HYDROLOGICAL PROCESSES
Hydrol. 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 variations
of the Yangtze River basin, China
Qiang Zhang,1,2,3* Vijay P. Singh,4 Chong-Yu Xu5 and Xiaohong Chen1,2,3
2
3
4
1
Department of Water Resources and Environment, Sun Yat-sen University, Guangdong, China
Key Laboratory of Water Cycle and Water Security in Southern China of Guangdong High Education Institute, Sun Yat-sen University, Guangdong, China
School of Geography and Planning, and Guangdong Key Laboratory for Urbanization and Geo-simulation, Sun Yat-sen University, Guangzhou, China
Department of Biological and Agricultural Engineering, and Department of Civil and Environmental Engineering, Texas A & M University, College
Station, TX, USA
5
Department of Geosciences and Hydrology, University of Oslo, Blindern, Oslo, Norway
Abstract:
Monthly sediment load and streamflow series spanning 1963–2004 from four hydrological stations situation in the main stem of
the Yangtze River, China, are analysed using scanning t-test and the simple two-phase linear regression scheme. Results indicate
significant changes in the sediment load and streamflow from the upper reach to the lower reach of the Yangtze River. Relatively
consistent positive coherency relations can be detected between streamflow and sediment load in the upper reach and negative
coherency in the middle and lower reaches. Interestingly, negative coherency is found mainly for larger time scales. Changes in
sediment load are the result mainly of human influence; specifically, the construction of water reservoirs may be the major cause
of negative coherency. Accentuating the human influence from the upper to the lower reach results in inconsistent correlations
between sediment load and streamflow. Decreasing sediment load being observed in recent years has the potential to alter the
topographical properties of the river channel and the consequent development and recession of the Yangtze Delta. Results of this
study are of practical significance for river channel management and evaluation of the influence of human activities on the
hydrological 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; the
Yangtze River basin
Received 30 October 2011; Accepted 17 February 2012
INTRODUCTION
The hydrological regimes of a river represent an
integrated basin response to climatic inputs, with
precipitation being dominant (Zhang et al., 2001; Zhang
et al., 2010). Statistical properties of streamflow variations are critical for the evaluation of regional availability
and variability of water resources (e.g. George, 2007;
Brabets and Walvoord, 2009; Chen et al., 2011; Zhang
et al., 2011a). Human activities heavily interfere with
hydrological processes, particularly sediment load. These
activities, especially the construction of dams and water
reservoirs, are primarily responsible for the reduction in
terrestrial sediments to coastal areas (James et al., 2006).
Walling and Fang (2003) pointed out that reservoir
construction was probably the most important influence
on land–ocean sediment fluxes. That is why a goal of the
International Geosphere Biosphere Program and its core
project, Land Ocean Interaction in the Coastal Zone, has
been to survey the terrestrial sediment supply to coasts
and analyse perturbations in this flux (Syvitski, 2003).
Sediment load and streamflow changes of large rivers
in China have been investigated a great deal. Analyzing
*Correspondence to: Qiang Zhang, Department of Water Resources and
Environment, Sun Yat-sen University, Guangzhou 510275, China.
E-mail: zhangqnj@gmail.com
Copyright © 2012 John Wiley & Sons, Ltd.
sediment load datasets from 1950 to 2005 from four
gauging stations in the main stem of Yangtze River,
Wang et al. (2007a,b) found distinct stepwise decreases
in sediment load, which they attributed to both natural
and anthropogenic impacts. Using the 1951–2004 time
series of annual sediment supply and coastal bathymetric
data, Yang et al. (2006) illustrated a significant
decreasing trend in riverine sediment supply since the
late 1960s and attributed this decreasing sediment to dam
construction. They indicated that the subsequent result
of decreasing sediment load may be responsible for
the recession of deltaic coast, which poses a great
challenge to coastal management. Zhang et al. (2006)
found that water reservoirs exerted more influence on
sediment transport than water discharge, and this
influence was more significant in the tributaries than in
the main stem of the Yangtze River. Analyzing annual
water discharge and sediment load series (from the 1950s
to 2004) from nine stations in the main channel and main
tributaries of the Zhujiang (Pearl River), Zhang et al.
