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

Contents 1

Contents1. Checklist .................................................................................................................... 2

2. Why is Good Irrigation Management Important? ................................. 4

3. Factors Influencing Crop Water Supply ...................................................... 6 • Cropwaterdemand ................................................................................................. 7 • Soilwatersupply ...................................................................................................... 8

4. Pulling these Factors Together ................................................................... 11 • Calculatingtriggerpointforaparticularcrop/paddockcombination .................... 12 • Prioritisingcrops .................................................................................................... 12 • Irrigatortype ......................................................................................................... 13

5. Tools for Monitoring Soil Moisture ............................................................ 14 • Soilwaterbudget .................................................................................................. 15 • Soilmoisturemeters .............................................................................................. 16

6. Improving Water Use on Farm ..................................................................... 19 • Matchingcropdemandtowater ........................................................................... 20 • Irrigatormanagement ........................................................................................... 20 • Costs ....................................................................................................................... 21

7. Glossary and Acknowledgements .............................................................. 22© Foundation for Arable Research (FAR)

DISCLAIMER

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

Know your soil • Availablewatercapacityofuppersoillayer • Stonecontentofuppersoillayer • Depthtogravellayer • Rootimpediments(cultivationpansandcompactsubsoils)

Determine key parameters for each paddock• Availablewatercapacity(availablewatertothecrop’srootdepth) • Triggerpoint-about50%ofavailablewatercapacity • Refillamount(howmuchwatershouldbeappliedbyirrigation?)

Evaluate distribution uniformity of irrigator

Effective irrigation aims to keep soil water content (SWC):• Abovetriggerpointsoyieldisnotbeinglost • Belowfieldcapacitysodrainageandleachingarenotoccurring

Schedule irrigation by either: • Useaschedulingserviceor • Calculatesoilwatercontentwithasoilwaterbudgetor • Useacropcalculator(whichprovidesadetailedwaterbudget)or • Measureyoursoilmoisturecontent

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

Soilmoisturemeasurementsmaybeusedtoindicatehowcloseasoilistothetriggerpointbutconsiderationofrootdepth,soilcharacteristicsandupcomingweathereventsarenecessarytodeviseanirrigationschedule.

4 Why is Good Irrigation Management Important?4

2. Why is Good Irrigation Management Important?• Crop

• Environment

• Cost

• Waterallocations

5Why is Good Irrigation Management Important? 5

CropEnsuringcropsarefullywateredhelpsmaximiseyieldandquality.

EnvironmentIftoomuchwaterisappliedthereisariskthatnutrientswilldrainintogroundwater.Ontheotherhand,withgoodirrigationmanagement,thepaddockisnotirrigatedtofieldcapacitysothatthereisspaceleftinthesoilprofiletoutiliserainfall.

Cost Arecentsurveyshowedtheaveragecapitalandoperatingcostofirrigationwas$2/mm/ha.Applyingjustenoughwatertomaximiseproductionmeansexcesswaterisnotpumped,thereforereducinglabourandenergycosts.Also,thecostofnutrientlossindrainagewillbeminimised.Goodirrigationsystemdesign(e.g.correctpipesizing)andmaintenancewillensureoperatingcostsareminimised.

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).

Waterstressoccurswhenacrop’srootsystemisunabletosupplyenoughwaterfromthesoiltomeetthedemandimposedbytheatmosphere.Inthesesituationsleafwaterpotentialdeclines(leavesbecomeflaccid)andstomataclosetomaintainasuitablewatercontent.StomatalclosurereducestheamountofCO2thatcanentertheleafwhichreducesphotosynthesis.Thereductioninphotosynthesiscausesareductioninthecrop’sbiomassproductionandsubsequentyield.Tomaximiseyieldsitisnecessarytoprovideenoughwatertoensurewatersupplyalwaysmeetscropdemand.

