1036 Clean – Soil, Air, Water 2012, 40 (10), 1036–1046

Zhiming Zhang1 Research Article Baoshan Cui1 Xiaoyun Fan1 Kejiang Zhang2 Wetland Network Design for Mitigation of Saltwater Hui Zhao3 Intrusion by Replenishing Freshwater in an Estuary Honggang Zhang1

In this paper, a wetland network is designed to mitigate saltwater intrusion based on 1State Key Joint Laboratory of Environmental Simulation and the relationship between river discharge and salinity in Modaomen waterway (MDMW), Pollution Control, School of the Pearl River estuary (PRE) of southern China. The designed network consists of Environment, Beijing Normal existing and expanded wetlands, river channels and their tributaries. The freshwater University, Beijing, P. R. China stored in wetlands can be diverted into river channels to adjust salinity gradients when 2Xinjiang Research Center of Water and Wastewater Treatment, Xinjiang saltwater intrusion reaches the predefined locations. In the MDMW, two exponential Deland Co., Ltd., Urumqi, P. R. China regression models are established between the freshwater discharge and the saltwater 3Institute of Mountain Hazards and intrusion length at both high water slack (HWS) and low water slack (LWS), respectively. Environment, Chinese Academy of The wetland network can effectively mitigate the saltwater intrusion when the fresh- Sciences, Chengdu, P. R. China water is diverted from wetlands into river channels according to the regressive models. The method of wetland network can save over 50% more freshwater (varying from 46.1 to 56.4% at HWS and from 53.4 to 61.8% at LWS) than the emergent water allocation schedule (EWAS) implemented in 2005 to protect against saltwater intrusion in the same area. Wetland network design facilitates water resource management in the PRE and can also be easily generalized to other estuaries.

Keywords: Discharge–salinity relationships; Ecosystem; Modeling; Storage capacity Received: January 11, 2012; revised: April 16, 2012; accepted: May 22, 2012 DOI: 10.1002/clen.201100735

1 Introduction and recharging freshwater in coastal aquifers [24, 25]. This process is usually time-consuming and negatively influences the surrounding An estuarine ecosystem, which features the land–ocean interactions ecosystems due to the pumped saltwater. With the relationships of an abundant population of flora and fauna, has the highest service between the freshwater discharge and salinity gradient, many values compared to other ecosystems [1]. Meanwhile, it is a sensitive efforts have also been employed to divert large quantities of fresh- ecosystem to the rapid industrialization and economic development water from upstream into estuarine or coastal zones to mitigate and sea level rise [2–5]. With the water shortages and sea level rise, saltwater intrusion. According to these relationships, an emergent researchers found that the salinity gradient was an important water allocation schedule (EWAS) has been implemented to mitigate factor in determining these estuarine ecosystem services [6–14]. saltwater intrusion since 2005 in Pearl River Basin, China [33, 34]. Over the past decades, salinity gradients and freshwater inflow in The frequency and scope of saltwater intrusion are serious due to the an estuary have been significantly disturbed by anthropogenic activi- drought in Pearl River Basin. Saltwater intrusion is a threat to the ties [15–19] and climate change [20]. For example, saltwater intrusion safety of water supply to the surrounding areas. Fortunately, will increase water salinity if less freshwater is discharged into authorities implement the EWAS which tried to divert the upstream estuaries arising from the dam construction projects [15, 16, 21, 22]. freshwater into the tidal rivers during dry season of 2005. There Freshwater in estuaries is also usually intercepted and used for is a significant negative correlation between the river discharge irrigation, maintenance of ecosystem integrity, and domestic and and water salinity in Pearl River estuary (PRE) [35]. The saltwater industrial purposes such that less freshwater is available for miti- intrusion will be mitigated by increased the river discharge, which gation of saltwater intrusion. can ensure water supply to the cities of Zhuhai and Macao. Although From the literature, researches about the saltwater intrusion have this schedule is effective, it is costly due to a lack of sufficient mainly focused on groundwater in coastal aquifers [23–25] and freshwater. surface water in river channels [26–32]. Previous studies indicated The freshwater discharge is significantly associated with the sa- the control effects of saltwater intrusion by pumping out saltwater linity gradient in many estuaries around the word. There are negative correlations between the saltwater intrusion length and river dis- charge [26, 27, 30, 36]. The saltwater intrusion follows a negative Correspondence: Professor B. Cui, State Key Joint Laboratory of power law to the river discharge in northern San Francisco Bay [37]. Environmental Simulation and Pollution Control, School of Environment, Becker et al. [26] compared three regression models (linear, expo- Beijing Normal University, No. 19 Xinjiekouwai Street, Beijing 100875, P. R. China nential, and power-law regressions) to determine the optimal E-mail: [email protected] relationship between the saltwater intrusion length and river dis- charge. Therefore, a simple and effective relationship between the Abbreviations: EWAS, emergent water allocation schedule; HWS, high water slack; LWS, low water slack; MDMW, Modaomen waterway; PRE, freshwater discharge and salinity gradient along the river channel Pearl River estuary should be established.

