Seasonal variation in natural recharge of coastal aquifers

Pauline N. Mollema & Marco Antonellini

Abstract Many coastal zones around the world haveirregular precipitation throughout the year. This results indiscontinuous natural recharge of coastal aquifers, whichaffects the size of freshwater lenses present in sandydeposits. Temperature data for the period 1960–1990 fromLocClim (local climate estimator) and those obtained fromthe Intergovernmental Panel on Climate Change (IPCC)SRES A1b scenario for 2070–2100, have been used tocalculate the potential evapotranspiration with theThornthwaite method. Potential recharge (difference be-tween precipitation and potential evapotranspiration) wasdefined at 12 locations: Ameland (The Netherlands),Auckland and Wellington (New Zealand); Hong Kong(China); Ravenna (Italy), Mekong (Vietnam), Mumbai(India), New Jersey (USA), Nile Delta (Egypt), Kobe andTokyo (Japan), and Singapore. The influence of variable/discontinuous recharge on the size of freshwater lenseswas simulated with the SEAWAT model. The discrepancybetween models with continuous and with discontinuousrecharge is relatively small in areas where the total annualrecharge is low (258–616mm/year); but in places withMonsoon-dominated climate (e.g. Mumbai, with rechargeup to 1,686mm/year), the difference in freshwater-lensthickness between the discontinuous and the continuousmodel is larger (up to 5m) and thus important to considerin numerical models that estimate freshwater availability.

Keywords Climate change . Coastal zones . Groundwaterrecharge . Numerical modeling . Saltwater–freshwaterrelations

Introduction

Aquifer salinization is a problem in many low-lying deltasaround the world (Barlow and Reichard 2010; Custodio

2010; Post 2005; Simmons 2005; Werner 2010) and thisproblem worsens due to rising sea levels and changingweather patterns (Maas 2007; Oude Essink et al. 2010;Vandenbohede et al. 2008; Bobba 2002). In most settings,natural conditions including geometry, lithology, andhydraulic conductivity of the aquifer and amount ofnatural recharge, as well as anthropogenic factors suchas drainage, determine the extent of salinization of coastalaquifers and the size of the remaining freshwater bodies.Local climate conditions and rainfall distribution also turnout to be important factors in places where dry and wetperiods alternate.

The aims of this study are: (1) to establish a set ofrepresentative areas around the world with currentlyalternating dry and wet periods throughout the year; (2)to establish how these dry periods may change in thefuture; (3) to determine what the influence of alternatingwet and dry seasons is on the size of freshwater lenses incoastal environments; and (4) to assess whether it isjustified to use a continuous recharge boundary innumerical studies of saltwater intrusion in areas withperiods of low or no natural recharge.

Salt transport in porous media is driven by variousprocesses: diffusion driven by concentration gradients,advection driven by hydraulic gradients, buoyancy drivenby fluid density gradients and convection driven bytemperature gradients. How all these processes lead to acertain salt distribution in realistic geologic settings is stillthe subject of many studies (Langevin et al. 2005:Schneider and Kruse 2005; Vandenbohede et al. 2011).Given enough time, salt and freshwater become distinctlyseparated stratified water bodies. The position of theinterface between salt and freshwater along coastlines hasbeen studied since the 1900s when Badon-Ghyben andHerzberg came up with a physical formula used to thisday (Herzberg 1901; Reilly and Goodman 1985). Sincethen, many analytical solutions have been found forparticular cases (e.g. Maas 2007) and many numericalmodels have simulated the problems for specific sites(Langevin et al. 2005; Oude Essink et al. 2010; Schneiderand Kruse 2005; Vandenbohede et al. 2011; Bobba et al.2000). Spatially changing recharge depending on land usehas been taken into account by some (e.g. Oude Essink etal. 2010; Schneider and Kruse 2005; Vandenbohede et al.2008) with the aim to determine the size and shape offreshwater bodies. Most models assume a continuousrecharge throughout the year but some studies in

Received: 14 May 2012 /Accepted: 10 February 2013Published online: 12 March 2013

* Springer-Verlag Berlin Heidelberg 2013

P. N. Mollema ()) :M. AntonelliniI.G.R.G. (Integrated Geoscience Research Group),University of Bologna,Via San Alberto 163, 48100, Ravenna, Italye-mail: pmollema@gmail.comTel.: +39-0544-937318Fax: +39-0544937319

Hydrogeology Journal (2013) 21: 787–797 DOI 10.1007/s10040-013-0960-9

temperate regions take account of seasonal variations(Eeman et al. 2012; Oude Essink et al. 2010; Michael etal. 2005). In order to study the effect of discontinuousrecharge, including periods with no recharge, using otherfactors that may influence the size of freshwater lenses,numerical experiments with the SEAWAT code wereperformed; SEAWAT simulates variable density ground-water flow in saturated porous media.

