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1、UndergroundwaterOfalltheearthswater97%isfoundintheoceans2%inglaciersonly1%onl.Ofthis1%almostall(97%)isfoundbeneaththesurfacecalledsubsurfaceundergroundwater.Mostofthiswatereventuallyfindsitswaybacktotheseaeitherbyundergr
2、oundmovementbyrisingintosurfacestreamslakes.Thesevastundergroundwaterdepositsprovidemuchneededmoisturefdryareasirrigateddistricts.Undergroundwateractsinsimilarwaystosurfacewateralsoperfminggeomphicwkasanagentofgradation.
3、Eventhoughmanhasbeenawareofsubsurfacewatersinceearliesttimesitsnatureoccurrencemovementgeomphicsignificancehaveremainedobscure.Recentlyhoweversomeanswershavebeenfoundtotheperplexingquestionsaboutundergroundwatersrelation
4、shiptothehydrologicalcycle.1.SourceofUndergroundWaterSincethedaysofVitruviusatthetimeofChristmanytheieshavebeenpresentedtoexplainthelargevolumeofwaterunderneaththeearthssurface.Onetheywasthatonlytheseacouldprovidesuchlar
5、gequantitiesthewatermovingundergroundfromcoastalareas.Vitruviuswasthefirsttorecognizethatprecipitationprovidedthemainsourceofsubsurfacewateralthoughhisexplanationsofthemechanicsinvolvedwerenotveryscientific.Histheynowfir
6、mlyestablishedistermedtheinfiltrationtheystatesthatundergroundwateristheresultofwaterseepingdownwardsfromthesurfaceeitherdirectlyfromprecipatationindirectlyfromstreamslakes.Thisfmofwateristermedmeteic.Averysmallproptiono
7、fthetotalvolumeofsubsurfacewaterisderivedfromothersources.Connatewateristhatwhichistrappedinsedimentarybedsduringtheirtimeoffmation.Juvenilewateriswateraddedtothecrustbydiastrophiccausesataconsiderabledepthanexamplebeing
8、volcanicwater.2DistributionofSubsurfaceWaterDuringprecipitationwaterinfiltratesintotheground.Undertheinfluenceofgravitythiswatertravelsdownwardsthroughtheminutepespacesbetweenthemit(3)CapillaryFringeImmediatelyabovethewa
9、tertableisaveryshallowzoneofwaterwhichhasbeendrawnupwardsfromthegroundwaterreservoirbelowbycapillaryfce.Thedepthofthiszonedependsentirelyonsoiltexturesoilswithminutepespacesbeingabletoattractmewaterfrombelowthansoilswith
10、largepespaces.Inthelattertypesofsoilthemolecularfcesarenotabietospanthegapsbetweensoilparticles.Thussy~ilsseldomexhibitanextensivecapillaryfringemergingfromsoilwaterthroughtothezoneofsaturation.(b)ZoneofSaturationThezone
11、ofsaturationistheareaofsoilrockwhosepespacesarecompletelyfilledwithwaterwhichisentirelydevoidofsoilair.Thiszoneistechnicallytermedgroundwatereventhoughthetermbroadlyincludeswaterinthezoneofaeration.Theupperlimitofthezone
12、ofsaturationisthewatertablephreaticsurface.Itisdifficulttoknowhowdeepthegroundwaterzoneextends.Althoughmostgroundwaterisfoundintheupperthreekmofthecrustpespacescapableofwaterretentionextendtoadepthof16km.Thisappearstobet
13、heupperlimitOfthezoneorockflowagewherepressuresaresogreatthattheycloseanyinterstitialspaces.Theupperlevelofthesaturatedzonecanbecompletelyplottedbydiggingwellsatvariousplaces.Studiessuggesttwoquiteinterestingpoints.(i)Th
14、ewatertablelevelishighestunderthehighestpartsofthesurfacelowestunderthelowestpartsofthesurface.Hillsmountainshaveahigherlevelphreatiesurfacethanvalleyslakes.Thereasonfthisisthatwatercontinuallypercolatingthroughthezoneof
15、aerationliftsthewatertablewhileseepagefromthegroundwaterzoneintocreekslakeslowersthelevel.(2)Thedepthofthe“WatertableDeiowthelsurfaceisgreatestinuplareaswherethewatermovesquitefreelydownhillundergravity.Closetostreamslak
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