Journal of Hydrology 403 (2011) 14–24

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Journal of Hydrology

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Modern recharge to fossil aquifers: Geochemical, geophysical,and modeling constraints

M. Sultan a,⇑, S. Metwally b, A. Milewski a, D. Becker a, M. Ahmed a, W. Sauck a, F. Soliman c,N. Sturchio d, E. Yan e , M. Rashed c, A. Wagdy f, R. Becker g, B. Welton a

a Department of Geosciences, Western Michigan University, Kalamazoo, MI, USAb Desert Research Center, El Matariya, Cairo, Egyptc Suez Canal University, Department of Geology, Ismalia, Egyptd Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL, USAe Environmental Science Division, Argonne National Laboratory, Argonne, IL, USAf Irrigations & Hydraulics Engineering Department, Cairo University, Giza, Egyptg University of Toledo, Department of Environmental Sciences, Toledo, OH, USA

a r t i c l e i n f o

Article history:Received 21 June 2010Received in revised form 4 February 2011Accepted 22 March 2011Available online 3 April 2011This manuscript was handled by L. Charlet,Editor-in-Chief, with the assistance ofPhilippe Négrel Associate Editor

Keywords:Sinai PeninsulaRechargeStable IsotopeSWATNubian AquiferGeophysics

0022-1694/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jhydrol.2011.03.036

⇑ Corresponding author. Address: Department of GeUniversity, 1903 W. Michigan Avenue, Kalamazoo, MI5487 (office), +1 269 387 5451, +1 269 387 5446 (lab

E-mail address: mohamed.sultan@wmich.edu (M.

a b s t r a c t

The Nubian Sandstone (NSS) aquifer of northeast Africa is believed to have been recharged in previouswet climatic periods in the Quaternary Period. While this is largely true, we show using the Sinai Penin-sula as our test site that the aquifer is locally receiving modern recharge under the current dry climaticconditions. The validity of the advocated model was tested using geophysical (conventional electricalresistivity [ER]) and isotopic (O, H) data, and estimates for modern recharge were obtained using contin-uous rainfall-runoff modeling over the period 1998–2007. Interpretations of ER profiles are consistentwith the presence of unconfined NSS aquifers flooring recharge areas at the foothills of the crystallinebasement in Sinai at Baraga (thickness: 20 to >188 m; resistivity: 16–130 X m) and Zalaga (thickness:27 to >115 m; resistivity: 3–202 X m). The isotopic composition (dD: �22.7 to �32.8‰; d18O: �4.47to �5.22‰) of groundwater samples from wells tapping the NSS aquifer underlying recharge areas is con-sistent with mixing between two endmembers: (1) fossil groundwater with isotopic compositions similarto those of the Western Desert NSS aquifer (dD: �72 to �81‰; d18O: �10.6 to �11.9‰), and (2) averagemodern meteoric precipitation (dD: �9.84‰; d18O: �3.48‰) in Sinai, with the latter endmember beingthe dominant component. A first-order estimate for the average annual modern recharge for the NSSaquifer was assessed at �13.0 � 106 m3/yr using the SWAT (Soil Water Assessment Tool) model. Findingsbear on the sustainable exploitation of the NSS aquifer, where the aquifer is being locally recharged, andon the exploitation of similar extensive aquifers that were largely recharged in previous wet climaticperiods but are still receiving modest modern meteoric contributions.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The deserts of Egypt, namely the Eastern Desert (ED), theWestern Desert (WD), and the deserts of the Sinai Peninsula (SP)(Fig. 1), are among the most arid deserts in the world (WD receiv-ing <5 mm/yr; ED: 25 mm/yr; SP: 40 mm/yr); however, the geo-logic evidence indicates that climate alternated between arid andwet periods throughout the Quaternary Period, with the last ofthe major wet periods occurring in the Holocene (9500–4500 yrBP). Beneath the surface of the deserts of Egypt and adjacent

ll rights reserved.

osciences, Western Michigan49008, USA. Tel.: +1 269 387); fax: +1 269 387 5513.Sultan).

portions of eastern Libya, northeastern Chad, and northwesternSudan (Fig. 1) lies an immense reservoir (>780,000 km3) of fresh-water in the Nubian Sandstone (NSS) aquifer system (Thorweihe,1990). The aquifer consists mainly of continental sandstones withintercalated shales of shallow marine and deltaic origin, uncon-formably overlying Proterozoic basement, and reaching a thicknessapproaching 3 km in the center of the basin (Hesse et al., 1987).The NSS aquifer is believed to contain fossil groundwater thatwas recharged in previous wet climatic periods by intensificationof paleomonsoons (Sarnthein et al., 1981; Prell and Kutzbach,1987; Yan and petit-Maire, 1994) or paleo-westerlies (Sultanet al., 1997; Sturchio et al., 2004).