This study is limited to 12 climate zones worldwidethat have one period during the year of very little or norecharge and that have a geology and geomorphologycontrolled by similar processes: low coastal plains, lowbeach-dune dynamics, and eustatic control on sedimenta-tion in the Holocene. These sites are Ameland (TheNetherlands), Auckland and Wellington (New Zealand),Hong Kong, Ravenna (Italy), Mekong (Vietnam), Mumbai(India) New Jersey coast (USA), Nile Delta (Egypt) Kobe(Japan), Tokyo (Japan) and Singapore.

The geology of the unconfined aquifers along low-lying coasts is essentially controlled by the local along-shore sediment dynamics and supply in the Holocene, bythe proximity to deltas and estuaries, by subsidence, bythe coastal tectonics, and by the sea-level changes fromthe last ice age to the present time (Hearty et al. 2007;Selley 2000; Stewart and Vita-Finzi 1999). Most of thelow-lying coasts considered were formed during and afterthe Holocene starting from the basal transgression(12,000 years ago) that culminated in the maximum seaingression 6,000 years ago and, as a result, they do havemany similarities (Allen 2003; Amorosi and Marchi 1999;Moura et al. 2007; Ronov 1994; Zinke et al. 2003). Thesesimilarities consists of a wedge of clastic deposits rangingfrom 10 to 50 m in thickness resting on pre-Holoceneclays or other older rock types; the wedge deposits aretypically well-sorted beach sand and dunes sand that nowform the coastal shallow aquifers (Amorosi and Marchi1999; Clemmensen et al. 2009) alternating with finersediments of deltaic or dune slack origin. Often in theseaquifers the presence of fossil Holocene transgression wateris encountered (Stuyfzand and Stuurman 2006;Vandenbohede et al. 2011). These coastal zones may havebarrier islands (New Zealand, The Netherlands, New Jerseycoast), recent dune belts along the coast (The Netherlands:Clemmensen et al. 2009) and/or older dune belts moreinland surrounded by lower-lying land (southern Adriaticcoast, Ravenna, Italy; Amorosi and Marchi 1999).

The model used in this study represents the Holoceneunconfined aquifers in each one of those situations: a1- km-wide sandy area with natural recharge surroundedby lower-lying saline groundwater or seawater. In partic-ular the model can be applied to the Quaternaryunconfined aquifers in the sandy beach dune deposits ofAmeland (Lammerts et al. 2001), the low-lying aquifersystem of the Okahukura Peninsula and Manukau low-lands (North Auckland, New Zealand; Crowcroft andSmaill 2001), the Waikanae, Rarangi and Te Horo beaches(Wellington, New Zealand; Ingham et al. 2006), theRavenna coast (Italy; Antonellini et al. 2008), the landreclaimed coastal zone of Hong Kong (Jiao et al. 2006),

the Mekong coastal floodplain (Vietnam; Benner et al.2008; Buschmann et al. 2008; Hanh and Furukawa 2007),the Mangrol-Chorwad coast (Mumbai, India; Desai et al.1979), the North African coast (Tunisia to Egypt; Kashef1983; Gaaloul and Cheng 2003), the New Jersey coastalplain (USA; Barlow 2003), the coasts of western Japan(Kobe) and Tokyo (Japan; Hiroshiro et al. 2006), andSingapore (Sien Lin et al. 1988).

It is acknowledged that a thorough quantification of theamount of freshwater in any location worldwide requiresmuch more local information then used in the simplemodels presented here. Therefore, it is emphasized that thepurpose of this study is to find out the influence of thedifferent climates, especially the occurrence of very dryperiods on freshwater lenses. At the same time, the studyseeks to verify whether it is justifiable to use a continuousrecharge boundary in saltwater-intrusion modeling studiesof areas with a variable climate. This can only be done byusing very simple models, keeping many variables fixed(e.g. geology, width of recharge area) and varying only theclimate and resulting recharge schemes and boundaryconditions in the models. Where this paper refers to placenames as ‘Wellington’ or ‘Mumbai’, it refers thereforerather more to ‘the climate of Wellington’ or ‘the climateof Mumbai’ than the precise setting of the aquifer.

Methods

Calculation of potential evapotranspirationand potential rechargeFor each of the 12 locations previously mentioned,average monthly rainfall data were obtained for the period1960–1990 with LocClim, a local climate estimator (FAO2002; Table 1). The average monthly temperature datafrom LocClim (FAO 2002) and those obtained from theSRES A1b scenario of the Intergovernmental Panel onClimate Change (IPCC 2007) have been used to calculatethe potential evapotranspiration with the Thornthwaite(1948) method adapted to the local latitude and averagemonthly hours of sunlight (Table 1). The potentialrecharge was defined as the difference between theprecipitation and potential evapotranspiration. When thedifference is less than zero, it is assumed that the potentialrecharge is zero. Although this definition of potentialrecharge is very simple (Healy 2010), it allows one tocompare the different climate scenarios for many placesaround the world for the present and future. Climate data(rainfall and temperature) for the period 2070–2100 at allaforementioned locations were extracted from IPCC SRESscenario A1b (Meehl et al. 2007). The future potentialrecharge is based on precipitation from climate models(IPCC 2007) and potential evapotranspiration calculatedwith the Thornthwaite (1948) formula and temperaturedata from coupled atmosphere-ocean global circulationmodels (GCMs; Meehl et al. 2007).