RootsCroprootdepthandrootarchitecturehasanimportantinfluenceonhowmuchsoiltheplantcanexploretoextractwater.Theavailablewatercapacity(AWC)ofasoildescribeshowmuchwatercanpotentiallybesuppliedbythesoilforagivenrootdepth.Forashallowrootedcrop,suchaspeas,theAWCislessthanadeeprootedcropsuchasautumnwheat.Thus,itisnecessarytoknowhowdeepacrop’srootswillgoandhowdeeptheyarewhentheirrigationseasonstarts.Croprootswillstopatanimpedingsoillayersuchasstones,ironpan,cultivationpanorwatertable.ThismustbetakenintoaccountwhencalculatingrootingdepthandAWC.Providedthereisnolimitationtorootingdepth,themaximumcroprootingdepthsarearound1.5mformaizeandwintercereals,1mforspringcereals,firstyearcloverandfirstyeargrassseedcropsand0.8mforpeas.Cropscantaketwotosevenmonthstogettothesedepths,thereforeirrigationwillneedtobeadjustedtoaccountforshallowerrootswhenthecropisyoung.

Providedsuitableconditionsprevail,plantrootscanextendthroughthesoilatratesofupto50cmdepthpermonthduringsummer(thisratecanbeusedforallcropsbutthedataismostrobustforcereals).Thisgrowthrateislessduringwinter(20cm/month)thuscropsowingtimewilleffecthowdeeptherootsareatanygiventime.Cropssowninspringandsummerwillreachtheirmaximumrootingdepthfasterthanautumnsowncrops(Table1).Thecurrentrootingdepthofyourcropwillaffecthowmuchirrigationyoushouldapplyandthereturnperiod.Shallowrootedcropsrequiremorefrequentirrigationwithlesswaterappliedeachtimethandeeperrootedcrops.

Table 1. Rooting depth (m) up to six months after sowing for a cereal crop sown at four different times

monthsaftersowing 1Mar 1May 1Aug 1Oct

1 0.5 0.3 0.2 0.4

2 0.8 0.5 0.5 0.9

3 1.0 0.7 0.9 1.4

4 1.2 0.9 1.1 -

5 1.4 1.2 1.6 -

6 1.6 1.6 - -

Crop coverCropcoverinfluencescroptranspirationandsoilevaporation,collectivelytermedevapotranspiration.Evapotranspiration(ET)isakeyelementofawaterbudgetandisreportedinmm.ClimaticconditionsthatdriveETaretemperature,radiation,humidityandwind.ETratescanreachashighas8mmperdayonhotwindydaysinsummer.DailyETvaluesareoftenpublishedinnewspapersandcanbeobtainedfromtheFARandNIWAwebsites(www.far.org.nzandwww.niwa.co.nz).

Evapotranspirationrepresentshowmuchwaterwillbetranspiredfromawellwateredcropthatisfullycoveringtheground.Acropwithanincompletegroundcoverdoesnotuseasmuchwater.Asanapproximaterule,acropwith50%groundcoverwilllosewaterat50%ofET,acropwitha25%groundcoverwilllosewaterat25%ofETandsoon.TheremainingportionofETdrivesevaporationfromthesoil.AbaresoilwilllosewateratasimilarratetoETifthesoilsurfaceiswet.Soforacropwith50%coversoonafterarainfallorirrigationevent,transpirationwillbe50%ofETandevaporationwillbe50%ofET.However,soilevaporationquicklydropsasthesoilsurfacedriesandwillbecomenegligibleafterafewdays.Formostarablecrops(cereals,peasetc),therateofwateruseisthesame.Theamountofwaterusedbyarablecropsisgenerallynotaffectedbycroptype.

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

Year Irrigatedyield (t/ha)

Dryland yield (t/ha)

Irrigationapplied(mm)

Responsetoirrigation (t/ha)

2009 9.0 5.1 315 3.9

2008 10.8 5.3 185 5.5

2007 8.0 8.1 80 -0.1

2006 7.5 5.4 230 2.1

2005 9.9 10.0 75 -0.1

2004 8.5 4.1 200 4.4

average 9.0 6.3 2.7

AnexampleofactualyielddecreaseduetodroughtisshowninTable2.AttheFARChertseyArableSitespringbarleyyieldwashalvedondrylandcomparedwithirrigatedintwoofthepastsixyears.