The progressive increase in Krypton-81 and Chlorine-36 agesfor groundwater from the southwest to the northeast parts of theWD of Egypt was interpreted to indicate that local rechargethrough intensified regional precipitation over the extensive NSS

Fig. 1. (a) Location map showing the distribution of Neoproterozoic outcrops, the overlying Phanerozoic rock units, and the Upper Jurassic to Lower Cretaceous MalhaFormation outcrops (primary recharge areas for the NSS aquifer) in the Sinai Peninsula (SP). Also shown are the locations of our groundwater samples (solid red triangles), andgeoelectric cross sections in Wadi Baraga (Box a) and Wadi Zalaga (Box b). Inset shows the study area (Box c); the areal extent of the NSS aquifer in Egypt, Sudan, Libya, andChad; and the distribution of deserts in Egypt, namely, the Western Desert (WD: west of the River Nile), the Eastern Desert (ED: east of the River Nile), and the Sinai Peninsula(SP: subtended by the Gulfs of Suez and Aqaba) Desert. (b) N–S trending cross section along line B–B0 plotted on Fig. 1a.

M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24 15

aquifer outcrops in southern Egypt and northern Sudan in theprevious wet climatic periods must have played a major role inthe recharge of the NSS aquifer (Sturchio et al., 2004). These

observations are at odds with earlier models that advocate thatrecharge was accomplished mainly by precipitation over moredistant mountains in Chad (e.g., Ball, 1927; Sanford, 1935).

16 M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24

In this manuscript we advocate that in dry climatic periods sim-ilar to that of today, the NSS aquifer in the WD of Egypt (Fig. 1) isreceiving no local recharge because there is negligible precipita-tion. That is apparently not the case in the ED and even less so inthe SP. In both areas, precipitation over the mountains is channeleddownstream over the NSS outcropping at the foothills of the RedSea Hills, providing ample opportunities for groundwater recharge.

In this manuscript we apply an integrated approach to test thevalidity of the advocated model in the SP. Specifically, we set out toaccomplish the following: (1) examine the areal extent and distri-bution at depth of NSS groundwater in recharge areas at the foot-hills of the basement complex, using geophysical investigations;(2) investigate the origin of, and modern contributions to, thegroundwater in the NSS by comparing the isotopic compositionsof groundwater from recharge areas to those of modern precipita-tion and fossil precipitation; and (3) estimate to the first order themagnitude of modern recharge, applying continuous rainfall runoffmodels.

2. Site description

Two main groups of rock units crop out in the SP: (1) Neoprote-rozoic (550–900 Ma) volcano-sedimentary basement rocks insouthern Sinai that are part of the Arabian–Nubian Shield Massif(Sultan et al., 1988; Stern and Kroner, 1993), and (2) overlyingthick Phanerozoic cover (Fig. 1). The major stratigraphic units inthe SP and their distribution at depth along a N–S-trending crosssection (B–B0) are shown in Fig. 1b. The basement complex isunconformably overlain by Upper Jurassic to Lower Cretaceous flu-viatile sandstone and conglomerate that belongs to the Malha For-mation of the Nubian Sandstone Group. This formation crops out atthe foothills of the basement complex, providing ample opportuni-ties for groundwater recharge for the NSS aquifer. The Phanerozoicsection is largely formed of Upper Cretaceous marine sandstone ofthe Matulla Formation, overlain by thick Tertiary limestonebelonging to the Thebes Group. During the Oligocene, tectonicmovement led to the elevation of the Arabo-Nubian Massif; inthe Miocene the SP landscape was shaped during a period of in-tense erosion, giving rise to numerous valley networks that werecarved into the elevated Red Sea hills (Said, 1993).

3. Modern recharge of the NA: geophysical constraints

Conventional Electrical Resistivity (ER) investigations were con-ducted to investigate whether groundwater accumulations are tobe found within the presumed NSS aquifer recharge areas at thefoothills of the basement complex in Central Sinai. Two areas wereselected: Wadi Baraga (Box a) and Wadi Zalaga (Box b) in Fig. 1. Weused vertical electrical soundings (VESs) with expanding electrodespacing (Schlumberger, 4 Array) for horizontal profiling along tran-sects using maximum current electrode separations (AB) of1400 m. ABEM SAS1000 was used for data collection during all ofthe field work. A total of 20 VESs were carried out in Wadi Baragaand its surroundings, and 14 VESs were carried out in Wadi Zalaga(Fig. 2). Topographic surveys were carried out with the purpose ofdetermining the location and ground elevation of the sounding sta-tions using a Magellan ProMark GPS unit. The RESIST (1988) andRESIX-IP (1988) software packages were used for quantitativeinversion of the obtained VESs’ measurements and for the estima-tion of apparent resistivity (rho: q) and thickness (h) values. Theestimated root mean square (RMS) errors for the 34 VESs acquiredranged from 2 to 6, indicating a good fit for the generated models.The geoelectrical cross sections shown in Fig. 3 were generatedfrom, and/or constrained by the following: (1) the modeled layersand their apparent resistivity and thickness values; (2) lithologic

