This paper assesses the impacts of the Three Gorges Dam, the South–North
Water Transfer Project and other water abstractions on the probability of
long-duration salt intrusions into the Yangtze River estuary. Studies of
intrusions of saltwater into estuaries are typically constrained by both
the short duration of discharge records and the paucity of observations of
discharge and salinity. Thus, studies of intrusions of saltwater into
estuaries typically seek to identify the conditions under which these intrusions
occur, using detailed observations for periods of 20–60 days. The paper
therefore first demonstrates a method by which to identify the conditions
under which intense intrusions of long-duration occur and then applies that
method to analyse the effect of the three projects. The paper constructs a
model of the relationship between salinity and discharge and then employs
Monte Carlo simulation methods to reconstruct the probability of observing
intrusions of differing intensities and durations in relation to discharge.
The model predicts that the duration of intrusions with chlorinity

Shanghai's industries and the more than 24 million people who live in the municipality now principally depend on the Yangtze River for their water supply. Yet, like other large estuaries (Gong, 2013), the estuary of the Yangtze River is affected by salt intrusions (Shen et al., 2003). The latest occurred in 2014 (CNTV, 2014), the longest since 1993. So two storages have been built in the estuary to store water against the threat of salt intrusions (see Fig. 1 for locations): the Chenhang Reservoir supplies about 23 % of the municipality's public water, and the Qingcaosha Reservoir supplies about 54 % (Li et al., 2014). Qingcaosha's storage capacity is said to be equivalent to 67 days of Shanghai's current consumption. The probability of long-duration salt intrusions in the Yangtze Estuary is thus of considerable social importance.

Map of Yangtze Estuary.

However, the discharge and tidal characteristics of the Yangtze Estuary are now being modified. Water is increasingly being abstracted from the river, for the South–North Water Transfer Project (SNWTP) as well as for more local uses (Chen, 2013; Zhang et al., 2012), tending to increase the length and the intensity of periods of low discharge (Li et al., 2014). The Yangtze River is increasingly being dammed for large hydroelectric projects that cumulatively can now store about 25 % of the annual flow; this capacity is planned to double by 2050 (Li et al., 2000). The largest of these hydroelectric dams is the Three Gorges Dam. As hydroelectric dams, these constructions have limited effects on annual discharge but do affect its seasonal distribution (Finlayson et al., 2013). All of these modifications will affect the probability of salt intrusions in the estuary and it is important to calculate their effects.

Intrusions of salt in estuaries have been widely studied, by both numerical
and theoretical methods. Most studies of the Yangtze Estuary have been
numerical (Kuang et al., 2009; Li et al., 2012; Liu et al., 2013;
Xue et al., 2009). However, Zhang et al. (2011)
applied Savenije's (1986, 1993) theoretical model. As Fig. 1 indicates, the
Yangtze Estuary is complex, with four outlets to the sea. Nevertheless,
these studies confirm that saltwater intrusions are primarily governed by a
combination of low discharge and spring tide conditions but also
influenced by wind speed and wind direction. The discharge of the Yangtze River is
highly seasonal, with average discharges below 12 000 m

The principal problem for these studies has been to examine the conditions
under which oceanic water intrudes far up the estuary. They seek to predict
an event – the observation of water with chlorinity exceeding 250 mg L

However, the duration of this event is also significant, especially for residents of Shanghai. An intrusion of a few days, no matter how intense, has little social significance, compared to an intrusion of a few months. Chen et al. (2013) do address duration, but they, like others (Chen et al., 2001; Gu and Yue, 2004; Wang et al., 2008; Yang, 2001; Zhao et al., 2009), identify a single threshold discharge below which intrusion is likely and above which intrusion is unlikely. In fact, salt intrusions may occur at a variety of discharges (Fig. 2), with a probability that varies inversely with discharge.

Chlorinity and discharge conditions in the periods of observation.

The difficulty is that, while long discharge records for many rivers are generally available, measurements of salinity are not routinely collected by state agencies. Salinity thus needs to be measured for a specific purpose, such as a research project. Measurements of salinity are therefore expensive and not available in long records. It is thus important to find a method for using historical data about discharge to generalise the limited observations of salinity and so obtain a long record of salinity, which identifies both the duration of a saline intrusion and the relative frequency of occurrence of such intrusions. It is this method that this paper presents and then employs to calculate both the probability of long-duration salt intrusions in the Yangtze River estuary and the impact of human modifications of the river on that probability.

