Hydrological Sciences-Jkmrnal-des Sciences Hydrologiques, 47( 1 ) February 2002
41
Large and episodic decrease of water discharge
from the Yangtze River to the sea during the dry
season
CHEN XIQING, ZHANG ERFENG
Stale Key Laboratory of Estuar'me and Coastal Research, East China Normal
Shanghai 200062. China
[email protected]
University,
XU JIANGANG
Geography Department, Nanjing University, Nanjing 210093,
China
Abstract A diagnostic study on the large and episodic decrease of discharge from the
Yangtze River to the sea is presented. No trend has been detected so far in the time
series of river flow recorded at Datong, but a large and short-term drop is evident
through analysing the yearly minimum discharges. Water abstractions in the middle
reach are evidently responsible for the large and episodic drop recorded at Datong.
Downstream of Datong the discharge further decreases due to water abstraction by
sluices and pumping stations, with a capacity comparable to the average discharge
from Datong in dry months. Such large-scale water diversions may greatly reduce the
water discharge into the sea during droughts, if countermeasures are not prepared. An
increasing seasonal variability will exacerbate the existing water and environmental
problems in the Yangtze Delta and estuary. An integrated management system is
necessary for sustaining sufficient freshwater discharge into the sea.
Key words Yangtze River; river runoff; water diversions; streamflow droughts; trend
detection; How regimes
Baisse importante et brève du débit du Fleuve Yangtze déversé en
mer durant la saison sèche
Résumé Une recherche diagnostique a été menée au sujet de la baisse importante et
épisodique du débit du Fleuve Yangtze déversé en mer. Jusqu'à présent, aucune
tendance n'a été détectée dans des séries temporelles de débit à Datong, mais l'analyse
des débits minimum annuels montre l'existence d'une baisse importante et brève, dont
les prélèvements d'eau sont évidemment responsables. Le débit continue de baisser en
aval de Datong à cause des prélèvements des canaux et des stations de pompage, dont
l'ampleur est équivalente au débit moyen des mois secs. Ces prélèvements d'eau à
grande échelle peuvent lourdement réduire le débit arrivant à la mer lors des
sécheresses, si des mesures palliatives ne sont pas mises en œuvre. L'augmentation de
la variabilité saisonnière va mettre plus encore en péril les ressources en eau et les
écosystèmes dans le delta et l'estuaire du Yangtze. 11 est donc nécessaire d'avoir une
gestion intégrée pour maintenir un débit d'eau douce déversé en mer suffisant.
Mots clefs Fleuve Yangtze; écoulement de cours d'eau; prélèvement d'eau; étiages; détection
de tendance; régimes hydrologiques
INTRODUCTION
Being the largest river in China, the Yangtze (Changjiang) covers an area of about
1.8 million km" with an average annual discharge into the sea of 28 600 m J s"1 between
1951 and 1999. The seasonal variability of water discharge is relatively small in
comparison with that of the large rivers in North China, such as the Yellow River.
Open for discussion until I August 2002
42
Chen Xiqing et al.
Human activities already have a strong influence on the hydrological processes in the
middle and lower reaches. The most notable is the large and episodic drop in flow and
water stage during the dry seasons. Even if it is not as extreme as in North China (cf.
Feng et al, 2001; Xia & Chen, 2001), it causes severe environmental, economic and
water supply problems. Meanwhile, global warming is expected to cause an increase of
the seasonal variability of discharge from the Yangtze into the sea.
DATABASE
The water discharge records are published by, or obtained from, the national hydrometric stations. Information about water abstractions along the river banks are based
on field surveys and documented material from local government agencies. The data of
water chloride concentrations in the Yangtze estuary are obtained from the local
waterworks. Some recent information comes from official reports.
DECREASE OF L O W FLOW RECORDED AT DATONG
The Datong hydrometric station (Fig. 1) is located at the tidal limit. It provides
continuous records since 1950 which allow the study of the changes in water discharge
from the Yangtze basin. The relationship between water levels and water discharge
downstream of Datong is influenced by tides and tidal flows. The three highly populated provinces in this area—Jiangsu, Anhui and Shanghai—have rapidly developed
industries and agriculture.
The monthly mean discharge at Datong shows large seasonal fluctuations, ranging
from 84 2 0 0 m V in August 1954 to 6730 m3 s~' in February 1963. Chen (1991)
in»
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Fig. 1 Geographical setting of the Yangtze River and its drainage basin, and the sites of
water abstractions downstream of the Datong hydrometric station.