and structural information (e.g., layer thickness and composition,faults, folds) extracted from field observations and/or bore holedata (5 wells); (3) published resistivity values (e.g., Sultan et al.,2009) for lithologies reported from field and/or bore hole data;and (4) measured depth to water table (e.g., B.W.1: 65 m, Z.W.1:2 m, and Z.W.3: 14 m; Fig. 2). Fig. 3c shows two examples, onefrom Wadi Baraga (VES B4) and the other from Wadi Zalaga (VESZ4), for modeling layer depth, thickness, and apparent resistivity,and the RMS values for the model.

In the Wadi Baraga area, two geoelectrical cross sections weregenerated, one trending N–S and the other trending NW–SE(Fig. 2a). Along these two cross sections, six geoelectrical layerswere recognized (Fig. 3a and b). These geoelectrical layers wereinterpreted as representing three lithologic sub-units in thesetwo geoelectrical cross sections. The uppermost unit was inter-preted as a highly resistive layer representing dry wadi fill deposits(q: 110–4896 X m) having thicknesses (h) ranging from 5 to 47 m.The upper highly resistive units are underlain by the water-bearingunit, the NSS. The saturated NSS unit has lower resistivity values(q: 16–130 X m) and ranges in thickness (h) from 20 to >188 m.The NSS is underlain by basement rocks of high-resistivity(q: 210–13432 X m) with varying depths ranging from 3 to122 m below ground surface.

In the Wadi Zalaga area (Fig. 2b), two geoelectrical cross sec-tions were generated, one trending N–S and the other NW–SE(Fig. 3d and e). Along these two cross sections, six geoelectrical lay-ers were observed. These layers were interpreted as representingthree lithologic sub-units. The uppermost unit was interpreted ashighly resistive layers representing dry wadi fill deposits(q: 384–2298 X m) having thicknesses (h) ranging from 10 to100 m. The upper highly resistive units are underlain by thewater-bearing NSS with lower resistivity values (q: 3–202 X m)and variable thickness (h) from 27 to >115 m. The saturated NSSunit is underlain by high-resistivity (q: 246–1732 X m) fracturedbasement rocks at depths ranging from 38 to 194 m below groundsurface.

The reported ER investigations support the suggestion thatgroundwater accumulations are found within the NSS aquifer re-charge areas at the foothills of the basement complex. Geochemicalmethods were then applied to address the question of whether thegroundwater in question is receiving contributions from modernprecipitation.

4. Modern recharge of the NA: geochemical constraints

Insights into the origin of the groundwater and the timing of itsrecharge were gained from analysis of its isotopic composition.Groundwater samples were collected in high-density polyethylenebottles, which were then tightly capped. Stable H and O isotope ra-tios were measured by standard methods of equilibration with H2

and CO2, respectively (Coplen, 1996; Nelson, 2000). Hydrogen andO isotope data are reported in terms of the conventional delta (d)notation, in units of per mil (‰) deviation relative to a standardreference, whereby

d;‰ ¼ ½ðRsample=RstandardÞ � 1� � 103

and R = 2H/1H or 18O/16O and the standard is Vienna Standard MeanOcean Water (Coplen, 1996). Precision of d2H values is ± 1.3‰ andthat of d18O values is ±0.2‰.

Fourteen groundwater samples were collected for isotopic anal-yses of H and O in H2O (Table 1) from open and productive wellstapping three types of aquifers: (1) fractured basement (samples:Bir Haroun, Bir Zeituna, Bir Halwagy, Bir Sahab, Bir Al Gufa, Bir AlSidra, Bir Nadia); (2) NSS unconfined aquifer cropping out at thefoothills of the basement outcrops (samples: Bir Zalaga 1, Bir

Fig. 2. False color Landsat TM image bands 2 (blue), 4 (green), and 7 (red) showing the distribution of VES locations (solid green triangles), well locations (open red circles)and geoelectric cross sections (blue and red lines). (a) Wadi Baraga area (Box a, Fig. 1). (b) Wadi Zalaga area (Box b, Fig. 1).

M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24 17

Zalaga 2, Bir Safra), areas that are considered here as potentialrecharge areas for the NSS aquifer; and (3) alluvial aquifers (Bir28, Bir Makateb, Bir 5 Makateb), all of which are believed to befed by fractured basement aquifer discharge into overlying alluvialsediments.