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Considering the water saving and the mitigation of saltwater 228N–228400 N, 1138 E–1138400 E) is one of the main outlets of the intrusion, how to allocate the upstream freshwater in the estuarine Pearl River (Fig. 1) [28, 50]. and coastal regions is another important question. Poulter et al. [4] The average annual runoff of MDMW is approximately presented a network-based method to identify the major shoreline 8.84 1011 m3 accounting for 28.2% of the total runoff of the entry points of saltwater to the drainage river networks. These Pearl River [46, 51]. The MDMW is a typical river channel with points are useful in the managing and monitoring of saltwater. intensive runoff and neap range in PRE. The great diurnal range The network-based method is nearly natural and cost-effective is approximately 0.86 m [51]. There are six fixed gauging stations [38]. With the application of graph theory to network analysis, (Baiqing, Daao, Zhuyin, Xihekou, Zhupaisha, and Guadingjiao) along wetland network approaches are widely used in watershed storm the MDMW from upstream to downstream. These stations are far water management [39, 40] and regional drought and flood control away from the mouth of the river inland with a distance of 0, 13.9, [38]. This methodology proves to be effective and has few negative 22.9, 29.6, 44.5, and 49.1 km, respectively (Fig. 1 and Tab. 1). effects on surrounding ecosystems. Typically, there are a number of wetlands which are connected by tributaries/river channels in an estuary. These wetlands include marsh, lakes, and reservoirs that 3 Methodology form a network, together with river channels, to regulate hydro- 3.1 Data collection logical processes [38–44]. In a network, the freshwater can be diverted into river channels from wetlands. The increased river In this study, the cross-sectional area and river width which were discharge then decreases the range of saltwater intrusion along defined at the tidal averaged water level were obtained from the river [26–30]. A new equilibrium can be established between measured data at these six fixed gauging stations. The cross-sectional water and salinity in river channels [24]. area and river width of river channels were used to determine the The objectives of this paper are as follows: (i) to investigate the shape of MDMW. The daily mean discharge at Wuzhou station was relationship between the freshwater discharge and the saltwater used to estimate the upstream freshwater discharge. Hourly average intrusion length; (ii) to design a wetland network to mitigate chlorosity of the six fixed gauging stations was used to calculate the saltwater intrusion; and (iii) to compare water allocations between river water salinity at each station. These data were obtained from network method and EWAS and evaluate the performance of wet- the publication [52]. Figure 1 illustrates the location of these land networks in different scenarios. stations.

2 Study area 3.2 Wetland network design The Pearl River is the third longest river in China. PRE is a multi- The framework to design a wetland network is illustrated in Fig. 2. A channel estuary with a drainage density of 0.68–1.07 km/km2 [45–47]. quantitative relationship between freshwater discharge and salinity There is a mixed diurnal and semi-diurnal tide with two troughs was identified, and then, the locations of wetlands were determined and two peaks in a day [48, 49]. The Modaomen waterway (MDMW, based on the distribution of water areas, which included ponds,

Figure 1. (a) Map of China. (b) The PRE and its river network. (c) The MDMW G1, G2, G3, G4, G5, and G6 are gauging stations named as Guadingjiao, Zhupaisha, Xihekou, Zhuyin, Daao, and Baiqing, respectively. S1 is Lianshiwan sluices.

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Table 1. Characteristics of eight segments in the MDMW

Segment Cross-section Distance Length of Qb) (km)a) segment (km) (m3/s)

– X1 50.4 – 1040 X1X2 X2 40.2 10.2 1360 X1X3 X3 40.2 10.2 1360 X2X4 and X3X4 X4 30.6 9.6 1750 X4X5 X5 22.0 8.6 2220 X5X6 X6 16.9 5.1 2600 X6X7 X7 12.7 4.2 3010 X7X8 X8 8.9 3.8 3510 a) Distance away from the cross section Xi (i ¼ 1,2,...,8) to the Figure 3. Variations of the cross-sectional area and the river width of Gudingjiao station (G1). the MDMW with distance away from the estuary mouth (Part of data from b) It is the freshwater discharge at the Wuzhou station. When Ou et al. [57]). the practical freshwater discharge equals the values given in the fifth column of freshwater discharge obtained from Eq. (8), the saltwater intrusion will not reach the corresponding segments.

From Eq. (1), a one-dimensional steady state advection-diffusion equation [27, 58, 59] was used to predict the saltwater intrusion as discarded ditches, and aquaculture water near the river banks. The follows: wetlands were then connected to the existing river channels to form @ðASÞ @ðAvSÞ @ @S a wetland network. Isolated river channels and wetlands were con- þ ¼ AKx (2) nected by new-building river segments. The discharge and the stor- @t @x @x @x age capacity of the wetlands were then evaluated. Finally, if the where S (%) is the salinity at the upstream boundary of the river, t (s) wetland network design was finished, the performance of wetlands is time, v (m/s) is velocity, and Kx is the longitudinal salinity distri- in different scenarios was then evaluated. bution coefficient that is independent of the distance from the mouth. Under steady state [27, 59], by introducing v ¼ Q/A, Eq. (2) 3.2.1 Relationship between freshwater discharge and can be written as: saltwater intrusion length QS dS ¼ K (3) Researches [10, 27, 53–55] found that the cross-sectional area of river A x dx channels exponentially reduced with increasing distance away 3 from the river mouth in some estuaries. This regression model where Q (m /s) is the river discharge from upstream. In this study, the was identified in the MDMW by Chen et al. [56] and Ou et al. [57] freshwater discharge (Q) was collected from the Wuzhou station and was given by: where the hydrological process was not influenced by the tides [49]. The Wuzhou station is approximately 340 km away from the river mouth (see Fig. 1b). Substituting Eq. (1) into Eq. (2), a new A ¼ A0 expðx=aÞ (1) equation can obtain the following [27]:

