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Introduction

Groundwater recharge from rainfall is the main sourceof recharge of the aquifer in the Gaza Strip area. Be-cause of its geographical location in the semi-arid zone,there are almost no other sources of groundwaterrecharge in the Gaza Strip. The average rainfall in thearea based on 20 years records amounts to 321 mm/a(PWA 2001). Since the total area of the Gaza Strip isabout 365 km2, the annual volume of rainfall is about111 million m3. Only part of this amount percolates tothe aquifer and the rest is evapotranspiration.

Identification of the net groundwater recharge isessential for groundwater modelling and water re-sources management. The calculation of the netgroundwater recharge is a big challenge for thehydrologist since there is no specific method to find

out the net recharge reliably. There are so manymethods for quantification of groundwater rechargefrom rainfall. Each method has its limitations anddifficulty in application. All the methods which havebeen used in this regard result in an estimation of theactual value.

Many stochastic parameters such as soil properties,topography, rainfall amount, evapotranspiration, anddepth to the water table affect the amount of perco-lated water. Because of uncertainty associated withestimation of these parameters, each method of esti-mation of groundwater recharge usually ends withdifferent result. Although many empirical formulaswere developed to find out the net groundwater re-charge, none of them have been shown to be efficientand accurate. Each method of recharge estimation hasits limitations in terms of applicability and accuracy.

Husam Baalousha Using CRD method for quantificationof groundwater recharge in the Gaza Strip,Palestine

Received: 13 February 2005Accepted: 12 June 2005Published online: 26 August 2005 Springer-Verlag 2005

Abstract Rainfall is the main sourceof groundwater recharge in the GazaStrip area in Palestine. The area islocated in the semi-arid zone andthere is no source of recharge otherthan rainfall. Estimation of ground-water recharge from rainfall is notan easy task since it depends onmany uncertain parameters. Thecumulative rainfall departure (CRD)method, which depends on the waterbalance principle, was used in thisstudy to estimate the net ground-water recharge from rainfall. Thismethod does not require much dataas is the case with other classicalrecharge estimation methods. TheCRD method was carried out usingoptimisation approach to minimise

the root mean square error (RMSE)between the measured and the sim-ulated groundwater head. The re-sults of this method were comparedwith the results of other rechargeestimation methods from literature.It was found that the results of theCRD method are very close to theresults of the other methods, butwith less data requirements andgreater ease of application. Based onthe CRD method, the annualamount of groundwater rechargefrom rainfall in the Gaza Strip isabout 43 million m3.

Keywords Groundwater recharge/

water budget

Rainfall/runoff

Arid regions Gaza Strip

Environ Geol (2005) 48: 889900DOI 10.1007/s00254-005-0027-x O R I G I N A L A R T I C L E

H. BaaloushaInstitute of Hydraulic Engineering andWater Resources Management,Aachen University of Technology(RWTH), Mies-van-der-Rohe-Str. 1,52056 Aachen, Germany

E-mail: baalousha@web.deTel.: +49-241-8027343Fax: +49-241-8022348

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Review of recharge estimation methods

Regarding the Gaza Strip area, many studies have beencarried out to estimate the groundwater recharge fromrainfall. Melloul and Bachmat (1975) have estimated thegroundwater recharge for the Gaza Strip using rechargecoefficients. They subdivided the area into three sub-

zones and computed the coefficients for each sub-zonebased on the soil type, and the average rainfall. Thecoefficients were estimated based on regression analysisbetween groundwater recharge and soil type. Accordingto their study, Melloul and Bachmat found that theannual groundwater recharge in the area equaled to41 million m3.

Another study was carried out in the area by IW-ACO and WRAP (IWACO and WRAP 1995) to esti-mate the net groundwater recharge based on chloridemass balance. The chloride mass balance method isbased on the assumption of conservation of massbetween the input of atmospheric chloride and the

chloride flux in the subsurface. This study was appliedto the northern part of the Gaza Strip, where the topsoil is composed mainly of sand dunes. In other words,that particular area has a high infiltration rate. Resultsof this study have showed that the amount ofgroundwater recharge is about 46 million m3 per year(IWACO and WRAP 1995). Table 1 summarises theresults of recharge estimation according to the differentmethods as discussed before.