The stable isotope ratios of H and O are given in Table 1 and areshown in Fig. 4, which includes data for samples from the presentstudy, as well as: (1) paleowaters from the NSS aquifers in the WDof Egypt and the Gulf of Suez (Sturchio et al., 1996); (2) NSS aquifergroundwater samples from the ED (Sultan et al., 2007); and (3)data for modern precipitation from Al Arish and Rafah (Fig. 1)(IAEA and WISER, 2008).

Examination of Fig. 4 and Table 1 shows that the hydrogen (dD)and oxygen (d18O) isotopic compositions of the groundwater sam-ples collected from the unconfined NSS aquifers in the rechargeareas cropping out at the foothills of the basement outcrops aresomewhat depleted (dD: �22.7 to �32.8‰; d18O: �4.47 to�5.22‰) compared to those collected from fractured basementoutcrops (dD: �19.9 to �23.2‰; d18O: �3.77 to �5.05‰) and thosefrom alluvial aquifers (dD: �22.7 to �23.4‰; d18O: �4.53 to�5.01‰), but they are less depleted than those reported from theGulf of Suez (Sturchio et al., 1996). The Gulf of Suez samples fromHammam Faraoun, Ayun Musa, Ain Sokhna, and HammamMusa (Fig. 1) are believed to represent groundwater in alluvialaquifers fed by ascending NSS groundwater accessing the sub-vertical faults defining the Gulf of Suez and its coastal plain(Sturchio et al., 1996).

The isotopic compositions of samples from the fractured base-ment and alluvial aquifers are similar to those of average modernprecipitation from Al Arish and Rafah (Fig. 1). These averages wereextracted from the reported isotopic compositions for cumulativesamples acquired on a monthly basis and weighted in proportionto the amount of recorded precipitation for each of the investigated

months. One of the explanations for the observed variations inisotopic compositions for our samples and those reported fromthe Gulf of Suez coastal plain is that they represent various degreesof mixing between two endmembers, one being fossil waters sim-ilar to those reported from the WD, deposited by intensified paleo-westerlies in previous cold and wet climatic regimes, and the otherbeing modern precipitation deposited in dry and warm climaticconditions (Sultan et al., 1997, 2011). If true, one might expect aprogressive depletion in the isotopic composition and a progres-sive increase in the age of the groundwater with distance fromthe recharge areas. That is apparently the case in the SP. The ana-lyzed NSS groundwater samples that were collected from locationsproximal to recharge areas (e.g., SN1-3, SN1-4, SN1-5; Fig. 1) areless depleted than those reported from discharge areas in the Gulfof Suez coastal plain (Fig. 4). Similarly, analysis of the reported dataindicates the radiocarbon ages increase with increasing distancefrom recharge areas in southern Sinai, and the oxygen and hydro-gen isotopic compositions of groundwater of the NSS aquifer areprogressively depleted with increasing distance from rechargeareas (JICA, 1999).

Additional support for the mixing hypothesis that we advocatecomes from examination of the variations in isotopic compositionsof groundwater from the SP, ED, and WD in relation to the amountof modern precipitation in these areas (Fig. 5). The greater theamount of modern precipitation, the greater the contribution frommodern recharge, and the less depleted the isotopic compositionsbecome. The isotopic compositions of our Sinai samples are theleast depleted (dD range from �19.9 to �32.8‰; d18O: �3.77 to�5.81‰) and the WD fossil groundwater samples are the most de-pleted, whereas the ED samples have isotopic compositions thatare intermediate between those of the WD and the SP. Hydrogenand oxygen isotopic compositions of the WD water samples aresimilar (dD: range from �72 to �81‰; d18O: �10.6 to �11.9‰)

Fig. 3. Geoelectric cross sections showing VES and well locations, and apparent resistivity, thickness, and distribution of the saturated and unsaturated rock units in the WadiBaraga (Box a, Fig. 1) and Wadi Zalaga (Box b, Fig. 1) areas. (a) S–N trending geoelectrical cross-section in the Wadi Baraga area. (b) SE–NW trending geoelectrical cross-section in the Wadi Baraga area. (c) Modeled layer depth, thickness, and apparent resistivity, and the RMS values for two locations (Wadi Baraga: VES B4; Wadi Zalaga: VESZ4). (d) S–N trending geoelectrical cross section along Wadi Zalaga. (e) SE–NW trending geoelectrical cross section along Wadi Zalaga.

18 M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24

over large areas and throughout various depths in the aquifer; theED fossil groundwater samples, on the other hand, have a wide

range in hydrogen and oxygen isotope ratios (dD range from �73to �19‰; d18O: �9.5 to �2.7‰).

Table 1Isotopic data for the SP groundwater samples from fractured basement, and from open and productive wells.