Water discharge from the Yangtze River to the sea during the dry season
43
Fig. 2 Variations of water discharge from 1950 to 1999 recorded at the Datong hydrometric station.
examined the long-term changes at Datong and found no significant trend in the annual
mean discharge. The changes of monthly mean discharge in the dry season, as
indicated by the February discharge shown in Fig. 2, also support this conclusion. The
Yangtze discharges have been assumed to be stable with regard to their annual and
monthly mean values over the past 50 years. However, the yearly minimum water
discharges reveal a striking phenomenon in that a large episodic drop appeared at the
end of January 1979. The water discharge dropped to 4620 mJ s"', remarkably lower
than the average long-term baseflow of about 6200 nr' s"1 since 1950 (Fig. 2). This
phenomenon highlights the importance of human impacts on the flow regime. Despite
a large drop in 1979, it has been difficult to test statistically if a significant difference
exists between the 1979 minimum and the other data (Fig. 2), because the records are
not long enough to detect natural periodic components. Nevertheless, the yearly minimum discharge, which was recorded one day earlier at the Wuhan (Hankou) station,
about 500 km upstream from Datong, provides strong evidence that water abstractions
during droughts are responsible for the large and episodic drop at Datong. The discharge decreased downstream from 5170 nr1 s"1 at Wuhan to 4620 nr' s"1 at Datong,
despite additional inflows from tributaries between these stations, such as the Poyang
Lake drainage system. A study of the water balance between Wuhan and Datong
indicated that water abstractions amounted to about 1750 m J s~' during this period
(Chen et al., 2001). From Datong downstream, the water discharge continually
decreases as a result of the tremendous water abstractions, as described below.
44
Chen Xiqing et al.
LARGE DECREASE FROM DATONG TO THE SEA
The tides in the East China Sea are semidiurnal, ranging from 2 to 3 m in general. The
tide waves can move upstream to Datong, about 680 km inland from the river mouth,
and cause daily fluctuations in water levels. Extensive flood plains lie in altitude
within the range of these daily tidal fluctuations. Sluices can divert a tremendous
volume of water to the low-lying area during high tides for use in agriculture,
industries and daily life. The large-scale sluices are often integrated with pumping
stations for water abstraction during low water periods. According to recent surveys
and statistics, the capacity of water abstractions downstream of Datong has increased
from about 5000 m3 s"1 in 1979 to about 7000 m3 s"1 by the end of 2000. (Table 1 and
Fig. 1).
The Wujiang Pivot (site 1—Table 1, Fig. 1) diverts water from the Yangtze to a
middle-sized tributary basin, with an irrigation area of about 5360 knr. The East Line
Water Diversion Project (site 2) extends 1150 km northwards from Yangzhou to
Tianjin in North China. Based on the Jiangdu Pivot built in 1977 at Yangtze (site 7),
the East Line will be expanded to about 1000 mJ s"1 in discharge capacity before 2030.
The Yangtze to Huaihe Line (site 3) and the Yangtze to Taizhou Line (Site 4) divert
water from the Yangtze to the Huaihe drainage basin just north of the Yangtze. In
addition, there are 20 pumping stations and 42 sluices that belong to the seven
administrations (sites 5-11). Each administration includes several cities and counties.
Geographically, the drainage area between Datong and the river mouth is confined
to a narrow zone, with little water input in the dry season, while the capacities of
pumping stations and sluices for water abstractions along the river banks downstream
of Datong total over 7000 m' s"1, comparable to the monthly discharge at Datong
during the dry season (Fig. 2). Most of the sluices and pumping stations are under the
control of local governments. The amount of water abstraction varies greatly with the
changing seasons and the changes in local climate. It is difficult to estimate in advance
how much water will be taken from the Yangtze. The unlimited water abstraction and
unpredictable water demands may cause large and episodic drops in water discharge to
the sea. In 1978, for example, the water abstraction below Datong was estimated at
Table 1 Capacities of water abstractions at sites downstream of Datong by the four major hydroengineering works (sites 1-4) and numerous sluices and pumping stations belonging to the seven
administrations (sites 5-11). See Fig. 1 for location of water transfer sites.
Water transfer site no.
4
5
6
7
8
9
10
11
Total
Water abstraction projects
Wujiang Pivot, Anhui Province
East Line Diversion (Phase 1)
Yangtze to the Huaihe Line
Yangtze to Taizhou Line (Phase 1)
Nanjing, Jiangsu Province
Zhenjiang, Jiangsu Province
Yangzhou, Jiangsu Province
Taizhou, Jiangsu Province
Wuxi, Jiangsu Province
Suzhou, Jiangsu Province
Nantong, Jiangsu Province
Capacity (m' s~
230
600
300
300
260
908
324
543
676
2500
434
7075
Water discharge from the Yangtze River to the sea during the dry season
i
45
,Fsb 1979
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t
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hn^niac
pudOHQ Alfcd
Tctihu Lake
Shanghai
Soir p in-
Nnnbui
• *.-/ s:
1
Hu/.hou
I!--! U
•
Jl:K1I¥
i
0
30 KM |
Fig. 3 The maximum water chloride concentration in the Yangtze estuary and the
delta area during the dry season of 1979.
over 30 x 109 rtr1. The Jiangdu Pivot at Yangzhou (site 7) alone extracted 6.3 x 109 mJ
water northwards to the Huaihe River basin between April and December 1978. It is
estimated that the water abstractions downstream of Datong during the dry months
between 1978 and 1979 are equivalent to an average discharge of more than 1000 mJ s"1.