The models used for extracting rainfall and temperaturedata at the center point of the grid closest to the localityconsidered were those provided by IPCC (2007). The

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Hydrogeology Journal (2013) 21: 787–797 DOI 10.1007/s10040-013-0960-9

Tab

le1

Mon

thly

precipitatio

nandpo

tentialevapo

transpirationforthe19

60–199

0period

from

LocClim

(FAO20

02)andthoseob

tained

from

theSRESA1b

scenario

(IPCC20

07)forthe

2070–210

0period

Month

Location

Latitu

deJan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Potentialevapotranspiratio

n1960–1990[m

m/m

onth]LocClim

FAO

(200

2)Ameland

54°N

3.92

85.93

220

.12

41.51

80.81

105.9

115

103.1

69.94

43.35

18.29

7.64

7Auckland

36°S

104

89.96

85.04

59.62

43.83

32.14

29.64

34.31

42.68

57.3

73.26

92.46

Hon

gKon

g22

°N30

.63

28.68

51.66

86.81

140.1

168.8

190.8

178

153.2

117

68.16

40.86

Kob

e34

°N8.72

79.78

822

.06

48.44

85.76

118

164.5

161.3

116.9

69.63

35.62

16.61

Mekon

gDelta

10°N

124.5

116.1

156.1

180.9

163.9

159.9

166.3

145.6

143.7

148.2

150.2

121.5

Mum

bai

18°N

72.47

75.77

120.7

161.5

208.9

186.8

165.8

150.2

138.5

147.5

115.9

89.24

New

Jersey

35°N

11.15

12.28

28.16

52.17

91.8

128

154.9

146.1

109.2

69.22

36.14

17.97

Nile

Delta

31°N

23.94

24.3

40.7

62.24

106.1

133.9

162

154.9

116.5

93.3

59.94

32.36

Ravenna

44°N

4.72

59.73

125

.548

.19

85.84

117.1

140.5

127.3

87.78

51.63

22.71

7.85

4Singapore

1°N

130.4

124.6

137.9

135.3

141.8

137.2

134.1

134.1

129.8

132.3

128

130.4

Toky

oBay

35°N

11.37

11.1

22.13

50.67

84.7

109.5

141.8

149.5

106.1

66.27

37.83

19.38

Wellin

gton

41°S

102.9

84.06

77.96

51.01

32.86

21.05

20.39

27.08

40.41

58.49

74.28

93.8

Precipitatio

n19

60–1

990[m

m/m

onth]LocClim

FAO

(200

2)D

AmelandNL

54°N

66.5

45.2

57.5

48.2

57.7

68.8

76.3

66.4

70.5

68.6

7775

.5Auckland

36°S

90112

142

129

120

179

151

146

130

116

8092

Hon

gKon

g22

°N23

4867

162

317

376

324

391

300

145

3527

Ravenna

44°N

56.8

56.5

65.9

67.5

65.9

58.8

49.8

62.2

71.2

80.6

97.4

73.8

Kob

e34

°N56

6912

015

113

719

919

118

424

016

410

070

Mekon

gDelta

10°N

133

1149

168

190

220

200

270

252

161

49Mum

bai

18°N

00

01

1456

375

253

331

764

163

New

Jersey

35°N

134.6

104.5

109

89.6

101.5

104.4

126.5

152.3

133.8

126.5

126.1

115.3

Nile

Delta

31°N

1813

85

30

01

04

611

Singapore

1°N

219

119

210

228

181

133

104

165

190

269

307

270

Toky

oBay

35°N

61.9

80.8

139.6

154.6

171.5

265.3

201

219.5

213.8

165.6

102

56.5

Wellin

gton

41°S

4727

5464

5856

7170

4470

4354

Potentialevapotranspiratio

nSRESA1b

scenario

2070–210

0[m

m/m

onth](IPCC20

07)