Specifically, the paper therefore addresses the following problems: how can the probability of long-duration (up to 60 days) salt intrusions in the Yangtze Estuary be identified? What is that probability? What is the effect on that probability of increasing water abstraction from the Yangtze Basin, the SNWTP and the construction of the Three Gorges Dam? It is our calculation of the probability of long-duration salt intrusions and our prediction of the impact on that probability of human modifications to discharge that sets this paper apart from other studies of salt intrusions in the Yangtze Estuary.

This paper is statistical and predictive. We do not model the estuary dynamics, either theoretically or numerically, but estimate statistical relations between discharge and salinity to identify the probability of long-duration salt intrusions. The paper draws on three sources of data.

First, it employs published data about discharge and salinity for various periods to estimate the relationship between discharge and salinity. The salinity data refer to the Gaoqiao gauging station in the estuary (Fig. 1), the nearest station to Qingcaosha, and are from Li et al. (2014). The discharges are measured at Datong, the nearest gauging station which has a long record of discharge data. The observations refer to the periods 1 January–30 April 1979 (120 observations), 13 February 1984–30 March 1987 (47 observations), 1 January–17 April 1987 (107 observations), 24 January–17 February 1999 (25 observations) and 16 February–4 March 2007 (17 observations). All 316 observations lie in periods in which the probability of low discharges and therefore of salt intrusions is high. Figure 2 illustrates the discharge and salinity conditions in these periods.

These observations are neither logically nor statistically independent. They
are not taken on a random sample of days between 1 January 1979 and 4 March 2007,
so they are not logically independent. A casual observation of Fig. 2,
confirmed by statistical tests, reveals that there is a pronounced lag
structure to the salinity measures, so they are not statistically
independent either. An appropriate estimate of the relationship between
discharge and salinity must recognise this non-independence of the
observations. Although an artificial neural network could be constructed to
model this relationship, we chose to estimate an equation of the form

Such a model has a straightforward, intuitive meaning. Observations are
numbered

There are four known sources of error in the data that have been used to
construct this equation. First, unknown, but variable, amounts of water are
extracted from the river or drain into the river between Datong and the
salinity gauging station at Gaoqiao. Zhang et al. (2012) estimate that net
abstractions are highest in periods of spring tide, in September, October
and November, and in years of drought (see also Dai et al., 2011). These differences
are systematic though with imperfectly understood characteristics, which
create errors of estimation. Secondly, the influence of spring and neap
tides is not included (Tong et al., 2010, illustrate these effects). However, the
partial autocorrelation plots of the residuals from Eq. (2) do not reveal any
significant autocorrelations beyond day 1. Third, there has been a rise in
the long-term level of the sea. Cai et al. (2009) estimate that the rate of
relative sea level rise in the Yangtze Delta has been accelerating and in
the past few decades was 6.6 mm yr

The second data set consists of the record of daily discharges at Datong.
These are available for the period 1950–2007 from the published
yearbooks of the Changjiang Water Commission, with gaps that were kindly
infilled by Klaus Fraedrich, and from 2004 to 30 September 2014 at the
website of the Changjiang Shuiwei Guangli Xitong (Changjiang Water Level
Management System;

Thirdly, the paper relies on published information about the volumes and operating rules of local abstractions, the SNWTP and the Three Gorges Dam. In each case, we investigate the effect of two scenarios: the first, “normal operating rules” which represent our best estimate of abstractions out of the basin and discharges from the Three Gorges Dam; the second, “conservative operating rules” which assume that abstractions and changes in discharge are less severe than under normal operating rules. In both cases, it is assumed that current plans for the years 2030–2035 are followed. The details of these rules and the sources from which they were calculated are described in Appendix A; their effect on discharge into the estuary, net of return flows, is illustrated in Fig. 3. The impact of these modifications on the probability of long-duration intrusions is then calculated as follows: first, for the 64 river years from 1950–1951 to 2013–2014, what is the probability of an intrusion of given duration and chlorinity? Second, if the Three Gorges Dam, SNWTP and local abstractions planned for 2030–2035 had operated from 1950–1951 to 2013–2014, what would have been the probability of an intrusion of given duration and chlorinity? The difference in probabilities is ascribed to the three modifications to the river.

