FOCUS Foundation For Arable Research FAR Irrigation Management for Cropping - A Grower’s Guide FOCUS • Why is Good Irrigation Management Important? • Factors Influencing Crop Water Supply • Pulling these Factors Together • Tools for Monitoring Soil Moisture • Improving Water Use on Farms ADDING VALUE TO THE BUSINESS OF ARABLE FARMING FAR would like to acknowledge the funding support of Issue 04 August 2010 ISSUE 4
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FOCUS - far.org.nz · 2. Background. This FAR Focus edition is part of a three year Foundation for Arable Research (FAR) and MAF . Sustainable Farming Fund (MAF SFF) funded project
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FOCUSF o u n d a t i o n F o r A r a b l e R e s e a rc h
FAR
Irrigation Management for Cropping - A Grower’s Guide
FOCUS
• Why is Good Irrigation Management Important?
• Factors Influencing Crop Water Supply
• Pulling these Factors Together
• Tools for Monitoring Soil Moisture
• Improving Water Use on Farms
A D D I N G V A L U E T O T H E B U S I N E S S O F A R A B L E F A R M I N G
FAR would like to acknowledge the funding support of
I s s u e 0 4 A u g u s t 2 0 1 0
I S S U E
4
2
BackgroundThis FAR Focus edition is part of a three year Foundation for Arable Research (FAR) and MAF Sustainable Farming Fund (MAF SFF) funded project titled “Improving Irrigation Management of Arable and Vegetable Crops”. The project has included; seminars, field days, FAR Arable Updates, irrigator evaluation case studies and the development of an irrigation scheduling software tool for arable crops, AquaTRAC™.
This booklet presents guidelines for optimal irrigation management of crops. It provides background on how plant and soil factors influence irrigation requirements and then provides practical detail on how to schedule irrigation, select a soil moisture meter and assess irrigator performance. Helpful tips are displayed in boxes throughout this booklet.
The booklet doesn’t cover irrigation system design. However, Irrigation New Zealand has developed an irrigation code of practice and irrigation design standards. This can be found at www.irrigationnz.co.nz/publications/code-of-practice
This project was jointly funded by the MAF Sustainable Farming Fund, FAR and Horticulture NZ
This publication is copyright to the Foundation for Arable Research and may not be reproduced or copied in any form whatsoever without written permission.
This FAR Focus is intended to provide accurate and adequate information relating to the subject matters contained in it. It has been prepared and made available to all persons and entities strictly on the basis that FAR, its researchers and authors are fully excluded from any liability for damages arising out of any reliance in part or in full upon any of the information for any purpose. No endorsement of named products is intended nor is any criticism of other alternative, but unnamed product.
FOCUSFARFOCUS
Checklist2
1. Checklist
Checklist 3
Know your crop• Maximumrootdepth(intheabsenceofimpediments) • Rootgrowthrateandtimetoreachmaximumrootdepth • Cropcover
Successful water scheduling using a water budget requires accurate records of:• Rainfall-howmuchandwhen • Irrigation-howmuchandwhen • Evapotranspiration-fromnewspaper,NIWAorFARwebsitesor usehistoricaveragesasacloseapproximation
A soil water budget together with weather forecasts can estimate when the trigger point will be reached next• SWCtomorrow=SWCtoday+rain+irrigation-drainage- evapotranspiration • DrainagewilloccurwhenSWCexceedsfieldcapacity
Water allocationsAsregulationsareintroduced,suchasrestrictingthequantityofwatertogrowers,itwillbecomemoreimportanttooptimiseuseofirrigation.
6 Factors Influencing Crop Water Supply6
3. Factors Influencing Crop Water Supply• Cropwaterdemand
• Soilwatersupply
7Factors Influencing Crop Water Supply 7
Crop water demandWhat crops use water forPlantsarecomposedof70to90%waterandplantcellsmustbewetfortheplanttogrowandfunctionproperly.Wateristakenupbythecrop’srootsystemtomaintainasuitablewatercontentintheplant.Theplanthastinyporesinitsleaves(calledstomata)thatitmustopentoallowcarbondioxide(CO2)toenterforphotosynthesis,theprocessthatdrivescropgrowthandyield.Anunavoidableconsequenceofphotosynthesisiswaterevaporatingfromtheplantcellsanddiffusingoutthroughtheopenstomata.Thislossofwateriscalledtranspirationand99%ofthewatertakenupbytheplantislostinthisway.Theamountofwaterlostbytranspirationdependsonatmosphericconditions.(Anincreaseintemperature,sunlightandwindspeedallcauseanincreaseintranspiration).