(2008c) demonstrated a significant decreasing sediment
load at some stations in the main tributaries, and since
the 1990s, more stations have witnessed significantly
decreasing sediment loads. It should be noted that the
influence of human activities or natural factors, such as
precipitation change, is subjected to different time scales,
for example, climate change usually influences the
HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER
hydrological processes at longer time scales as compared
with that of human activities, such as the construction of
water reservoirs. The previous studies seem to have
mostly ignored this important consideration, which is the
major 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 and
ecological environmental conservation within China.
Numerous water reservoirs have been built in the Yangtze
River basin, and most of these reservoirs are located in the
tributaries. Xu (2005) stated that up to the end of 1980s,
there were 11 931 water reservoirs in the upper Yangtze
River basin with a total storage capacity of
2.05 1010 m3. The construction of the Three Gorges
Dam started on December 14, 1994, and ended in 2009
with a total storage capacity of 39.3 109 m3. The
construction of Gezhouba Dam started in May 1970,
and its operation started in December 1988, with a total
storage capacity of 1.58 109 m3. The building of water
reservoirs has greatly altered the hydrological processes
in the Yangtze River basin (Zhang et al., 2006). It should
be noted here that, when compared with the Three-Gorge
Dam, the Gezhouba Dam is much smaller in terms of
volume capacity (1.58 109 m3 of Gezhouba Dam vs
39.3 109 m3 for Three Gorges Dam). It has only a
limited influence on hydrological processes except
sediment transport several years afterwards. However, it
92 E
96 E
100 E
445
may have considerable impacts on ecological environment such as the migratory fishes from the middle and
lower 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 as
follows: (1) to investigate abrupt changes in sediment
load and streamflow at different time scales, (2) to
investigate 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 Yangtze
River were obtained collected from the Changjiang
(Yangtze) Water Resource Commission, which firmly
controls the quality of data before release. Locations of
the stations are shown in Figure 1. There were no missing
data in the streamflow series, but missing data were found
in the sediment load series in 1979 at the Yichang station
and in 1965, 1966, 1968, 1976 and 1978 at the Datong
station. No missing data can be found in the sediment
load and streamflow series at other stations considered in
this study. The existence of missing data results in a
decrease in the sample size available for analysis. To
make full use of the data without loss of its statistical
properties, missing values were filled in with multi-annual
mean sediment load of the respective individual months.
104 E
108 E
116 E
112 E
120 E
32 N
Nanjing
TGD
2
GZB
3
Yichang
36 N
Shanghai
4
Wuhan
1
28 N
Chongqing
24 N
Yayanjiang R.
Danjiangkou
water reservoir
Hanjiang R.
Minjiang R.
Dongting Lake
Jialingjiang R.
Poyang Lake
Wujiang R.
Taihu Lake
Jinshajiang R.
CHINA
Huanghe R.
Hydrological station
1 Pingshan st. 4 Datong st.
Yangtze R. basin
2 Yichang st.
3 Hankou st.
Gezhouba Dam
(GZB)
Three Gorges Dam (TGD)
Figure 1. Location of the Yangtze River basin and the hydrological stations
Copyright © 2012 John Wiley & Sons, Ltd.
Hydrol. Process. 27, 444–452 (2013)
446
Q. ZHANG ET AL.
Table I. Sediment load and streamflow data at four hydrological
stations of the Yangtze River basin
Station
name
Pingshan
Yichang
Hankou
Datong
Drainage area
(km2)
Streamflow
series
Sediment load
series
485 099
1 005 501
1 488 036
1 705 383
1963–2004
1963–2004
1963–2004
1963–2004
1963–2004
1963–2004
1963–2004
1963–2004
METHODOLOGY
Zhang et al. (2009) revised a method developed by Lund
and Reeves (2002) for detecting abrupt hydrological
changes, and the revised method was applied in this
study. The original method (Solow, 1987; Easterling and
Peterson, 1995; Lund and Reeves, 2002) can be written as
m1 þ a1 t1 þ et
(1)
Xt ¼
m2 þ a2 t2 þ et
where Xt is the dependant variable representing a hydrometeorological series; m1 and m2 are the mean values of
the two sub-series, respectively, divided by c, the
assumed change point; a1 and a2 are the regressive
coefficients of the two sub-series, respectively; t1 and t2
are 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 the
total length of the two sub-series; and et is mean zero
independent random error with a constant variance.
