Soil-Water Management

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Title:
Soil-Water Management
Physical Description:
Fact sheet
Creator:
Hornsby, A.G.
Publisher:
University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
Place of Publication:
Gainesville, Fla.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Melanie Mercer.
Publication Status:
Published
General Note:
"December 1992"
General Note:
"RF-AA005"

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University of Florida Institutional Repository
Holding Location:
University of Florida
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All rights reserved by the submitter.
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IR00004396:00001


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Soil-WaterManagement1 TheInstituteofFoodandAgriculturalSciencesisanEqualOpportunity/AffirmativeActionEmployerauthorizedtoprovideresearch,educational informationandotherservicesonlytoindividualsandinstitutionsthatfunctionwithoutregardtorace,color,sex,ornationalorigin. FloridaCooperativeExtensionService/InstituteofFoodandAgriculturalSciences/UniversityofFlorida/JohnT.Woeste,DeanA.G.Hornsby2PHYSICALCONCEPTSRELATED TOSOIL-WATERMANAGEMENTThemovementandretentionofwaterinsoilsare controlledprimarilybyphysicalpropertiesofsoils. Soilstructureisdefinedasthephysicalconstitutionof asoilmaterialasexpressedbysize,shapeand arrangementofthesoilparticlesandassociatedvoids. Soilparticlesincludemineralandorganicmaterialas primaryandsecondaryoraggregateunits.Thesize distributionoftheseparticlesinfluencesthesize distributionofporesandthetotalporespace(see I-7.3).Porosity(P)ofsoilcanbecalculatedas: whereBDisthebulkdensity(thedrymassofthe natural,undisturbedsoilperunitvolume)andPDis theparticledensity(massperunitvolumeofthesolid particleswithoutvoids)(SeeSoilScienceFactSheet, SL-37).Porosityisexpressedasavolumepercent. Waterisheldwithinthevoidspacebyadhesive andcohesiveforces.Thewatermoleculesadhere (adhesiveforce)tosoilparticlesurfacesandare attractedtootherwatermoleculesbycohesiveforces. Theseforcescontrolthewettingofsoilsbywaterand thecontinuityofwaterinthesoilpores.Asoilis consideredsaturatedwhenalltheporespacesare filledwithwater.(SeeSoilScienceFactSheets,SL-37 &SL-38).WATERHOLDINGCAPACITYDuetothevoidspacesinsoils,thereexistsa capacitytostorewaterforusebyplants.Duringand immediatelyfollowingrainfallorirrigationthesoil poresarefilledwithwater.Withtime,gravity drainageresultsinadecreaseinsoil-watercontent. Theinitialrapiddrainageleadstoacondition describedas"fieldcapacity."Althoughwidelyused, "fieldcapacity"isnoteasilydefinedsinceitdepends notonlyonthesoiltextureandstructurebutalsoon thedrainagecharacteristicofthesoilprofile. Fieldcapacityhasbeendefinedasthewater contentatwhichtheinitialrapidgravitydrainage ceasesorbecomesnegligible.Forfinetexturedsoils thismaytakeseveraldays,however,forcoarse texturedsoils(sands)theconditionoccursinoneday orless.Fieldcapacitycanbemeasuredonlyinthe field.However,itisoftenestimatedforwell-drained soilsfromlaboratorymeasurementsondisturbedsoil samples:1/10-barwatercontentforsandysoil,and 1/3-barwatercontentformedium-orfine-textured soils. Plantsextractwaterfromthesoiltomeet physiologicalneeds.Asthesoildriesoutplantsmust expendmoreenergytotakeupsufficientwaterfor theirneeds.Asaplant'sdemandforwaterexceedthe soil'scapacitytoprovidethewater,theplantwill showsymptomsofwilt.Thesoilwaterwilleventually bedepletedtothepointthatthewiltedplantswillnot recover.Thisconditioniscalledthe"permanent wiltingpoint"or"permanentwiltingpercentage" (PWP)andisexpressedasthewatercontentofthe soilatwhichthatconditionoccurs.Fromlaboratory studiesthewatercontentatPWPhasbeenfoundto 1.ThisdocumentwaspublishedDecember1992asRF-AA005,FloridaCooperativeExtensionService.Formoreinformation,contactyour countyCooperativeExtensionServiceoffice. 2.ExtensionSoilScientist,CooperativeExtensionService,InstituteofFoodandAgriculturalSciences,UniversityofFlorida,Gainesville.