Ameland

54°N

10.16

12.76

25.76

47.18

88.43

113.7

123.8

111.1

76.39

47.79

21.77

12.92

Auckland

36°S

115.2

99.73

93.95

65.37

47.85

35.1

32.45

37.51

46.58

62.56

80.28

101.9

Hon

gKon

g22

°N35

.56

33.29

62.2

110.3

187.4

231.8

265.8

246.6

210.1

155.2

85.44

48.58

Ravenna

44°N

20.93

21.54

31.05

45.97

80.77

129

182.4

158.2

95.18

61.48

35.73

22.79

Kob

e34

°N11.35

12.44

26.21

56.69

102.4

144.3

207.7

204.5

145.1

83.19

41.65

19.88

Mekon

gDelta

10°N

176.5

165.1

228.4

271.7

239.9

234

244

209.6

208.3

215.3

220.9

171.9

Mum

bai

18°N

94.59

100.5

167.7

234.3

312.9

275.2

238.5

213.8

197.1

212.7

162.8

120.2

New

Jersey

35°N

17.03

18.3

26.5

5899

.14

152

186.1

175.5

121.5

72.11

40.08

25.1

Nile

Delta

31°N

27.3

27.75

47.08

73.72

130.5

168.7

208.1

199

146.6

115.3

71.85

37.31

Singapore

1°N

170.2

163.7

181.2

178.1

186.9

180.9

175.6

175.6

170

172.9

167.3

170.2

Toky

oBay

35°N

14.66

14.31

26.77

59.42

100.4

126.2

167.3

178.6

128.8

78.45

44.35

23.34

Wellin

gton

41°S

111.7

91.17

84.36

55.02

35.7

23.36

22.81

29.83

43.8

63.1

80.17

101.5

Precipitatio

nSRESA1b

scenario

2070–210

0[m

m/m

onth](IPCC20

07)

AmelandNL

54°N

73.15

49.72

58.94

49.41

59.14

63.64

70.58

61.42

72.26

70.32

78.93

83.05

Auckland

36°S

91.8

114.2

139.9

127.1

118.2

170.1

143.5

138.7

128.1

114.3

78.8

93.84

Hon

gKon

g/Macau

22°N

21.85

45.6

6716

231

739

4.8

340.2

410.6

300

145

3525

.65

Ravenna

44°N

63.2

49.5

52.1

48.5

44.5

36.7

23.5

22.8

38.1

70.2

77.1

69.4

Kob

e34

°N56

6912

015

114

6.6

212.9

204.4

196.9

256.8

175.5

100

70Mekon

gDelta

10°N

11.7

2.7

10.45

46.55

159.6

209

242

220

283.5

264.6

169.1

44.1

Mum

bai

18°N

00

01.07

14.98

602.4

804.6

570.3

339.2

68.48

17.12

3.21

New

Jersey

35°N

152.1

118.1

109

89.6

101.5

107.5

130.3

156.9

133.8

126.5

126.1

130.3

Nile

Delta

31°N

14.76

10.66

6.56

4.1

2.46

00

0.82

03.28

4.92

9.02

Singapore

1°N

208.1

113.1

210

228

181

139.7

109.2

173.3

190

269

307

256.5

Toky

oBay

35°N

61.9

80.8

139.6

165.4

183.5

283.9

215.1

234.9

228.8

165.6

102

56.5

Wellin

gton

41°S

4727

5464

5856

7170

4470

4354

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Hydrogeology Journal (2013) 21: 787–797 DOI 10.1007/s10040-013-0960-9

monthly rainfall flux and air temperature anomalies withrespect to 2000 AD were extracted and averaged from anensemble of models for the 30-year period 2070–2100(IPCC 2007). In order to assess the effect of current andfuture recharge patterns on the size of freshwater lensessurrounded by saltwater, the development of freshwaterlenses was simulated, in all 12 locations under twopotential recharge schemes based on the climate data for1960–1990 and on the climate data from the IPCC SRESA1b.

Numerical flow and transport modelThe simulations described in this paper were performedwith SEAWAT (Langevin et al. 2007). This computerprogram is based on the groundwater flow codeMODFLOW (Harbaugh et al. 2000) and the solutetransport code MT3DMS (Zheng and Wang 1999)designed to simulate three-dimensional variable-densitygroundwater flow coupled with multi-species solute andheat transport using the finite difference solution to theflow and transport equations. The solver used is apreconditioned conjugate-gradient (PCG) solver. Fluiddensity can be calculated using concentrations from oneor more solute species. It is therefore able to simulatedensity driven flow and solute transport.

Model area, mesh and boundary conditionsThe two-dimensional model area consists of a 7-km-longaquifer slice, 10 m wide and 60 m deep (Fig. 1) of whichthere are 189 columns, 25 layers and 4,725 cells. The areaof recharge in the centre extends from x03,000 m to x04,000 m. Columns are 100 m wide except in the centrefrom X02,700 to X04,400 where columns are 12.4 mwide. The uppermost layers down to 30 m depth are 1.5-m-thick and the remaining layers are 6 m thick.

The model area is considerably larger than the zone ofrecharge to eliminate the effects of the boundaries andensure hydrostatic conditions at the edges of the model.The grid courant number was set to 0.75 to minimizenumerical oscillations. The aquifer hydraulic conductivityin all models is set to 6.9E-4 m/s representing beach, duneor river sand bodies deposited during the Holocenetransgression. The vertical conductivity is one third ofthe horizontal conductivity. The effective porosity is takento be 30 %, longitudinal dispersion 2 m, and thedispersivity ratios are Dx/Dy010 and Dx/Dz0100.