Timing of irrigation in relation to crop growth stageDoesitmakeadifferencewhenawatershortageoccurs?Inotherwords,doesgrowthstagematter?Untilabout35yearsago,therewasaverystrongfeelingthattherewerestagesofgrowthatwhichsensitivitytowaterdeficitswasmuchgreaterthanothers.Morerecentworkhasnotsupportedthatidea.IrrigationresearchinNewZealandoverthepast20yearshasfoundlittleevidenceofcriticalgrowthstagesforirrigationinmostofthecropstested.Anincreaseintheamountofwaterstressdecreasedyieldandtheyieldreductionforagivenamountofwaterstresswasthesameregardlessofwhenitoccurred.Theonlyexceptionsappeartobemaizeinwhichdroughttoleranceincreasedthroughtheseason,andwhitecloverinwhichsomewaterstressisneededatsomegrowthstagestopreventleavesovertoppingtheflowersandreducingyield.

Theapparentpresenceofcriticalstages(e.g.floweringinpeas)canbeexplainedbythecrop’spatternofevapotranspirationdemand.Inhotweatheracropwithfullcoverwilllosewateratafasterrateandgobeyond

triggerpointfasterthanacropwithpartialcoveroracropincoolweather.Therefore,anygrowthstagethatoccurswhenacrophasfullcoverorathottertimesoftheyearwillappeartobemoresensitivetowaterstress.Thisisbecausethewaterstressarrivesandincreasesinseverityfaster,notbecausetheparticulardevelopmentphaseismoresensitivetowaterstress.Fortheexampleoffloweringinpeas,itappearsmostsensitivetowaterstressbecauseitoccurswhenthecrophasfullcoverandatahottimeoftheyear.Foranycropitcouldbesaidthatthemostsensitivestagesarethosewhenthecrophasthehighestevapotranspirationdemand.

Soil water supplyAvailable water capacity (AWC)Theavailablewatercapacity(AWC)ofasoildescribeshowmuchwatercanpotentiallybesuppliedbythesoilforagivenrootdepth.AWCisthedifferencebetweenfieldcapacityandpermanentwiltingpoint.

• Fieldcapacity(FC)isthesoilwatercontentatitsfullestpointafterthe largestporeshavedrainedandistheupperlimitofwaterstoragein thesoil.

• Permanentwiltingpoint(PWP)isthelowerlimitforcropwater extractionandisthepointwhenthegrowthofacropcompletelystops duetowaterstressandispermanentlywiltedthroughouttheday.

Fieldcapacityandpermanentwiltingpointaremeasuresofsoilwatercontent(SWC)andarebothmeasuredinmm.ThusAWCismeasuredinmmalso.Thesethreemeasuresofsoilwatercontent(FC,PWPandAWC)areneededtocalculateimportantfactorsforefficientirrigation.Theseinclude:

• Thetriggerpoint(TP),thesoilwatercontentwhenwaterstressoccurs andcropgrowthdeclines.Thiscanbeestimatedas50%ofavailable watercapacity(AWC).Thetriggerpointcanbeusedasthesoilwater contentatwhichirrigationisneeded.

• Readilyavailablewater(RAW)istheamountofwaterinasoilbetween thetriggerpointandfieldcapacity.Ifirrigatingwhenthesoilwater contentisatthetriggerpoint,RAWishowmuchisneededtore-wet thesoiltofieldcapacity,althoughirrigatingtofieldcapacityis notrecommended.

Soilmoisturedeficit(SMD)isthedifference(mm)betweenthesoilwatercontentandfieldcapacity.Soilwatercontent(SWC)istheactualamountofwater(mm)heldinthesoil.Thiscanbeestimatedfromasoilwaterbudgetormeasuredthroughavarietyofsoilmoisturemonitoringdevices(seesection5).Giventheimportanceofsoilavailablewatercapacityinirrigation,itisimportantyoudetermineAWCforyoursoiltype.AWCisastaticsoilproperty,sodeterminingAWCisaone-offexercise.

Mostsoilsarelayersofdifferentmaterials,andthedepthandtypeofthosematerialswillaffectAWC.TodetermineAWC,digaholetothemaximumrootdepthanddescribethedepthandthetexturesofthoselayers.UsethevaluesinTable3tocalculateAWC.