ID Name Location Lat Long Altitude (m) Description DWT (m) dD ‰ d18O‰

SN1-2 Bir 28 Al Tur 28.37431 33.56984 90 Productive well (Alv)a �22.3 �5.22SN1-3 Bir Zalaga 1 W. Zelega 29.05652 34.40927 Open well (NSS)b 7 �22.7 �4.53SN1-4 Bir Zalaga 2 W. Zelega 29.00975 34.38137 Open Well (NSS)b 5 �31.0 �5.44SN1-5 Bir Safra Barga area 28.75737 34.34483 Open well (NSS)b 7 �26.0 �4.47SN1-6 Bir W. Marra W. Marra 28.79905 34.24910 Production well (NSS)b 27 �32.8 �5.22SN1-7 Bir Haroun W. El Sheik 28.56807 33.96538 1507 Open Well (FB)c 13 �20.5 �4.37SN1-8 Bir Zeituna W.i El Sheik 28.59463 33.99162 1439 Open Well (FB)c 30 �20.1 �4.50SN1-9 Bir Halwagy W. El Sheik 28.66773 33.99033 1299 Open Well (FB)c 55 �19.9 �3.77SN1-10 Bir Sahab W. El Sheik 28.71512 33.77945 887 Open Well (FB)c 44 �22.5 �4.49SN1-11 Bir Al Gufa W. El Sheikh 28.69378 33.69315 754 Open Well (FB)c 45 �20.8 �4.72SN1-12 Bir Al Sidra W. El Sheikh 28.90050 33.47115 429 Open pit (FB)c 20 �22.5 �4.89SN1-13 Bir Makateb W. Feiran 28.79910 33.46438 328 Open well (Alv)a �23.4 �5.05SN1-14 Bir Nedia W. El Sheikh 28.77593 33.52240 399 Productive well (FB)c 63 �23.2 �5.05SN1-15 Bir 5 Makateb W. Feiran 28.78778 33.42263 268 Productive well (Alv)a 70/80 �23.0 �5.01

a Alluvial sediments.b Nubian sandstone.c Fractured basement.

Fig. 4. Comparison between stable isotope ratios [hydrogen (dD) vs. oxygen (d18O)] for groundwater samples from the SP with NSS aquifer paleowaters from the WD(Sturchio et al., 2004), the ED (Sultan et al., 2007), and the Gulf of Suez (Sturchio et al., 1996), and data for modern precipitation from El Arish and Rafah (IAEA and WISER,2008). Also shown is the global meteoric water line (solid line): dD = d18O + 10 (Craig, 1961).

M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24 19

The progressive enrichment in the isotopic composition of theNSS aquifer groundwater from the SP to the ED to the WD is hereinterpreted to indicate variable degrees of mixing between fossilwater that precipitated during wet climatic periods and meteoricprecipitation deposited during the intervening dry climatic peri-ods (e.g., present climate). This hypothesis is supported by thepatterns of modern precipitation. Currently, rainfall over theNSS outcrops (recharge areas) in southern Sinai is considerable(�100 mm/yr) compared to that over their counterparts in theWD, which hardly receive any precipitation (0–5 mm/yr) (EMA,1996; Legates and Wilmott, 1997; Nicholson, 1997). Precipitationin the ED is intermediate between that reported for the WD andthat for the SP.

5. Modern recharge of the NA: modeling constraints

We attempted to quantify modern recharge contributions to thefossil water of the NSS aquifer of the SP. The adopted methodolo-gies were those of Milewski et al. (2009a,b) that compensate foruncertainties arising from scarcity of one or more of the followingdata sets: temporal and spatial rainfall depths, stream flow data,and field data. A catchment-based, semi-distributed hydrologicmodel was developed for continuous simulation of surface runoff

and potential recharge to the groundwater system. For this pur-pose the Soil and Water Assessment Tool Model (SWAT) wasutilized.

5.1. Model construction

The SWAT model provides a continuous simulation of the over-land flow, channel flow, transmission losses, evaporation on baresoils and evapotranspiration on vegetated canopy, and potentialrecharge to the shallow alluvial aquifers (Arnold and Fohrer,2005; Arnold et al., 1998). SWAT was selected because it is a con-tinuous model, allowing rainfall-runoff and groundwater-rechargeestimates to be made over extended periods of time; it is also com-patible with GIS data formats, allowing us to import the existingGIS databases for the SP into the model.

5.2. Database generation using GIS

The initial step in the development of our hydrologic model wasthe generation of a database incorporating digital mosaics fromvarious sources. Recently-released GIS-based SWAT modules(ArcSWAT: SWAT, 2007) can readily display and query geospatialinformation in a GIS environment and use GIS databases as inputs

Fig. 5. Average annual precipitation extracted from TRMM 3B42.v6 3-hourly data acquired (1998–2007) over Egypt showing the lowest precipitation in the WD, moderateprecipitation in the ED, and the highest precipitation in the SP. Also shown are the distribution of the meteorological stations and TRMM stations in the SP and the ED.