The vertical east and west boundaries as well as thelower boundary of the model are no flow boundaries. Theupper boundary in the centre of the model is a rechargeboundary, either a continuous recharge or a discontinuousrecharge, with four stress periods for each simulated year.On the two sides of the recharge area, the upper boundaryis defined by a constant head (0 m) boundary and aconstant concentration (30 g/l) boundary.

The initial salt concentration in all models is homog-enous and equal to 30 g/l, representing a saline environ-ment of either the sea, lagoons, or a low-lying

mechanically drained area (polder) with saline groundwa-ter; these can be mixed with some freshwater either fromrivers or rainfall.

In the models with a continuous recharge boundary, theamount of recharge is in fact the potential annual recharge,based on precipitation and potential evapotranspirationvalues averaged over the whole period (1960–1990 or2070–2100). In the discontinuous model, each modeledyear is divided into four seasons, the actual divisiondepending on each site, so that the dry months aregrouped together (Table 3). This results in four stressperiods for each year. The simulated time is 50,005 days.Three simulations with an artificial climate were run to beable to compare better the effect of continuous anddiscontinuous recharge on the size of the freshwater lens.These models have a continuous recharge of 325, 407 and1,686 mm/year to be able to compare with models ofAmeland, New Jersey and Mumbai that simulate discon-tinuous recharge (Fig. 2b). In this way the models with an‘artificial’ continuous recharge together with the modelswith a ‘real’ continuous recharge (Singapore 1,167 mm,Kobe 616 mm, Tokyo 889 and 955 mm) cover the wholerange of annual recharge modeled: from 0 to1,686 mm/year.

The thickness of the fresh–brackish water lens in theoutcome of the numerical experiments was defined as thedepth of the iso-salinity concentration contour of 2 g/l inthe middle of the recharge area. Water with a saltconcentration of 2 g/l is brackish water according to some(Stuyfzand 1989) but in areas of low annual recharge suchas the Mediterranean where the fresh-brackish waterlenses along the coast mostly support the (natural)vegetation, many plant species survive well in water withsalinity up to 3 g/l (Antonellini and Mollema 2010).

Results and discussion

According to the historical climate data and climatemodels, Mumbai has the highest annual potential rechargein the 1960–1990 period (1,686 mm) and also in the futurescenario (1,392 mm, Fig. 3). The lowest potential rechargeof 0 mm now and in the future occurs in the Nile Delta.The most dramatic reduction of potential annual rechargeby the end of this century will occur at lower latitudes(Mumbai, Singapore, Hong Kong and Mekong), which isdue to a combination of reduced rainfall and increasingtemperatures and evapotranspiration rates. The mostpronounced change in length of the dry period occursfor Kobe (Japan) where it goes from zero months with norecharge up to two dry months in a row, and forSingapore, where it goes from zero dry months to fourdry months in a row, thus changing the recharge from acontinuous to a discontinuous pattern, even though thetotal annual amount of recharge remains practically thesame (Table 2).

The numerical simulations show that, in general, thethickness of coastal freshwater lenses increases with totalannual potential recharge, as expected. The depth of the

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fresh–saltwater interface ranges from 3.8 m in Wellingtonto 25.0 m in Mumbai (Fig. 2a, Table 3). The freshwaterlens becomes thicker when the recharge is appliedcontinuously, rather than discontinuously (Fig. 2b); com-pare for example Singapore and Hong Kong for the 1960–1990 scenario (Fig. 2a). Although Singapore has only aslightly higher annual potential recharge (1,167 mm/year)than Hongkong (1,114 mm/year), the fact that it is

distributed more evenly throughout the year causes theresulting freshwater lens to be much thicker. Similarly, thefreshwater lens of Kobe under the current continuousrecharge of 615 mm/year is much larger (17.4 m) than thefuture freshwater lens under a future discontinuousrecharge of 616 mm/year (14.6 m). In Fig. 2b, thedifference between the depth of freshwater lenses withcontinuous recharge or discontinuous recharge is

Fig. 1 Total potential annual recharge, defined as annual precipitationminus annual potential evapotranspiration, as a function of latitude, for the 12locations studied. Both the reference period 1960–1990—LocClim (FAO 2002) and the IPCC SRES A1b scenario 2070–2100 are indicated

Fig. 2 Model grid and boundary conditions

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Hydrogeology Journal (2013) 21: 787–797 DOI 10.1007/s10040-013-0960-9

illustrated with some artificial climate schemes. Thediscrepancy between continuous and discontinuous re-charge in terms of change in lens thickness [m] is smallerin areas where the total annual recharge is lower as in