Relations between streamflow and sediment load were
analysed by using a coherency technique, which entails
two steps: scanning t-test and coherency analysis based
on the scanning t-test. In the scanning t-test (Jiang et al.,
2002), statistic t(n, j) is defined as the difference of the
subsample averages between every two adjoining subseries of equal subseries size (n) expressed as
scales longer than 30 years. For shorter subsample sizes,
the critical values are overly restrictive. Because the
significance level varies with n and j, the test statistic was
normalized, to make values comparable, as
tr ðn; jÞ ¼ t ðn; jÞ=t0:05
(3)
when tr(n,j) > 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. Finally,
the coherency of abrupt changes between two series u and
v was defined as
trc ðn; jÞ ¼ sign½tru ðn; jÞtrv ðn; jÞfjtru ðn; jÞtrv ðn; 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 the
same direction, whereas if trc(n, j) < 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
regarded as an indication of the interaction between these
two series on decadal and basin scales.
RESULTS AND DISCUSSIONS
Changes in sediment load and streamflow at Pingshan
station
Pingshan station is located upstream to the Gezhouba
Dam and Three Gorges Dam (Figure 1). Streamflow has
been increasing at a time scale of > 128 months. The
decrease and increase can be found to be intermittent at
different time scales (Figure 2). Regions dominated by
significant change points also are found to distribute
sporadically within the time scales versus time space.
Comparatively, patterns of distribution of change points
of sediment load are in approximate agreement with those
1=2
1=2
2
2
(2) of streamflow. Increasing tendency is found mainly at
t ðn; jÞ ¼ xj2 xj1 n sj2 þ sj1
longer time scales, and the abrupt changes in sediment
load at shorter time scales, such as < 64 months, are
where
relatively complicated. For sediment load and streamflow
j1
jþn1
j1
X
X
X
changes,
the long-term tendency is dominated by the
1
1
1
2
xj1 ¼
xðiÞ; xj2 ¼
xðiÞ; s2j1 ¼
xðiÞxj1 ; increasing trend, and these results are good line with
n i¼jn
n i¼j
n1i¼jn
those by Zhang et al. (2006). Earlier study suggested that
jþn1
X
1
2
the river suspended sediments are mainly from the upper
s2j2 ¼
xðiÞ xj2 ;
Yangtze River Catchment (Pan, 1999); the sediments
n 1 i¼j
from Jinshajiang River alone accounts for about 39.4% of
in which n is the subsample size varying as n = 2, 3,. . . , that in Yichang station (Xu, 2005). Over-exploitation in
< N/2 or may be selected at suitable intervals; and the upper Yangtze Catchment led to an increasing trend of
j = n + 1, n + 2 ,. . . , N–n + 1 is the reference time point. It sediment load in the Pingshan station (Pan, 1999).
should be noted that hydrological series are usually autoFigure 3 shows abrupt changes in streamflow (upper
correlated. Thus, the Table-Look-Up Test (von Storch panel of Figure 3) and sediment load (lower panel of
and Zwiers, 1999) was adopted to modify the significance Figure 3). Different changing properties of streamflow
criterion of statistic t(n, j) based on lag-1 autocorrelation can be identified during different time intervals. Specifcoefficients of the pooled subsample and the subsample ically, before 1965, streamflow was characterized to be
size n. Criterion t0.05 for the correction of the dependence increasing. Decreasing streamflow can be observed
was employed to determine significant changes in time during 1965 and the early 1970s. The 1970s and 1980s
Copyright © 2012 John Wiley & Sons, Ltd.
Hydrol. Process. 27, 444–452 (2013)
447
streamflow (Figure 4) also indicates positive relations
between sediment load and streamflow by positive
coherency at various time scales.
