediment load in the middle andlower Yangtze River reaches are impacted by thetrapping effects of the water reservoirs in the middleYangtze River basin, for example, the Gezhouba Damand the Three Gorges Dam. Massive trapping effects ofwater reservoirs of the Gezhouba Dam and the ThreeGorges Dam directly cause a significant decrease insediment load in the middle and the lower YangtzeRiver basin. The scouring of the riverbed in the lowerYangtze River leads to moderate changes in sedimentload. The Three Gorges Dam still causes a considerabledecrease in sediment load, resulting in a decrease insediment load since about 2000. However, changes instreamflow seem mainly to be the result of changes in

Hydrol. Process. 27, 444–452 (2013)


precipitation. Comparison between the results ofprecipitation changes across the Yangtze River basin(Zhang et al., 2008a) and abrupt changes in streamflowcorroborates the tremendous impact of precipitationchanges on the hydrological processes. Coherencyanalysis investigates abrupt changes of streamflowversus sediment load relations on different time scales,and this is another novelty of this study when comparedwith standing research.

3. Streamflow and sediment load are influenced byclimate change and human activities, and theirinfluence varies with time scales. Besides, influencesof water reservoirs on sediment load changes alsoare affected by balance-effects of the storage ofwater reservoirs (e.g. Want et al., 2007b). Therefore,retention effects of water reservoirs on sediment loadare relatively complicated because the reservoirs act assinks of sediment load. However, the storage capacityis decreasing with the operation of the waterreservoirs. New reservoirs are completed; meanwhile,the old reservoirs will no longer have storage forsediment deposition. In this case, the retention capacityof water reservoirs for sediments is decreasing.Therefore, it is difficult to exactly estimate the totalannual deposition in so many reservoirs within theYangtze River drainage basin (Yang et al., 2002). Anunderstanding of changes in the hydrological regime atdifferent time scales and possible underlying causes isof scientific and practical significance.

ACKNOWLEDGEMENTS

This work is financially supported by Program for NewCentury Excellent Talents in University (the FundamentalResearch Funds for the Central Universities), the NationalNatural Science Foundation of China (Grant No.: 41071020;50839005), the Project from Guangdong Science andTechnology Department (Grant No.: 2010B050800001;2010B050300010), and by a grant from the Research GrantsCouncil of the Hong Kong Special Administrative Region,China (Project No. CUHK405308). Cordial gratitude shouldbe extended to the Changjiang the Changjiang (Yangtze)Water Resource Commission for providing the data analysedin this study. Our cordial gratitude also should be owed to theeditor-in-chief, Prof. Dr Malcolm G. Anderson, and threeanonymous reviewers for their professional and pertinentsuggestions and comments, which are greatly helpful forfurther improvements of the quality of this manuscript.

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