Ameland (difference of 2.6 m) and larger in places with aMonsoon-dominated climate such as Mumbai (4.7 mdifference, Fig. 2b). The difference between the resultsof the continuous and discontinuous models can be

Fig. 3 Results of the numerical simulations. a The depth to the 2-g/l contour line of the fresh–brackish water lens is shown for the 12locations for the 1960–1990 (LocClim FAO 2002) data and those obtained from the SRES A1b scenario (IPCC 2007) for the period 2070–2100. b Discontinuous and continuous models with similar annual recharge. A few simulation results are added to represent artificialclimates with continuous recharge

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explained by the fact that under discontinuous recharge,the mixing zone between saline and freshwater becomeslarger than under continuous recharge. This is in agree-ment with observations made by Eeman et al. (2012) ondetailed models of mixing zones. In many of the studyareas (Ameland, Mekong, Mumbai, Hong Kong andRavenna), the future total annual recharge is less thantoday’s. Consequently, the thickness of the freshwater lensat these localities will also be smaller: 1.2 m difference forAmeland, 5.2 m difference for Hong Kong, 10.8 mdifference for Mekong, 3 m difference for Mumbai, and1.2 m for Ravenna (Fig. 2a; Table 3).

In view of climate change, an increasing use ofgroundwater reserves is foreseen by the IPCC studies(Kundzewicz et al. 2008) since water demand willincrease, and more variability in precipitation and runoffwill occur. At the same time, in large parts of the world,groundwater recharge is thought to decrease, for examplein large parts in South America and the southMediterranean (Kundzewicz et al. 2008); however, it isrecognized that even current groundwater levels andgroundwater recharge are not well known. Other studiesforesee a reduction of freshwater lenses in specific placesalthough in some cases future sea-level rise is consideredas the only cause for the reduction in size of the waterlenses. For example, while the freshwater lenses on smallislands in the Laccadive Sea, India, are thought todecrease from 25 to 10 m by an increase of 0.1 m of sealevel (Bobba et al. 2000), the modeled freshwater lensunder a similar climate (Mumbai) is decreasing from 25 to22 m because of the change in recharge only; thus, the twoeffects, sea-level rise and changing recharge patterns, willboth reduce the size of freshwater lenses. The freshwaterlens on the eastern part of Ameland where the width of theisland is more or less as wide as the modeled rechargezone in this study, the freshwater lens reaches a depth of−5 to −10 m relative to mean sea level (MASL) where theinterface is defined by the 1 g/l iso concentration contour(Pauw et al. 2012). This is very similar to the modeledthickness of 10 m under the LocClim (FAO 2002)

scenario. Numerical models of the island of Texel (theNetherlands), with a similar setting as the study siteAmeland, show that the polder areas on the island will beaffected strongly by sea-level rise that will enhance seepageof saline water, but the freshwater in the dunes will be lessaffected by sea-level rise (Pauw et al. 2012); however, inthese models, changing recharge patterns are not taken intoaccount. Faneca Sanchez et al. (2012) also concluded, basedon modeling of the North Netherlands, that in the future,groundwater levels will vary considerably throughout theyear because of increasing rainfall in winter and this will leadto an increasing salt load. Several studies on the southern PoDelta in Italy, the region of the study site Ravenna, show thatmeasured salinity of the groundwater is very high and thatthe freshwater lenses are very thin when they occur(Antonellini et al. 2008; Antonellini and Mollema 2010;Mollema et al. 2013). This is because most of the land issubsiding and drainage is needed to prevent flooding(Giambastiani et al. 2007). Water-budget studies show thathardly any freshwater reaches the water table, not evenleaving much for pine tree evapotranspiration in the sandydunes (Mollema et al. 2012;Mollema et al. 2013). At the fewplaces that are not mechanically drained, at the mouth of theRiver Bevano for example, there are freshwater lenses whosesize is comparable to the modeled freshwater lens(Antonellini et al. 2010).

In New Jersey, Auckland and Wellington, it is thoughtthat the annual recharge will increase in the future and thenumber of dry consecutive months will supposedlydecrease, which will cause the freshwater lenses toincrease in thickness. Similarly, the freshwater lenses inTokyo will also increase since the annual recharge isgoing to increase and the recharge pattern will remaincontinuous (Table 3; Fig. 2a). The Nile Delta will receiveno potential recharge now or in the future and so rain-fedfreshwater lenses are unlikely to exist at all in the NileDelta. This is confirmed by observations of Sherif (2003),which suggest that freshwater in the Nile Delta is onlypresent thanks to infiltration of irrigation water throughditches and rice fields.