Change in discharge caused by normal and conservative operating rules for Three Gorges Dam, the South–North Water Transfer Project and local abstractions.

Parameter estimates and goodness of fit.

SE: Standard error; df: degrees of freedom;

The principal limitation of this analysis is the short duration of the time series of daily flows. There are only 64 river years of data from 1 January 1950 to 30 September 2014. Long periods of saline intrusion are relatively rare events, perhaps occurring no more than five times per century, so this record is clearly an insufficient basis from which to draw robust conclusions about the relationship between low frequency, long-duration intrusions and river discharge.

The model estimates and goodness of fit criteria are contained in Table 1.
The effect of all variables is significant at

Distribution of residuals, with normal fit.
Note: one-sample Kolmogorov–Smirnov test

The model predicts a close relation between discharge and mean salinity,
especially when discharge falls for long periods below about
10 000 m

Frequency distribution of years, classified by number of days with high chlorinity.

This model, with its known frequency distribution of residuals (Fig. 4), is
therefore used to simulate salinity intrusions. These simulations follow
Monte Carlo methods, in which the frequency distribution of residuals is
repeatedly sampled, in conjunction with the other parameters of Eq. (2), in
order to identify the probability that intrusions of given intensities and
durations will occur. The results are first illustrated in detail for 1962,
the year with the longest-duration intense intrusion on record. Figure 6
reveals the frequencies with which salinities of 250, 400 and 500 mg L

Frequency distributions of numbers of consecutive days with
chlorinity above 250 (upper left), 400 (upper right) and 500 mg L

Correlations between probabilities of chlorinity above stated levels for stated lengths of time and selected discharge characteristics, annual scale.

Note:

Similar calculations have been made to determine the probability of
occurrence of various durations of intrusions of 250, 400 and 500 mg L

Figure 7 reveals in more detail how the probability of observing
duration–chlorinity pairs depends on the duration of discharge below
8000 m

Relationship between long-duration, high-intensity intrusions and duration of low discharges.

The upper graph in Fig. 7 illustrates how the probability of 30-, 40- and 50-day
intrusions of

The central graph in Fig. 7 illustrates how intrusions of chlorinity

This information is summarised in Table 3.

Probability of intrusion duration and intensity in relation to length of periods of low discharge.

The net effect of the three modifications is to reduce the discharge of the Yangtze River at the estuary for most of the year (Fig. 3). The operations of the Three Gorges Dam cause net decreases in discharge when it is being filled (October and early November), but nil effects or net increases throughout the rest of the year, including in the periods of lowest flow (December–February). However, these effects are offset by abstractions for the middle and eastern routes of the SNWTP and by abstractions in the delta region. In aggregate, discharge is reduced throughout the year, except when normal operating rules raise discharges from the Three Gorges Dam above inflows (in April and May).

The consequence is an increase in the risk of long-duration salt intrusions.
We illustrate the calculations by examining the probabilities of intrusions
of chlorinity

Probability of intrusions of

More light is shed on the risk of intrusions in Fig. 8. These graphs reveal
the proportion of years in which the probability of an intrusion of
chlorinity

Impact of normal and conservative operating rules for the Three Gorges Dam, South–North Water Transfer Project and local abstractions on probabilities of intrusions of 30, 50 and 60 days.

Unlike previous studies of intrusions of saltwater into the estuary of the
Yangtze River, this paper has sought to identify the conditions under which
intense intrusions of long duration occur. Constrained by both the shortage
of the discharge record and the paucity of observations of discharge and
salinity, the paper has constructed a model of the relationship between
salinity and discharge and then employed Monte Carlo simulation methods to
reconstruct the probability of observing intrusions of differing intensities
and durations in relation to discharge. The model predicts that the duration
of intrusions with chlorinity

In 51 of the 64 years analysed, the probability of an intrusion of at least
30 days at chlorinity

In 26 years, the probability of an intrusion of at least 30 days at
chlorinity

Existing estimates in the literature of discharges at which salinity
intrusions occur in the Yangtze Estuary are point estimates: they seek to
identify a discharge below which intrusions are likely and above which
intrusions are unlikely. Serious intrusions occurred near the Chenhang Reservoir
in 1978–1979, 2001–2002 and 2006–2007, when the maximum average monthly discharges
in January and February were 7103, 10 165 and 11 777 m