Pingshan: streamflow
128
64
Sediment load and streamflow changes at Yichang station
32
16
8
256
1970
1975
1980
1985
1990
1995
2000
1985
1990
1995
2000
Pingshan: sediment load
128
64
32
16
8
1965
1970
1975
1980
Standardized streamflow
(m3/s)
Figure 2. Change points on different time scales of streamflow and
sediment load variations of the Pingshan station. Dashed lines show
decreasing trend after the change point, and solid lines indicate increasing
trend after the change point. Thick solid and dashed lines denote
significant change points. The meanings of the line styles are the same for
subsequent figures. The x-axis denotes time scales with unit of months;
first, we should identify the change points in the plot, that is, the regions
circled by thick dashed or solid contours, and then read the time scales
from x-axis and then the time when the change point occurred from the yaxis. In so doing, the time scales and the time when the change point
occurred can be read from the plot. The same procedure will be followed
in the subsequent figures
128
64
32
16
1965
2
1970
1975
1980
1985
1990
1995
2000
Figure 4. Coherency between sediment load and streamflow series at the
Pingshan station
1
0
−1
−2
1960
6
Standardized sediment load
(kg/s)
256
8
3
1970
1980
1990
2000
4
Time scales (months)
Time scales (months)
1965
The upper panel of Figure 5 shows abrupt changes in
streamflow at the Yichang station. Figure 6 displays
abrupt behaviours of streamflow and sediment load using
linear regressive technique. Increasing tendency can be
found at time scales of > 128 months. Changing characteristics of streamflow at time scales of < 128 months
are complicated. Significant change points are identified
in about 1975, the mid-1980s and the early 1990s.
Comparison between the upper panel of Figure 5 and the
upper panel of Figure 2 indicates similarity in streamflow
changes at the Yichang and Pingshan stations. The
differences, if any, between streamflow changes at the
Yichang and the Pingshan stations should be due to the
streamflow from the tributaries between the Pingshan and
Time scales (months)
256
2
256
Yichang: streamflow
128
64
32
16
8
1965
0
−2
1960
1970
1980
1990
2000
Figure 3. Linear trends of time intervals separated by change points for
the streamflow and sediment load series at the Pingshan station
are featured by increasing streamflow. Decreasing
streamflow is found after 2000. Abrupt changes in
sediment load shown in the lower panel of Figure 3 are
similar to those of streamflow changes, implying that the
transport of sediment load is subjected mainly to the
hydrodynamic characteristics of the river channel (Lu
et al., 2003). Coherency analysis of sediment load and
Copyright © 2012 John Wiley & Sons, Ltd.
Time scales (months)
Time scales (months)
HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER
256
1970
1975
1980
1985
1990
1995
2000
1985
1990
1995
2000
Yichang: sediment load
128
64
32
16
8
1965
1970
1975
1980
Figure 5. Change points on different time scales of streamflow and
sediment load variations of the Yichang station. Dashed lines show
decreasing trend after the change point, and solid lines indicate increasing
trend after the change point. Thick solid and dashed lines denote
significant change points. The meanings of the line styles are the same for
subsequent figures
Hydrol. Process. 27, 444–452 (2013)
Q. ZHANG ET AL.
4
3
2
1
0
−1
−2
−3
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
6
4
2
0
result of the trapping effect of the Three Gorges Dam.
Streamflow was not consistently decreasing or increasing
before the early 1990s and has exhibited moderate
variations since the 1990s. Changes in sediment load
were different from those of the streamflow, being
dominated by a general decreasing trend. A sudden
decrease was observed after the early 1980s and ~2000.
Results of coherency analysis (Figure 7) indicated
negative relations between streamflow and sediment load
after the early 1980s on a time scale of > 64 months,
showing tremendous impacts of the trapping effect of the
Gezhouba Dam on the sediment transport. No negative
relations were found during other time intervals on
different time scales, particularly after the late 1990s,
which should be caused by concordant changes in the
sediment load and streamflow, that is, decreasing tendency,
although the magnitudes of changes were different.