2 where A (m ) is the cross-sectional area of the river channels and A0 is aQ lnðS=S0Þ¼ ðÞexpðx=aÞ1 (4) the cross-sectional area at river mouth (G1). x (km) is the length of the KxA0 cross-section away from the river mouth. a (km) is the convergence where S (%) indicates the salinity at the river mouth. The saltwater length which is the length scale of the exponential function against 0 intrusion length of the PRE was set as x ¼ x , where the salinity (S) measured data. It can be seen very clearly in Fig. 3 that convergence L was 0.5%. The threshold of salinity (S ¼ 0.5 %) at the upstream length (a) can be determined by the regression model (Eq. (1)). boundary of the river was extracted from the Standards for Drinking Water Quality in China (GB5749-2006). The saltwater intru- sion length is then determined by: A K lnð0:5=S Þ x ¼ aln 1 0 x 0 (5) L aQ

where xL (km) is the saltwater intrusion length.

In Eq. (5), the negative ln(0.5/S0) indicated that the saltwater

intrusion length (xL) increased with greater cross-sectional area of

the river mouth (A0) and with the higher mixing coefficient (Kx) and decreased with the increase of the river discharge (Q). In this paper, the salinity was converted from the chlorosity using Han’s function, which indicated the salinity was proportional to the chlorosity [60]. The maximum and minimum values were used to calculate the Figure 2. The flow chart of designing a wetland network for mitigation of saltwater intrusion length at high water slack (HWS) and low water saltwater intrusion. slack (LWS), respectively [61]. This model might underestimate the

ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com Clean – Soil, Air, Water 2012, 40 (10), 1036–1046 Wetland Network Design for Mitigation of Saltwater Intrusion 1039 range of saltwater intrusion [27, 62, 63]. Thus, a parameter (f) was used to calibrate Eq. (5). The calibrated intrusion length is given as: xL0 ¼ fxL (6) where xL0 (km) is the calibrated saltwater intrusion length. The regression model between upstream freshwater discharge and the calibrated saltwater intrusion length was then determined by:

ciQ xLi ¼ ximaxe i (7) where xLi (km) is the saltwater intrusion length relative to the freshwater discharge (Qi), i ¼ 1, 2 assigned to the two different states of HWS and LWS, respectively, ci is a positive constant, and xi max (km) is the maximum saltwater intrusion length for each state.

If the saltwater intrusion length (xLi) was known, the upstream freshwater discharge (Qi) can then be determined by: 1 xLi Qi ¼ ln (8) ci ximax

3.2.2 Determining the location of wetlands In PRE, some tributaries had been used to store freshwater during the wet season, the stored freshwater would be employed to irrigate cropland during the dry season [51]. Sluices had been set between these tributaries and the main channel. These sluices would store freshwater during the wet season and prevent saltwater from enter- ing the tributaries during the dry season. Sluices were detected at the range of saltwater intrusion in the MDMW. Each sluice was located at the confluence between the tributaries and the main channel of MDMW (Fig. 1). Eight major confluences were selected Figure 4. Layout of the designed wetland network, where G1, G2, G3, G4, G5, and G6 are stations of Guadingjiao, Zhupaisha, Xihekou, Zhuyin, along the MDMW. Then eight cross-sections of MDMW were deter- Daao, and Baiqing, respectively; S1 is Lianshiwan sluices; W1 to W8 are mined according to location of the eight confluences (Fig. 4). Finally, eight wetlands which are designed to mitigate saltwater intrusion; X1X2, the MDMW was divided into the eight segments (Fig. 4). Their X1X3, X2X4, X3X4, X4X5, X5X6, X6X7, and X7X8 are eight segments. characteristics are given in Tab. 1. The wetlands used for water storage were designed around X1–X8 (see Fig. 4). The location of wetlands could change according to the land use types nearby the than the threshold (0.5%), the stored freshwater could be diverted to confluences (Fig. 4). The distances to the river mouth (G1) were the river channels to decrease water salinity. considered to the saltwater intrusion length. The ecological service value of wetlands is almost two times greater than that provided by the water areas [1, 64]. Therefore, 3.2.4 Wetland discharge (Qt) and storage capacity (SC) the water areas around the MDMW could be transformed into wet- The costs of constructing wetlands included earthwork, land occu- lands from the view point of ecology. The initial locations of the pation, labor force, management, and auxiliary facilities [65]. Most of selected wetlands were located nearby the confluences. They were them were connected with the storage capacity of wetlands, which adjusted according to the distribution of water areas. In this paper, can be determined by: the water areas were transferred to wetlands with storage capacity. The wetlands were connected with the MDMW by tributaries at the 0 SC ¼ 3600Qtt (9) confluences, where the sluices were set to prevent saltwater intru- sion. With these sluices managers can control the switch of wetlands where SC (m3) is the storage capacity of wetlands, and 3600 is a unit depending on the range of saltwater intrusion. 3 conversion constant, Qt (m /s) the discharge from wetlands to river channels, and t0 (h) is the time to replenish water. Each wetland was 3.2.3 Integrating and forming the wetland network assumed that it can replenish water to the river channels about The wetlands, river channels, and tributaries formed a topological 5 h/day at a certain rate. The aim of the water allocation was to keep structure, i.e., a wetland network. As the water level in river channel the salinity at an acceptable level at the water intake of S1 [66]. rose during wet season, freshwater would flow into the designed Sufficient freshwater at S1 was required to ensure water supply to wetlands through the tributaries. On the other hand, the upstream the cities of Zhuhai and Macao. The EWAS was implemented to stored freshwater would be replenished into the designed wetlands decrease the salinity in PRE. This strategy was effective to mitigate through the transfer tributaries during the dry season (see details in the saltwater intrusion [35, 36, 66, 67]. In this study, the corresponding Section 4.5). Conversely, if the salinity of the river water was greater discharge was used to calculate the storage capacity of the wetlands.