The second principal result of the paper is the measurement of the effect of
the Three Gorges Dam, the SNWTP and local water abstractions in the delta on
the probability of long-duration intrusions. The calculations were presented
for intrusions of chlorinity

Others have also pointed out that the Three Gorges Dam, the SNWTP and local abstractions will affect the probability of salt intrusions into the Yangtze Estuary. In general, the operations of the Three Gorges Dam tend to reduce the probability of saline intrusions, since under normal operating rules the reservoir discharges more than inflow during the periods of lowest natural discharge in December–February (An et al., 2009). However, the SNWTP and local abstractions both reduce discharge into the estuary, tending to create a “high likelihoods” of saltwater intrusions in December–February of a dry year and in January and February of a normal year (Chen et al., 2013); likewise, Zhang et al. (2003, 2012) estimate the effects on discharge of local abstractions, though without calculating their impacts on the probability of an intrusion of saltwater. None of these, however, has calculated the changes in the probability of intrusions caused by these abstractions nor sought to apply their methods to the entire historical record of discharge and much less to identify the likely duration of saline intrusions.

The literature does identify other conditions besides discharge that affect the occurrence of salt intrusions, notably tide and wind conditions (Kuang et al., 2009; Li et al., 2012; Liu et al., 2013; Xue et al., 2009; Zhang et al., 2011). As noted in Appendix A, Zhang et al. (2012) also demonstrate that water abstractions from the Yangtze River below the gauging station at Datong are a significant proportion of total discharge, especially if years of relatively low discharge are also years of low rainfall in the estuary region. These are factors that an analysis of discharge data for the entire period 1950–2014 cannot take into account. In addition, there are concerns that rainfall patterns in the Yangtze Basin may be changing under the influence of climate change (Jiang et al., 2008; Tao et al., 2012), which may alter the discharge characteristics of the river; furthermore, sea levels off the Yangtze Estuary have been rising and this will have effects similar to a reduction in discharge (Cai et al., 2009). The paper has not accounted for these factors.

Finally, we should comment on our choice of method. The risk of using a
statistical model is that it works in the calibration period but fails
outside that time; we sought to minimise this risk by calibrating the model
with data from five separate years from 1979 to 2007 and spanning
discharges in the entire range 6000–16 000 m

This paper has demonstrated a new method for calculating the probability of
occurrence of long-duration salt intrusions of specified chlorinity. The
method shows that the relationship between discharge and the intensity and
duration of salinity intrusions is probabilistic and continuous. At
discharges

Furthermore, they will become even less rare as the Three Gorges Dam, the SNWTP and local water abstractions in the delta begin to affect discharge into the Yangtze Estuary. The proportion of years for which an intrusion is more likely than not rises from 0.22, 0.05 and 0.03 for 30-, 50- and 60-day intrusions, respectively, to 0.42–0.56, 0.20–0.36 and 0.13–0.27, depending on the operating rules of the three projects. However, these predictions do not account for ongoing changes in precipitation or rises in sea level, both associated with climate change. If climate change does not have the effect of increasing discharge in the winter months, then operating rules will have to be revised during years of low discharge or Shanghai will have to find alternative sources of water to prevent the disruptions to supply that these calculations predict.

The largest dam on the Yangtze is the Three Gorges Dam. Other dams are being built above Three Gorges, but we assume that any effects of their operation on discharge at Datong will be regulated through the Three Gorges Dam. There are also dams on other tributaries which join the Yangtze below the Three Gorges Dam, but their effect is not separately considered here, though it is present in the discharge record.

