Slah Nasri

Groundwater geochemistry of a small reservoir catchment in Central Tunisia

AbstractDue to the scarcity of water resources in semiarid sedimentary basins, hill reservoirs are often constructed torechargegroundwaterandlimitrunoffinducedwaterloss.TheimpactofsuchreservoirsongroundwaterchemistryisinvestigatedintheaquifersoftheElGouazinewatershed,CentralTunisia.Threegroundwatertypesarerecognised,Ca–HCO 3 ,Na–ClandCa–SO 4 .Thestrongsimilaritybetweenhostrockandgroundwaterchemistriesindicatessig-nificantrock–waterinteraction.Aflowpath,alongwhichthechemicalcompositionofthegroundwaterevolves,canbeidentified using the contrast in stable isotope signature between upstream and downstream groundwater. Shallowupstreamgroundwaterisrechargedbytheinfiltrationofrainwaterwiththerateofrechargestronglylinkedtothepermeability of the host lithology. Calcium and HCO 3 are supplied to an alluvial aquifer from a more rapidlyrechargedlimestoneaquiferwiththeconcentrationofCaandHCO 3 ionsdecreasingbydilution.ThealluvialaquiferisalsoenrichedinCaandSO 4 duringthedownstreamflowofgroundwaterthroughgypsiferousmaterials.Thereisevi-dence of mixing between meteoric groundwater and evaporated reservoir water. Below the reservoir and partlyresponsibleforreservoirleakageisasandyaquifer,formedbyweatheringanderosionofasandstonehostwhichalsosupplieswatertothealluvialaquifer.# 2002ElsevierScienceLtd.Allrightsreserved.1.IntroductionA perennial problem for farmers and herdsman insemiaridareaswithoutpermanentriversisthescarcityofwaterowingtothelowanderraticrainfall.Inthelastcentury,globalpopulationexpansionhastremendouslyincreased the pressure on water resources. To securewater supply and intensify agricultural production,water harvesting is traditionally used to combat theproblemsofvaryingrainfallratesandpopulationden-sities.One technique of water harvesting, floodwater har-vesting(FWH),alsocalledlargecatchmentwaterhar-vesting or spate irrigation, is widely developed. Whenfloodwaterisharvestedinthestreambed,thewaterflowisdammedandstoredintemporaryorpermanentsur-face reservoirs. Water is forced to infiltrate and,asaresult,torechargetheaquifer,withrechargeconcentratedmainly in the valley downstream from the reservoir(Prinz,1999).FWHisarelativelyinexpensivetechniqueprovidingwaterforthedevelopmentofagricultureandagroforestry, and reducing the use of other valuablewater resources such as groundwater. Water supply isnotalwaysevenandsecureduringtheyeardependingontheclimaticvariability.Largeevaporationlossesandlossofstoragebysiltationandpollutionarethemainlimitations of FWH. Social problems like conflictbetweenwaterusersareafurtherdrawbacktothisstyleofwaterharvesting.IntheMediterraneanbasin,waterharvestingplayedan important role in ancient civilisations such as theRomanandEgyptianempires(Prinz,1999).Duringthe20thcentury,moderntechniques(groundwaterdrilling,largereservoirconstructionetc.),whicharefavouredbygovernmentpolicy,slowlyreplacedtraditionalharvest-ing techniques. High demands for water during theagricultural growing season, which coincides with thedryperiodoftheMediterraneanclimate,hasresultedinthe depletion of the groundwater table in some areas.Thelowrenewalrateofgroundwaterresourcesandthe

Apport d’une caractérisation du réseau hydrographique sur la forme des crues à l’échelle d’un petit bassin-versant en zone semi-aride

Cet article a pour but d’apprecier l’impact d’une prise en compte du reseau hydrographique dans la definition des crues reconstituees par un modele hydrologique a base physique en zone semi-aride. Pour ce faire, sur le petit bassin-versant cultive de Fidh-Ali, localise au centre de la Tunisie, nous avons mis en œuvre un modele utilisant l’equation de Green et Ampt comme fonction de production, bien adaptee aux conditions de ruissellement et d’infiltration en zone semi-aride, et l’equation de l’onde cinematique comme fonction de transfert. Le modele utilise permet le decoupage du bassin-versant en panneaux trapezoidaux hydrologiquement homogenes. Afin d’ameliorer la reconstitution des crues, nous avons individualise les chemins de l’eau en panneaux rectangulaires ayant les principales caracteristiques topographiques du lit de l’oued Fidh-Ali et de ses principaux affluents. Le calage et la validation du modele hydrologique ont ete realises pour des conditions d’etats de surface des sols favorables au ruissellement, avant les labours d’automne. Un modele de bilan hydrique a permis de determiner l’humidite initiale des sols avant chaque averse. La prise en compte du reseau hydrographique, par une representation simplifiee dans un modele a base physique semi-distribue tel que le modele « abc », permet finalement, sur la base d’un calage du debit maximum de crue, d’ameliorer le critere de Nash de 0,8 a 0,95 et de 30 % l’estimation du volume ruissele. Le gain est donc significatif, malgre les autres sources d’imprecision liees a la connaissance de la repartition spatiale des pluies et a l’estimation du debit maximal observe, connu avec une precision de 5 % sur le bassin-versant de Fidh-Ali.