The causes and impacts of water resources crises in the Pearl River Delta

Journal of Cleaner Production 177 (2018) 413e425

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The causes and impacts of water resources crises in the Pearl River Delta Bingjun Liu*, Sihan Peng, Yeying Liao, Weili Long Department of Water Resources and Environment, Sun Yat-sen University, Guangzhou, 510275, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 September 2017 Received in revised form 21 November 2017 Accepted 23 December 2017 Available online 27 December 2017

Owing to its combination of superior natural conditions and abundant resources, the Pearl River Delta (PRD), China, is densely populated, with concentrated industry and rapid development. The PRD is also one of the most threatened estuaries in the world, as it has been exposed to an increasing series of crises related to water resources. Water conflict, water pollution, and saltwater intrusion have all increased dramatically in the past few decades, which pose a threat to regional water security and impose constraints on regional development. The goal of this paper is to analyze the causes and impacts of some of the water resource crises faced by estuaries under dramatically changing environments, based on studies in the PRD. The PRD is centered around the Pearl River Estuary (PRE) and is the second most economically active district in China. Rapid economic development and a population boom have resulted in an average annual growth rate of 1.43% in water resource demand, which has further accelerated water pollution by a substantial growth in sewage discharge. Water pollution results in the impairment of the ecological functions of the water, and the standard rate of water function areas in the PRD remains under 50%. Large-scale and uneven sand excavations not only decreased the river discharge from upstream but also allowed more tidal prisms to enter, which triggered severe saltwater intrusion. Meanwhile, sea-level rise and changing wind patterns also contribute to increasingly severe saltwater intrusion, which was characterized by gradual increases in salinity, as well as more prolonged periods of higher salinity that exceeds acceptable thresholds occurring earlier in the year. This study is intended to bring attention to these challenging issues and provide some of the information needed to advance research into sustainability assessment of the water resources. © 2017 Published by Elsevier Ltd.

Keywords: Estuaries Changing environment Water resource crises Pearl River Delta

1. Introduction As the most productive natural habitats in the world, estuaries provide important services that support economic activities and societies (Xu et al., 2016). Estuaries worldwide have been subject to rapid development and are among the most densely populated regions in the world. It is estimated that roughly 40% of the world's population, about 2.8 billion, currently live within 100 km of the coast, and there will be 4 billion people living along the world's coasts by 2100 (CIESIN, 2012). In recent years, intensive anthropogenic activities, including land reclamation for urban or industrial use, sand excavation, and dam construction (Sun et al., 2012; Chuai et al., 2014; Meyers et al., 2014; Anthony et al., 2015; Paiva et al., 2016; Andrews et al., 2017); and external natural

* Corresponding author. E-mail address: [email protected] (B. Liu). https://doi.org/10.1016/j.jclepro.2017.12.203 0959-6526/© 2017 Published by Elsevier Ltd.

phenomena, such as sea-level rise (Werner and Simmons, 2009; Hussain et al., 2014; Cui et al., 2015), have seriously affected the natural circulatory functions of many estuaries around the world (Frihy, 2003; Crossland et al., 2005; Kim et al., 2006; Jarvie et al., 2012; Statham, 2012; Abdrabo and Hassaan, 2015; Luan et al., 2016). The impairment of estuarine circulatory functions results in a high risk of water resource crises in estuaries, such as water pollution, flood hazards, loss of wetland habitats, eutrophication, and saltwater intrusion (Wang et al., 2013; Maskell et al., 2014; Watanabe et al., 2014; Gross and Hagy, 2017). These crises can cause immense loss of property and human life, and they have made estuaries more susceptible to environmental degradation (Branch, 1999; Elliott et al., 2014; Ribeiro et al., 2015). For instance, a great reduction in river discharge into the Yangtze River Estuary brought about a prolonged intrusion of saltwater into the estuary, which strongly influenced all of Shanghai's waterworks along the Huangpu River. A devastating algae bloom, which was the result of excess N and P nutrient concentrations, caused a water supply crisis

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that affected two million people in Wuxi City in 2007 (Chen et al., 2001; Li et al., 2009). Heavy metal pollution emergencies in the tributaries of the Pearl River, such as the North River's Cd pollution crisis in 2005, the Duliu River's As spill in 2007, the South Pan River's Cr contamination in 2011, and the Longjiang River's Cd emergency in 2012, have severely impacted upon the water supply and environmental health of these rivers. Situated in the vicinity of the Pearl River Estuary (PRE), the Pearl River Delta (PRD) is surrounded by several cities: Guangzhou, Shenzhen, Zhuhai, Foshan, Huizhou, Dongguan, Zhongshan, Jiangmen, and Zhaoqing, and is contiguous to Hong Kong and Macao. Its geographical advantages, reinforced by economic reform policies, have made the PRD a core economic region in Guangdong Province. With 4.27% of the total population (58.74 million people in 2015) and 0.57% of the land area (54,754 km2) of China, the PRD has contributed disproportionately to China's economy, providing 9.12% of the national gross domestic product (GDP) in 2015 ($996.14 billion). Rapid social and economic development, as well as population growth, in the PRD has led to intensive anthropogenic disturbance to the water resource system, which has become increasingly prominent in recent years. On the one hand, the population boom and massive economic growth have contributed to continuously increasing water demand and the release of wastewater into rivers without effective purification treatment (Yang et al., 2017; Zhang et al., 2017). On the other hand, the rapid uneven degradation of the riverbed has triggered severe saltwater intrusions in the last two decades. As the source of sand and gravel for Hong Kong, large-scale excavation of river sand has occurred in the PRD since the mid-1980s, and the annual volume of dredged sand has been about 70 million m3 for the last 15 years. However, the amount of sand supplied from upstream is not sufficient to replace the dredged sand (Zhang and Deng, 2010), and the degradation of the riverbed has led to changes in the hydrodynamics of the entire river network. Uncontrolled sand excavation has become one of the most significant anthropogenic effects on river discharge, water levels and divided flow ratio (DFR) between various watercourses of the PRD since the 1980s, which has ultimately aggravated the intrusion of saltwater (Lv and Du, 2006; Luo et al., 2007; Zhang et al., 2009). The effects of sea-level rise and wind on saltwater intrusion were also found in the PRD. The severity of saltwater intrusion is largely related to the expected rate of sea-level rise, which severely affected the freshwater resources. The winter prevailing wind can distinctly enhance the intrusion of saltwater into the PRE (Huang et al., 2004; Zhou et al., 2012a; Wang et al., 2012). A large number of studies have investigated estuarine water resource crises, as well as relevant issues such as hydrology, sedimentation, saltwater intrusion, water quantity and quality, and

ecosystems (Table 1). The goal of this paper is to present a comprehensive explanation for the causes and impacts of water resource crises in the PRD. The findings will provide important information and function as reference material for the management of water resources in the PRD, and for research into typical estuaries around the globe.