20 M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24

to the SWAT model. We generated digital mosaics that were usedas model inputs. These mosaics covered the entire SP and includedthe following digital data sets: (1) temporal, calibrated rainfall data(3-hourly precipitation data: 1998–2007) extracted from thezsatellite-based Tropical Rainfall Measuring Mission (TRMM) data;(2) a geologic mosaic from two geologic maps, each covering 2�

latitude by 3� longitude (scale 1:500,000); (3) land-use mapsextracted from the U.S. Geological Survey (USGS) 1 km global LandUse and Land Cover database generated from Advanced Very HighResolution Radiometer (AVHRR) data (acquisition date: April1992–March 1993); (4) a mosaic of three quadrants (each covering5� by 6�) from the NASA Landsat GeoCover Dataset 2000 (Landsat

M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24 21

GeoCover Orthorectified Thematic Mapper Dataset 2000; spatialresolution: 15 m) (Tucker et al., 2004); (5) climatic parametersincluding solar radiation, wind speed, air temperature, and relativehumidity obtained from the Egyptian Meteorological Authority’sClimatic Atlas (EMA, 1996); and (6) a digital elevation model mo-saic from 37 Advanced Spaceborne Thermal Emission and Reflec-tion Radiometer (ASTER) scenes at 30 m resolution. The data setsdescribed above, originally in various projections, were co-regis-tered to a reference map (NASA Landsat GeoCover Dataset 2000)and re-projected to a common projection (UTM – Zone 36,WGS84).

Average monthly climatic data (e.g., minimum temperature,maximum temperature, solar radiation, and wind speed) were ex-tracted from the Global Historical Climatology Network (GHCN)global climatic dataset (EarthInfo, 1998–2005) and the EgyptianMeteorological Authority (EMA, 1996).

To alleviate problems arising from the paucity of rain gauges,and their general distribution in areas of low elevation, procedureswere developed to extract precipitation data from satellite-basedTRMM precipitation sensors as opposed to precipitation from raingauge data. We used the 3-hourly data from the TRMM datasetswith a spatial resolution of 0.25� � 0.25� that is available for thestudy area from 1997 to present. Because TRMM can underesti-mate precipitation in arid environments by 15–30% (Chiu et al.,2006; Chokngamwong and Chiu, 2006), the TRMM-based precipi-tation for the study area was calibrated against rain gauge datausing procedures described in Milewski et al. (2009a).

5.3. SWAT model setup and calibration

The hydrologic model of the SP was constructed within theSWAT framework to simulate the hydrologic processes using itsphysically-based formulations. Initial losses and direct overlandflows in the Hydrologic Response Units (HRUs) were estimatedusing the US Department of Agriculture, Soil Conservation Service,method (SCS, 1972), a method that was successfully applied toephemeral watersheds in southwestern United States that bearsome resemblance in their climatic, hydrologic, topographic, land-scape, and soil and land-use types to watersheds in the SP (Gheithand Sultan, 2002; Osterkamp et al., 1994). The bulk of the physicalproperties of the HRUs in each sub-catchment was extracted fromdatabases that we generated for soils, land cover, and land-usetypes throughout our previous studies (Gheith and Sultan, 2001,2002; Milewski et al., 2009a). The soil types were extracted fromthe 1:500,000 geologic map series (Klitzsch et al., 1987a–e), andthe land-use maps were derived from the USGS 1 km global LandUse and Land Cover database that was generated from AVHRR data(Anderson et al., 1976). Evaporation on bare soils and transpirationon vegetated canopy was calculated using the Penman–Monteithmethod (Monteith, 1981). A simplified top soil profile was em-ployed in the model with soil properties dictated by the assignedland-use and soil type. In our case, the ‘‘Southwestern US AridRange’’ provided by the SWAT database was the selected land-use type across the entire study area.

Channel flows were estimated using the Muskingum routingmethod (McCarthy, 1938), whereby Manning’s coefficient for uni-form flow in a channel was used to calculate the rate and velocityof flow in a reach segment for a given time step. Channel flowswere subject to transmission losses, a partitioning that dependson the channel geometry, upstream flow volume, duration of flow,bed material size, sediment load, and temperature (Neitsch et al.,2005). We assumed negligible losses from channel flows to tran-spiration or evaporation for the following reasons: (1) vegetationis scarce or absent under the prevailing arid to hyper-aridconditions; (2) flows are short-lived, typically not lasting for morethan a day, with cloudy conditions typically prevailing throughout

storm events; and (3) alluvial deposits flooring the valleys havehigh hydraulic conductivities.

Simulations were performed at daily time steps, the smallesttime steps allowed by SWAT, using cumulative 3-hourly TRMMdata over periods of 24 h and applying monthly average valuesfor temperature, wind speed, relative humidity, and solar radiationas daily estimates.