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For all wetlands, the discharge (Qt) can be calculated by: water allocation method in the emergency scenario can be estimated by: Qti ¼ QXj QXi; i < j; i ¼ 1; 2; :::; 7; j ¼ 2; 3; :::; 8 (10)

F ¼ ðÞV2 V1 =V2 100 (12) where Qti is the discharge from wetland Wi to the river channels, and

QXi and QXj are the freshwater discharges flowing through Xi and Xj, 3 where F (%) is the percentage of water saving, V1 (m ) the daily water respectively. Substituting Eq. (8) into Eq. (10), the discharge of wet- 3 allocation by the network, and V2 (m ) is the daily water allocation by lands can then be determined by the saltwater intrusion length. the EWAS at Wuzhou station. The values of V1 and V2 can be deter- There were many tributaries along the segments from Wuzhou mined by station to MDMW (only main tributaries are considered and shown in Fig. 1b and Tab. 2). The freshwater discharge decreased from V1 ¼ 3600 T ðÞQL2 þ QL1 (13a) Wuzhou station to MDMW due to the freshwater flow into these tributaries. Seepage and evaporation of river water was not con- sidered in this paper. The allocated percentages of freshwater from f f V2 ¼ 3600 T Q Q n ¼ 2; 3; ...; 8 (13b) main channels to these tributaries were determined using the n1 n method presented by Ou et al. [57]. Therefore, the average freshwater where 3600 is a unit conversion constant, and T (h) is the minimum discharges flowing through the cross-sections can be calculated by: time of taking water from S1 (5 h/day). Thus, T ¼ 5h.QL1 and QL2 are

QXi ¼ Q ðÞ1 a i ¼ 1; 2; ...; 7 (11) the sum discharges from all wetlands to river channels at HWS f and LWS, respectively. Qn represents the freshwater discharge at where Q is the freshwater discharge at Wuzhou station, and a (%) is Wuzhou station when the saltwater arrived at the cross-section Xn, the allocated percentage from the main river channels to tributaries. n ¼ 2,3,...,8. Allocated percentages (a) were obtained from literature [57], which was used to adjust river freshwater discharges at different cross- sections (see Tab. 2). 3.4 Cost-benefit analysis The paired branches (e.g., X1X2 and X1X3, X2X4 and X3X4) were Wetlands with storage capacity in the wetland network were trans- integrated in the calculation, simplifying the computation and ferred from water areas (e.g., ponds, discarded ditches, and aqua- enhancing the overall performance of the saltwater intrusion model culture water). The wetlands and major river channels (MDMW) were [61]. Thus, the average length of X1X2 and X1X3 (10.2 km, Tab. 1) was connected by existing tributaries. There were few new river channels used to calculate the discharge (QXi) at X2 and X3. The sum discharges to be built. Water control structures (sluices) had been built at the (QX2 þ QX3) was calculated by combining the paired branches into a confluences. The cost of the wetland network can be estimated by the single one (see Tab. 1). The upstream freshwater discharges (Q)were costs of construction and land acquisition and is given as: allocated to the paired branches based on the ratio of the average tidal .X X discharge of gauging station G5 and G6 (QX2/QX3 ¼ 48:52), which were C ¼ Cc þ Cl ¼ q vi þ p Ai=TL 365 (14) obtained from the Hydrological Year Book of 2006 (published by the Hydrological Bureau of the Ministry of Water Resources of China). where C (RMB/day) is the cost of the wetland network, Cc (RMB/day)