Yield response to water stressWaterstressinplantsreducesstomatalopeningandthereforephotosynthesis.Thisleadstoareductionincropgrowthandthereforeloweryields.Waterstressisinfluencedbysoilwatercontent(SWC).Whenthesoilwatercontentisclosetoitsfullpoint(fieldcapacity),watersupplycanmeetdemandandacropcangrowatitsmaximumrate.Asthesoildries,therootscaninitiallygetenoughwaterfromthesoilfortheplanttocontinuegrowingatitsmaximumrate.Howeveratacertainpoint,calledthetriggerpoint(TP),therootscannolongerextractenoughwater,supplywillfallbelowdemand,thecropbecomeswaterstressedandgrowthdecreases(Figure1).Asthesoilcontinuesdrying,growthcontinuestodeclineuntileventuallyitstopsatwhatisoftencalledthepermanentwiltingpoint(PWP)orlowerlimit.
8 Factors Influencing Crop Water Supply8
Crop
yield
Soilwatercontent
Non-limitedgrowth Water-limitedgrowth Nogrowth
Wet Dry
WiltingpointFieldcapacity
Triggerpoint
Figure 1. Diagrammatic yield responses of crops to soil moisture deficits
Ifsoilwatercontentismaintainedabovethetriggerpointthecropwillalwaysbegrowingatitsoptimumandyieldswillnotbelimitedbywaterstress.YieldwillbereducedbelowpotentialwheneverSWCfallsbelowtriggerpoint.ThesizeofthereductionincreasesthefurthertheSWCfallsbelowthetriggerpointandthelongertheSWCremainsbelowthetriggerpoint.(Any yield lost from water stress cannot be regained!)
Table 2. Spring barley response to irrigation at the FAR Chertsey Arable Site
Timing of irrigation in relation to crop growth stageDoesitmakeadifferencewhenawatershortageoccurs?Inotherwords,doesgrowthstagematter?Untilabout35yearsago,therewasaverystrongfeelingthattherewerestagesofgrowthatwhichsensitivitytowaterdeficitswasmuchgreaterthanothers.Morerecentworkhasnotsupportedthatidea.IrrigationresearchinNewZealandoverthepast20yearshasfoundlittleevidenceofcriticalgrowthstagesforirrigationinmostofthecropstested.Anincreaseintheamountofwaterstressdecreasedyieldandtheyieldreductionforagivenamountofwaterstresswasthesameregardlessofwhenitoccurred.Theonlyexceptionsappeartobemaizeinwhichdroughttoleranceincreasedthroughtheseason,andwhitecloverinwhichsomewaterstressisneededatsomegrowthstagestopreventleavesovertoppingtheflowersandreducingyield.
Soil water supplyAvailable water capacity (AWC)Theavailablewatercapacity(AWC)ofasoildescribeshowmuchwatercanpotentiallybesuppliedbythesoilforagivenrootdepth.AWCisthedifferencebetweenfieldcapacityandpermanentwiltingpoint.
Table 4. The AWC to 1.0 m of a silt loam soil with different depths to gravel, and the necessary interval between irrigation events at different evapotranspiration (ET) rates assuming no stones in the upper layer
Maximum daily rate of water uptakeSupplyofwatertothecropisdependentontheavailablewatercapacity(AWC)ofthesoilandtheratethatcroprootscanextractit.Ageneralruleisthatacropcanextractamaximumof10%oftheAWCeachday.SoifacropisgrowinginasoilwithanAWCof50mmitwillbeabletoextractamaximumof5mm/dayofwater.Thefollowingdaytherewillbe45mmofavailablewater(assumingnorainfallorirrigation)andonthisdaythecropwillbeabletoextract4.5mmofwater.Thereforeforthesoiltomeetwaterdemandforoptimumyield,AWCmustbe10timesgreaterthandailyET.IfwetaketheETdataforLincoln(Figure2),theAWCthatmustbepresenttomeetcropdemandrangesfrom10to50mmandpeaksat80mmfordayswhenETis8mm/day.ETatLincoln(whichisrepresentativeofmuchofCanterbury)exceeds6mm/dayon5%ofdays.Thismeansweshouldaimtoensurethereisalwaysatleast60mmAWCinthesoilandwecancalculateappropriatetriggerdeficitsfromthis.
