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Water Management Best Management Practices for Phosphorus Control on Organic Soils: Minimizing Water Table Fluctuations
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STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/IR00001224/00001
 Material Information
Title: Water Management Best Management Practices for Phosphorus Control on Organic Soils: Minimizing Water Table Fluctuations
Series Title: Water Management Best Management Practices for Phosphorus Control on Organic Soils series
Alternate Title: Water Management Best Management Practices for PhosphorusControl on Organic Soils: Minimizing Water Table Fluctuations
Water Management BMPs on Organic Soils: Minimizing Water Table Fluctuations
Abbreviated Title: Minimizing Water Table Fluctuations
Physical Description: Fact Sheet
Creator: Bottcher, Del
Izuno, Forrest T.
Publisher: University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
Place of Publication: Gainesville, Fla.
Publication Date: 1992
 Subjects
Subjects / Keywords: Phosphorus -- Environmental aspects -- Florida -- Everglades
Everglades Agricultural Area
Water in agriculture -- Environmental aspects -- Florida -- Everglades
Water quality management -- Florida -- Everglades
Spatial Coverage: Everglades Agricultural Area
 Notes
Abstract: Background -- Optimal water table -- Allowable water table fluctuations -- Temporal water table control -- Spatial water table control
Acquisition: Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Diana Hagan.
Publication Status: Published
General Note: "Original publication Date December 1992. Reviewed July 2002."
General Note: "AGR56"
 Record Information
Source Institution: University of Florida Institutional Repository
Holding Location: University of Florida
Rights Management: All rights reserved by the submitter.
System ID: IR00001224:00001

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page2 sametimeitisincreasingtheriskofwaterstress.In additiontothelowerwatertableadverselyimpacting cropproduction,theadditionalaeratedsoilvolume willincreasesoilmineralizationratesandrelated nutrientreleases.Downwardwatertablefluctuations, therefore,shouldbepreventedifatallpossible. Upwardfluctuationsofthewatertable,onthe Table1. Minimumwatertabledepthsformaximum yieldsintheEAA(adaptedfromSnyderetal.,1978 and1987,andCoale,1988). Crop WaterTableDepth cm.in. SnapBeans 45.7-61.018-24 Cabbage 45.7-61.018-24 Cauliflower 61.024 Celery 61.0-76.224-30 SweetCorn 76.2-91.430-36 Lettuce 45.7-61.018-24 Onions 45.7-61.018-24 Peas 45.7-61.018-24 Potatoes 45.7-61.018-24 Tomatoes 45.7-61.018-24 Escarole 61.0-76.2est.24-30est. Endive 61.0-76.2est.24-30est. Radishes 35.6-40.6est.24-30est. Parsley 35.6-40.6est.24-30est. Sod 45.7-61.0est.18-24est. Sugarcane 61.0est.24est. otherhand,cansaturateaportionoftherootzone whichwilllimitmineralization,butcanalsoadversely impactcropgrowth.Theimpactoftemporaryroot saturationoncropgrowthisafunctionofthecrop, temperature,soil,cropmaturity,aswellasofthe degree,frequency,anddurationofsaturation.Table 2providestherelativemaximumtimetoallowfor thefulldrainageoftheactiverootofmajor EvergladesAgriculturalAreacropsafterarainfall event.Thetablereflectsthemostcropsensitive condition,soadjustmentstotheprovidedvalues shouldbemadebasedonindividualfarming conditions,ifknown.Asshown,vegetablesarevery sensitivetowetsoilconditions,comparedtosodand especiallytosugarcane.Table2alsoreflectsthe potentialurgencyofdroppingthewatertablebased onthepercentoftherootzonesaturatedaftera drainageevent.Sinceahigherwatertabledoeshave theadvantageofreducingmineralizationofthe Table2. Maximumallowabletime(days),asa functionofthepercentofrootzonesaturated,tofully draintherootzoneafterarainfallevent1 Crop 100% Saturated 50% Saturated 25% Saturated Vegetables 0 .5 1 Sod 2 4 8 Sugarcane 5 9 14 1Currentdatadoesnotexistforthesecrops. Thevaluesweregeneratedbytheofthe EAAEnvironmentalProtectionDistrictand IFASexperts.Theyshouldbeconsidered advisoryonlyandshouldbeusedwith caution. soil,thedraw-downofanupwardfluctuationshould bedelayedtothemaximumallowabletimeinTable 2.Thispracticewillalsoreducepumpingvolumes. Obviously,knowledgeoftheactualwatertable locationinthefieldwillbeneededformanagement. Theuseofwatertablewells,therefore,ishighly recommended. Wesuggestthatindividualgrowersexperimenton smallplotstodeterminethesaturationsensitivityfor theirindividualcropsbecausesaturationsensitivity canvarysignificantlybetweenfarmsduetothe parametersindicatedabove.Forexperimental procedures,pleasecontactyourIFASwaterextension specialist.TEMPORALWATERTABLECONTROLTemporalwatertablecontrolmeanskeepingthe watertableascloseaspossibletotheoptimalwater tableovertime.Temporalvariationscanbestbe managedbyimprovingtheoperationalschedulesfor bothdrainagepumpsandirrigationinputs. Operationalschedulesneedtotakeintoaccountthe followingparameters: *predictedrainfall,