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Soil-WaterManagement Page2 beessentiallyequivalenttothewatercontentat15 barssoilmoisturetension. Sincegravitydrainagetofieldcapacityoccurs relativelyrapidly,wateravailabletoplantsisgenerally consideredtobethatbetweenfieldcapacityand PWP.Thispropertyiscalled"AvailableWater Capacity"(AWC).AWCcanbecalculatedasfollows: where: AWC=AvailableWaterCapacityincm BD=Bulkdensity(gms/cm) T=Thicknessofrootzoneincm FC=FieldCapacity(%byweight) PWP=PermanentWiltingPercentage(%by weight) DW=DensityofWater(gms/cm) ThusAWCisexpressedincmofwaterintheroot zoneofthickness-Tcm. Figure1providesanexampleoftherelationships betweenFC,PWP,andAWCforsoilsofthree textures.NotethattheAWCismuchgreaterforthe siltloamthanforthesandyloam.Thisdifferenceis primarilyduetothedifferencesinwatercontentat fieldcapacity.SandshaveverylowAWC.Table1 givestypicalAWCdataforselectedFloridasoils. Note: Theconceptsoffieldcapacityandavailable watercapacityholdforwelldrainedandmoderately welldrainedsoils.However,forpoorlydrainedsoils careshouldbeusedininterpretingtheseparameters. Table1. ClassificationandSoil-WaterPropertiesofSelectedFloridaSoils SoilSeriesClassification NaturalDrainage AvailableWaterCapacity Hydraulic Conductivity* 0-6" 6-12" 12-24" (inches) (in./hr.) Lakelandsand(TypicQuartzipsamment) Excessive 0.39 0.38 0.74 24.2 Paolasand(spodicQuartzipsamment)Excessive 0.140.080.21 36.0 Gainesvillesand(TypicQuartzipsamment) Good 0.65 0.57 0.90 8.3 Orangeburgfinesandyloam (TypicPaleudult) Good0.730.912.13 1.4 Tavaresfinesand(TypicQuartzipsamment) Moderatelygood 0.39 0.27 0.43 8.2 Goldsborosand(AquicPaleudult) Moderatelygood0.420.421.38 0.2 Electrafinesand (ArenicUlticHaplohumod) Somewhatpoor 0.32 0.22 0.41 <0.1 Sparrfinesand(RossarenicPaleudult)Somewhatpoor0.670.651.20 <0.1 Myakkafinesand(AericHaplaquod) Poor 0.68 0.54 0.47 3.6 Bladensand(TypicAlbaquult) Poor 0.720.721.34 <0.1 Placidloamysand(TypicHumaquept) Verypoor 2.09 1.69 1.03 7.0 Lauderhillmuck(LithicMedisaprist)Verypoor 2.412.425.77 24.6 *Leastpermeablehorizonwithinpedondepth(80in.).(FromCarlisleandHallmark,1977)

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Soil-WaterManagement Page3 Figure1. RelationshipsbetweenFieldCapacity,PermanentWiltingPercentage,andAvailableWaterCapacity forthreesoiltexturalclasses. MEASUREMENTOFSOILWATERCONTENTSoil-watercontentcanbemeasuredbydirectand indirectmethods.Themostcommonandleast expensiveisthegravimetricmethod.Thismethod consistofweighingoutapproximately20gramsof moistsoil,dryingitinanovenat105degreesCuntil thereisnoweightchangethenweighingforthefinal overdryweight.Thegravimetricwatercontent (WCg)isthendeterminedasfollows: andisexpressedasaweightpercent.Sometimesitis convenienttoexpressthewatercontentonavolume basis(WCv).Thisrequiresknowledgeofthebulk densityofthesoilwhenthesamplewastaken.The relationshipbetweenWCgandWCvisasfollows:

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Soil-WaterManagement Page4 whereWCvisexpressedasavolumepercent.Itis importanttonotethatonemustknowwhether water-contentdataareexpressedonavolumetric basisoronagravimetricbasissincetheyaredifferent andcouldleadtoseriouserrorsifused interchangeably. Indirectmethodsofsoil-watercontent measurementincludeneutronscattering,gamma-ray attenuation,andtensiometry(seeSmajstrlaand Harrison,1982).Thenuclearmethodsrequire expensiveelectronicinstrumentationandalicenseto usetheradioactivesource.Theuseoftensiometers requiresalaboratorydeterminedwater-releasecurve totranslatefromsoil-watertensionvaluestowater contentsonavolumebasis(SeeSoilScienceFact Sheet,SL-38). Formostapplicationsthedirectgravimetric methodissufficientandtheleastexpensive.(See FloridaIrrigationGuide,USDA-SCS,1982Chapter IIandAppendixA).SOIL-WATERPOTENTIAL ANDMOVEMENTWatermovesinsoilsinresponsetopotential (energy)gradients.Theseenergygradientsarisefrom severalcausessuchasgravitionalforces,capillary forces,andosmoticforces.Suchenergygradientscan beexpressedaspressureperunitarea(positiveor negative).Insoilsthesoil-waterpotentialisexpressed asanegativepressurehead,asa"suction",orasa "tension"andcanbeexpressedintermsofpressure (millibars,bars,psi,etc.)orintermsofequivalent depthofwater(inchesorcentimetersofwater).[One barpressure1034cmequivalentwaterheight14.7 poundspersquareinch(psi).] Thewatercontentofasoilatagiventimeis relatedtothepressureheadwhichisactinguponthat soil.Thewatercontentatagivenpressurehead differswithsoilsofdifferenttexturesdueto differencesincapillaryforces.Thefinerthetexture ofthesoilthesmallertheporesresultingingreater capillaryforces.Thisresultsinahigherwater contentsinfinertexturedsoilsthancoarsertextured soilsatthesamepressurehead.Figure2depictsthis relationshipforsomeFloridasoils.(SeeSoilScience FactSheet,SL-38andCarlisleandHallmark,1977). Figure2. Waterretentionatselectedtensionsfor surfacehorizonsforfourFloridasoils. Tensiometersmeasurethepressureheadofwater insoil.Sincetheabilityofplantstoremovewater fromsoilsisrelatedtothesoil-waterpressure,the tensiometerisplacedinthesoilatadepthrelevantto therootingpatternofthecropbeinggrownandis usedtomeasuredepletionofmoisture.Theamount ofdepletionallowedwilldependonthesoil,thecrop beinggrownandtheirrigationsystem. Movementofwaterinsoilisimportantfor drainage,cropuptake,andnutrientandpesticide leaching.Overirrigationcanleadtoleachingof fertilizers(especiallynitrogenandpotassium)to depthsbelowtheactiverootzone,therebyeffectively removingthenutrientsfromaccessbythecrop. Likewisesoil-appliedpesticidesmaybeleachedifthey arefairlywatersolubleandhavelowsorptionvalues (SeeSoilScienceFactSheetSL-40.Forthesereasons knowledgeofwatermovementisimportantnotonly forcropproduction,butalsoforenvironmentaland publichealthconsiderations.

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Soil-WaterManagement Page5REFERENCESV.W.CarlisleandC.T.Hallmark,1977.Drainage CharacteristicsandAvailableWaterCapacitiesof VariousFloridaSoils.ProceedingoftheFlorida Turf-GrassManagementConference,Vol. 25:108-115. USDA/SCS.1982.FloridaIrrigationGuide, Gainesville,Florida. J.Bouma,P.S.C.RaoandR.B.Brown.1982.Basics ofSoil-WaterRelationships-PartI.Soilasa PorousMedium.SoilScienceFactSheet,SL-37. J.Bouma,R.B.BrownandP.S.C.Rao.1982.Basics ofSoil-WaterRelationships-PartII.Retentionof Water.SoilScienceFactSheet,SL-38. J.Bouma,R.B.Brown,P.S.C.Rao.1982.Basicsof Soil-WaterRelationships-PartIII.Movementof Water.SoilScienceFactSheetSL-39. A.G.SmajstrlaandD.S.Harrison.1982. MeasurementofSoilWaterforIrrigation ManagementIFASCircularNum.532. P.S.C.Rao,andA.G.Hornsby.1989.Behaviorof PesticidesinSoilsandWater.SoilScienceFact SheetSL-40(Revised).