2. Study area and data The PRD is located in the downstream alluvial plain of the Pearl River from 21
300 N to 23
420 N, and from 112
260 E to 114
240 E (Fig. 1), covering an area of about 3000 km2. As the second largest river in China (Zhao et al., 2014), the Pearl River has three major tributaries: the West River, the East River, and the North River. The runoff discharges into the South China Sea through eight outlets, which are, in sequence from east to west, the Humen, Jiaomen, Hongqimen, Hengmen, Modaomen, Jitimen, Hutiaomen, and Yamen Waterways. The PRD lies in a subtropical region with features of a subtropical monsoon climate. Annual precipitation ranges from 1200 to 2200 mm, and precipitation from AprileSeptember accounts for 82%e85% of the yearly total. Owing to its high precipitation, the PRD is considered to have one of the world's most complicated fluvial networks. The dense river network provides favorable conditions for the development of irrigation agriculture and transportation, laying a solid foundation for economic growth and social development. The development and the growth rate of the economy and society of the PRD were analyzed and compared to those of several developed regions in the world. The socioeconomic data used in this paper were taken from the Guangdong Statistical Yearbook, the Tokyo Statistical Yearbook, the World Bank Database, the Food and Agriculture Organization of the United Nations Database, and the Bureau of Economic Analysis of the United States. These data included measurements of GDP, industrial structure, and population from 1980 to 2015. Data about the utilization of water resources included water resources demand, water use index, and water quality. The hydrological, terrain and meteorological data used in this paper included salinity, saltwater intrusion extent, river discharge, tidal elevation, river channel bathometry, and wind direction. These data were all obtained from the Hydrological Bureau of Guangdong Province and the Hydrological Yearbook. The reliability and homogeneity of the data series were strictly checked before they were released. The salinity series and saltwater intrusion extent series were observed at 17 stations along the Modaomen Waterway (MW) from October 2000 to March 2012. Daily upstream river discharge was measured at the Sanshui and Makou stations from 1965 to 2012. Daily tidal data from 1965 to 2012, as recorded at the Sanzao

Table 1 Summary of previous studies about relevant issues of water resource crises. Topic

Findings

Water pollution

Water pollution changes the environment conditions Changes in channel River channels were significantly undercut by geometry large-scale sand excavation Degradation of the riverbed lead to changes in hydrodynamics Saltwater intrusion Decrease of river discharge from upstream can aggravate saltwater intrusion Sea-level rise is responsible for saltwater intrusion Previaling wind can enhance saltwater intrusion Effects of saltwater intrusion on ecosystem

References Fang et al. (2003); Zhang et al. (2017); Yang et al. (2017) Luo et al. (2007); Wu et al. (2014); Zhang et al. (2010) Lv and Du (2006); Luo et al. (2007); Zhang et al. (2009); Zhang et al. (2010); Zhang and Deng (2010) Lv and Du (2006); Becker et al. (2010); Yuan et al. (2016) Huang et al. (2004); Werner and Simmons (2009); Gong and Shen (2011); Zhou et al. (2012a); Hussain et al. (2014); Hussain and Javadi (2016); Wong et al. (2003); Gan et al. (2004); Ji (2008); Wang et al. (2012); Zhou et al. (2012b); Zheng et al. (2014) Long et al. (2013); Xie et al. (2014); Liu et al. (2014); Pettit et al. (2016); Yin and Harrison (2008)

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Fig. 1. Map of the PRD and its river network.

tide station, the closest tide gauge station to the PRE (approximately 16 km from the PRE outlets), were used to represent the sea level of the South China Sea (Zhou et al., 2012b). The stream bed geometry included data from 1990 and 2005. Wind conditions and directions were observed and recorded at the outlet of the PRE, covering the period from 2000 to 2012. The variable sets collected for this study are listed in Table 2. 3. Research methodology The study mainly consists of three parts. In the first part, we described the phenomena and impacts of water resources crises in the PRD with Mann-Kendall trend tests (MK tests). In the second

part, we analyzed the causes of the water resources crises in both aspects of anthropogenic activities and external natural forcings with Spearman's rank correlation coefficients. In the third part, we put forward corresponding suggestions to alleviate the water resources crises. The MK test was applied to distinguish whether the salinity and tidal elevation were experiencing natural fluctuations, or were undergoing certain trends. The MK test (Mann, 1945; Kendall, 1957) is a non-parametric test used to detect statistically significant trends in time series, as recommended by the World Meteorological Organization (Hamed, 2008). The MK test has been widely used with many hydroclimatic series, such as temperature, rainfall, river flow, and water quality (Hamed, 2009; Ishak et al., 2013; Dinpashoh

Table 2 Summary table for the variables. Variable type

Variable

Timescale

Source

Socioeconomic data

GDP Industrial structure Population Water resources demand Water use index Water quality Salinity Saltwater intrusion extent River discharge Tidal elevation River channel bathometry Wind direction

1980e2015 (Every-five-years)

Guangdong Statistical Yearbook; Tokyo Statistical Yearbook; World Bank Database; Food and Agriculture Organization of the United Nations Database; Bureau of Economic Analysis of the United States Hydrological Yearbook of Guangdong Province

Water resources data

Hydrological data

Terrain data Meteorological data

1980e2015 (Every-five-years) 2008e2015 (Yearly) 2000e2015 (Monthly) 2000e2015 (Monthly) 1965e2015 (Daily) 1965e2015 (Daily) 1990 and 2005 (Yearly) 2000e2012 (Yearly)

Hydrological Bureau of Guangdong Province; Hydrological Yearbook of Guangdong Province

Hydrological Bureau of Guangdong Province Hydrological Bureau of Guangdong Province

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et al., 2014; Tan et al., 2017). For a time series X ¼ fx1 ; x2 ; …; xn g, the Mann-Kendall test statistic S is given by:

4. Results and discussion 4.1. Water resource crises faced by the PRD

S¼

n1 X

n X

  sgn xj  xi

(1)

i¼1 j¼iþ1

where sgnðxj  xi Þ is the sign function as:

8   < þ1 xi < xj sgn xj  xi ¼ 0 xi ¼ xj : 1 x > x i j

(2)

The standard normal test statistic Z is computed as follows:

8 S1 > > pffiffiffiffiffiffiffiffiffiffiffiffiffiffi if S > 0 > > > VarðSÞ > > < Z¼ 0 if S ¼ 0     > > > > Sþ1 > > > : pffiffiffiffiffiffiffiffiffiffiffiffiffiffi if S < 0 VarðSÞ

(3)

The variance is computed as:

VarðSÞ ¼

nðn  1Þð2n þ 5Þ 

Pm

i¼1 ti ðti

18

 1Þð2ti þ 5Þ

(4)

where n is the number of data points, m is the number of tied groups and ti denotes the number of ties of extent i. A tied group is a set of sample data having the same value. Positive or negative values of Z indicate increasing trends or decreasing trends. The significance of trend was determined at 95% confidence limit (a ¼ 0.05). The Spearman rank correlation coefficient was used to quantify how well the GDP and water resources demand, river discharge and saltwater intrusion extent, as well as the average monthly tidal elevation and salinity, monotonically depended on each other. The Spearman rank correlation coefficient (Spearman, 1904, 1906) is a non-parametric measure of statistical dependence, used for evaluating the degree of linear association or correlation between two independent variables (Gautheir, 2001; Zhang et al., 2016). It has become one of the most widely used non-parametric statistics in the scientific literature, used in statistics, medicine, epidemiology, biology, hydrology, and other fields (Carroll, 1990; Borkowf, 2002; Grimaldi et al., 2011). Assume that we measure two traits Xi and Yi on each of n subjects and convert them to ranks xi and yi . The Spearman rank correlation coefficient rs is given as follows:

rs ¼

covðx; yÞ

(5)

sx sy

where covðx; yÞ is the covariance of the rank variables, and sx and sy are the standard deviations of the rank variables. Only if all n ranks are distinct integers, the correlation coefficient can be computed using the formula:

rs ¼ 1  6

X

d2   n n2  1

(6)

where d is the difference between the two ranks of each observation, and n is the number of observations. The Spearman rank correlation coefficient lies in the ranges 1  rs  1, where 1 means a significant negative correlation, 0 means no link, and 1 means a significant positive correlation.