Chen et al. (2001) describe the plans for the operation of the Three Gorges
Dam. The reservoir is planned to store water from October each year, tending
to decrease flows below the dam. In a dry year, the water storage process
may have to be extended to November. Then, during the dry season (December–April), the water level in the reservoir will need to be dropped to meet
the needs of the hydropower plant. The plan is that minimum water levels
will be attained at the end of September. Guo et al. (2011; their Figs. 3, 8)
provide more details: in June, July and August, the reservoir water level is
maintained at 145 m a.s.l. to provide for flood control during the wet season.
In October, the water level is raised to 175 m a.s.l., the planned maximum
level, and it is maintained at as high a level as possible (and above
155 m a.s.l. to facilitate navigation) until the end of April, when it is again
reduced to 145 m a.s.l. According to Zhang et al. (2012, Table 3), the Three
Gorges Dam holds

Given estimates that it takes 14 days for water to flow from the dam site to Datong, Zhang et al. (2012) estimate that the reservoir's operation would be as follows: in a dry year (e.g. 2001–2002) fill from 15 September to 31 October and in an extremely dry year (e.g. 1978–1979) fill from 1 September to 15 November.

We assume the following. Under normal rules, the reservoir is filled in
October, which reduces flow at Datong from 15 October to 14 November by
8000 m

Zhang et al. (2012) state that the dam first began to affect discharges in 2006–2007, when experiments began to fill it. These experiments continued until 2010. Thereafter, the dam has functioned normally. Therefore, the assumed modifications to discharge at Datong are not applied to years 2010–2011 onwards.

The South–North water diversion project (SNWTP) project involves three routes: a western route, a middle route and a third route, taking water from the Yangtze to northern China. The western route is still in the planning stages and the volume of water to be diverted is still uncertain; it will not be operation until at least 2020. It is not considered any further here, though it can be assumed that it will exacerbate the situation described in this paper. The central and the eastern routes are now operating, however.

The eastern route is planned to transfer 8.9, 10.6 and
14.8 billion m

We assume as follows. By 2030 the eastern route will be transferring 900 m

It is planned that the middle route will divert
12–14 billion m

In addition to the SNWDP, water is also diverted from the lower Yangtze (below Datong) to cities and agricultural areas. The history, current status and some projections of these diversions are comprehensively described by Chen et al. (2001) and Zhang et al. (2003, 2012). Zhang et al. (2003) provide historical data about the capacity of water extractions from 1958 to 2000 and also indicate net water extractions for November, January and March in the 1980s and by 2030. Chen et al. (2001) provide a summary of the findings of Zhang et al. (2003). Zhang et al. (2012) update the information provided by Zhang et al. (2003); in addition, they provide estimates of daily net abstractions for the dry seasons of 4 years.

These data indicate that the capacity to extract water from the Yangtze
downstream of Datong has increased rapidly since the 1950s. However, the
available data do not indicate that the net quantity of water abstracted
from the Yangtze has increased over time, controlling for discharge, tides
and local precipitation. Regression estimates of net abstractions (according
to Zhang et al., 2012, their Fig. 4a) indicate that, net of tidal conditions,
these depend on the sequence of the observation within the river year
(tending to decrease and then rise), on the discharge (tending to rise with
discharge) and on the year (tending to decrease slightly over time). These
data imply that the estimates of discharge at Datong are biased estimates of
discharges at the estuary and also that there is no evidence that this bias
has changed over time.
However, Chen et al. (2013) indicate that new abstractions are planned.
These imply an additional net reduction in discharge of 670 m

There are three modifications to the discharge of the Yangtze River at its estuary, estimated under two conditions – conservative rules and normal rules. The discharge at Datong is modified by the operation of the Three Gorges hydropower station (except during the years after 2010–2011, when it was actually in operation) and by the operation of the middle route of the SNWDP); both have normal year and conservative rules of operation. Below Datong, additional water is extracted by the eastern route of the SNWDP and by more local abstractions (the latter of which also have normal and conservative rules of modification). The resulting implications for discharge at the estuary are illustrated in Fig. 8 of the paper.

A spreadsheet (.xlsx), containing the observations on discharge and salinity, the discharge data and the assumed abstractions, is available upon request to the corresponding author.

M. Webber performed the calculations and wrote the paper; M. T. Li, J. Chen, B. Finlayson, D. Chen and Z. Y. Chen provided the data; all authors contributed to the design of the research and commented on the results and conclusions.

The authors gratefully acknowledge ARC research grant DP110103381, which supported the research reported in this paper. They are grateful to Klaus Fraedrich (University of Hamburg), Ma Ji and Xiong Xianzhe (University of Melbourne), who kindly supplied some of the data, and to two referees for their comments. Edited by: P. Regnier