The resultant saltwater intrusions brought about serious adverse consequences for agriculture, industries and drinking water over the Yangtze Delta (see Fig. 3).
The relationship between the Datong discharge and Wusong chlorinity, as shown
in the upper-left panel in Fig. 3, also reflects the large drop in water discharge into the
sea during the dry season of 1979. The Wusong Waterworks is located at the outlet of
the Huangpu River that extends from Taihu Lake to the Yangtze estuary. The monthly
mean data are used to smooth the effect of daily tidal fluctuation. The two points
corresponding to the data of February and March 1979 deviate markedly from the
others, reflecting a great drop in freshwater discharge into the sea as a result of water
abstractions.
LATEST HYDROLOGICAL INFORMATION
After the 1998 flood (Zong & Chen, 2000), the water level in the middle reach of the
Yangtze River dropped rapidly. This process began in November 1998 and lasted until
April the following year. Many areas that had been affected by the floods now had to
combat the severe droughts. Until late February 1999, the depth of the navigation
channel at several sections was only 1.8-2.5 m. This was the first time that water
transportation in the middle reach was prevented due to the insufficient water depth.
46
Chen Xiqing et al
Beside the heavy siltation in the channel following the floods, the low discharge was
also a major cause of the extremely shallow water. The precipitation in 1999 over the
middle basin, and the Yangtze as a whole, did not noticeably deviate from the average
value, albeit with large regional variations. The Dongting, the Poyang and the
Hanjiang are the three large drainage systems in the middle Yangtze River basin. From
October 1998 to April 1999 precipitation over the Hanjiang drainage basin was
25-40% lower than the average, but, over the Poyang drainage system, a basin
including five rivers, it was 25-40% higher (Chen et al., 2000). The monthly discharge
from Yichang from January to April 1999 was close to its long-term average.
Therefore, the rapid drop was largely attributed to the effects of the reservoir in the
middle basin. According to data from the Yangtze Water Resource Commission, the
water volumes in the reservoirs in the Dongting and Poyang drainage systems were
4.37 x 109 m3 and 3.16 x 109 m3 lower, respectively, at the end of 1998 than in 1997.
The Hanjiang River basin received much more precipitation after the 1998 floods, but
reservoir managers tended to stock up as much water as possible for the coming dry
season. Such behaviour may greatly increase the seasonal variability of water discharge into the main Yangtze River.
The Danjiangkou Reservoir (Fig. 1) on the upper Hanjiang River was initially built
for hydropower, flood control and irrigation. It will become a water source for the
Middle Line Water Diversion Project by upgrading its volume capacity. The Middle
Line extends 1241 km from Danjiangkou to Beijing and Tianjin, with a designed
discharge of 630 nr1 s"1. During the 2000 drought, hydropower production was interrupted in order to supply water to the fluvial plains downstream. The water level in the
reservoir was 4-5 m lower than the dead water level (139 m). Therefore, the water
supply to the East Line is not enough during severe droughts. The water diversion can
markedly reduce the water discharge from the Hanjiang into the main Yangtze during
the dry season.
From December 1998 to March 1999, the monthly mean discharge at Datong
varied between 9100 and 11 000 m J s" 1 . The average water chlorinity at Baozhen
(Fig. 3) in the estuarine island rose to 1595 ppm. The saltwater strongly affected the
Lingqiao Waterworks at Pudong Area (Fig. 3) from January to April. The water production in this waterworks only reached 60% of the designed capacity in 1999. Beside
a low discharge from Datong, the water abstractions downstream of Datong are clearly
responsible for the strong saltwater intrusion in the estuary. Jiangsu Province alone
abstracted 11.48 x 109 m J of water from the main Yangtze in 1999.
In spring 2000, the Yangtze basin was again affected by a severe drought. The
Datong discharge dropped to only 10 000-20 000 m J s"1 in May 2000, considerably
lower than the historical minimum that occurred in 1975. The extremely low discharge
from the upper basin caused a sharp decrease in sediment discharge to the estuary. The
local residents between Changshou and Taicang (Fig. 3) were surprised to find that the
normally turbid water had become very clear.
FUTURE DEVELOPMENT
The future changes in water discharge into the sea during the dry season will depend
on two factors. One is the drainage basin management, the other is the global and/or
regional climate change.