4.1.1. Rapid increase in water resources demand The demand for water resources in the PRD has undergone sustained massive growth in recent decades. Table 3 shows the increase in water resources demand since 1980, reaching 24.95 billion m3 in 2005, with an average annual growth rate of 2.39%. Although water resources demand declined by 7.9% between 2005 and 2015, the overall trend was still one of growing demand from 1980 to 2015, with an average annual growth rate of 1.43%; by contrast, the demand for water resources in the USA has declined in the fluctuations since the 1980s. Rapid growth in the demand for water resources has brought about conflicts between available water supplies and the water use sectors, as well as competition for water use between different sectors. Because traditional agricultural production has gradually given way to manufacturing and service industries in the PRD, the industrial consumption of water resources has surpassed that of agriculture. Industry has become the main user of water, accounting for 48% of the total water consumption in 2015. The percentage of water used by agriculture of the PRD has displayed an obvious negative trend, decreasing from 88% in 1980 (12.22 billion m3) to 32% in 2015 (7.31 billion m3), in contrast, the USA, a large industrial country, has maintained a steady level of agricultural water consumption of about 35%. 4.1.2. Low water efficiency In the PRD, low water efficiency is very common in industry, agriculture, and nearly every aspect of social activity and daily life. It can be observed from Table 4 that, although the water use indices in the PRD declined markedly from 1980 to 2015, there remains a gap comparing with international level. Water consumption per unit of GDP fell to 228 m3/104 USD in 2015, which was more than three times and five times the figures for Japan (165 m3/104 USD) and Israel (100 m3/104 USD) in 2009, respectively. The water consumption per unit of industrial output of the PRD was about 250 m3/104 USD in 2015, whereas that of Japan was 88 m3/104 USD and that of Israel was only 23 m3/104 USD in 2009. It is obvious that there is high potential for improvement in water efficiency for the PRD on a comprehensive level, and in industry in particular. 4.1.3. Water pollution The growing demand for water resources has greatly increased sewage discharge, which has led to water pollution and qualityrelated issues. In accordance with the Surface Water Quality Standards of the People's Republic of China, river quality in the PRD is divided into six categories: Class I, Class Ⅱ, Class III, Class Ⅳ, Class V and Less V Class. Water quality worsens from Class I to Less V Class, and water with a quality standard of Class V and Less V Class is regarded as polluted (Zhu et al., 2002). It can be seen from Fig. 2

Table 3 Water resources demand of the PRD. Year

Water resources demand (billion m3)

Industrial water resources demand (%) Agriculture

Industry

Household

1980 1985 1990 1995 2000 2005 2010 2015

13.81 14.57 16.77 19.46 22.02 24.95 24.76 22.71

88 86 71 58 45 35 31 32

6 8 22 34 44 44 44 48

5 6 7 8 11 22 25 20

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Table 4 The trends of water use index of the PRD. Year

Water consumption per capita (m3)

Water consumption per unit of GDP (m3/104 USD)

Urban household water use (L/p$d)

Rural household water use (L/p$d)

Water consumption per unit of industrial output (m3/104 USD)

1980 1985 1990 1995 2000 2005 2010 2015

758 733 743 759 684 523 441 387

6079 6170 5864 4125 2335 1188 770 228

147 164 194 211 296 252 230 202

98 96 110 116 95 165 153 149

376 470 1028 852 522 459 453 250

that rivers with a quality of Class V or below represent a large proportion, making up about 35.27% of the total, even though rivers with better quality (quality of Class III and above) have exhibited a slight upward trend from 2008 to 2015. Water pollution always results in the impairment of the ecological functions of a body of water. Particularly in recent decades, aquatic ecological environments have increasingly been threatened because the ecological functions of water have not been sufficiently acknowledged or protected. As can be easily seen in Table 5, although the standard rate of water function areas in the PRD has improved from its low point, from 27% in 2008 to 44% in 2015, it has still less than 50%.

less than 250 h (about 10 days) during the first three years. However, it exceeded 700 h (about 30 days) from 2003 to 2005, reached 1500 h (about 63 days) during 2005e2006, and surpassed 1800 h (about 75 days) during 2009e2010 and 2011e2012. Furthermore, the onset of threshold-breaking conditions at Pinggang station have occurred earlier with each passing year. Prior to 2004, these conditions were mostly confined to the winter, from December until March, but they now strike significantly earlier, often occurring between November and February. For example, salinity exceeded the threshold as early as October in 2009 and 2011. 4.2. Causes of water resource crises in the PRD

4.1.4. Increasingly severe saltwater intrusion Saltwater intrusion is the encroachment of saline water into freshwater upper regions of an estuary (Zhou et al., 2012b). As the main source of drinking water in the PRD, the Modaomen Waterway (MW) carries the largest portion of the river flow. It has also suffered from increasingly severe saltwater intrusion in recent years. Saltwater intrusion in the MW has affected the security of the regional water supply and destroyed the habitats of native plants and affected their growth, which has resulted in changes to the dominant communities in the estuary (Long et al., 2013; Xie et al., 2014; Liu et al., 2014; Pettit et al., 2016). A long-term survey of daily salinity in the dry season from 2000 to 2012 at Pinggang station, where the pumping station for the supply of water to Macao and Zhuhai is located, was conducted to analyze the characteristics of saltwater intrusions. The results showed that the worsening saltwater intrusion over this 12-year period was chiefly characterized by gradually rising salinity, as well as prolonged and seasonally advanced durations of salinity that exceeded the acceptable threshold for drinking purposes of 250 mg/L, according to the National Hygienic Standard for Drinking Water (GB 5749-2006). The salinity time series was dominated by an increasing trend according to the MK test (P < .001) (Fig. 3a), and the periods of excessively high salinity became more prolonged and more strongly influenced by seasonality (Fig. 3b). The time where the salinity levels were above the threshold at Pinggang station was

4.2.1. Intensive anthropogenic activities 4.2.1.1. Rapid economic and social development. The favorable geographical location of the PRD, along with economic and political reforms, made the PRD the southern gateway for Chinese international trade and one of the earliest regions to open up to the outside world (Shen et al., 2006), which attracted a large number of surplus laborers from around the country. As listed in Table 6, the population of the PRD more than tripled between 1980 and 2015, increasing from 18.21 million to 58.74 million. The GDP of the PRD has sustained double-digit growth rates over the last few decades, increasing from $9.01 billion in 1980 to $996.14 billion in 2015, with an average annual growth rate of 14.39%. Since 1980, the PRD has experienced a rapid transition from an agriculture-based economy to an increasingly industrialized and technologically-based economy. The GDP of primary industries in the PRD has undergone a substantial decrease, falling by an average of 8.92% annually, whereas the GDP of secondary industries decreased gradually after a period of rapid growth, and the GDP of tertiary industries has displayed a significant growth trend. The respective percentage shares of GDP of the primary, secondary, and tertiary industries changed from 47.17%, 32.67%, and 20.16% in 1980, respectively, to 1.79%, 43.58%, and 54.63% in 2015, respectively. Compared with

Table 5 Standard rate of water function areas in the PRD based on full factorial measure. Year

Total water function areas

Qualified water function areas

Standard rate (%)

2008 2009 2010 2011 2012 2013 2014 2015

81 85 89 85 95 121 137 137

22 38 32 38 33 47 61 60

27 45 36 45 35 39 45 44 Fig. 2. The river water qualities in the PRD.