Example1.AWCto1mofasoilthatis60cmofsiltloamovergravels wouldbe:

(160mm/m(siltloam)x0.6m)+(55mm/m(gravels)x0.4m)=118mm

Example2.AWCto80cmofasoilthatis30cmofsiltloamoverclayloamwouldbe:

(160mm/m(siltloam)x0.3m)+(130mm/m(clayloam)x0.5m)=113mm

9Factors Influencing Crop Water Supply 9

Table 3. AWC of different soil textures

Soiltexture AWC(mm/m)

Clay 110-150

Silt 155-165

Sand 65-110

Gravels 50-60

Stones 0

Foreachlayer(otherthangravels),itisimportanttodeterminetheamountofstones.StonesholdnowaterandhencehaveadilutioneffectonAWC.AsoilthatishalfstoneswillonlyhavehalftheAWC.Thevolumeofstonescanbedeterminedbydiggingoutaknownvolumeofthesoillayer(e.g.areaof20cmx20cmand30cmdeep),separatingoutthestonesandmeasuringtheirvolume(theeasiestwayistomeasurehowmuchwatertheydisplacefromafullbucket).Thevolumeofstonesisexpressedasa%ofthetotalvolumethatwasdugout.

Thedeeperthesoil,thehighertheAWCandthelongerittakestoreachthetriggerpoint(Table4).Onashallowstoneysoil(egLismore),irrigationisrequiredatleastweeklytopreventlossofyieldinmidsummer,whereasonadeepsiltloamsoil(egTempleton),irrigationcanbeextendedouttoeverytwoweeksormorewithoutyieldloss.

Tip

AnotherwaytocalculatehowmuchplantavailablewaterthesoilholdsistouseaneutronprobeorAquaflextomeasuretheSWCwhenthesoilisatfieldcapacityduringwinter,andagainwhenthesoilisdryandthecropisatpermanentwiltingpoint.Acommercialirrigationmonitoringservicecanalsodothis.

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

Depthtogravel(m)

AWCto1.0mdepth(mm)

RAWfromFCtoTP(mm)

DaysfromFCtoTP@

3mm ET/day

DaysfromFCtoTP@

5mm ET/day

DaysfromFCtoTP@

7mm ET/day

0.2 72 36 12 7 5

0.4 94 47 16 9 7

0.6 116 58 19 12 8

0.8 138 69 23 14 10

1.0 160 80 27 16 11

Insomesituationscultivationpansorcompactedsubsoilswillstoprootpenetration.Thisreducesrootdepthtothedepthoftheimpededlayer,sothepresenceofimpededsoillayersmustbeknowntoaccuratelyestimatecroprootdepthandAWC.

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

tion(

mm)

1Jul07 30Sep07 30Dec07 30Mar08 29Jun08

7

6

5

4

3

2

1

0

Figure 2. Average daily evapotranspiration rates (mm) for Lincoln.

Soil quality and its influence on available water capacityOneofthemostimportantaspectsofsoilqualityisitsimpactonsoilAWC.Inasoilwithnolayersthatrestrictrootgrowth,theAWCisdeterminedbytherootingdepthofthecrop.InFigure3a,theAWCfordeeprootingwheatis200mmgrowinginadeepsiltloamsoilbutonly80mmforshallowrootedpotatoes.

ThepresenceoflimitinglayersrestrictsrootingdepthandaltersAWC.InFigure3b,alimitinglayerofgravelshasbeenintroducedat60cm.ThegravelshaveverylowAWCandtheimpactonAWCatthisdepthhasagreatereffectonthedeeperrootedcrop(wheatAWC=145mm)thantheshallowrootedcrop(potatoAWC=75mm).Ifgravelswereshallowerortherewerestonesintheuppersoillayer,theAWCwouldbeevenless.

Plantscanonlyaccess10%oftheAWCintheirrootzoneeachday.

TocalculatetheAWCneededtoavoidyieldloss,multiplytheETx10.

Onlightorshallowsoilsthepotentialyieldmaynotbeachievablebecauseevenwhenthesoilisatfieldcapacity,theAWCmaybelessthan10xET.

10 Factors Influencing Crop Water Supply10

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

SoilcompactionisanotherexampleofalimitinglayerandeffectsAWCintwoimportantrespects.Firstly,soilcompactionreducesporespacebetweensoilaggregatesthatareimportantforwaterstorage.Secondly,compactionalsoreducestheabilityofplantrootstoexplorethesoil.InFigure3c,acompactedsubsoilat30cmhasbeenintroducedandAWChasbeendramaticallyreducedforallcrops.Potatocropsinparticularareverysensitivetocompactionanditiscommonforthemtoonlypenetrateto30cmdepth,andinthiscase,resultsinanAWCofonly40mm.Thiscropwillbehighlypronetowaterstressandwillnotachievemaximumyieldbecausesomemoisturestressisinevitable.