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page3 *actualrainfall(measuredonfarm), *pump/irrigationcapacities, *cropsusceptibilitytowaterstress, *hydrauliccapacityofditch/channelsystem, *in-fieldaswellasditch-waterlevels,and *seepage. Pumpoperationscheduleswillneedtovary accordingtotheseparametersinasophisticated fashion.Forexample,highdischargeratesmaybe necessaryatthebeginningofhighdischargeevents, whereasinsmallerstormeventspumpstart-upmay needtobedelayedtodetermineifitisevennecessary topump.Inallcases,itiscriticalthatthe operationalscheduleterminatedrainagedischarge beforethewatertableisdroppedbelowtheoptimal level. Temporalwatertablecontrolcanbestbe achievedbydevelopingrelationshipsbetweenfarm inflowandoutflowratesversusthewatertable responseinteriortoafield.Thesewatertable responserelationshipscanbedeterminedbyplotting pumpandirrigationflowratesagainstwatertable levelsrecordedwithinthefields.Examplesoftypical responsecurvesareprovidedinFigure1.Themost usefulwatertableresponserelationshipswouldbefor thetwoextremeswherethereiseitherthemaximum (wetanddraining)orminimum(dryandirrigating) availablewaterconditioninthesoilprofile(defined laterinthissection).Theearlyconditionisdepicted inFigure1.Fieldditchwaterlevelscanbeusedas roughestimatesofin-fieldwatertables,butusingdata fromwatertablewellsinthefieldsisstrongly recommended.Additionally,placingseveralwater tablerecordersthroughoutthefarmwillallowforthe determinationofthespatialvariationofwatertable responsesacrossafarm(seeSpatialWaterTable Control). Itisimportanttonoteherethatthewatertable responsecurveforbothdrainageandirrigationwill besignificantlyaffectedbyseepageintoafarm.In severeseepageproblemareas,irrigationinputmay neverbeneededbecauseirrigationdemandcanbe metorexceededbyseepage(requiringpumpage duringirrigation).Duringstormdrainage,higher dischargeratesmustbeusedtocompensateforthe additionalwater.Similarwatertableresponsecurves asthosedepictedinFigure1canbeachievedforhigh seepageareas,butatahighwatermanagementcost. Oncethewatertableresponserelationshipsare known,awaterbudgetaccountingprogramfortheinfieldrootzoneshouldbedeveloped.Thisbudget musttakeintoaccounttheevapotranspiration(ET) andrainfall(actualand/orpredicted),aswellasthe watertableresponsetoinflowsoroutflows.The watertablemovement(WTm)inresponsetorainfall andETcanberoughlyestimatedbythefollowing relationship(unitsareininches): Equation1 Note:The7-inchresponsecoefficientcanvaryfrom 5to9,dependingonthesoilproperties.Duetoits relativelylowsensitivitytowatermanagementcontrol criteria,however,7shouldworkwellformost conditions. Thisrelationshipwouldmeanthatoneinchof raincouldraisethewatertableapproximately7 inches.Thekeywordshereare"couldraise"because aportionoftherainfallorETcouldpossiblybe utilizedtoreplaceorremoveavailablewaterinthe aeratedsoilprofilewithoutdisplacingthewatertable. Inotherwords,ifthesoilisverydry,thenabout.51.0inchofrainfallmaybeneededtore-wetitbefore thewatertablewillrise.Conversely,about.5-1.0 inchofETmayhavetooccurbeforethewatertable willdrop.TheamountofrainfallorET"leftover" afterfillingtheneedsofavailablewaterinthesoil profileiscalled"excess"rainfallorET.Thestandard irrigation"accountingmethod"canbeusedtokeep trackoftheavailablewaterinthesoilprofile. Theaccountingmethodusesthefollowing relationship(unitsareininches): Equation2