The adopted model parameters for this study were extractedfrom our previous investigations in the Wadi Girafi watershed,an E–W trending, medium-sized (area: 3656 km2) watershed thatcollects precipitation from the highlands of central Sinai andflows eastwards towards Israel (Milewski et al., 2009a). WadiGirafi was selected for the following reasons: (1) in Sinai, thestream flow data needed for calibration purposes is only availablefor the Wadi Girafi watershed, where the Israeli Hydrologic Ser-vice collected (from 1998 to 2006) stream flow data at the outlet(Bottleneck station) of the Wadi Girafi watershed; and (2) thereare similarities in physical catchment descriptors (e.g., geography,climate, catchment size, topography, geology, vegetation, etc.) ofWadi Girafi and those of the investigated watersheds, and thusthe key sensitive model parameters (e.g., SCS curve numbers,groundwater water delays, etc.) that were extracted for the WadiGirafi could be readily transferred to the similar proximal inves-tigated watersheds. The procedures for calibrating (coefficient ofdetermination, R2: 0.86; coefficient of efficiency: 0.85) the WadiGirafi watershed were described in detail in Milewski et al.(2009a).

5.4. Model results

Continuous rainfall-runoff models were simulated from 1998 to2007 for 20 watersheds in the SP (Fig. 6) using model parametersextracted from the calibrated Wadi Girafi watershed (Milewskiet al., 2009a). All of the selected watersheds are located at the foot-hills of the Precambrian mountains and encompass Nubian aquiferoutcrops. Watersheds to the north that have NSS outcrops of lim-ited areal extent were excluded; thus our average annual modernrecharge estimates for the NSS aquifer in the SP should be consid-ered conservative estimates.

The SWAT model results for each of the investigated 20 water-sheds, including the average annual amount of precipitation, ini-tial losses, surface runoff, total watershed transmission losses,and transmission losses over the Nubian outcrops throughoutthe investigated period are summarized in Table 2. Nubian aquiferrecharge was assumed to be equal to the estimated transmissionlosses, consistent with findings from similar arid environmentsthat showed minimal recharge contributions from the initiallosses (Bazuhair and Wood, 1996; Dettinger, 1989; Flint et al.,2000).

Inspection of Table 2 shows that the total area of the modeledwatersheds in the SP is 14,209 km2, approximately 22.5% of thearea of the SP, whereas the Nubian outcrops covered approxi-mately 8% of the area of these watersheds. First-order estimatesfor NSS recharge for each of the investigated watersheds were esti-mated by multiplying the watershed transmission losses by theproportion of NSS outcrops in the watershed. The average annualmodern recharge for the NSS in the SP was estimated at13.0 � 106 m3 from the sum of annual transmission losses for 20watersheds for each of the investigated years (1998–2007) aver-aged by the number of years (10 years).

6. Summary and conclusion

We conducted an integrated study using geophysical, geochem-ical, field, and remote sensing data sets, GIS technologies, andcontinuous rainfall-runoff modeling to examine the validity of a

Fig. 6. False-color Landsat TM image bands 2-4-7 for southern Sinai showing the 20 watersheds (red outlines) encompassing NSS outcrops (yellow outlines) that weremodeled using SWAT continuous rainfall-runoff models.

Table 2SWAT model results for the investigated watersheds from 1998–2007.

Watershed Area of watershed Area of Nubian Precipitation Initial losses Surface runoff Watershedtransmission losses

Nubiantransmission losses

km2 km2 � 106 m3 � 106 m3 (%) � 106 m3 (%) � 106 m3 (%) � 106 m3 (%)