Table 2 Summary climate data: annual potential recharge [mm] and number of consecutive dry months for the period 1960–1990 (Loc-Clim; FAO 2002) and the IPCC SRES A1b scenario for the period 2070–2100 (Thornthwaite 1948)

Annual potential recharge1960–1990 [mm] LocClim(FAO 2002)

Annual potential recharge2070–2100 [mm] SRESA1b scenario (IPCC 2007)

Number of consecutive monthswith no potential recharge1960–1990 LocClim (FAO 2002)

Number of consecutivemonths with no potentialrecharge 2070–2100SRES A1b scenario(IPCC 2007)

Ameland NL 325 285 4 5Auckland NZ 615 673 3 3Hong Kong 1114 690 5 4Ravenna IT 285 191 6 5Kobe 616 615 0 2Mekong Delta 745 135 5 9Mumbai 1,686 1,492 8 8New Jersey 407 609 4 3Nile Delta 0 0 12 12Singapore 1,167 459 0 4Tokyo 889 955 0 0Wellington 77 159 8 5

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Hydrogeology Journal (2013) 21: 787–797 DOI 10.1007/s10040-013-0960-9

Tab

le3

Mod

elinpu

trecharge,division

oftheseason

s.Notethatvalues

areallinmm/yeareven

thou

ghtherecharge

isapplieddu

ring

partof

theyear.T

heresulting

thickn

essof

thefresh–

brackish

water

lens

ordepthto

2-g/lsalin

itycontou

rlin

ein

themiddleof

therecharge

area

isalso

repo

rted

Mod

elDiscontinuo

us(D

isc)

orcontinuo

us(Con

t)recharge

Season1

recharge

[mm/year]

Season2

recharge

[mm/year]

Season3

recharge

[mm/year]

Season4

recharge

[mm/year]

Ann

ual

recharge

[mm]

Depth

to2-g/l

isosalinity

contou

r[m

]

Divisionof

season

sa

AmelandNL

Disc

765

270

026

132

510

ndj-fm

a-mjj-aso

A1b

:AmelandNL

Disc

540

90

594

285

8.5

ndj-fm

a-mjj-aso

AucklandNZ

Disc

048

71,21

575

361

514

.7nd

j-fm

a-mjj-aso

A1b

:AucklandNZ

Disc

050

11,25

593

067

315

.3nd

j-fm

a-mjj-aso

Hon

gKon

gDisc

02,05

62,36

249

1,114

19.8

jfm-amj-jas-on

dA1b

:Hon

gKon

gDisc

069

1,38

11,30

269

014

.6jfm-amj-jas-on

dKob

eCon

t61

661

661

661

661

617

.4NA

A1B

:Kob

eJP

Disc

791

831

4679

661

514

.6jfm-amj-jas-on

dMekon

gDelta

Disc

020

71,17

71,58

874

515

.8djf-mam

-jja-son

A1b

:Mekon

gDisc

00

053

513

55.0

ndj-fm

a-mjj-aso

Mum

bai

Disc

00

5,90

479

51,68

625

.0djf-mam

-jja-son

A1b

:Mum

bai

Disc

00

4,95

957

01,49

222

.0djf-mam

-jja-son

New

Jersey

USA

Disc

813

0117

721

407

11.0

jfm-amj-jas-on

dA1b

:New

Jersey

USA

Disc

340

116

015

360

914

.1djf-mam

-jja-son

Nile

Delta

Con

t0

00

00

0NA

A1b

:Nile

Delta

Con

t0

00

00

0NA

Ravenna

ITDisc

389

00

634

258

7.8

jfm-amj-jas-on

dA1b

:Ravenna

ITDisc

370

100

384

191

6.6

jfm-amj-jas-on

dSingapo

reCon

t1,16

71,16

71,16

71,16

71,16

724

.9NA

A1b

:Singapo

reDisc

503

312

01,02

645

911.8

djf-mam

-jja-son

Tok

yoBay

Con

t88

988

988

988

988

921

.8NA

A1b

:Tok

yoCon

t95

595

595

595

595

522

.3NA

Wellin

gton

NZ

Disc

020

284

077

3.8

djf-mam

-jja-son

A1b

:Wellin

gton

NZ

Disc

037

409

188

159

6.1

ndj-fm

a-mjj-aso

and

jNov

ember,Decem

ber,Janu

ary,

fmaFebruary,

March,April,

mjjMay,June,July,asoAug

ust,September,Octob

er;no

tethat

consecutive-mon

thgrou

ping

scanvary

NAno

tapplicable

794

Hydrogeology Journal (2013) 21: 787–797 DOI 10.1007/s10040-013-0960-9

Under the IPCC A1b climate scenario, only Tokyo andSingapore appear to change from a continuous to adiscontinuous recharge regime, while in the other loca-tions only the amount of annual recharge will change, inmost cases reducing the size of the freshwater lenses. Insettings with lower latitudes such as Mumbai, MekongDelta, and Hong Kong, this change will be more dramaticwith greater loss of freshwater.