The problem with the previously-mentioned methodsis that their accuracy is poor. Since the calculation ofrecharge requires many uncertain parameters such soiltype, land use, and hydrogeological properties, the re-

sults have a high degree of uncertainty.In this study, another method was used to estimatethe net groundwater recharge from rainfall. This methodis called cumulative rainfall departure (CRD) and itdepends on less uncertain data than other methods. Thedata required by the CRD method are: monthly rainfallrecords, measurements of groundwater levels, aquiferstorativity, abstraction records, and lateral inflow andoutflow.

Methodology

The cumulative rainfall departure method (CRD)

The CRD method was proposed first by Bredenkampet al. (1995). The theory behind cumulative rainfalldeparture depends on the water balance in the aquifer.

This balance can be expressed as follows:

R Qp Qout Qin DhiA S; 1

where:

R: the total recharge to the aquifer [L3/T],Qp: the discharge from pumping [L

3/T],Qout: the lateral outflow [L

3/T],Qin: the lateral inflow [L

3/T],Dhi: the change in groundwater level in a time per-

iod i [L],A: the area of study [L2] and,S: the aquifer storativity.

When the pumping rate in the above equation isknown, the change in aquifer storage accounts for thenet balance between the inflow and outflow to theaquifer. Hence, the cumulative rainfall departure can beexpressed by the relation between rainfall and ground-water level fluctuations. It is assumed that there is alinear relationship between the monthly change in waterlevel and the cumulative rainfall departure. Bredenkampet al. (1995) has expressed this relation between rainfalland CRD in the following form:

CRDi CRDi1 Pi C; 2

where:

CRDi: the cumulative rainfall departure (mm) formonth i [L];

CRDi-1: the cumulative rainfall departure (mm) formonth i-1 [L];

Pi: precipitation (mm/month) [L/T];C: a cut off value (below which no recharge oc-

curred) (mm) [L].

This equation illustrates the relation between twoCRD amounts in two successive time periods. Thegeneral equation, which is used to compute the net re-charge from the CRD method can be written in thefollowing form (Bredenkamp et al. 1995):

hi hi1 R

SQin OoutSA

Qp

SA; 3

where:

R: effective recharge = fraction of precipitation(m) [L];

Qin: inflow into aquifer (m3/month) [L/T];

Qout: outflow from aquifer (m3/month) [L/T];

Table 1 Some results of groundwater recharge in the literature

Source Method Value(million m3/year)

Fink 1970 Change in aquifer storage 3367Melloul and

Bachmat 1975Recharge coefficients 41

IWACO andWRAP 1995

Chloride mass balance 46

CAMP 2000 Land use and rechargecoefficients

37

CAMP 2000 Groundwater modelling 4045

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Qp: withdrawal from aquifer (m3/month) [L/T];

hi: head (m) in month I [L];hi-1: head previous month [L];S: storativity;A: area of study [L2].

The pumping rate and precipitation records areusually known with good accuracy. Thus, if the lateralinflow and storativity are known, the only unknown inthe above equation is R, which is the net recharge.Application of the CRD method can be done by plottingof the monthly measured groundwater level versus thecomputed level. The latter term, as it appears in Eq. 3,can be computed as a summation of the groundwater

recharge, pumping, and the groundwater lateral inflowand outflow. Given the value of the storativity, the fitbetween the computed and observed groundwater levelcan be assessed. The best estimate of recharge can beobtained when the best fit between the computed andmeasured groundwater head is achieved.

The best fit can be obtained by minimisation of the

root mean square error (RMSE) between the measuredand calculated groundwater heads. An optimisationprocedure is used with the help of Excel to minimise theerror and to get the best fit between the two curves.