Evap
otra
nspira
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mm)
1Jul07 30Sep07 30Dec07 30Mar08 29Jun08
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6
5
4
3
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1
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Figure 2. Average daily evapotranspiration rates (mm) for Lincoln.
Soil quality and its influence on available water capacityOneofthemostimportantaspectsofsoilqualityisitsimpactonsoilAWC.Inasoilwithnolayersthatrestrictrootgrowth,theAWCisdeterminedbytherootingdepthofthecrop.InFigure3a,theAWCfordeeprootingwheatis200mmgrowinginadeepsiltloamsoilbutonly80mmforshallowrootedpotatoes.
Figure 3. Field capacity and permanent wilting point (PWP) for a range of crop/soil combinations. Numbers above each of the PWP lines represent the available water capacity (AWC) for the three crops specified
4. Pulling these Factors Together• Calculatingtriggerpointforparticular crop/paddockcombination
• Prioritisingcrops
• Irrigatortype
12 Pulling these Factors Together12
Calculating trigger point and refill amount for a particular crop/soil type combination50% x AWCByknowingacrop’srootingdepth,andthetypeanddepthofthevarioussoillayersintherootingzone,theAWCcanbeestimatedforeachcrop/soiltypecombinationasfollows:
Allowing for variabilityToscheduleirrigationoptimallyitisnecessarytoknowthevariabilityinsoilphysicalpropertiesacrossapaddock.Forexample,ifirrigationisscheduledbasedonaheavierpartofthepaddock,thecroponlightersoilmaynotbeirrigatedadequately.Inthepastthewaterholdingcapacityofsoilwasmeasuredbytakingcores.Thesepointmeasurementsgaveinformationforaspecificlocationbutnotthewholepaddock.Technologyisnowavailabletomapsoilwaterholdingcapacityoveranentirepaddock.Anelectromagneticsensordraggedoverapaddockmeasuressoilelectricalconductivitytoadepthofabout1.5m,whichiscloselyrelatedtosoiltextureandthereforewaterholdingcapacity.Mapsgeneratedfromthisdatashowthevariabilityinsoilphysicalpropertiesacrosstheareameasured.InNewZealandthereiscurrentlyonecommercialserviceavailableforthismapping(http://www.gpsfarmmap.com/en/index.php).Cropyieldmapscanalsoindicatewheresoiltypesmaybedifferent.
Prioritising cropsWhich crop should take priority if there is insufficient water?Asthesoildriesfromthetriggerpointtowardwiltingpoint,potentialyielddecreasesinastraightline(Figure1).Theslopeofthislinedescribestherelativeyielddecline(RYD).Thebiggerthepotentialyields,thegreatertherelativereductionsinyieldafterTP.Therateofyieldlosshasbeenassessedforarangeofcropsandisdescribedasapercentageofpotentialyieldpermmofsoilwaterdeficit(SWD)abovetheTP(Table6).
13Pulling these Factors Together 13
Table 6. Relative yield decline for a range of crops
Soil water budgetSofarwehavecoveredhowsoilandcropfactorswillinfluenceavailablewatercapacity,triggerpointandrefillamount.Thisinformationcanthenbeusedforirrigationscheduling.Themosteffectiveirrigationschedulingwillmeasureorcalculatesoilwatercontent(SWC)eachdaytodeterminewhenSWCwillreachthetriggerpoint.Irrigationwillthenbeappliedonthisdayattherefillamount.ThispracticewillkeeptheSWCabovetriggerpointsoyieldismaximisedbutmaintainSWCbelowfieldcapacitytominimisetheriskofdrainageandsubsequentleaching.
Forecasting soil water content (SWC) using a water budget is based on the following equation:SWCtomorrow=SWCtoday+irrigation+rainfall-evapotranspiration-drainage
Example of a soil water budgetAnexampleofasoilwaterbudgetispresentedinFigure4.Awheatcropwasplantedon1August.Thesoilhas0.6mofsiltloamoverlyinggravelsandarootdepthof1.0mgivinganAWCof120mm,aTPof60mm,andarefillamountof50mm.