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page4 Figure1. Typicalresponserelationshipbetweenthefarmlevelinflowandoutflowtothein-fieldwatertable

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page5 Thetotalavailablewaterisapproximatelyequal tothedifferencebetweenthefieldcapacityandthe wiltingpointofthesoilmultipliedbythedepthofthe aeratedsoil. Usingtheabovewaterbudgetinformation, irrigationandpumpschedulingdecisionscanthenbe optimizedforwatertablecontrol.Irrigation scheduling,drainage/pumpoperationsorpredictedvs observedrainfallshouldbeused.IrrigationSchedulingIrrigationschedulingshouldbebasedonsetting inflowratestomatchfarm-wideETratesonce availablewaterhasbeenexhausted.Thiscouldbe doneoperationallybyobservingthein-fieldwater tablelevelsand"accounting"forthecurrently availablewater.Then,usingEquation2,anestimate canbemadeofthetimewhentheexcessavailable waterwillbecomedepleted.Takingtheestimated timetodepletioninconjunctionwiththewatertable responsecurve(Figure1),thecorrecttimetoinitiate irrigationcanbecalculated. Therateoffarmlevelirrigationinflowcanbe roughlyestimatedbypredictedETrates.Continuous finetuningbaseduponobservedin-fieldwatertable levels,however,willbethebestprocedurefor maintainingoptimalwatertablesafterirrigationhas beeninitiated. Duringirrigation,theavailablewaterinthesoil profileisnormallyatitslowestlevel.Thesoil, therefore,willhavethecapacitytostoreabout.5-1.0 inchesofrainfallbeforeexcesswaterwillcausethe watertabletorise(MalaikaandBottcher,1988). However,sincetherootswillnowhavethisadditional availablewatertouse,continuingirrigationwillcause watertablestorise.Irrigation,therefore,shouldbe immediatelyterminatedafteranysignificantrainfalls (lessthan.2inches)inordertopreventupwardwater tablefluctuationswhichcouldresultinadditional futurepumpingdemands.Thetimeuntilre-initiating irrigationcanbecalculatedbythesameprocedure describedabove.DrainageorPumpOperationsDrainageorpumpoperationstoremoveexcess rainfallcanbescheduledinasimilarfashionto irrigation.Now,however,thepotentialriseinthe watertableduetomeasuredorpredictedrainfall mustbeconsideredintheschedulingofthepump(s). Duetothetimedelaysbetweenpumpstart-upand watertableresponseinthefield(Figure1),itis normallynotpracticaltouseonlytheobservedinfieldwatertablelevelsascontrolguides.Theactual orpredictedrainfall,therefore,shouldbeemployed toestimatethewatertablerisebyusingEquation1. Onceagain,theamountofavailablesoilwater storage,asdeterminedbythe"accountingmethod" describedabove,mustbesubtractedfromtherainfall beforeuseinEquation1.Thepredictedwatertable risecanthenbecomparedtothewaterresponse curve(Figure1),thecropsaturationtolerance(Table 2),andthepredictedETfortheallowablesaturation period.Thiscomparisonshouldbemadeinthe followingfashion: Step1. Obtainthepredictedwatertablelevelfrom Equation1usingtheexcessrainfall(predictedor observed)anduseitinTable2toestimatethe allowabletimeneededtoreturnthewatertableto optimallevels. Step2. DeterminethevolumeofETthatwilloccur beforethecropexperiencessaturatedwaterstressby multiplyingtheestimatedETrate--basedoncrop andseason(SeeJones,etal.,1988)--bythe allowablerecoverytimeobtainedinstep1.IftheET volumeexceedstheexcessrain,pumpsshouldnotbe turnedonandestimatesforfutureirrigation schedulingshouldbemade.IftheETvolumeisless thantheexcessrainfall,pumpingshouldbeinitiated immediatelyandrunonlyaslongasneededto removethedifferencebetweentheexcessrainfalland theETvolumecalculated.Removingthiswateras quicklyaspossiblebyusingfullpumpcapacitywill typicallyprovideforlowerphosphorusconcentrations inthedischargedwater. Step3. Repeatedcalculationswillbeneededbecause ofthevariabilityofrainfall.Eachadjustmentwill requiretherepetitionofsteps1and2witha continuoustrackingofallowablerootsaturation.By thispointithasbecomeapparentthatthese continuousandfrequentadjustmentswillbecomevery complicatedovershorttimeperiods.Itis recommended,therefore,thataportablecomputerbe programmedwiththeappropriatealgorithms.Such aprogramisnotcurrentlyavailable,butispresently beingdevelopedbytheInstituteofFoodand AgriculturalSciencesandshouldbeavailablesoon.