Soil managementSoilswhichhavebeendirectdrilledforanumberofyearsdifferfromconventionallycultivatedsoilsintermsofaggregatesize,aggregatestability,porosityandorganicmatter.ThesefactorsinfluencesoilAWC.TrialsattheFARChertseyArableSiteandtheMillenniumTillageTrial(MTT)atLincolnshowthattheseparametersaretypicallyimprovedinthetopsoilascultivationintensityisdecreased.IntheMTT,thesoilstructuralconditionscorecard(SCS)wasusedtoevaluatethestructuralconditionofsoilsineachofthetillagetreatments.Scoresrangedfrom1to10,where1waspoorestand10thebestsoilstructuralcondition.Aftersevenyearsofcontinuouscropping,boththeminimumandno-tillagetreatmentsmaintainedSCSwellwithintheoptimumrange(5-10).HowevertheSCSofintensivetillagesoilsfellbelowthepreviouslyestablishedcriticallimitof5from2005onwards.

Cultivationalsocausesmoisturelossfromthesoil(aeration,warming,residueburial)andreducedcultivationtechniqueshelpretainsoilmoisture,thekeyreasonforadoptionofdirectdrillingindryenvironmentssuchasAustralia.Remedialcultivationsuchasrippingsoilswithacompactedlayermayallowrootstoaccessagreatervolumeofsoilmoisture.

Sources and further readingMcLaren,R.G.andCameron,K.C.(1997)SoilScience:Sustainableproductionandenvironmentalprotection.OxfordUniversityPress

White,J.andHodgson,J.(2002)NewZealandPastureandCropscience.OxfordUniversityPress

11Pulling these Factors Together 11

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:

1. Estimatetherootdepthofthecrop(RDinmetres)consideringtime sinceplantingandthepossiblepresenceofimpededlayers.Thiscan beestimatedfromsowingtime(seesection3andTable1)orby diggingaholetofindhowfardownrootshavegrown.

2. CalculatetheAWCofeachlayerofsoilwithinthatrootdepth,adjusting non-gravellayersforstonecontent(seesection3andTable3).

3. SumtogethertheAWCofeachlayerwithintherootzone.

FromthetotalAWC,thetriggerpoint(TP)andrefillamount(RA)foreachpaddockcanthenbecalculatedas:

• Triggerpoint(TP)=AWCx0.5 • Refillamount(RA)=TPx0.8

Thiscalculationforrefillamountwillprovideenoughwatertotakethesoilfromtriggerpointto90%offieldcapacity.Thiswillensurenodrainageoccursfromirrigationandthereissomecapacityinthesoiltoabsorbanyrainfallthatmayoccursoonafterirrigation.

ExamplecalculationofAWCforasiltloamsoilwithgravelfrom0.6mandacropwitharootingdepthof1.5m

AWCsiltloam=160x0.6=96mm

AWCgravels=55x0.9=49.5mm

AWC=96+49.5=145.5mm

TP=145.5x0.5=72.7mm

RA=72.7x0.8=58mm

Maximum daily rate of water uptake

OnsoilswithalowAWC,thetriggerpointmayneedtobereducedwhenETishigh.

Theprevioussectiondescribesthecommonlyusedruleforirrigationschedulingusingatriggerdeficitat50%ofAWCforagivencrop/soilcombination.Thisrulewillensureadequateirrigationpracticesforsoil/cropcombinationsthathaveanAWCof120mmorgreaterbecause50%oftheAWCwillleaveatleast60mmofavailablewaterinthesoil.Becauseplantscanonlyextract10%ofAWCperday,then60mmofsoilwaterwillprovideenoughmoisturewhenETisupto6mm/day(typicalETratesforsummer).IfETexceeds6mm/dayand/ortheAWCislessthan120mm,thenwaterstressislikely.Thusthetriggerpointshouldbeadjustedsothereisalways60mmofsoilwaterremaininginthesoil(Table5).