is the cost of construction, Cl (RMB/day) is the land acquisition 3.3 Contrast of water allocation compensation, q (RMB/(year m3)) is the cost of construction per cubic meters in one year in China (according to Xu et al. [67], q is The EWAS to mitigate saltwater intrusion had been implemented 0.67 RMB/year), v and A is the SC and area of wetland Wi, in the Pearl River Basin for many years. The merit of a network i i respectively, p (103 RMB/m2) is the compensation of unit area of water areas in China (the value of p is extracted from the Land Table 2. Average water allocated river discharge and percentage between Compensation Protection Standards of Guangdong Province in 8 and 15 February, 2001 in the West River Delta (The data is extracted from 2005), TL is the land contracting time of aquaculture (30 years for the Figure of Ou et al. [57]) water areas in China), and 365 is a unit conversion constant. The Gauging station at Average allocated Average water water areas were roughly 2 m depth according to the field study. main tributaries river discharge allocated The S1 was a major water resource to ensure water supply for the 3 (m /s) percentage (%) cities of Zhuhai and Macao. With the aim of the wetland network designed, the method from [68] was used to estimate the water Tianhe 931.0 42.0 Nanhuaa) 759.0 34.3 supply values of wetland network, which can be calculated by: Jiangmena) 63.0 2.8 b) Muzhoukou 201.0 9.1 W ¼ pzRz þ pmRm (15) Zhuzhouc) 47.5 2.1 d) Denglongshan 214.0 9.7 where W (RMB/day) is the water supply values of the wetland net- a) They are located at upstream of X1–X8 and uses for calculating work, and pz and pm are the prices of water in Zhuhai and Macao, 3 the discharge of W1–W8. respectively. pz and pm were 1.83 RMB/m (www.zhpi.gov.cn/ (in b) 3 It is located at upstream of X3–X8 and uses for calculating the Chinese)) and 0.98 RMB/m [69, 70] in 2005. Rz and Rm are the daily discharge of W3–W8. water requirements of Zhuhai and Macao. According to the Planning c) It is located at upstream of X4–X8 and uses for calculating the discharge of W4–W8. and Construction of Water Supply of Zhuhai (www.zhuhai-water. d) It is located at downstream of all the cross-sections and does not com.cn/ (in Chinese)), the daily water requirement of Zhuhai is use in this paper. 1.20 106 m3/day, and Macao is approximately 0.18 106 m3/day.

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The ecosystem service added values were evaluated by:

DZ ¼ Zw Zl ¼ Aw Bw Al Bl (16) where DZ (RMB/day) is the ecosystem service added values of the wetland and Zw and Zl (RMB/day) are the total service values 2 of wetlands and water areas. Aw and Al (m ) are total areas of wetland and water areas. Aw and Al are assumed to be the same. Bw and Bl (RMB/(m2 day)) are the average service values of wetlands and water areas obtained from Costanza et al. [1]. If DZ > 0, the wetland network had a positive effect on the surrounding ecosystems; DZ < 0 meant the wetland network had negative effects.

Therefore, the net benefit (Nb) of the wetland network designed can be calculated by

Nb ¼ W þ DZ C (17) if Nb > 0, the wetland network designed is cost-effective.

4 Results 4.1 Relationship between freshwater discharge and saltwater intrusion length The relationship between freshwater discharge and saltwater intrusion length obtained from the regression analysis at HWS and LWS in dry season was given as:

0:0007Q 1 2 xL1 ¼ 104:13e ðR ¼ 0:8276Þ; for HWS (18a)

0:0008Q 2 xL2 ¼ 56:27e 2 ðR ¼ 0:5702Þ; for LWS (18b) where xL1 and xL2 is the saltwater intrusion length at HWS and LWS, Figure 5. Land use types around the MDMW in 2000. The labels are same respectively. The saltwater intrusion length was dependent on the as Fig. 4. Water areas include ponds, discarded ditches, and aquaculture water. river discharge from upstream. Therefore, the upstream freshwater discharge can be written as: during wet season. The storage function of the wetland network 1 xL1 Q1 ¼ ln ; for HWS (19a) was employed to mitigate saltwater intrusion. According to the 0:0007 104:13 regression model between the saltwater and discharge, the saltwater intrusion length would be shortened because extra freshwater was 1 x allocated into river channels. The saltwater intrusion length was Q ¼ ln L2 ; for LWS (19b) 2 0:0008 56:27 inversely proportional to the freshwater discharge. Therefore, the aim of mitigating saltwater intrusion could be achieved by the Equations (19a) and (19b) can be used to determine the freshwater wetland network. discharge, once the saltwater intrusion length was specified.

4.2 The wetland network for mitigating saltwater 4.3 Identification of wetland discharge (Qt) and intrusion storage capacity (SC) The land use types around the MDMW in 2000 are illustrated in From Eq. (11), the freshwater, which flowed into the MDMW, could be Fig. 5. The initial locations of the designed wetlands just considered calculated. If the freshwater discharge at Wuzhou station was the distances from the selected sites to the confluences. The 1040 m3/s, the saltwater would reach the cross-section X1 at HWS locations of the designed wetlands could be adjusted according to (Eq. (19a)). There were two major tributaries between Wuzhou the distribution of water areas in Fig. 5. station and the cross-section X1 (see Fig. 1b and Tab. 2). According A wetland network in the MDMW was designed by integrating the to the average water allocated percentages of Nanhua and Jiangmen wetlands, river channels, and tributaries to mitigate saltwater intru- (34.3 and 2.8%, see Tab. 2), the volume of freshwater allocated sion (see Fig. 6). Wetland W8 was removed because it was located at to tributaries was removed from the total. The corresponding dis- the downstream of the sluice S1 named Lianshiwan, which was a charge (QX1), which could pass through cross-section X1, was calcu- 3 3 major water intake of Zhuhai and Macao. This network could allo- lated using Eq. (11): QX1 ¼ 1040 m /s (1–34.3%–2.8%) ¼ 654 m /s. cate freshwater to river channels when the saltwater intrusion Similarly, if the saltwater reached X2 and X3, the upstream fresh- occurred. The freshwater could be stored in wetland network water discharge should be no less than 1360 m3/s. The corresponding