Sediment load and streamflow changes at Hankou station
the Yichang stations, such as the Wujiang River
(Figure 1). Besides, the storage capacity of the river
channel between these two stations should be one of the
factors causing differences in streamflow changes at the
Pingshan and Yichang stations.
Changes in sediment load are subjected to different
patterns of changes when compared with those at the
Pingshan station. Several time intervals were identified to
be dominated by different changing properties of
sediment load. Specifically, significant change points
were detected in the early 1970s, the early 1980s and the
late 1990s. The construction of the Gezhouba Dam started
in 1970, and its operation started in the early 1980s with a
total storage of 1.58 109 m3, which has been exerting a
tremendous influence on sediment transport (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). These water reservoirs on
the mainstream and tributaries of the Yangtze River have
been trapping large amounts of sediment and have given
rise to significant abrupt changes in the sediment load in
the early 1980s and the mid-1980s (Yang et al., 2006).
The effects of water reservoirs, that is, the Gezhouba and
the Three Gorges Dam water reservoirs, are reflected by
abrupt changes in streamflow.
Since the start of the construction of the Three Gorges
Dam in 1994 and its completion in late 2008 with a total
storage capacity of 3.93 1010 m3, the period of 2003–
2006 was decided as the post-TGD (Three Gorges Dam)
by Chen et al. (2008). Changes in annual sediment load
indicate a decrease in the load since 2003 (Chen et al.,
2008). The late 1990s were identified as the significant
change point, and the sediment load has obviously been
decreasing since this change point, which should be the
Copyright © 2012 John Wiley & Sons, Ltd.
Time scales (months)
Figure 6. Linear trends of time intervals separated by change points for
the streamflow and sediment load series at the Yichang station
Figure 8 shows abrupt changes in sediment load and
streamflow at the Hankou station (see Figure 1 for its
256
128
64
32
16
8
1965
1970
1975
1980
1985
1990
1995
2000
Figure 7. Coherency between sediment load and streamflow series at the
Yichang station
Time scales (months)
−2
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
256
Hankou: streamflow
128
64
32
16
8
1965
Time scales (months)
Standardized sediment load (kg/s)
Standardized streamflow (m3/s)
448
1970
1975
1980
1985
1990
1995
2000
1985
1990
1995
2000
256 Hankou: sediment load
128
64
32
16
8
1965
1970
1975
1980
Figure 8. Change points on different time scales of streamflow and sediment
load variations of the Hankou station. Dashed lines show decreasing trend
after the change point, and solid lines indicate increasing trend after the change
point. Thick solid and dashed lines denote significant change points. The
meanings of the line styles are the same for subsequent figures
Hydrol. Process. 27, 444–452 (2013)
449
HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER
2
1
0
Sediment load and streamflow changes at Datong station
Time scales (months)
Changes in streamflow were evidently similar to those
at the Yichang and Hankou stations in terms of abrupt
changes (Figure 11). No large tributaries and no massive
streamflow input were available in the lower Yangtze
River basin, which should contribute to the similarity of
streamflow changes at the Yichang, Hankou and Datong
256
128
64
32
16
8
1965
−1
256
3
2
1
0
−1
−2
−3
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Figure 9. Linear trends of time intervals separated by change points for
the streamflow and sediment load series at the Hankou station
Copyright © 2012 John Wiley & Sons, Ltd.
1975
1980
1985
1990
1995
2000
Datong: streamflow
128
64
32
16
8
1965
−2
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
4
1970
Figure 10. Coherency between sediment load and streamflow series at the
Hankou station
Time scales (months)
3
Coherency analysis (Figure 10) shows that on a longer
time scale, such as > 64 months, streamflow has a
negative relation with sediment load since the mid1970s. This should be the result of the construction of
Gezhouba Dam, which decreased the sediment load in the
lower river reach and caused inconsistent relations
between sediment load and streamflow on longer time
scales. On shorter time scales of < 64 months, however,
positive coherency was still observed. Therefore, sediment transport load also is heavily influenced by
hydraulics on shorter time scales.