In the following sections, application of the CRDmethod to a case study is presented. To see the effec-

Fig. 1 Location map of theGaza Strip

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tiveness of the CRD method, its results were compared

with the results of other methods of recharge estimation.

Case study

The study area

The Gaza Strip area is part of the Palestinian occupiedterritories, located between longitudes 31 and 25 N, andlatitudes between 34 and 20 E. It is a coastal area lo-cated along the eastern Mediterranean Sea (see Fig. 1).The length of the Gaza Strip is about 40 km, while thewidth varies from 6 to 12 Km. Thus, the total area of the

Gaza Strip is about 365 km

2

. Because of its geograph-ical location, the Gaza Strip forms a transitional zonebetween the semi-humid coastal zone in the north, thesemi-arid loess plains in the east, and the arid Sinaidesert in Egypt. The area consists of a littoral zone, astrip of younger dunes situated on top of a system ofolder Pleistocene beach ridge, and more to the east,gently sloping alluvial and loessial plains. The GazaStrip is densely populated since about 1 million inhab-itants (PCBS 2000) live in this small area. Therefore thepopulation density in the area is the highest in the world.

Geology of the area

The aquifer system in the area of study is part of thecoastal plain, which extends from Haifa city in the northto Sinai desert in the south and covers an area of about2,000 km2 (Metcaf and Eddy 2000). The coastal plain ischaracterised by flat relief, and is bounded to the east bythe foothills of the mountain belt. This plain is narrow inthe north and gets wider in the south with an averagewidth of about 13 km. Calcareous sandstone and gravel

from the Pleistocene age and recent Holocene sand

dunes are the main water bearing layers in the aquifer.Some silts, clay, and conglomerate do exist in the aquiferformation. As shown in Fig. 2, the aquifer is mainlyphreatic and its thickness varies from a few meters at theeast of the Gaza Strip to some 170 m at the shoreline.The aquifer overlies thick impermeable marine clay ofthe Neogene age called the Saqiyia Formation.

Rainfall

The rainfall is an important component of groundwaterrecharge in the area since runoff is almost negligible.Although there is a wadi (seasonal stream) running fromIsrael across the Gaza Strip and draining into theMediterranean sea, dams were built just behind theborder of the Gaza Strip (United Nations 2003; ARIJ1995). As a result, no flow takes place in this wadianymore. There are eight meteorological stations in thearea at which the rainfall amount is recorded on dailybasis. According to the records of the Ministry ofTransportation for the period from 1980 to 2000, theannual average rainfall varies from 433 mm/a in thenorth to 236 mm/a in the south. The absolute maximumannual rainfall in the entire area of the Gaza Strip wasrecorded in the hydrological year 1994/1995. At thatyear, the annual average rainfall was 567 mm. On theother hand, the absolute minimum annual rainfallamounts to 114 mm.

Different methods have been developed for the esti-mation of areal rainfall from raingauge measurements. Itwas found that the Thiessen method gives consistentlybetter and more accurate results than other methods(Melesse et al. 2003). Therefore, the Thiessen methodwas used to obtain the weighting coefficients for thespatial distribution of rainfall in this study. With the help

Fig. 2 Typical hydrogeologicalcross-section

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of GIS, Thiessen polygons were constructed to find outthe spatial distribution of rainfall. Figure 3 illustrates thelocations of the meteorological stations and averagerainfall in each polygon based on the Thiessen method.

Water table fluctuations

The groundwater level monitoring network in the GazaStrip has some 200 piezometers distributed in the entirearea of the Strip. Distribution and allocation of these

piezometers was done in such a way that they are awayfrom any operating pumping well in order to ensure thatthe static water level can be monitored. This networkwas constructed a long time ago and maintained by thePalestinian Water Authority (PWA). Historical recordsof water levels are available for the past 30 years onmonthly basis. Therefore, groundwater level records inthe period from 1980 to 2000 were used with thoserainfall records for the same period to carry out theCRD method. It should be noticed that the heads usedin the CRD method are the water table since the aquifer

Fig. 3 Rainfall, meteorologicalstations, and Thiessen polygons

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is phreatic. Following the construction of Thiessenpolygons, the monthly records of water levels at eachpolygon were averaged to be used with the CRDmethod. In general, the aquifer in the Gaza Strip isshallow with depths to the water table varying from afew meters to 30 m at some particular locations.