This simplified model is designed to capture the effectsof discontinuous recharge on an idealized aquifer. In manycoastal low-lying areas the actual size of the freshwaterbodies may be smaller than expected on the basis ofnatural recharge alone due, for example, to drainage and/or subsidence (Giambastiani et al. 2007; Oude Essink etal. 2010; Antonellini and Mollema 2010; Mollema et al.2013). Upwelling of saltwater (seepage) creates a largemixing zone, thereby also reducing the actual size offreshwater bodies (Eeman et al. 2012; La Licata et al.2011). The freshwater lenses, on the other hand, may alsobe larger than expected based on natural recharge alonebecause of irrigation surplus infiltration (Greggio et al.2012; Sherif 2003). Other factors that may influence thesize of freshwater lenses in coastal zones, besides achange in the recharge pattern, are the presence of lowpermeability layers (Post and Simmons 2010) and highersea levels (Meehl et al. 2007; Oude Essink et al. 2010;Maas 2007; Pauw et al. 2012).

The definition of potential recharge that is used in thispaper is fairly simple. The calculation of the potentialevapotranspiration with the Thornthwaite method is basedon standard climate variables only. This study does not takeinto account an exact water balance of the soil, whichshould include evaporation from the soil, evaporation andtranspiration from possible vegetation, evaporation fromthe water table if it is shallow, processes in the unsaturatedzone such as capillary rise, changes in soil moisture storageand more (Carrera-Hernández et al. 2012; Healy 2010).

Perhaps future studies will show that one needs to lookat daily or even hourly water balances to have a correctidea of natural recharge. The numbers of rainstormsand their intensity are very difficult to predict. Theseparameters not only have an immediate influence onthe soil moisture and irrigation needs, as noted byMollema et al. (2012), but also influence the amountof water that eventually reaches the water table.Historical data suggest that the number of rainy dayshas been decreasing for example in Italy and in Europe(Brunetti et al. 2006; Klein Tank et al. 2002; Marletto2010), which may not be a negative factor in areaswith warm (Mediterranean) climates: perhaps a fewheavy rainstorms during the summer cause more waterto infiltrate into the soil and underlying aquifer thanmany light storms where the water evaporates beforehitting the ground (Mollema et al. 2012)

The future climate data are based on models thatinclude uncertainties. However, all the simplificationswere needed to be able to compare the influence of manydifferent climate scenarios. Hopefully, this study willcontribute towards a better understanding of the

relationship between climate, groundwater recharge andfresh groundwater availability.

Conclusions

Many places around the world have irregular precipitationthroughout the year. This results in discontinuous naturalrecharge of coastal aquifers, which affects the size offreshwater bodies in coastal zones. Temperature andprecipitation data for the period 1960–1990 from LocClim(FAO 2002) and those obtained from the IPCC SRES A1bscenario have been used to calculate the potential evapo-transpiration with the Thornthwaite (1948) method, both forthe period 1960–1990 and for the year 2100, for 12 sitesaround the world. Potential recharge was defined as thedifference between the precipitation and potential evapo-transpiration on a monthly basis. Nine out of the 12 sites—Ameland (The Netherlands), Auckland and Wellington(New Zealand); Hong Kong; Ravenna (Italy), Mekong(Vietnam); Mumbai (India), New Jersey coast (USA); NileDelta (Egypt)—have a period in which the potential rechargeis small or zero and so have a discontinuous natural rechargeof the aquifer; Kobe and Tokyo (Japan) and Singapore do notexperience months without recharge and so have continuouspotential recharge throughout the year.

Mumbai has the highest annual potential recharge in the1960–1990 period (1,686 mm) and also in the future A1bscenario for the year 2100 (1,392 mm). The lowest potentialrecharge of 0 mm now and in the future occurs in the NileDelta. The most dramatic reduction of potential annualrecharge by the end of this century will occur at lowerlatitudes (Mumbai, Singapore, Hong Kong and Mekong).The most pronounced change in length of dry period occursfor Kobe (Japan) where it goes from zero months with nopotential recharge up to two dry months in a row, and forSingapore where it goes from zero dry months to four drymonths in a row, even though the total annual amount ofrecharge remains practically the same. The numericalmodels show that higher annual recharge rates producethicker freshwater lenses. Models where the recharge isapplied continuously throughout the year result in thickerfreshwater lenses than models with the same amount ofpotential recharge applied discontinuously. The differencein freshwater quantity under continuous and discontin-uous recharge is relatively small in areas where the totalannual recharge is low (Wellington, Ravenna,Ameland); however, in places with Monsoon-dominatedclimate such as Mumbai, the difference between thediscontinuous and the continuous model is large.Therefore, this study shows that the alternation of dryand wet periods, especially in areas with high annualprecipitation, should be taken into account to be able toestimate the quantity of freshwater in coastal aquifers.

Acknowledgements This study was funded by CIRCLE-MED forthe Waterknow project and by the Regione Emilia Romagna (Italy).Thanks go to Tanja Zegers and Jan Kees Blom for reviews of anearlier draft of this paper.

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