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discharge (QX2 þ QX3), which could pass through X2 and X3, was calculated using Eq. (11):

3 3 QX2 þ QX3 ¼ 1360 m =s ð1 34:3% 2:8%Þ¼855 m =s

The additional freshwater discharge, i.e., 1360 1040 ¼ 320 m3/s, should be allocated to the river channel at the Wuzhou station. However, the corresponding additional freshwater discharge (Eq. (10), 3 Qt1 ¼ 855 654 ¼ 201 m /s) should be allocated to the MDMW from W1. The freshwater discharges through each cross-section were calcu- lated and are listed in Tab. 3. If these freshwater discharges were supplied into the MDMW, saltwater would not pass through the corresponding cross-sections. Freshwater discharges were in short supply during dry season. Therefore, additional discharges should be diverted into river channels to mitigate saltwater intrusion. The additional discharges (output discharges) diverted from wetlands

into the MDMW are given in Tab. 4. The output discharge (Qt) should be 201, 101, 109, 205, 196, 212, and 258 m3/s for W1–W7 at HWS,

respectively (see Tab. 4). At LWS, the output discharge (Qt) should be greater than 57, 170, 191, and 223 m3/s for W4–W7, respectively. The minimum storage capacity of wetlands at HWS was also listed in Tab. 4. Because the maximum saltwater intrusion length at LWS was 24.1 km, based on Eq. (18b), this length was away from X1, X2, and X3. Thus W1–W3 were not at work. The results were also shown in Tab. 4.

4.4 The water resources of designed wetlands The allocation time of freshwater from wetlands to river channels during the dry season was no less than 5 h/day. This allocation time should maintain the salinity content at an acceptable level at S1 and Figure 6. The wetland network of the MDMW for mitigating saltwater ensure water supply for Zhuhai and Macao. The minimum discharge intrusion. The labels are same as Fig. 4. T1, T2, T3, and T4 are the of Wuzhou station was 1440 m3/s at the period of the emergent water tributaries for supplementing water to wetlands. allocation in 2005. The saltwater intrusion length was at most 38 km at HWS and 17.8 km at LWS (Eq. (18)). Wetlands (W1 at HWS, W1, W2,

Table 3. Average freshwater discharges allocated to different cross-sections

Cross-section The freshwater discharge at Wuzhou station (m3/s)

1040 1360 1750 2220 2600 3010 3510

X1 654 855 – – – – – X2 – 353 455 – – – – X3 – 379 487 – – – – X4 – – 942 1147 – – – X5 – – – 1147 1344 – – X6 – – – – 1344 1556 – X7 – – – – – 1556 1814

Table 4. Discharge and storage of wetlands at different saltwater intrusion length

3 a) 3 b) 3 c) 3 xL1 (km) xL2 (km) Q (m /s) QL1 (m /s) QL2 (m /s) SC (m ) Wetlands

6 50.4 xL1 > 40.2 24.1 xL2 > 19.0 1040 Q < 1360 201 0 5.76 10 W1 6 40.2 xL1 > 30.6 19.0 xL2 > 13.9 1360 Q < 1750 101 0 5.58 10 W2 6 40.2 xL1 > 30.6 19.0 xL2 > 13.9 1360 Q < 1750 109 0 8.46 10 W3 6 30.6 xL1 > 22.0 13.9 xL2 > 9.5 1750 Q < 2220 205 57 6.84 10 W4 6 22.0 xL1 > 16.9 9.5 xL2 > 7.0 2220 Q < 2600 196 170 7.38 10 W5 6 16.9 xL1 > 12.7 7.0 xL2 > 5.1 2600 Q < 3010 212 191 9.00 10 W6 6 12.7 xL1 > 8.9 5.1 xL2 > 3.4 3010 Q < 3510 258 223 5.76 10 W7 a) Additional discharge from wetlands to river channel at HWS. b) Additional discharge from wetlands to river channel at LWS. c) Storage capacity of wetlands at HWS.