Lateral inflow and outflow

Lateral inflow is an important parameter to the overallwater balance in the Gaza Strip. The amounts of lateralinflow and outflow are subject to change from one year

to another due to different hydrogeological parameters(rainfall, pumping, etc). However, the groundwater lat-eral inflow and outflow can be estimated based on dif-ferent approaches. Since the groundwater level for thearea of study is monitored monthly, a groundwater levelcontour map can be created. Therefore, the lateral in-

flow can be computed based on the difference betweencontours. Another approach for estimation of lateralinflow is the use of groundwater models. Coupling ofresults from different sources in the literature was donein order to get the best estimate of this amount. TheCoastal Aquifer Management Program (CAMP) for theGaza Strip has built an integrated groundwater numer-

Table 2 Hydrogeological parameters based on analysis of pumping tests data

Well Id. Lithology Ta Kb Storativiy

L/159A Calcareous sandstone and sand 2,6403,290 3070 510)3

P/124 Calcareous sandstone and sand 8451,330 15 810)3

C/128 Calcareous sandstone and sand 8682,000 27 510)4

A/180 Sand dunes 3,4607,960 140 0.03

R/162L Coarse sand and gravel 3,3004,600 70 610)3

Information from Israeli resources (16 wells)Calcareous sandstone and sand 2,9009,000 2990 10)210)3

Calcareous sandstone and sand 10100 10)3

Fig. 4 CRD fitting diagram for Bait Hanon

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Fig. 5 CRD fitting diagram for Bait Lahia area

Fig. 6 CRD fitting diagram for Jabalya area

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Fig. 7 CRD fitting diagram for Gaza area

Fig. 8 CRD fitting diagram for Nussierat area

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Fig. 9 CRD fitting diagram for Dier Albalah area

Fig. 10 CRD fitting diagram for Khan Yunis area

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ical model for the area of study (Metcalf and Eddy2000). The results of this model, in addition to all otherdata in the literature (PWA 2001) were manipulated andused to obtain the values of lateral inflow to the aquifer.

Hydraulic parameters

Hydraulic properties are usually obtained by carryingout pumping tests. Many of these tests have been carriedout in the Gaza Strip. In addition to the old pumpingtest data, the Palestinian Water Authority (PWA) hascarried out different new pumping tests as a part of theCAMP project (Metcaf and Eddy 2000). Moreover,there are some old data from Israeli sources regardingthe hydraulic properties in the Gaza Strip (Fink 1970;Melloul and Bachmat 1975; Yakirevich et. al. 1998)besides the data from the literature.

Table 2 summarises the results of the pumping teststhat were carried out in the area. Values of transmis-sivity range between 700 and 5,000 m2/day. The corre-sponding values of hydraulic conductivity range between20 and 80 m/day. Storativity values range between0.0005 and 0.03 as shown in the table. Complete statis-tical analysis for hydraulic parameters of the Gaza Striparea and rainfall analysis can be found in the literature(Baalousha 2004).

Application of the CRD method

Based on the polygons created by the Thiessen methodaccording to meteorological stations shown in Fig. 3,

the CRD model was applied on 8 sub-zones. Theabstraction data and rainfall records were obtained fromthe Palestinian Water Authority and Ministry of Agri-culture (PWA 2001) for the period from 1980 to 2000 onmonthly basis. In an Excel spreadsheet, the monthlyrainfall data were listed against the correspondinggroundwater level data for each sub-zone, and plottedagainst time (Figs. 4, 5, 6, 7, 8, 9, 10, 11). The ground-water level has then been simulated based on Eq. 3, afteridentification of storativity, total inflow and outflow forthe simulated period. The resulting groundwater leveldata from simulation has been plotted on the same axesas the rainfall and the measured groundwater data.

