Water diversion and sea-level rise: Potential threats to freshwater supplies in the Changjiang River estuary
Introduction
Salinity is a basic measure of water quality in estuarine environments and in particular it determines whether or not estuary water can be used for potable water supply (Huang and Foo, 2002, Hong and Shen, 2012). Large variations in salinity occur in estuaries where riverine freshwater meets and interacts with saltwater to form strata with fresh overlying salt and areas with a range of different salinities produced by mixing (Schroeder et al., 1990, Shen et al., 2003, Gong et al., 2013). Freshwater discharge from upstream, tidal range, physical configuration of the estuary, and in the longer term, sea-level rise, interact to determine where, and for what periods of time in the tidal cycle, salinity will limit the usability of the estuarine water (Chen et al., 1991, Caitlin et al., 2009, Wolanski and McLusky, 2011). For rivers with a large seasonal variation in freshwater discharge, as is the case for the Changjiang, the season with lowest flow will be vulnerable to higher salinities, and this may be compounded if management activities serve to reduce the freshwater discharge still further.
The Changjiang River mouth is a huge cone-shaped estuary with three major islands located there, trending en-echelon towards the southeast (Fig. 1). The river mouth is divided into the South and North Branches, separated by Chongming Island (CM), and the South Branch is divided into the South and North Channels, separated by Changxing (CX) and Hengsha (HS) Islands. The South Channel is further divided into the South and North passages separated by Jiuduansha (JDS) Island (Fig. 1). In the very long term the river mouth has been shifting southeastward via intensive sediment dynamical processes (Chen et al., 1988, Li et al., 2011), so at present 95% of the Changjiang discharge is via the South Branch, and the North Branch is being gradually silted up (Chen et al., 1988, Dai et al., 2013).
The hydrodynamics and morphology of the Changjiang estuary are such that there are unusual circumstances relating to the intrusion of saltwater (Mao et al., 2000, Xue et al., 2009). Saltwater intrudes into the estuary in the winter low flow period through both the South and North Branches (Fig. 1). The saltwater intrusion in the South Branch is opposed by the main freshwater discharge of the river, and therefore weakens generally around the middle of Changxing Island (Fig. 1). Saltwater can also intrude into the North Branch, passing right around the northwest end of Chongming Island and into the South Branch, meeting the freshwater flow upstream of the saltwater intrusion that comes directly into the South Branch (Zhang et al., 2012).
Recent changes to the supply of freshwater to Shanghai have involved building infrastructure in the Changjiang estuary to replace most of the water previously taken from the Huangpu River basin due to severe water pollution (Finlayson et al., 2013) (Fig. 1). The Qingcaosha reservoir has been built adjoining the northeastern shore of Changxing Island (Fig. 1). It has a water surface area of 70 km2 and a storage capacity of 4.38 × 108 m3. Although this reservoir site is known to be affected by saltwater intrusion in the winter season, the storage holds 68 days supply of water for Shanghai. This will allow the reservoir to continue supplying freshwater from the storage for the duration of a saltwater intrusion event with a return period of 100 years (Le, 2012). Two small-scale intake points with reservoirs, Dongfengxisha and Chenhang, are also located in the Changjiang River mouth (Fig. 1). These together can supply 70% of the freshwater demand of Shanghai city as at 2012.
The conditions for which this system was designed are changing and will continue to change in the future. This will be a great concern as new projects are built to divert water from the upper Changjiang into northern China where water shortage has occurred due to over irrigation and urbanization etc. (Zhang et al., 2003, Wang et al., 2008), and particularly, as the sea level is rising and the delta is sinking (Chen et al., 2001). These diversions have reduced the discharge in the winter season, causing more severe saltwater intrusions. The situation is exacerbated during the dry season such as occurred in 2006 (Chen and He, 2009, Zhu et al., 2010). These factors tend to suggest that the storage capacity of 68-days may not be enough to protect the water supply to Shanghai in the future. In this paper we examine the historical record of discharge in relation to the water diversion projects and sea-level rise to determine the severity of this threat to future water supply to Shanghai, taking account of the saltwater incursions directly into the South Branch and via the North Branch around Chongming Island. We present a scenario of the sustainability of the freshwater sources for Shanghai given its present population of >23 million, and the lack of alternative water sources given the heavy pollution of local water bodies in the Changjiang delta region (Finlayson et al., 2013).
Section snippets
Discharge
Daily discharge recorded at Datong station (Fig. 1) was sourced from the hydrology yearbooks of the Changjiang for the periods 1950–1986 and 1997–2011 (The Changjiang Water Conservancy Committee, 1950–1986, 1997–2011). Datong is the furthest downstream hydrological gauging station, but is 600 km upstream of the river mouth. Daily discharges are shown in Fig. 2A. We define the wet season as May 1–Oct. 30 and the dry season as Nov. 1–April 30. We have grouped daily discharge into 4 types, i.e.
Discharge effects
The daily discharge recorded at Datong Station (Fig. 2A) averages 25,500 m3 s−1 and shows no trend through time. The 4 types of discharge described above, i.e. high flow, normal flow, low flow and extreme low flow, are all >15,000 m3 s−1 during the wet season (Fig. 2B). This contrasts with the dry season when the extreme low flow discharge is always below 15,000 m3 s−1, and the low flow and normal flow discharges are below 15,000 m3 s−1 for 70% and 50% of the dry season respectively (Fig. 2C).
Freshwater sources – dry season shortage
The Changjiang is now the main source for the supply of freshwater to Shanghai and is likely to become more important in the future. Demand is rising as the population (presently >23 million) grows, the local sources are being heavily polluted, and there are threats developing in the Changjiang system in the dry season related to water diversions and saltwater intrusions (Chen et al., 2001, Gong et al., 2013). Undoubtedly, there is sufficient freshwater available in the Changjiang basin as the
Future scenarios
The ultimate purpose of this study is to understand the nature of the salinity distribution and its persistence over time in the Changjiang River mouth. This is of particular concern given that only 68 days supply of freshwater can be stored in Qingcaosha reservoirs in the Changjiang estuary. Therefore, our scenario study only applies to Gaoqiao station (Qingcaosha reservoir). The two low flow types (‘low flow’ and ‘extreme low flow’) are considered here in relation to future discharge,
Conclusions
This hydrological study focuses on the future availability of freshwater in the Changjiang estuary, on the basis of an examination of the available data: discharge, salinity, water diversion projects and sea level rise. Owing to the pressure from increasing population and heavy pollution of surface water available locally, the city of Shanghai, with >23 million people, has become increasingly dependent on the Changjiang estuary. The new Qingcaosha reservoir in the river mouth area was designed
Acknowledgements
This research is supported by the Australian Research Council (Grant No: P110103381), the China National and Natural Science Foundation (Grant No. 41271520), the Creative Research Groups of the China Natural Science Foundation (Grant No. 40721004), and The Chongming Tourism Administration (Grant No. 48102860).
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