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Fig. 3. The characteristics of saltwater intrusion in the dry season from 2000 to 2012 at Pinggang station.

some metropolitan regions in the world, the PRD has faster population growth and faster economic growth. The PRD maintained a high economic growth rate even during the financial crisis of the late 2000s, when the economies of many developed countries stopped growing or even shrank. The annual population growth of Guangzhou and Shenzhen, two main economic regions in the PRD, were 3.22% and 5.56% in 2015, respectively, which far surpassed those of the Tokyo (1.02%), New York City (1.72%), and Hong Kong (0.89%). The annual GDP growth of Guangzhou and Shenzhen reached 6.95% and 7.97%, respectively, whereas those of the Tokyo, New York City, and Hong Kong were 2.11%, 2.76%, and 2.36%, respectively. The great population pressure and rapid expansion of the economy of the PRD resulted in rapid growth in the demand for water resources. The correlation coefficient rs of 0.983, which is significant at the 95% probability level, proves that GDP and water resources demand were closely correlated before 2005, when the economy was in its extensive stage (Fig. 4a). Because the development of the economy promoted advancements in technology and the spread of high-efficiency equipment and behaviors, the demand for water resources has been declining since 2005. The correlation coefficient rs of 0.955, which is significant at the 95% probability level, also reflects that there was a remarkable negative correlation between the GDP and the water resources demand after 2005 (Fig. 4b). 4.2.1.2. Rapid changes in channel geometry. The river channels upstream of the PRD and in the estuary have been significantly

undercut by large-scale sand excavation since the 1980s. Before the 1980s, anthropogenic activities had a relatively small impact on the river channels in the PRD, but the initiation of large-scale sand excavation in the river channels in the mid-1980s and a boom in dredging in the 1990s (Luo et al., 2007) dramatically increased this impact. Fig. 5 shows the geometry of the North and West Rivers in the 1990s and 2000s. The main streams of the North and West Rivers generally experienced vertical erosion. Since 1990, both the North River and the West River have experienced drastic undercutting. The increases in the mean depth of the riverbeds of the North River and the West River from the 1990s to the 2000s were 131.39% and 47.17%, respectively. In response, the main streams of the North and West Rivers have generally narrowed, with mean channel widths decreasing by 40.45% and 21.60%, respectively. The reductions in the width-todepth ratios of the rivers were 66.65% and 39.87%, respectively, which reflects how the waterways of the North and West Rivers are becoming narrower and deeper due to sand excavation, and the North River has suffered more intense riverbed undercutting than the West River. Large-scale and uneven sand excavation has led to imbalances in the development of sedimentation and spatial differentiation of hydrodynamic regimes, which significantly changed the dominant runoff processes (Chun et al., 2002). The influence that the changes in channel geometry had on the DFR, a ratio used to analyze the flow distribution of the braided river, was studied to analyze the effect of anthropogenic activities on runoff processes. This was done on the basis of the yearly time series for runoff at Sanshui and

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Table 6 Economic and social development of the PRD. Year

1980 1985 1990 1995 2000 2005 2010 2015

Population (million people)

18.21 19.88 23.70 32.92 42.90 45.47 56.16 58.74

GDP ($ billion)

9.01 16.21 21.05 48.81 101.73 223.15 572.57 996.14

Industrial structure (%) Primary Industry

Secondary Industry

Tertiary Industry

47.17 34.54 15.27 8.50 5.44 3.05 2.14 1.79

32.67 33.88 43.86 48.66 47.60 50.69 48.36 43.58

20.16 31.58 40.86 42.84 46.96 46.25 49.50 54.63

Fig. 4. Relationship between GDP and water resources demand of the PRD.

Makou stations, the main stream-flow stations for the North River and the West River, respectively (Fig. 6a). Before the 1980s, the DFR at Sanshui and Makou stations changed slightly. Since the 1980s, especially after the 1990s, the DFR at Sanshui station has undergone an evident uptrend, whereas that of Makou station has experienced a sharp decline, both with a change of 7% between 1990 and 2012. Since the upstream river discharge changed little, the decrease in the DFR at Makou station (the main stream-flow station of the MW) indicated that the downstream river discharge decreased during the dry season. There is a consensus among researchers that the decrease of upstream river discharge

had an adverse impact on saltwater intrusion processes in coastal aquifers, because fresh water from upstream can reduce salinity within the estuary (Becker et al., 2010; Yuan et al., 2016). The dilution effect of upstream discharge on saltwater intrusion can be confirmed by the relationship between the saltwater intrusion extent in the MW and the river discharge at Makou station (Fig. 6b). The Spearman rank correlation coefficient rs of 0.758, which is significant at the 95% probability level, reflects how there is a strong negative correlation between the river discharge and saltwater intrusion extent. That is, a decrease in river discharge at Makou station aggravated saltwater intrusion downstream.

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Fig. 5. Geometrical change of the North River and the West River a), c), e): The North River; b), d), f): The West River.

Changes in channel geometry in the estuary also had an adverse impact on saltwater intrusion processes in coastal aquifers (Luo et al., 2007; Zhang and Deng, 2010). As shown in Table 7, the MW was divided into three subsections in this study: an upper subsection from Baiqingtou to Zhuyin; a middle subsection from Zhuyin to Zhupaisha; and a lower subsection from Zhupaisha to Hengqin. The upper subsection has suffered more intense riverbed undercutting than the middle subsection and the lower subsection, with changes in mean depth of 2.94 m, 2.78 m, and 2.31 m, respectively. The thalweg of the upper subsection, which defines the lowest points along the length of a watercourse, changed from 9.59 m in 1990 to 13.40 m in 2005, whereas the respective changes in the middle subsection and the lower subsection were 12.09 m in 1990 to 15.54 m in 2005 and 8.67 m in 1990 to 11.83 m in 2005, respectively. Uneven sand dredging along the waterway, with more intense riverbed erosion in the upper subsections, directly caused a constant decrease of the channel slope and even allowed a negative gradient, resulting in the inability of more saline, denser water to drain. The significant resulting increase in the salinity of the whole Pearl River network aggravated the saltwater intrusion in the PRD.

4.2.2. External natural forcings 4.2.2.1. Sea-level rise. Since the 20th century, sea-level rise has been detected worldwide (Gornitz et al., 1982; Holgate and Woodworth, 2004; Church and White, 2011), and it has also been an important topic in the PRD for several decades. Huang et al. (2004) predicted that there would be a rise in sea level of close to 30 cm at the mouth of the PRD by 2030. A study by He et al. (2014) showed that the regional sea level had increased at a rate of 4.08 mm/yr from 1959 to 2011. In this study, the average monthly tidal elevation at Sanzao station was used to study the sea-level rise in the PRD between 1965 and 2012. As shown in Fig. 7a, the monthly average tidal elevation displayed an increasing trend with a simple linear regression slope of 0.0115 according to the MK test (P < .001), which means that a progressive rise in sea level occurred in the PRD, and that sea level increased at a rate of about 1.4 mm/yr over the past decade. This increase largely coincides with existing research findings (Gray, 2007; Church and White, 2015; Deconto and Pollard, 2016). It has been proven that sea-level rise is partially responsible for the erosion and inundation of the land surface and increased flooding of coastal land (Nicholls and Cazenave, 2010; Hussain and

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Fig. 6. The change of river discharge downstream into the MW and its correlation with saltwater intrusion processes.