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page6 CheckwithyourCooperativeExtensionSpecialiston itsavailability. Theaboveprocedurewillrequiresignificant trainingofstaffandon-farmexperiencebeforeitwill becomefullyfunctional.Intheinterimperiod,itis suggestedthatatleastautomatic"cut-off"controlsbe placedonallfarmpumpstoassurethatover-drainage isreducedtoaminimum.A"cut-off"floatcanbe installedatawaterlevelinthemainfarmcanalno morethan0-6inchesbelowoptimalin-fieldwater tablelevels.Automatic"on"switchescanalsobeused toinitiateorre-initiatepumpage.Suchautomated systemswillprimarilyservetoprotectagainstpump operatorsfailingtoturnoffpumpsbeforesignificant over-drainagehasoccurred.Notethatfloatcontrol systemsarepronetofailurewithoutregular maintenanceandshouldnotbeconsidereda replacementforassigninganoperatorthejobof periodicallycheckingthepump. Anoptimallydesigneddrainagesystemwouldnot requiremultiplepumpcyclestoremoveexcess rainfall.Multiplepumpcyclingisanindicationof insufficienthydrauliccapacity,e.g.waterlevel gradientsneededtomovewatertothepumpstation wouldbeexcessive.Datahaveshownthatwater pumpedearlyinastormistypicallyofbetterquality thanwaterpumpedlaterinthestormprocess. Therefore,gettingtheexcessrainfalloutasquicklyas possiblewithoutover-drainingthefieldsisimportant. Obtainingsufficienthydrauliccapacityisfurther discussedinalatersection.UseofPredictedvsObservedRainfallUseofpredictedvsobservedrainfallneedstobe understoodintermsofwhentouseoneoverthe other.Observedrainfallshouldalwaysbeused wheneverpossiblebecauseitobviouslyrepresentsthe realsituation.However,itmaybecomenecessaryto initiatepumpingbasedon predicted rainfallifthe crop'swatersaturationstresssensitivityissuchthata delayingainingwatertablecontrolthroughuseof observeddatacouldcausecropdamage.Typically, predictedstormsoflessthan1inchofrainfallrequire noprepumping foranycrops.Stormsbetween1-3 incheswillonlyimpactvegetableswhilestorms greaterthan3inchescouldpotentiallyimpactall crops.Theproceduredescribedearlier,however, shouldbeusedtodeterminethepotentialforthe occurrenceofadetrimentalimpact.Itisimportantto notethatthesensitivityofthewatertablevaries seasonallyduetocroprotationsanddifferentgrowth periods.Fallowperiods,forexample,haveno saturationlimitations,exceptforlandpreparation needs.SPATIALWATERTABLECONTROLSpatialwatertablecontrolimplieskeepingthe watertablethroughoutthefarmasuniformas possibleatanygiventime.Variablewatertables acrossafarmaretypicallytheresultofanuneven groundsurface,inadequatehydrauliccapacityofthe primaryfarmcanalsystemandfieldditches,and/or poorculvertmaintenanceand/ormanagement.Allof theseconditionscancauseexcessivesoil mineralizationandrelatedphosphorusreleases.UnevenGroundSurfacesUnevengroundsurfacescanberesponsiblefor variablesoilmoistureconditionsandrelatedhighP lossesacrossafarmorwithinafield,evenifa uniformwatertableismaintainedthroughoutthe canal/ditchsystem.Laserlevelingisthebestwayto eliminatethesesoilsurfaceundulations.However,if yourfarmhasasignificantelevationchangefromone sideofthepropertytotheother,thencontrolculverts willbeneededtoseparatethelandintoan appropriatenumberoflargeblockswithinwhichthe soilcanbeeconomicallylaser-leveled.Boosterpumps willbeneededtomovewaterintheupslopedirection betweeneachoftheblocks.Sinceirrigationflowrate requirementsarelessthanfordrainage,itisusually mosteconomicaltohavethelandslopingtowardthe maindrainagepumpstationsothatonlyirrigation wouldhavetobehandledbytheboosterpumps.It ispossibleinsomesituationstoreleasetheirrigation waterdirectlyintothefarm'shighestelevationblock andtointhiswayeliminateanyinternalbooster pumps.Todothis,howeverthecanal,pump,and culvertsystemmustbedesignedwithsufficientflow capacity.InadequateHydraulicCapacityInadequatehydrauliccapacitycancausenonuniformdrainageand,toalesserdegree,irrigation acrossafarm.Typically,under-drainage(poor) occursinareaslocatedfurtherawayfromthepump station,whileareasnearerthepumpbecomeoverdrainedasdepictedinFigure2.Thisover-drainage