Wardan 1161.23 14.53 97.89 86.78 0.89 3.66 3.74 7.46 7.62 0.09 0.10Gharandal 882.39 76.99 83.65 79.23 0.95 0.97 1.16 3.45 4.12 0.30 0.36Tayiba 356.22 132.33 32.77 27.65 0.84 4.59 14.00 0.54 1.64 0.20 0.61El-Gart 728.84 55.35 91.62 83.05 0.91 3.80 4.15 4.76 5.19 0.36 0.39Sidri 1074.61 40.23 111.44 76.40 0.69 6.52 5.85 28.51 25.58 1.07 0.96Feiran 1806.47 23.65 64.49 29.16 0.45 6.65 10.31 28.69 44.48 0.38 0.58Durba 124.47 7.79 2.96 2.63 0.89 0.04 1.39 0.30 10.00 0.02 0.63‘Araba 66.03 10.22 1.80 1.36 0.75 0.07 4.03 0.37 20.77 0.06 3.21Awag 1943.14 0.81 73.65 46.42 0.63 6.86 9.31 20.36 27.65 0.01 0.01Near Bir Taba 90.27 31.47 14.21 11.34 0.80 2.35 16.51 0.53 3.70 0.18 1.29Bir Merakh 49.96 18.51 8.77 4.80 0.55 0.58 6.60 3.39 38.68 1.26 14.33South of Bir Merakh 37.49 15.11 6.58 1.06 0.16 3.34 50.75 2.19 33.21 0.88 13.39Near G. Ghazlani 46.06 8.41 8.09 3.80 0.47 1.62 19.98 2.67 33.04 0.49 6.03North of Ain Quseiyib 30.93 9.92 2.72 1.54 0.56 0.66 24.28 0.52 19.26 0.17 6.18Ain Quseiyib 54.30 4.45 3.19 2.19 0.69 0.83 25.92 0.17 5.34 0.01 0.44El-Mahash 46.97 6.87 2.76 1.17 0.43 0.53 19.32 1.05 38.15 0.15 5.58El-Malha 50.27 8.29 2.98 1.32 0.44 0.50 16.88 1.16 38.89 0.19 6.42South of El-Malha 50.39 12.26 2.45 1.68 0.69 0.25 10.02 0.52 21.19 0.13 5.16Watir 3531.35 338.00 208.35 187.09 0.90 2.12 1.02 19.14 9.19 1.83 0.88Kid 2082.30 293.67 117.44 63.99 0.54 16.68 14.20 36.77 12.52 5.19 4.42

Total Nubian per year 1108.87 12.96Total Nubian (1998–2007) 1108.87 129.65

22 M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24

M. Sultan et al. / Journal of Hydrology 403 (2011) 14–24 23

widely accepted notion that the NSS aquifer of northern Africa(Egypt, Sudan, Libya, Chad) was exclusively recharged by fossilwater in previous wet climatic periods in the Quaternary Period.Our working hypothesis is that while this is largely true for manyof the areas occupied by the NSS aquifer, this might not be true inareas of relatively higher modern precipitation. One of these areas,our test area, is the SP. In dry climatic periods similar to that of to-day, the precipitation over the mountains in Sinai is channeleddownstream over NSS Group exposures at the foothills of the crys-talline basement hills in southern Sinai, providing ample opportu-nities for groundwater recharge. In other words, the groundwaterin the NSS aquifer in areas of relatively higher precipitation repre-sents a mixture of fossil water and modern precipitation. The advo-cated model is supported by interpretations of our geophysical(conventional electrical resistivity) investigations and geochemicaldata. The interpretation of our geophysical data is consistent withthe presence of unconfined NSS saturated aquifers flooring re-charge areas at the foothills of the crystalline basement in the SPat Baraga and Zalaga areas. The isotopic compositions (O, H) forour groundwater samples from wells tapping the NSS aquiferunderlying recharge areas could be accounted for as mixtures oftwo endmembers: fossil groundwater with isotopic compositionssimilar to that of the WD NSS aquifer, and modern meteoric precip-itation, with the latter endmember being the dominant compo-nent. This interpretation is supported by the observed increase inmodern precipitation from west to east that could explain the pro-gressive west to east enrichment in the isotopic composition of theNSS aquifer groundwater. Sinai receives the highest amount of pre-cipitation and its groundwater samples are the most enriched; theWD receives the least amount of precipitation and its samples arethe most depleted, while the ED receives intermediate amounts ofprecipitation and the isotopic compositions of its samples are alsointermediate.

Conservative estimates for the modern contributions to the NSSaquifer in southern Sinai were obtained using continuous rainfall-runoff models for the watersheds that crosscut the NSS outcropsand act as potential recharge domains for the NSS aquifer. Adopt-ing methodologies and calibration parameters from previousinvestigations (Milewski et al., 2009a), and using SWAT rainfall-runoff simulations for the period 1998–2007, we estimated theaverage annual modern recharge for the NSS aquifer at�13.0 � 106 m3. Results highlight the importance of modern localrecharge of the NSS aquifer and bear on its sustainable exploitationin the SP and elsewhere where the aquifer is being locally re-charged. Our findings are also relevant to other similar large fossilaquifer systems worldwide that were largely recharged in previouswet climatic periods yet are still receiving modest modern mete-oric contributions.

Acknowledgements

Funding was provided by the United Nations Development Pro-gramme (UNDP) and the Global Environmental Facility (GEF) Inter-national Water Program, the National Science Foundation (NSF)Science and Technology Grant (OISE-0514307), the NATO ScienceFor Peace Program (SfP 982614), and the US–Egypt Science & Tech-nology Program’s Junior Scientist Development Visit Grant, andsupported by the U.S. Department of Agriculture, all awarded toWestern Michigan University. We thank the administration ofCairo University and the Ministry of Water Resources and Irriga-tion for the logistical support provided.

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