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W3, and W4 at LWS) would not be used for mitigating saltwater 4.5 Comparison of water allocation between intrusion. Because saltwater could not reach the cross-sections (X1 at wetland network and EWAS HWS, X1, X2, X3, and W4 at LWS). When the discharge was 1440 m3/s at Wuzhou station, there was Wetland W1 would not be used to mitigate saltwater intrusion at the longest saltwater intrusion length not only at HWS but also at HWS. W1, W2, and W3 would not be used at LWS (see Section 4.4). LWS during dry season of 2005. The additional discharges should The results of comparing water allocation based on the network divert from wetlands into the MDMW to achieve the aim of this method with the EWAS in 2005 are shown in Tab. 6. Generally, the paper. According to Eqs. (10), (11), (18), and (19), the additional network method would save approximately a half of the freshwater discharges from W2–W7 to river channels were 81, 86, 205, 196, compared to the EWAS (varying from 46.1 to 56.4% at HWS and from 212, and 258 m3/s at HWS, respectively; in addition, the additional 53.4 to 61.8% at LWS). This result might be caused by the difference of discharges from W5–W7 to the corresponding segments were 145, water allocation at HWS and LWS. The percentage of saved water at 191, and 223 m3/s at LWS, respectively. HWS is less than that at LWS (Tab. 6). Less freshwater was required Some tributaries (T1, T2, T3, and T4, see Fig. 6) could be used to at the LWS than at the HWS (Tab. 6) to prevent saltwater intrusion transfer upstream freshwater into wetlands to ensure the water from the S1. resources of wetlands. These tributaries were defined as the transfer tributaries in this paper. Input discharges from upstream through 4.6 Cost-benefit analysis transfer tributaries were employed to replenish wetlands that ensure water supply of output discharge. The ratio of input dis- According to Eqs. (14),(15) and (16), the costs of the wetland charge to output discharge was 5:24 (5 h/24 h). Therefore, the input network were 0.13 106 RMB/day, and water supply values were discharges were 17, 18, 43, 41, 44, and 54 m3/s from transfer tribu- 2.04 106 RMB/day. The ecosystem service added values of the wet- taries to W2–W7, respectively (Tab. 5). The wetlands will be replen- land network were 0.14 106 RMB/day. The net benefits of the ished by freshwater through the transfer tributaries, and the wetland network were 2.05 106 RMB/day. Thus, the wetland allocation discharges of T1, T2, T3, and T4 were 181, 139, 98, and network was cost-effective and had social and ecological benefits. 54 m3/s, respectively. Particularly, saltwater cannot arrive at X2 and X3 at LWS during dry season of 2005. W1–W3 can be replenished by freshwater in the main river channel-MDMW. 5 Discussion When freshwater was allocated to these designed wetlands 5.1 Relationship between freshwater discharge through the transfer tributaries according to the input discharges in Tab. 5, the minimum discharge of Wuzhou station necessary to and saltwater intrusion length maintain the salinity of river water was 1728 m3/s obtained from A simple model is used to investigate the saltwater intrusion in Eq. (11). The additional discharge, 288 m3/s (1728 1440 ¼ 288 m3/s), MDMW. Although a negative exponential relationship between could be supplied by the upstream reservoirs. These reservoirs had the freshwater discharge from upstream and saltwater intrusion been used in the EWAS in the PRE [35, 36]. length at both HWS and LWS is identified, this relationship may

Table 5. Input and output discharges of the designed wetlands around the MDMW

Wetlands Output discharge (m3/s) Input discharge SC (m3) Discharge Transfer (m3/s) (m3/s) tributaries HWS LWS

W2 81 0 17 5.58 106 –– W3 86 0 18 8.46 106 –– W4 205 0 43 6.84 106 181 T1 W5 196 145 41 7.38 106 139 T2 W6 212 191 44 9.00 106 98 T3 W7 258 223 54 5.76 106 54 T4

Table 6. Comparison of water allocation obtained from the network method and the EWAS in 2005

3 3 3 xL1 (km) Segments Q (m /s) V1 (network method) (m ) V2 (EWAS in 2005) (m ) F (%)

HWS LWS HWS LWS

6 6 40.2 xL1 > 30.6 X2X3, X2X4 1440 Q < 1750 3.01 10 0 5.58 10 46.1 – 6 6 30.6 xL1 > 22.0 X4X5 1750 Q < 2220 4.72 10 0 8.46 10 56.4 – 6 6 6 22.0 xL1 > 16.9 X5X6 2220 Q < 2600 6.59 10 2.61 10 6.84 10 48.4 61.8 6 6 6 16.9 xL1 > 12.7 X6X7 2600 Q < 3010 7.25 10 3.44 10 7.38 10 48.3 53.4 6 6 6 12.7 xL1 > 8.9 X7X8 3010 Q < 3510 8.66 10 4.01 10 9.00 10 48.4 55.4 Total 149.30 106 303.26 106 50.8

ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com 1044 Z. Zhang et al. Clean – Soil, Air, Water 2012, 40 (10), 1036–1046 underestimate the range of saltwater intrusion [27, 62]. The results the wetlands, tributaries, and the river channels work together to obtain from this model are calibrated using the contour map of the mitigate saltwater intrusion. saltwater intrusion, which is created by the interpolation method of Inverse Distance Weighting. Salinity can be converted to the contour 5.3 Management implications map of the saltwater intrusion using the ArcGIS 9.2 software. The calibrated regression coefficients are increased from 0.77 to 0.83 at The connectivity and storage functions of a wetland network are HWS and from 0.48 to 0.57 at LWS. The calibrated saltwater intrusion used to mitigate saltwater intrusion. Wetlands are important com- length is more accurate than the one obtained from the model. The ponents of estuaries that can be easily connected by river channels to calibrated length is then used to determine wetland attributes such form a wetland network. A wetland network has been applied to as discharge and storage capacity. decrease adverse effects on ecosystems through such means as watershed storm water management [39, 40] and region drought and flood control [38]. Replenishing freshwater stored in a 5.2 Mitigation of saltwater intrusion by wetland wetland network to river channels can mitigate saltwater intrusion. However, the connectivity of wetlands can be damaged due to urban network development and other anthropogenic activities. Regional develop- At HWS, the freshwater discharge at Wuzhou station was more than ment will lead to a change in channel morphology [71, 72]. The 1440 m3/s when the EWAS was implemented in 2005. Saltwater flows wetlands from water areas together with the existing river network into X2X3 and X2X4. The sluices near W2 and W3 are opened and are connected to each other to control the salinity level in water. freshwater is allocated into X2X3 and X2X4 on the rate of 81 and An estuary is located at the interface zone between terrestrial and 86 m3/s. These values are the minimum discharge necessary to pre- aquatic ecosystems. Sufficient freshwater is required to maintain its vent the saltwater from entering into X2X3 and X2X4. Similarly, ecological functions and local development. The basic function of other wetlands along the MDMW can divert freshwater from W4–W7 wetlands is to store water. The wetlands involved in a wetland on the rate of 205, 196, 212, and 258 m3/s, respectively. This fresh- network play an important role in the protection of an estuarine water will keep saltwater out of the corresponding segments on the ecosystem [5], which is at the risk of sharp shrink and functional basis of the relationship between the freshwater discharge and degradation [73, 74]. The wetland network method provides a better saltwater intrusion length. Finally, the salinity of water at S1 is less solution to protection of saltwater intrusion in the PRE. The similar than the threshold (0.5%). method can also be used for storm water management. The frame- The saltwater intrusion length at LWS is less than that at HWS, work of wetland network design presented in this paper is a good which indicates that replenish freshwater into river channels at example of the implementation of the ‘‘returning farmland to lake’’ LWS is the optimal option to prevent saltwater intrusion. The policy in China. saltwater intrusion length was approximately 17.8 km away from The existing and reconstructed wetlands and river channels are the mouth at LWS during the dry season in 2005. Therefore, the connected each other to form a wetland network, which is cost- saltwater can not reach the cross-section X5 (approximately 22.0 km effective and has significant ecological and environmental benefits, away from the mouth). Thus, W1, W2, W3, and W4 are not at work e.g., water purification, biodiversity protection, and water resource during LWS. Other wetlands (W5–W7) divert freshwater into conservation. The positive effects of the designed wetland network the MDMW on the rate of 145, 191, and 223 m3/s, respectively. on the surrounding ecosystems are identified in this paper. W1 is reserved for spring tides or storm surges, and if the saltwater intrusion is close to the cross-section X1, it will be used 6 Conclusions to divert water to the river channels. The output discharges of wetlands (W1–W7) are given in Tab. 4. A wetland network is designed by integrating existing wetlands and To ensure sufficient freshwater discharges from wetlands to river river channels from water areas to mitigate saltwater intrusion in channels during the dry season, the storage capacity of wetlands is the PRE. A methodology of the wetland network design is presented determined based on Eq. (9) with the given time (5 h/day) at HWS. In and well-tested in a river estuary (MDMW). This study focuses on this paper, it is considered that the discharge from wetlands to river the network design and the determination of the discharge and channels at LWS is less than that at HWS (see Tab. 4). However, the storage capacity of wetlands. The wetland network can effectively EWAS recommended by Chen and coworkers [33] Wen et al. [36] and mitigate saltwater intrusion when the calculated discharge is Han and Cheng [60] to mitigate saltwater intrusion in the Pearl River diverted from wetlands into river channels according to the relation- Basin, they did not consider this difference. The wetland network ship between the freshwater and the saltwater intrusion length method is water saving compared to the EWAS (see Tab. 6). Therefore, which follows an exponential law. The difference of water allocation diverting water into river channels at LWS will prolong the time of at HWS and LWS is considered in this paper. The saltwater intrusion replenishing water and save water. length at LWS is less than that at HWS, which indicated that replen- Replenishing freshwater to river channels is an effective mean for ish freshwater into river channels at LWS is the optimal option to mitigation of saltwater intrusion based on Eq. (18). Once the fresh- prevent saltwater intrusion. Compared with the EWAS implemented water discharge from upstream is sufficient, the saltwater will ebb in 2005 to prevent saltwater intrusion in the same area, the network from the MDMW. In this paper, the freshwater stored in the wetland method can save >50% more freshwater (varying from 46.1 to network is used to replenish the freshwater to the river channels 56.4% at HWS, and from 53.4 to 61.8% at LWS). The wetland network during dry season. Each wetland involved in the network replenishes is cost-effective and has social and ecological benefits. The design of freshwater to the adjacent river segment with the corresponding a wetland network facilitates water resource management in the discharge. The salinity of river water is then reduced to the prede- PRE and can also be easily generalized for application to other fined level (0.5%). Generally, all the components of the network, e.g., estuaries.

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