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page7 Figure2. Twocorrectivetechniquesforpoorwatertableuniformityacrossafarmduetoinadequatehydraulic capacityoffarmcanals

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page8 ofareascanresultinexcessivesoilmineralizationand relatedphosphoruslosses.Inadequatehydraulic capacitycanalsoresultinaslower,"pulsing"typeof watertabledrawdownwhichcanproducehigher phosphorusconcentrationsinthedrainagewater. Variabilityofthewatertablesacrossafarmcanbe managedbydesigningsufficientflowcapacityinthe farmcanal/ditchsystemandmaintainingand managingflashboardculvertsinfeeder/fieldditches. Inadequatefieldditchspacingasdepictedin Figure3and4canbeanotherhydrauliclimitation. Ifthesoilis"tight"duetoalowhydraulic conductivity,significantwatertablevariationsbetween thefieldditchescanoccurforlongperiodsoftime afterastorm.Theonlywaystoincreasethemid-field watertabledrawdownistodropthefieldditchesvery lowortoshortenthedistancebetweenditches.The droppingofthefieldditchwaterlevelsisnotadvised becauseofthesevereover-drainagewhichwilloccur neartheditchesbeforethemid-fieldlevelsdrop.It isrecommended,therefore,thattheditchspacingbe setappropriatelytoassuresufficientdrainage.The rateofwatertabledropatmid-fieldasafunctionof ditchspacingcanbecalculatedbyusingoneof severaldrainageequationsorcomputermodels.An agriculturalordrainageengineeringexpertshouldbe consultedtocompleteadrainagespacinganalysis. Adequatehydrauliccapacityoftheprimary canal/ditchsystemcanbedeterminedbyeithera computerhydraulicanalysisofthesystemorbyfield measurementsofwaterlevelsacrossthefarmduring apumpevent.Becauseirrigationflowratesareabout onethirdofdrainageflowrates,onlydrainageneed beconsideredforsizingthecanal/ditchsystem. Thecanalsystemshouldbedesignedtoprovide minimallysufficientdrainageforthefieldatthe furthestflowdistancefromthefarmpumpwithout droppingthewatertablesinthefieldsnearestthe pumpbymorethanafewinches.Thedrainage responserelationshipproceduredescribedina previoussectionofthisdocumentwillprovidethe necessaryassessmentinformationfordrainage capacity. Inadequateflowcapacityinacanalsystemcanbe correctedbyincreasingthesizeofthecanals/ditches and/orbyusingboosterpumpsatspecificlocations throughoutthesystem.Figure2showshowthe increasedcanalcapacityandboosterpump arrangementwouldenhancewatertableuniformity acrossthefarm.Thelocationandnumberofbooster pumpsandthesizingofcanals/ditcheswillrequirean engineeringanalysisofthecanalsystemwhichis beyondthescopeofthisguide. In-fieldwatertablenon-uniformitycanbe partiallycompensatedforwithoutincreasingcanal systemcapacitybyrestrictingtheflowfromfield ditches.Thiscanbestbeaccomplishedbyusing culvertswithflashboardrisers.Theboardsinthe culvertsclosesttothepumpstationshouldnotbe pulledbelowafewinchesoftheoptimalwatertable duringadrainageevent.Thisallowsthemainfeeder canalstodropsignificantlywithoutrapidlydraining thefieldsnearestthepump.Experiencewillhaveto beobtainedforeachindividualfarmsysteminorder todeterminetheappropriateboardsettings throughoutthefarmthatwillprovidethemost consistentuniformity.Thisprocedureismorelabor intensiveandprovideslesswatertablecontrolthan otherprocedureswhichincreasethehydraulic capacityofthedrainagesystem.Therefore,thisisnot theidealwaytogainuniformity,butitcanbeuseful whentheflashboardculvertsarealreadyinplace. Thisis,however,onlyatemporarymeasuretobe useduntilmoreappropriatecontrolmeasurescanbe implemented.IrrigationUniformityIrrigationuniformitycanbebestcontrolledbythe appropriateuseofflashboardculvertsand/orlaser leveling.Itisessentialthatthegroundsurfacebeas uniformaspossibletomaintainoptimalwatertables throughoutafarm.Therearenowatermanagement practicesthatcancorrectforvariablegroundsurfaces withinawatermanagementunitorcontrolblock. IrrigationinflowsmustexactlymatchthefarmET lossesorelsethewatertableswilleitherbegintorise orfall.ThedynamicchangesofETdemandsover relativelyshorttimeperiodscreatetheneedfor continuouscontrolofinflows.Optimalwaterlevels aretypicallymanagedeitherbyregulatingtheinflow ratesbyautomaticinflowcontrolorbyusing flashboardculvertsandare-cyclingcanalsystemas depictedinFigure5.Regulatedinflowcontroloffers thelowestlaborcostandthelowestpotentialwater dischargesfromthefarm.Itdoes,however,require averylevelfarmwithsufficienthydraulicditch capacitytoassurenomorethanafewinchesofwater tablevariationacrossafarmorafarmblock.