Table 7 Change of the geometrical parameters of the MW. Subsection

Geometrical parameters

1990

2005

Variation

Baiqingtou to Zhuyin (Upper subsection)

Mean depth of Thalweg (m) Mean depth of Thalweg (m) Mean depth of Thalweg (m) Mean depth of Thalweg (m)

5.30 9.59 6.68 12.09 4.99 8.67 5.66 10.11

8.24 13.40 9.46 15.54 7.30 11.83 8.33 13.59

2.94 3.81 2.78 3.45 2.31 3.16 2.67 3.48

Zhuyin to Zhupais (Middle subsection) Zhupaisha to Hengqin (Lower subsection) Total

the riverbed (m) the riverbed (m) the riverbed (m) the riverbed (m)

Javadi, 2016). It has also been confirmed to have an important role in aggravating saltwater intrusion (Gong and Shen, 2011; Lv and Du, 2006), because it will increase the hydraulic head of the saltwater body in coastal boundaries and result in upstream movement of the saltwater-freshwater interface, which is followed by a significant depletion in the quantity of freshwater (Sherif and Singh, 1999). The effects of sea-level rise on saltwater intrusion in the PRD can be proven in our study with the Spearman rank correlation coefficient (Fig. 7b). The correlation coefficient rs of 0.856, which was significant at the 95% probability level, proved that the average monthly tidal elevation and the average monthly salinity were decisively correlated, which reflects the fact that the salinity increased with the strengthened tidal dynamics. The higher the tidal elevation was at the outlet, the more severe the saltwater intrusion was into the PRD.

4.2.2.2. Changes in wind directions. Using wind direction observation data recorded in Macao (the outlet of the PRD) during the 2000e2012 dry seasons, we analyzed the trend of wind directions in the PRD. As can be seen in Fig. 8, northerly and southeasterly winds have occurred frequently from 2000 to 2007 in the MW, the frequencies of which were about 50% and 20%, respectively. However, obvious northeasterly migration has occurred since 2008, with the frequency of north-northeasterly and northeasterly winds increasing from about 4% to about 50%. It has been established that transport and mixing processes can be significantly affected by distinct wind patterns if an estuary is wide enough (Gan et al., 2004; Ji, 2008). The prevailing downstream winds can enhance the seaward surface low-salinity water, causing the high-saline water underneath to accelerate landward as a compensation current. This can become a great driving force in

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Fig. 7. Sea-level rise and its correlation with saltwater intrusion processes.

saltwater intrusion. The rivers in the PRD run mainly from north to south, so the northeasterly migration of monsoon winds has accelerated the vertical mixing between the subsurface saline water and the surface shelf water in the PRD, which has contributed to severe saltwater intrusion (Wong et al., 2003; Zhou et al., 2012b; Zheng et al., 2014). 5. Management suggestions Successful practices have been applied in many water resources crises-affected regions in the world (Abarca et al., 2006; Koussis et al., 2010; Qiu and Zhu, 2013; Werner et al., 2013). In order to relieve the further pressure on the security of water resources in the PRD, there are some measures presented to alleviate the water resources crises. Integrated water resources management (IWRM) has been proven to be an effective measure to mitigate the effects of saltwater intrusion in the estuary, and has been carried out in the PRD for the last ten years. IWRM guarantees the water supply safety of Macao and Zhuhai by storing fresh water during the wet season and dispatching it into the estuary during periods of salt tides through a water transmission system. The water transmission system consists of the Zhuzhoutou, Pinggang, and Guangchang pumping stations, the Zhuyin, Dajingshan and Fenghuangshan reservoirs for temporary water storage, and a water transmission pipeline. The control of channel geometry is typically aimed at alleviating the saltwater intrusion and guaranteeing the security of water

supply. In the PRD, comprehensive measures, such as administrative authorization, institutional support, and approach improvement, are required to control the indiscriminate sand mining along the estuary. On the one hand, authorities should begin to establish strict regulations and policies to tighten controls on the volumes of sand excavated from river channels and prohibit all sand exploitation within certain important channels. On the other hand, enterprises must pay to acquire a license for every ton of dredged sand because of the limited sand resources. Price leveraging is considered to be an important tool in water demand management, including promoting water conservation and improving water use efficiency (Lu et al., 2015; Zhou et al., 2015). It was reported that the price structure of the urban water supply has been adjusted by setting the urban water price using a ladder pricing model in Guangzhou since 2012, which has somewhat affected residential water consumption. Exploitation and utilization of unconventional water resources, such as seawater, storm water, intermediate water, and sewage, is also a good way to solve the crisis of water shortage. Several countries, such as Cyprus, France, Greece, Spain, Italy and Portugal, have developed standards specifically for wastewater reclamation (Pintilie et al., 2016). There is also a great market potential for reclaimed water in the PRD. Domestic sewage discharge increased by around 14% from 2.61 billion m3 to 2.97 billion m3 between 2005 and 2015, and the treatment rate of sewage discharge has increased rapidly and reached about 94% by 2015.

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423

Fig. 8. Directions of the wind at the outlet of the PRD.

6. Summary and conclusions Estuaries, the hosts of critical exchanges of materials, energy, and information, play a significant supporting role in economic development. High-intensity anthropogenic activities and natural mechanisms have made estuaries ecologically fragile areas that are

under pressure from frequent water resource crises, and conditions have become increasingly dire in recent years. This study examined the causes and impacts of water resource crises in the PRD, which lies in one of the most developed estuaries in the country. Since the implementation of economic and political reforms in the 1970s, the population and the economy of the PRD

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have experienced a phase of rapid growth, which has led to serious disturbances in the water environments. The great population pressure and rapid growth of the economy of the PRD have been responsible for a dramatic increase in the demand for water resources, and this has contributed to a series of water shortages and incidents of water pollution. Large-scale and uneven sand excavation has significantly decreased the river discharge from upstream into the MW and weakened the dilution effects of freshwater on salinity along the MW. It has also created a vulnerable environment that allows more tidal prisms to enter channels with impaired ability to drain saltwater, which has triggered severe saltwater intrusion and threatened regional water supplies and ecological security. External natural factors, such as sea-level rise and changes in predominant wind directions, also have negatively impacted upon the estuary. Sea level in the PRD has increased by about 1.4 mm/yr over the past decade, which has strengthened the tidal dynamics and gradually worsened the effects of saltwater intrusion. More frequent northeasterly winds have also been partly responsible for the intrusion of saltwater into the PRD over the past decade. In recent years, comprehensive measures have been introduced to alleviate the water resources and its negative effect, including integrated water resources management, control of channel geometry, price leveraging and exploitation and utilization of unconventional water resources. This study has shown that water resource crises are global phenomena that have occurred in estuaries worldwide. The intensification of human activities and natural forcings, which have resulted in water resource crises, have put water resource systems in increasingly serious danger. Even though some control measures have been implemented in the PRD, challenges still remain because the management measures are insufficient to address the problems produced by recent changes in the environment. Hopefully, the causes and impacts noted in this paper will be focus points for sustainable development in estuaries, which can be adapted to other areas. Acknowledgement The research in this paper is fully supported by the National Key Research and Development Program of China (2017YFC0405905 and 2016YFC0401305), the National Natural Science Foundation of China (Grant No. 91547108), the Open Research Foundation of Key Laboratory of the Pearl River Estuarine Dynamics and Associated Process Regulation, Ministry of Water Resources([2017]KJ07), and the Fundamental Research Funds for the Central Universities. We also thank Enago (www.enago.cn) for the English language review. References
zquez-Sun ~e
, E., Carrera, J., Capino, B., Ga
mez, D., Batlle, F., 2006. Abarca, E., Va Optimal design of measures to correct seawater intrusion. Water Resour. Res. 42 (9), 203e206. https://doi.org/10.1029/2005WR004524. Abdrabo, M.A., Hassaan, M.A., 2015. An integrated framework for urban resilience to climate change e case study: sea level rise impacts on the Nile Delta coastal urban areas. Urban Clim. 14, 554e565. https://doi.org/10.1016/ j.uclim.2015.09.005. Andrews, S.W., Gross, E.S., Hutton, P.H., 2017. Modeling salt intrusion in the San Francisco Estuary prior to anthropogenic influence. Continent. Shelf Res. https://doi.org/10.1016/j.csr.2017.07.010. Anthony, E.J., Brunier, G., Besset, M., Goichot, M., Dussouillez, P., Nguyen, V.L., 2015. Linking rapid erosion of the Mekong River delta to human activities. Sci. Rep. 5, 14745 https://doi.org/10.1038/srep14745. Becker, M.L., Luettich, R.A.J., Mallin, M.A., 2010. Hydrodynamic behavior of the Cape Fear River and estuarine system: a synthesis and observational investigation of discharge-salinity intrusion relationships. Estuar. Coast Shelf Sci. 88 (3), 407e418. https://doi.org/10.1016/j.ecss.2010.04.022. Borkowf, C.B., 2002. Computing the nonnull asymptotic variance and the asymptotic relative efficiency of Spearman's rank correlation. Comput. Stat. Data Anal. 39 (3), 271e286. https://doi.org/10.1016/S0167-9473(01)00081-0. Branch, G., 1999. Estuarine vulnerability and ecological impacts: estuaries of South Africa. In: Brian, R. (Ed.), Allanson and Dan Baird. Trends in Ecology & Evolution,