Water discharge from the Yangtze River to the sea during the dry season
Al
The drainage basin planning and management in the past largely focused on local
flood protection and irrigation. Countermeasures should be prepared for droughts,
including the control of interbasin water diversion. The potential problems of water
diversion are the impacts this might have on the lower Yangtze. The Chinese government decided to speed up the interbasin water diversion projects after the prolonged
and widespread drought in North China in 1999/2000. There is an urgent need for an
integrated management to sustain sufficient water discharges into the sea. Legislation
is necessary to control the operations of interbasin water diversions (Chen, 2000).
There were 45 657 reservoirs of different scales in the Yangtze drainage basin in
1999. The total volume capacity is as high as 151.9 x 109 m', equivalent to over 50%
of the total discharge in the dry season between December and April. Moreover, a
large number of reservoirs are under construction or planned to be built in coming
years, including the gigantic Three-Gorges Reservoir. The available data indicate that
the total water volume in reservoirs before the end of the flood season varies greatly
and depends on the changes of climate. Most of the reservoirs are located in the middle
Yangtze between Yichang and Datong. They were constructed for local irrigation,
flood control, hydropower and so on. Reservoir management can play an important
role in the future in controlling the water discharges into the main Yangtze.
The decrease in the capacity of reservoirs has become a widespread problem. The
Gongzui and Tongjiezi, for example, are the two large reservoirs in the mid-lower
reaches of the Dadu River (Fig. 1), used for hydropower production, flood control and
irrigation. The Gongzui Reservoir had a designed volume capacity of 373.7 x 106 m J .
This decreased rapidly by 63.8% from 1971 to 1992 (Huang, 1999). The Tongjiezi
Reservoir is about 32 km downstream from the Gongzui Reservoir, with a volume
capacity of 210 x 106 m3. The hydropower plant at this reservoir began operation in
April 1992. By 1998 the volume capacity had decreased by 51.7% (Linghu, 1999).
Deforestation in tributary basins can be a major cause for a decreasing trend in
water discharge into the main Yangtze during the dry season, as illustrated by the
example of the Minjiang River (Chen et al,, 2001). Furthermore, rapid siltation in
reservoirs may greatly reduce the capability for regulating the water discharge.
Agreements are required between the hydropower corporations and local governments
before water discharges may be released by reservoirs during droughts, but they are
often limited to the tributary basins, with little attention being given to the discharges
into the main Yangtze.
The upper basin from the headwater to Yichang, where the high plateau,
mountains and deep valleys characterize the landforms, contributes to about 50% of
the total discharge. The meltwater from glaciers and snow are an important supply to
the upper Yangtze in the form of surface water and groundwater. Due to global
warming, the mountainous glaciers in the headwater area showed a retreating tendency, ranging from a few metres to 25 m a year, between 1970 and 1990. Meanwhile,
the water levels of many lakes had declined rapidly.
After the 1998 floods on the Yangtze, the Chinese government began to take
drastic action to protect forests in the upper basin. A large natural conservation area
was created in the headwater area on the Qinghai-Tibet Plateau, though it is still a long
way from providing the environmental remediation.
The water discharge into the sea during the dry season varies greatly from year to
year, but the low discharges are more frequent. For example, about 50% of the January
48
Chen Xiqing et al.
discharges at Datong are between 8000 and 10 000 mr s"1, although levels fluctuated
from 7220 to 24 700 mJ s"1 (Chen, 1998). In contrast, the total capacity of water
abstractions is as high as 7000 nr s"' downstream of Datong. The capacity of water
abstractions in the middle basin is yet to be investigated. There is an urgent need for an
inventory of water abstractions in the framework of integrated drainage basin management. Although the Three-Gorges Reservoir, when completed, may increase discharge
downstream by about 1000-2000 m' s"1, the water diversions may pose additional
influences on discharge into the sea if effective countermeasures are not taken. An
increasing variability in freshwater and sediment discharges into the sea will greatly
influence the estuarine and coastal ecosystems (Chen & Zong, 1998). A proposal to
control the water discharge from the Yangtze to the sea has received much attention
from the Chinese Association of Science and Technology (Chen & Chen, 2000).
Acknowledgements This paper is a part of studies "On the changes of water discharge
in dry season from the Yangtze into its sea: history and trend" funded by the Natural
Science Foundation of China (Grant no. 49971071). The Key Sciences and
Technology Development Foundation established by the State Education Ministry of
China also supported this study (Grant no. 98042) under the title "Changes of water
discharge from the Yangtze into the sea under the background of the south to north
water diversions and its impact on freshwater resources in the Yangtze estuary". The
authors are also grateful for support from another key project, also supported by the
Natural Science Foundation of China (Grant no. 49736220), which studies the material
flux from the Yangtze into the sea.
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Received 3 November 2000; accepted 27 August 2001