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page9 Figure3. Influenceofadditionalditchesfordrainagecontrol.

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page10 Figure4. Influenceofadditionalditchesfordrainagecontrol.

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page11 Regulatedinflowsforwatertablemaintenance canbeachievedbyusingautomaticallycontrolledgate structuresorpumps.Bothgatesandpumpswould utilizeafloatcontrolsystemtoactivatethem.For optimalmanagement,a"smart"controller-programmableforvariablyregulatingflowratebased onmaincanalwaterlevels--canbeemployed.The useofapumpmanagerisnormallyinsufficientto properlycontrolpumpoperationsfortheregulated inflowprocedure. Whenfarmslopeuniformityand/orautomated inflowcontrolarenotavailable,flashboardculverts canbeused.Theseflashboardculvertscanbe operatedatthefieldditchleveloratalargerblock level.Arecyclingirrigationsystemisdepictedin Figure6.Waterisfed(typicallybygravity,but sometimespumped)intothefeedendofthefield canal/ditchandspillsovertheflashboardsattheother endoftheditch.Thisallowstheflowrateintothe feedercanal/ditchtoremainrelativelyconstantwhile theflowovertheboardsvariesaccordingtotheET demandinthefield. Themanagementconcernwiththissystemisthe problemofdealingwiththereturnflowintothe collectorditch.Thiscanbeeasilyhandledbyafairly smallpumpwhichmaintainsthecollectorditch's waterlevelbelowtheflashboardelevation.Thepassthroughwaterismostreadilymanagedbybeing pumpedoffthefarm.Itcanlaterbeusedagainby anyonealongthecanalsystem.However,toprevent thisirrigationthrough-flowwaterfrombeingcredited againstyourdrainagedischarge,youshouldpumpit intotheinletbasinofthemainirrigationinlet structure.Thisprocedurewillassurethatthe through-flowwaterreturnstoyourfarm.Monitoring ofitsdischarge,thus,maynotbenecessary. Flashboardculvertswillobviouslyrestrictwater flowduringdrainageeventsunlesstheboardsare removed.Boardswouldnothavetoberemovedif adequatehydrauliccapacityexistsinthefeedside canal/ditch.This,however,wouldrequirealarger canal/ditch.

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WaterManagementBMPsonOrganicSoils:MinimizingWaterTableFluctuations Page12 Figure5. Irrigationwatertablecontrolsystemusingflashboardculvertsandareturnsystem.