14, p. 499. https://doi.org/10.1016/S0169-5347(99)01732-2 (12). Carroll, Y.Y.J., 1990. A diagnostic for heteroscedasticity based on the spearman rank correlation. Stat. Probab. Lett. 10 (1), 69e76. https://doi.org/10.1016/01677152(90)90114-M. Chen, X., Zong, Y., Zhang, E., Xu, J., Li, S., 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. Geomorphology 41 (2), 111e123. https:// doi.org/10.1016/S0169-555X(01)00109-X. Chuai, X., Huang, X., Wang, W., Zhao, R., Zhang, M., Wu, C., 2014. Land use, total carbon emissions change and low carbon land management in coastal Jiangsu, China. J. Clean. Prod. 103, 77e86. https://doi.org/10.1016/j.jclepro.2014.03.046. Chun chu, L.I., Lei, Y., Wei, H.E., Dai, Z., 2002. Evolutional processes of the pearl river estuary and its protective regulation and exploitation. J. Sediment. Res. 3, 44e51. Church, J.A., White, N.J., 2011. Sea-level rise from the late 19th to the early 21st century. Surv. Geophys. 32 (4), 585e602. https://doi.org/10.1007/s10712-0119119-1. Church, J.A., White, N.J., 2015. A 20th century acceleration in global sea-level rise. Geophys. Res. Lett. 33 (1), 313e324. CIESIN, 2012. Center for International Earth Science Information Network (CIESIN). http://sedac.ciesin.columbia.edu/es/papers/Coastal_Zone_Pop_Method.pdf. Crossland, C.J., Kremer, H.H., Lindeboom, H.J., Marshall Crossland, J.I., Le Tissier, M.D.A., 2005. Coastal Fluxes in the Anthropocene. Springer, Berlin, p. 232. Cui, L.F., Ge, Z.M., Yuan, L., Zhang, L.Q., 2015. Vulnerability assessment of the coastal wetlands in the Yangtze Estuary, China to sea-level rise. Estuarine. Coast. Shelf Sci. 156 (5), 42e51. https://doi.org/10.1016/j.ecss.2014.06.015. Deconto, R.M., Pollard, D., 2016. Contribution of Antarctica to past and future sealevel rise. Nature 531 (7596), 591e597. https://doi.org/10.1038/nature17145. Dinpashoh, Y., Mirabbasi, R., Jhajharia, D., Abianeh, H.Z., Mostafaeipour, A., 2014. Effect of short- term and long- term persistence on identification of temporal trends. J. Hydrol. Eng. 19 (3), 617e625. https://doi.org/10.1061/(ASCE)HE.19435584.0000819. Elliott, M., Cutts, N.D., Trono, A., 2014. A typology of marine and estuarine hazards and risks as vectors of change: a review for vulnerable coasts and their management. Ocean Coast Manag. 93 (93), 88e99. https://doi.org/10.1016/ j.ocecoaman.2014.03.014. Fang, Z.Q., Cheung, R.Y.H., Wong, M.H., 2003. Heavy metals in oysters, mussels and clams collected from coastal sites along the Pearl River Delta, South China. Environ. Sci. 15 (1), 9e24. Frihy, O.E., 2003. The Nile delta-Alexandria coast: vulnerability to sea-level rise, consequences and adaptation. Mitig. Adapt. Strategies Glob. Change 8 (2), 115e138. https://doi.org/10.1023/A:1026015824714. Gan, J., Ingram, R.G., Greatbatch, R.J., Baaren, T.V.D., 2004. Variability of circulation induced by the separation of Gaspe Current in Baie des Chaleurs (Canada): observational studies. Estuar. Coast Shelf Sci. 61 (3), 393e402. https://doi.org/ 10.1016/j.ecss.2004.06.009. Gautheir, T.D., 2001. Detecting trends using spearman's rank correlation coefficient. Environ. Forensics 2 (4), 359e362. https://doi.org/10.1006/enfo.2001.0061. Gong, W., Shen, J., 2011. The response of salt intrusion to changes in river discharge and tidal mixing during the dry season in the Modaomen Estuary, China. Continent. Shelf Res. 31 (7), 769e788. https://doi.org/10.1016/j.csr.2011.01.011. Gornitz, V., Lebedeff, S., Hansen, J., 1982. Global sea level trend in the past century. Science 215 (215), 1611e1614. https://doi.org/10.1126/science.215.4540.1611. Gray, V., 2007. Climate change 2007: the physical science basis. Summary for policy makers. Int. Panel Clim. Change Fourth Assess. Report Ipcc Secretariat 54 (4), 44e45. Grimaldi, S., Kao, S.C., Castellarin, A., Papalexiou, S.M., Viglione, A., Laio, F., et al., 2011. 2.18 e statistical hydrology. Treat. Water Sci. 33 (470), 479e517. https:// doi.org/10.1016/B978-0-444-53199-5.00046-4. Gross, C., Hagy, J.D., 2017. Attributes of successful actions to restore lakes and estuaries degraded by nutrient pollution. J. Environ. Manag. 187 (122) https:// doi.org/10.1016/j.jenvman.2016.11.018. Hamed, K.H., 2008. Trend detection in hydrologic data: the ManneKendall trend test under the scaling hypothesis. J. Hydrol. 349 (3e4), 350e363. https:// doi.org/10.1016/j.jhydrol.2007.11.009. Hamed, K.H., 2009. Exact distribution of the mann-kendall trend test statistic for persistent data. J. Hydrol. 365 (1), 86e94. https://doi.org/10.1016/ j.jhydrol.2008.11.024. He, L., Li, G., Li, K., Shu, Y., 2014. Estimation of regional sea level change in the Pearl River Delta from tide gauge and satellite altimetry data. Estuar. Coast Shelf Sci. 141 (141), 69e77. https://doi.org/10.1016/j.ecss.2014.02.005. Holgate, S.J., Woodworth, P.L., 2004. Evidence for enhanced coastal sea level rise during the 1990s. Geophys. Res. Lett. 31 (7), 157e175. https://doi.org/10.1029/ 2004GL019626. Huang, Z., Zong, Y., Zhang, W., 2004. Coastal inundation due to sea level rise in the Pearl River Delta, China. Nat. Hazards 33 (2), 247e264. https://doi.org/10.1023/ B: NHAZ.0000037038.18814.b0. Hussain, M.S., Javadi, A.A., Robin, V., 2014. Numerical modelling of the impacts of sea level rise on seawater intrusion in unconfined coastal aquifers. In: Uk Conference of the Association for Computational Mechanics in Engineering. ACM. http://hdl.handle.net/10871/18649. Hussain, M.S., Javadi, A.A., 2016. Assessing impacts of sea level rise on seawater intrusion in a coastal aquifer with sloped shoreline boundary. J. Hydro-Environ. Res. 11, 29e41. https://doi.org/10.1016/j.jher.2016.01.003. Ishak, E.H., Rahman, A., Westra, S., Sharma, A., Kuczera, G., 2013. Evaluating the

