Large and episodic decrease of water discharge from the Yangtze River to the sea during the dry season

Hydrological Sciences Journal 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 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China cxqing@ihw.com.cn 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; flow 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. Il 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 2 with an average annual discharge into the sea of 28 600 m 3 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 1 August 2002 转载

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 LOW 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 84 200 m 3 s -1 in August 1954 to 6730 m 3 s -1 in February 1963. Chen (1991) 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 46000 24000 Discharge (m 3 s -1 ) 42000 38000 34000 30000 26000 22000 18000 1950 1960 Annual Mean Discharge 1970 1980 1990 Year 2000 Discharge (m 3 s -1 ) 22000 20000 February Mean Discharge 18000 16000 14000 12000 10000 8000 6000 Year 4000 1950 1960 1970 1980 1990 2000 12000 11000 10000 Yearly Minimum Discharge Discharge (m 3 s -1 ) 9000 8000 7000 6000 5000 1979 4000 1945 1955 1965 1975 1985 Year 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 m 3 s -1, remarkably lower than the average long-term baseflow of about 6200 m 3 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 m 3 s -1 at Wuhan to 4620 m 3 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 3 s -1 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 m 3 s -1 in 1979 to about 7000 m 3 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 km 2. 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 m 3 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 3 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. Water abstraction projects Capacity (m 3 s -1 ) 1 Wujiang Pivot, Anhui Province 230 2 East Line Diversion (Phase I) 600 3 Yangtze to the Huaihe Line 300 4 Yangtze to Taizhou Line (Phase I) 300 5 Nanjing, Jiangsu Province 260 6 Zhenjiang, Jiangsu Province 908 7 Yangzhou, Jiangsu Province 324 8 Taizhou, Jiangsu Province 543 9 Wuxi, Jiangsu Province 676 10 Suzhou, Jiangsu Province 2500 11 Nantong, Jiangsu Province 434 Total 7075

Water discharge from the Yangtze River to the sea during the dry season 45 Fig. 3 The maximum water chloride concentration in the Yangtze estuary and the delta area during the dry season of 1979. over 30 10 9 m 3. The Jiangdu Pivot at Yangzhou (site 7) alone extracted 6.3 10 9 m 3 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 m 3 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 10 9 m 3 and 3.16 10 9 m 3 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 m 3 s -1. During the 2000 drought, hydropower production was interrupted in order to supply water to the fluvial plains downstream. The water level in 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 3 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 10 9 m 3 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 3 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 47 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 10 9 m 3, 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 10 6 m 3. 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 10 6 m 3. 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 at 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 had shown 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 m 3 s -1, although levels fluctuated from 7220 to 24 700 m 3 s -1 (Chen, 1998). In contrast, the total capacity of water abstractions is as high as 7000 m 3 s -1 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 3 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. REFERENCES Chen Dekun, Sun Jichang, Liu Jinping, Zhou Guoliang & Chen Shu e (2000) Annual Report on Water Regime in 1999. China Water Resource and Waterpower Publishing House, Beijing (in Chinese). Chen Xiqing (1991) Sea-level changes since the early 1920s from the long records of two tidal gauges in Shanghai, China. J. Coastal Res. 7(3), 787 799. Chen Xiqing (1998) Changjiang Yangtze River Delta, China, J. of Coastal Res. 14(3), 838-858 Chen Xiqing (2000) Transboundary river, interbasin water transfer and the fundamental problems about the water diversion projects in China. Resources and Environment in the Yangtze basin 9(1), 92 97 (in Chinese). Chen Xiqing & Chen Jiyu (2000) Proposal to study and control the decrease tendency in discharge of the Changjiang River entering the sea in dry season (initially published by the Chinese Association of Science and Technology as an official document to the Central Committee of Communist Party of China and the State Council). Science & Technology Review 2000(2) (Serial no. 140), 39 40 (in Chinese). Chen Xiqing & Zong Yongqiang (1998) Coastal erosion along the Changjiang River deltaic Shoreline, China: history and prospective. Estuarine, Coastal and Shelf Science 46, 733 742. Chen Xiqing, Yongqiang Zong, Zhang Erfeng, Xu Jiangang & Li Shijie (2001) Human impacts on the Changjiang (Yangtze) River basin, China, with special reference to the impacts on the dry season water discharges into the sea. Geomorphol. 41(2 3), 111 123. Feng, Q., Cheng, G. D. & Endo, K. N. (2001) Towards sustainable development of the environmentally degraded River Heihe basin, China. Hydrol. Sci. J. 46(5), 647 658. Huang Guohui (1999) Reservoir sedimentation and turbine abrasion at Gongzui hydropower station. Sichuan Water Power 18(4), 33 36 (in Chinese). Linghu Kehai (1999) Reservoir sedimentation at Tongjiezi hydropower station. Sichuan Water Power 18(4), 33 36 (in Chinese). Xia Jun & Chen, Y. D. (2001) Water problems and opportunities in the hydrological sciences in China. Hydrol. Sci. J. 46(6), 907 921. Zong Yongqiang & Chen Xiqing (2000) The 1998 flood on the Yangtze, China. Natural Hazards 22, 165 184. Received 3 November 2000; accepted 27 August 2001