Time scales (months)
Standardized sediment load (kg/s)
Standardized streamflow (m3/s)
location). In general, similar changes can be found in
streamflow when compared with those at the Yichang
station in terms of the time when the change points occur.
Changes in streamflow at the Pingshan station are
different from those at the Yichang and Hankou stations.
These results clearly show different influencing factors
for streamflow changes at the foregoing three hydrological stations, which should be attributed to the uneven
spatial distribution of precipitation changes (Zhang et al.,
2008a). These results also indicate that streamflow in the
Yangtze River is influenced by climate change, such as
precipitation, but not by anthropogenic factors, such as
the construction of water reservoirs. Differences in
patterns of streamflow changes at the Hankou station,
when compared with those at the Yichang station, also
should be caused by streamflow from the Hanjiang River
and hydraulic regulation of the river channels between the
Yichang and Hankou stations.
It can be observed from the lower panel of Figure 8
that the sediment load is dominated by a decreasing
tendency except for a short time interval, that is, 1985 to
early 1990s, which is characterized by increased
sediment load. Figure 9 intuitively clarifies the changes
in sediment load and streamflow. Streamflow follows a
relatively complicated changing pattern, decreasing after
the early 1990s, which should be caused by the
decreasing precipitation in the upper Yangtze River
basin (Zhang et al., 2008a). Changes in sediment load are
dominated by a decrease, and this decrease is abruptly
evident after 2000, which should be attributed to the
construction of the Three Gorges Dam because of the
abrupt change stemming from the construction of the
Three Gorges Dam, as reported in earlier studies (e.g.
Zhang et al., 2008b).
1970
1975
1980
1985
1990
1995
2000
1985
1990
1995
2000
loadload
Datong:sediment
sediment
256 Datong:
128
64
32
16
8
1965
1970
1975
1980
Figure 11. Change points on different time scales of streamflow and
sediment load variations of the Datong station. Dashed lines show
decreasing trend after the change point, and solid lines indicate increasing
trend after the change point. Thick solid and dashed lines denote
significant 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.
Standardized sediment load (kg/s)
Standardized streamflow (m3/s)
stations. In addition, this result further confirms the
conclusion that changes in streamflow are mainly the
result of precipitation changes. Sediment load has
generally been decreasing particularly since 2000
(Figures 12 and 13). The mid-lower basin is a kind of a
sediment sink (Chen, 1996). Increasing evident increase
of the sediment load at the middle and the lower Yangtze
River basin when compared with the upper Yangtze River
basin is partly because of an important sink that
significantly decreases the sediment discharge from the
Yangtze River into the sea (Chen et al., 2005). It is
estimated that the annual sand extraction amounted to
about 40 million tons in the early 1980s and increased to
about 80 million tons in the late 1990s. In this case,
sediment load changes are the integrated results of climate
changes and human activities. However, the decrease in
sediment load at the Datong station in recent decades has
been moderate when compared with that at the Yichang
and Hankou stations, which should be caused by the
erosion of riverbed. The scouring process caused by the
decreased sediment load from the upper and the middle
4
3
2
1
0
−1
−2
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
6
4
2
0
−2
1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Time scales (months)
Figure 12. Linear trends of time intervals separated by change points for
the streamflow and sediment load series at the Datong station
256
128
64
32
16
8
1965
1970
1975
1980
1985
1990
1995
2000
Figure 13. Coherency between sediment load and streamflow series at the
Datong station
Copyright © 2012 John Wiley & Sons, Ltd.
Yangtze River caused a moderate decrease, although the
decreasing tendency was still evident. The time interval
after 2000 witnessed a significant decreasing sediment
load. The decreasing sediment since 2000 can be
attributed to the trapping effect of the Three Gorges
Dam. Streamflow at the Hankou station had been
generally increasing, but it has been decreasing since
2000. In the lower Yangtze River basin, precipitation has
been increasing, and it is particularly true for precipitation
maxima and precipitation intensity (Zhang et al., 2008a,
2011b). Spatial patterns of precipitation regimes over the
Yangtze River basin result in spatial distribution of
changes in streamflow.