B. Liu et al. / Journal of Cleaner Production 177 (2018) 413e425 non-stationarity of australian annual maximum flood. J. Hydrol. 494 (12), 134e145. https://doi.org/10.1016/j.jhydrol.2013.04.021. Jarvie, H.P., Jickells, T.D., Skeffington, R.A., Withers, P.J.A., 2012. Climate change and coupling of macronutrient cycles along the atmospheric, terrestrial, freshwater and estuarine continuum. Sci. Total Environ. 424, 252e258. https://doi.org/ 10.1016/j.scitotenv.2012.07.051. Ji, Z.G., 2008. Hydrodynamics and Water Quality: Modeling Rivers, Lakes, and Estuaries. Wiley-Interscience. https://doi.org/10.1002/9780470241066. Kendall, M.G., 1957. Rank correlation methods. Biometrika 44 (1/2), 298. https:// doi.org/10.2307/2333282. Kim, T.I., Choi, B.H., Lee, S.W., 2006. Hydrodynamics and sedimentation induced by largescale coastal developments in the Keum river estuary, Korea. Estuar. Coast Shelf Sci. 68, 515e528. https://doi.org/10.1016/j.ecss.2006.03.003. Koussis, A.D., Georgopoulou, E., Kotronarou, A., Mazi, K., Restrepo, P., Destouni, G., et al., 2010. Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: application to the Akrotiri basin and aquifer. Cyprus. Hydrol. Sci. J. Des Sci. Hydrol. 55 (7), 1234e1245. https://doi.org/10.1080/02626667.2010.512469. Li, X.Z., Ülo, M., Ma, Z.G., Yue, J., 2009. Water quality problems and potential for wetlands as treatment systems in the Yangtze River Delta, China. Wetlands 29 (4), 1125e1132. https://doi.org/10.1672/08-205.1. Liu, M., Guo, Z., Chen, Q., 2014. Impact of unusual saltwater intrusion on aquatic plant communities in Pearl River Estuary (in Chinese). Environ. Sci. Technol. 22 (3), 128e138. Long, A., Sun, L., Shi, R., Zhou, W., Dang, A., 2013. Saltwater intrusion induced by a complete neap tide and its effect on nutrients variation in the estuary of Pearl River, China. J. Coast Res. 29 (5), 1158e1168. https://doi.org/10.2112/JCOASTRESD-12-00182.1. Lu, Z., Wei, Y., Xiao, H., Zou, S., Ren, J., Lyle, C., 2015. Trade-offs between midstream agricultural production and downstream ecological sustainability in the Heihe River Basin in the past half century. Agric. Water Manag. 152, 233e242. https:// doi.org/10.1016/j.agwat.2015.01.022. Luan, H.L., Ding, P.X., Wang, Z.B., Ge, J.Z., Yang, S.L., 2016. Decadal morphological evolution of the yangtze estuary in response to river input changes and estuarine engineering projects. Geomorphology 265, 12e23. https://doi.org/ 10.1016/j.geomorph.2016.04.022. Luo, X.L., Zeng Eddy, Y., Ji, R.Y., Wang, C.P., 2007. Effects of in-channel sand excavation on the hydrology of the Pearl River delta, China. J. Hydrol. 343 (3), 230e239. https://doi.org/10.1016/j.jhydrol.2007.06.019. Lv, A., Du, W., 2006. The mechanism analysis of salt intrusion in Modaomen Estuary (in Chinese). Guangdong Water Res. Hydropower 5, 50e53. Mann, H.B., 1945. Nonparametric tests against trend. Econometrica 13 (3), 245e259. https://doi.org/10.2307/1907187. Maskell, J., Horsburgh, K., Lewis, M., Bates, P., 2014. Investigating riveresurge interaction in idealised estuaries. J. Coast Res. 30 (2), 248e259. https://doi.org/ 10.2112/JCOASTRES-D-12-00221.1. Meyers, S.D., Linville, A.J., Luther, M.E., 2014. Alteration of residual circulation do large-scale infrastructure in a coastal plain estuary. Estuar. Coast 37 (2), 493e507. https://doi.org/10.1007/s12237-013-9691-3. Nicholls, R.J., Cazenave, A., 2010. Corrections and clarifications: sea-level rise and its impact on coastal zones. Science 329 (5992), 628e628. http://www.jstor.org/ stable/40799668. Paiva, B.P., Schettini, C.A.F., Pereira, M.D., Siegle, E., Miranda, L.B., Andutta, F., 2016. Circulation and suspended sediment dynamics in a tropical estuary under ^nc 88 (3), 1265e1276. different morphological setting. An. Acad. Brasil. Cie https://doi.org/10.1590/0001-3765201620150620. Pettit, N.E., Bayliss, P., Bartollo, R., 2016. Dynamics of plant communities and the impact of saltwater intrusion on the floodplains of Kakadu National Park. Mar. Freshwater. Res. https://doi.org/10.1071/MF16148. Pintilie, L., Torres, C.M., Teodosiu, C., Castells, F., 2016. Urban wastewater reclamation for industrial reuse: an LCA case study. J. Clean. Prod. 139, 1e14. https:// doi.org/10.1016/j.jclepro.2016.07.209. Qiu, C., Zhu, J.R., 2013. Influence of seasonal runoff regulation by the three gorges reservoir on saltwater intrusion in the changjiang River Estuary. Continent. Shelf Res. 71 (6), 16e26. https://doi.org/10.1016/j.csr.2013.09.024. Ribeiro, D.C., Costa, S., Guilhermino, L., 2015. A framework to assess the vulnerability of estuarine systems for use in ecological risk assessment. Ocean Coast Manag. 119, 267e277. https://doi.org/10.1016/j.ocecoaman.2015.05.022. Shen, J., Feng, Z., Wong, K.Y., 2006. Dual-track urbanization in a transitional economy: the case of Pearl River delta in south China. Habitat Int. 30 (3), 690e705. https://doi.org/10.1016/j.habitatint.2005.04.003. Sherif, M.M., Singh, V.P., 1999. Effect of climate change on sea water intrusion in coastal aquifers. Hydrol. Process. 13 (8), 1277e1287. https://doi.org/10.1002/ (SICI)1099-1085(19990615)13, 8%3C1277::AID-HYP765%3E3.3.CO;2-N. Spearman, C., 1904. The proof and measurement of association between two things. Am. J. Psychol. 15 (1), 72e101. https://doi.org/10.2307/1412159. Spearman, C., 1906. ‘footrule’ for measuring correlation. Br. J. Psychol. 2 (1), 89e108. https://doi.org/10.1111/j.2044-8295.1906.tb00174.x. Statham, P.J., 2012. Nutrients in estuaries d an overview and the potential impacts of climate change. Sci. Total Environ. 434 (18), 213e227. https://doi.org/10.1016/ j.scitotenv.2011.09.088. Sun, Z., Huang, Q., Opp, C., Hennig, T., Marold, U., 2012. Impacts and implications of major changes caused by the Three Gorges Dam in the middle reaches of the Yangtze River, China. Water Resour. Manag. 26 (12), 3367e3378. https://doi.org/ 10.1007/s11269-012-0076-3.