When to stop irrigatingThescenariooutlinedaboveassumesthatcropsrequireoptimalwatersupplyatalltimes.Somecropsbenefitfromaperiodofmoisturestresswhenirrigationshouldnotbeapplied(e.g.whitecloverseedcrops).Tocalculatewhenirrigationshouldbestoppedtoproduceastressperiod,identifywhentheperiodofstressiswanted.UsethewaterbudgettoworkoutwhenirrigationshouldbestoppedtoensuretheSWCwillbebelowTPatthattime.Thewaterbudgetcanbeusedinasimilarmannertodeterminewhenirrigationshouldbestoppedattheendoftheseason.Estimatewhenthecropwillcompletegrainfill,identifythispointonthewaterbudgetandstopapplyingirrigationsoSWCisreachingTPataboutthistime.Thewheatcalculatorforecaststhedategrainfilliscomplete.
Soil Water MeasurementAsoilwaterbudgetisinexpensiveandreasonablyaccurateifitisdonecarefully.However,errorscanaccumulateovertimesosoilmoisturemeasurementsareagoodwayofgettingthebudgetbackontrack.Therearearangeofdevicesthatcanbeusedincluding:
Time Domain Reflectometry (TDR)TheTDRmethodinvolvesconnectingasmallelectronicdevicetoapairofmetalrodswhichareinsertedintothesoil.Therodsareusuallyabout5mmindiameterandnormallynolongerthan70cm.Goodcontactwiththesoiliscritical.Theelectronicdeviceemitsapulseofelectromagneticradiation,similartoaradarpulsewhichtravelsdowntherods.TheTDRdevicethenmeasuresthetimetakenforthepulsetobereflectedbackfromtheendoftherodsandthisisrelatedtothesoilwatercontent.Usedinresearch,especiallyforclose-to-surfacesoilmeasurements.
6. Improving Water Use on Farms• Matchingcropdemandtowatersupply
• Irrigatormanagement
20 Improving Water Use on Farms20
Matching crop demand to water supply (sowing time/crop rotation)GenerallywhenETishighinsummeritisdifficultforirrigationsystemstokeepupwithcropdemand.Arablegrowerscanmanagethissituationbyincludingcropsthatdon’tallrequirefullirrigationduringthisperiod.Forexample,whitecloverrequireslesswateruptoandduringfloweringandpastureintherotationmaybesacrificedtofullyirrigatemorevaluablecrops.
Irrigator managementi. Sources of lossWecanuseourknowledgeofcrops,soilsandsoilwatertodeterminewhentoirrigateandhowmuch.Thefinalpartofthepackageistoensuretheirrigatorisapplyingtherightamountofwaterandthatnoneofthewaterbeingappliedislost.
ii. Distribution uniformity (DU)AsystemwithalowDUmeansthatapaddockisbeingwateredlessuniformly(evenly)thanasystemwithahighDU.DifferenttypesofirrigatorshavedifferentinherentDUs,withcentrepivotsandlateralmoveshavingadesignefficiencyofaround80-90%androtatingboomsaround60-70%.ExamplesofirrigatoruniformitiesfoundinarecentsurveyaregiveninTable8,theseshowthattheextraamountofwaterneededtoensurethatthewholepaddockreceivesthedesiredamountofwaterrangesfrom17%to122%morethanwhatwasactuallyapplied.
Table 8. Water applications and distribution uniformities measured on six irrigation systems, together with the amount of water which would need to be applied to cover whole paddocks with the intended amount.
Sources and Further readingMartin,R.J.,Thomas,S.M.,Zyskowski,R.F.,andStevens,D.R.Irrigationefficiencies.ProvidedbySIDE-SouthIslandDairyEvent,NewZealand.
A D D I N G V A L U E T O T H E B U S I N E S S O F A R A B L E F A R M I N G
Foundation for Arable Research PO Box 80 Lincoln 7640 New Zealand Phone: +64 3 325 6353 Fax: +64 3 325 6354 Email: [email protected] Web: www.far.org.nz