The relation between groundwater level and rainfall,which was described before in Eq. 3, is assumed to belinear. Thus far, the figure for each sub-zone representsthe measured and simulated groundwater head. The laststep is the optimization procedure to get the best fitbetween the measured and the simulated groundwaterlevel. The optimization process is controlled by calcu-lation of the root mean square error between the mea-sured and simulated values. The best fit was achieved atthe minimum RMSE. Once the minimum value of the

Fig. 11 CRD fitting diagram for Rafah area

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RMSE has been achieved, the best estimate of rechargecan be read from the model based on Eq. 3. The valuesof the RMSE for each sub-zone are given in Table 3.The whole process of optimization does not take morethan one minute for each sub zone.

Results

Table 3 summarises the results of the CRD model.Figures 4, 5, 6, 7, 8, 9, 10, 11 show the fitting of theobserved and modelled groundwater levels based on therecharge cumulative rainfall departure model.

The CRD method depends on different parameters asthey appear in Eq. 3: rainfall, measured groundwaterlevel, storativity, lateral flow, and pumping. It was foundfrom sensitivity analysis that the estimated value of re-charge is dependent on storativity more than otherparameters such as lateral flow. Hence, reliable values ofstorativity should be input into the model. If the stor-ativity value is far from the real value, assuming that

other parameters are correct, no fitting of the model canbe achieved. Thus, fitting of curves along with RMSEcalculation can eliminate the uncertainty in storativity.It was found from sensitivity analysis that the change inthe lateral flow has negligible effects on the estimatedrecharge value. This is obvious because the computedrecharge based on Eq. 3 is dependent on lateral flow andpumping divided by the area, which is the sub-zone area.

The average groundwater recharge as a percentage ofrainfall in the entire area of the Gaza Strip is calculatedas 36.74%. Since the average annual rainfall is 321 mm/a, and the area of Gaza Strip equals 365 km2, the cal-culated recharge value from rainfall amounts to

43.29 million m3 per year. Comparing this result withthe results in the literature (Table 1), it is clear that theresults obtained using the CRD method are reasonableand close to the results obtained by other methods fromthe literature.

Conclusion

The cumulative rainfall departure method (CRD) is agood, cheap, and efficient tool to estimate the netgroundwater recharge. From the results obtained in thisstudy, it is clear that the CRD method can be appliedeasily with a minimum amount of data in comparison toother recharge estimation methods. The advantages ofthis method are that the only needed data are the rain-fall, abstraction data, storativity of the aquifer, and thelateral inflow and outflow.

Moreover, the CRD method is independent of thecoefficients which are needed for other methods of re-

charge estimation, and consequently, has less sources ofuncertainty. The optimisation procedure followed in thismethod promotes and facilitates the fitting of the actualfield measurements of groundwater head with the com-puted data. From sensitivity analysis, it is found that theestimated recharge based on the CRD method isdependent on the storativity value more than otherparameters. The value of groundwater recharge ob-tained by the CRD method is comparable to the resultsof the coefficient method and seems very reasonablewhen applied to the groundwater model for the GazaStrip.

Limitations

The CRD method can be used only for unconfinedaquifers since it depends on fluctuations of the watertable. To obtain accurate results using the CRD method,values of storativity, monthly records of water table andrainfall should be known with reliable accuracy. Thismethod is best suited for aquifers with high storativity.Uncertainty of the estimated recharge based on theCRD method increases as the depth to the water tableincreases and vice versa.

Table 3 Results of recharge modelling

Sub-Zone CRD RMSE

Bait Hanon 38.1% 0.0291Bait Lahia 37.7% 0.0291Jabalia 36.3% 0.0406Gaza 34.8% 0.0374

Nussierat 31.64% 0.0266Dier Balah 35.3% 0.1559Khan Yonis 33.6% 0.0177Rafah 41.1% 0.0601Average 36.74 0.05045

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