425

Tan, X., Gan, T.Y., Shao, D., 2017. Effects of persistence and large-scale climate anomalies on trends and change points in extreme precipitation of Canada. J. Hydrol. 550, 453e465. https://doi.org/10.1016/j.jhydrol.2017.05.028. Wang, B., Zhu, J.R., Hui, W., Yu, F.J., Song, X.J., 2012. Dynamics of saltwater intrusion in the modaomen waterway of the Pearl River estuary. Sci. China Earth Sci. 55 (11), 1901e1918. https://doi.org/10.1007/s11430-012-4371-x. Wang, C., Sun, Q., Wang, P., Hou, J., Qu, A., 2013. An optimization approach to runoff regulation for potential estuarine eutrophication control: model development and a case study of yangtze estuary, China. Ecol. Model. 251 (251), 199e210. https://doi.org/10.1016/j.ecolmodel.2012.12.026. Watanabe, K., Kasai, A., Antonio, E.S., Suzuki, K., Ueno, M., Yamashita, Y., 2014. Influence of salt-wedge intrusion on ecological processes at lower trophic levels in the Yura Estuary, Japan. Estuar. Coast Shelf Sci. 139 (4), 67e77. https:// doi.org/10.1016/j.ecss.2013.12.018. Werner, A.D., Simmons, C.T., 2009. Impact of sea-level rise on sea water intrusion in coastal aquifers. Grundwasser 47 (2), 197e204. https://doi.org/10.1111/j.17456584.2008.00535.x. Werner, A.D., Bakker, M., Post, V.E.A., Vandenbohede, A., Lu, C., Ataie-Ashtiani, B., et al., 2013. Seawater intrusion processes, investigation and management: recent advances and future challenges. Adv. Water Resour. 51 (1), 3e26. https:// doi.org/10.1016/j.advwatres.2012.03.004. Wong, L.A., Chen, J.C., Xue, H., Dong, L.X., Su, J.L., Heinke, G., 2003. A model study of the circulation in the Pearl River Estuary (PRE) and its adjacent coastal waters: 1. Simulations and comparison with observations. J. Geophys. Res. Oceans 108 (C5), 249e260. https://doi.org/10.1029/2002JC001451. Wu, Z., Milliman, J.D., Zhao, D., Zhou, J., Yao, C., 2014. Recent geomorphic change in LingDing bay, China, in response to economic and urban growth on the Pearl River delta, southern China. Global Planet. Change 123, 1e12. https://doi.org/ 10.1016/j.gloplacha.2014.10.009. Xie, W., Zhang, C.L., Zhou, X.D., Wang, P., 2014. Salinity-dominated change in community structure and ecological function of Archaea from the lower Pearl River to coastal South China Sea. Appl. Microbiol. Biotechnol. 98(18) (7971) https://doi.org/10.1007/s00253-014-5838-9. Xu, X., Li, X., Chen, M., Li, X., Duan, X., Zhu, G., Feng, Z., Ma, Z., 2016. Land-oceanhuman interactions in intensively developing coastal zone: demonstration of case studies. Ocean Coast Manag. 133, 28e36. https://doi.org/10.1016/ j.ocecoaman.2016.09.006. Yang, Y.Y., Liu, W.R., Liu, Y.S., Zhao, J.L., Zhang, Q.Q., Zhang, M., et al., 2017. Suitability of pharmaceuticals and personal care products (ppcps) and artificial sweeteners (ass) as wastewater indicators in the pearl river delta, south China. Sci. Total Environ. 590e591, 611e619. https://doi.org/10.1016/j.scitotenv.2017.03.001. Yin, K., Harrison, P.J., 2008. Nitrogen over enrichment in subtropical Pearl River Estuarine coastal waters: possible causes and consequences. Continent. Shelf Res. 28 (12), 1435e1442. https://doi.org/10.1016/j.csr.2007.07.010. Yuan, R., Zhu, J., Wang, B., 2016. Impact of sea-level rise on saltwater intrusion in the Pearl River Estuary. J. Coast Res. 300 (2), 477e487. https://doi.org/10.2112/ JCOASTRES-D-13-00063.1. Zhang, W., Yan, Y., Zheng, J., Li, L., Dong, X., Cai, H., 2009. Temporal and spatial variability of annual extreme water level in the Pearl River Delta region, China. Global Planet. Change 69 (1e2), 35e47. https://doi.org/10.1016/ j.gloplacha.2009.07.003. Zhang, W., Ruan, X., Zheng, J., Zhu, Y., Wu, H., 2010. Long-term change in tidal dynamics and its cause in the Pearl River Delta, China. Geomorphology 120 (3), 209e223. https://doi.org/10.1016/j.geomorph.2010.03.031. Zhang, X., Deng, J., 2010. Affecting factors of salinity intrusion in Pearl River estuary and sustainable utilization of water resources in Pearl River delta. Sustainability in Food and water. Springer Netherlands 18, 11e17. https://doi.org/10.1007/97890-481-9914-3_2. Zhang, W.Y., Wei, Z.W., Wang, B.H., Han, X.P., 2016. Measuring mixing patterns in complex networks by spearman rank correlation coefficient. Phys. A Stat. Mech. Appl. 451, 440e450. https://doi.org/10.1016/j.physa.2016.01.056. Zhang, G., Bai, J., Rong, X., Zhao, Q., Jia, J., Cui, B., et al., 2017. Heavy metal fractions and ecological risk assessment in sediments from urban, rural and reclamationaffected rivers of the pearl river estuary, China. Chemosphere 184, 278e288. https://doi.org/10.1016/j.chemosphere.2017.05.155. Zhao, Y., Zou, X., Cao, L., Xu, X., 2014. Changes in precipitation extremes over the Pearl River basin, southern China, during 1960e2012. Quat. Int. 333 (4), 26e39. https://doi.org/10.1016/j.quaint.2014.03.060. Zheng, S., Guan, W., Cai, S., Wei, X., Huang, D., 2014. A model study of the effects of river discharges and interannual variation of winds on the plume front in winter in Pearl River Estuary. Continent. Shelf Res. 73 (2), 31e40. https:// doi.org/10.1016/j.csr.2013.11.019. Zhou, W., Luo, L., Xu, H.Z., Wang, D.X., 2012a. Saltwater intrusion in the Pearl River estuary during winter. Aquat. Ecosys. Health Manag. 15 (1), 70e80. https:// doi.org/10.1080/14634988.2012.649238. Zhou, W., Wang, D., Luo, L., 2012b. Investigation of saltwater intrusion and salinity stratification in winter of 2007/2008 in the Zhujiang River Estuary in China. Acta Oceanol. Sin. 31 (3), 31e46. Zhou, Q., Wu, F., Zhang, Q., 2015. Is irrigation water price an effective leverage for water management? an empirical study in the middle reaches of the Heihe River Basin. Phys. Chem. Earth Parts A/b/c s 89e90, 25e32. https://doi.org/ 10.1016/j.pce.2015.09.002. Zhu, Z., Deng, Q., Zhou, H., Ouyang, T., Kuang, Y., Huang, N., Qiao, Y., 2002. Water pollution and degradation in Pearl River delta, south China. Ambio 31 (3), 226e230. http://www.jstor.org/stable/4315241.