CONCLUSIONS
Analysis of monthly streamflow and sediment load data
from four hydrological stations along the mainstem of the
Yangtze River shows abrupt changes. The following
conclusions are drawn from this analysis:
1. Climate change and human activities induce changes in
streamflow and sediment load at different time scales.
Generally, the changes of hydrological regime influenced by climate factors, on a long-term basis, was
comparable to the changes by anthropogenic factors.
However, hydrological changes caused by human
activities, such as the construction of water reservoirs,
can reflect the anthropogenic influence on a longer time
scale. Trapping by water reservoirs can decrease
sediment load, and the decrease in sediment load can
occur for a considerably long time. In this sense, the
influence of water reservoirs on the hydrological
regime, particularly the sediment load and subsequent
effects, can be far reaching. Besides, the abrupt
behaviours of sediment load and streamflow are
presented in time versus temporal scales showing
abrupt changes on different time scales, this is the
novel points when compared with research on similar
topics within the Yangtze River basin.
2. Coherency analysis shows that sediment transport in
the upper Yangtze River heavily depends on the river
hydraulics, as reflected by the positive coherency
between sediment load and streamflow at the Pingshan
station. Changes in sediment load in the middle and
lower Yangtze River reaches are impacted by the
trapping effects of the water reservoirs in the middle
Yangtze River basin, for example, the Gezhouba Dam
and the Three Gorges Dam. Massive trapping effects of
water reservoirs of the Gezhouba Dam and the Three
Gorges Dam directly cause a significant decrease in
sediment load in the middle and the lower Yangtze
River basin. The scouring of the riverbed in the lower
Yangtze River leads to moderate changes in sediment
load. The Three Gorges Dam still causes a considerable
decrease in sediment load, resulting in a decrease in
sediment load since about 2000. However, changes in
streamflow seem mainly to be the result of changes in
Hydrol. Process. 27, 444–452 (2013)
HYDROLOGICAL PROCESSES AT DIFFERENT TIME SCALES IN THE YANGTZE RIVER
precipitation. Comparison between the results of
precipitation changes across the Yangtze River basin
(Zhang et al., 2008a) and abrupt changes in streamflow
corroborates the tremendous impact of precipitation
changes on
the hydrological processes. Coherency
analysis investigates abrupt changes of streamflow
versus sediment load relations on different time scales,
and this is another novelty of this study when compared
with standing research.
3. Streamflow and sediment load are influenced by
climate change and human activities, and their
influence varies with time scales. Besides, influences
of water reservoirs on sediment load changes also
are affected by balance-effects of the storage of
water reservoirs (e.g. Want et al., 2007b). Therefore,
retention effects of water reservoirs on sediment load
are relatively complicated because the reservoirs act as
sinks of sediment load. However, the storage capacity
is decreasing with the operation of the water
reservoirs. New reservoirs are completed; meanwhile,
the old reservoirs will no longer have storage for
sediment deposition. In this case, the retention capacity
of water reservoirs for sediments is decreasing.
Therefore, it is difficult to exactly estimate the total
annual deposition in so many reservoirs within the
Yangtze River drainage basin (Yang et al., 2002). An
understanding of changes in the hydrological regime at
different time scales and possible underlying causes is
of scientific and practical significance.
ACKNOWLEDGEMENTS
This work is financially supported by Program for New
Century Excellent Talents in University (the Fundamental
Research Funds for the Central Universities), the National
Natural Science Foundation of China (Grant No.: 41071020;
50839005), the Project from Guangdong Science and
Technology Department (Grant No.: 2010B050800001;
2010B050300010), and by a grant from the Research Grants
Council of the Hong Kong Special Administrative Region,
China (Project No. CUHK405308). Cordial gratitude should
be extended to the Changjiang the Changjiang (Yangtze)
Water Resource Commission for providing the data analysed
in this study. Our cordial gratitude also should be owed to the
editor-in-chief, Prof. Dr Malcolm G. Anderson, and three
anonymous reviewers for their professional and pertinent
suggestions and comments, which are greatly helpful for
further improvements of the quality of this manuscript.
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