Table 5. Calculating the trigger point for a range of soil AWC

SoilAWC(mm) TP(mm)so60mmsoilmoistureremainsinsoil

40 CropwillbeunderwaterstressatET>4mm

80 20

120 60

160 100

200 140

Table5showsthatforasoilwithanAWCofonly40mm,thecropwillbewaterstressedwhenETexceeds4mm/dayevenifthesoilisatfieldcapacity.UsingdatafromtheLincolnweatherstation,ETwouldexceed4mm/dayonapproximately25%ofdaysinatypicalseason.Usingtheequationsoutlinedinsection4,therefillamountiscalculatedas16mm,whichwouldrequirefrequentirrigation(every4daysifETwere4mm/d).Manyirrigationsystemsdonotallowthisfrequencyofirrigationsoitislikelythatsupplywillnotmeetdemandattimesduringtheseasonandyieldwillbelessthanthepotentialmaximum.Thishelpsexplainwhycropyieldsarealwaysloweronveryshallowsoils(lowAWC)evenwhentheyarefullyirrigated.

Allowing for variabilityToscheduleirrigationoptimallyitisnecessarytoknowthevariabilityinsoilphysicalpropertiesacrossapaddock.Forexample,ifirrigationisscheduledbasedonaheavierpartofthepaddock,thecroponlightersoilmaynotbeirrigatedadequately.Inthepastthewaterholdingcapacityofsoilwasmeasuredbytakingcores.Thesepointmeasurementsgaveinformationforaspecificlocationbutnotthewholepaddock.Technologyisnowavailabletomapsoilwaterholdingcapacityoveranentirepaddock.Anelectromagneticsensordraggedoverapaddockmeasuressoilelectricalconductivitytoadepthofabout1.5m,whichiscloselyrelatedtosoiltextureandthereforewaterholdingcapacity.Mapsgeneratedfromthisdatashowthevariabilityinsoilphysicalpropertiesacrosstheareameasured.InNewZealandthereiscurrentlyonecommercialserviceavailableforthismapping(http://www.gpsfarmmap.com/en/index.php).Cropyieldmapscanalsoindicatewheresoiltypesmaybedifferent.

Allowingforvariabilityintheweatherincludesleavingaround20mmsoilmoisturedeficitafterirrigationincaseitrains(unlesstheAWCisverylowasdescribedintheprevioussection).

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

Cropspecies %decreaseinpotentialyieldforeachmmbelowthetriggerpoint

Maize*,oats,potatoes,sweetcorn 0.10%

Ryegrassseed 0.14%

Barley,peasandwheat 0.25%

*maize sensitivity to drought decreases with time because the TP increases as the root system develops

Theformofyieldlosscanchange.Forinstance,earlydroughtstressinbarleyreducesgrainnumbermostlythroughthelossoftillers,whereaslatedroughtstressresultsinsmall,pinchedgrainwithhighscreeninglosses.

Theyieldresponsetoirrigationistheresultofavoidingasituationinwhichthetriggerpointisexceeded.Inroundterms,a50mmirrigationonaveryhealthywheatcrop(potentialyield13t/ha)appliedjustbeforetriggerpointisreachedis13t/hax50mmx0.25%=1.63t/ha.Assumingirrigationcosts$100/ha($2/mm)andthewheatisworth$300/t,thereturnwouldbe$480/ha,anetgainof$380/ha.

Forthesameirrigationona2t/haryegrassseedcrop,theyieldresponsewouldbe2t/hax50mmx0.14%=0.14t/ha.Atavalueof$2,200/t,thereturnwouldbe$308/ha,andreturnnettoirrigationof$208/ha.Inthisexample,iftherewasinsufficientwateritwouldpaytoirrigatethewheataheadoftheryegrass.

ThereisaspreadsheetontheFARwebsitewhereyoucandothiscalculationforarangeofcrops.www.far.org.nz.

Tip

Ifthesoilwatercontentisbelowthetriggerpointandtherearetwowarmsummerdays(equivalenttoETof10mm)therewillbeapotentialyieldlossof1to3%dependingonplantspecies.

Irrigator typeOnsoilswithlowAWCandhotdryweatherwithhighET,irrigationwillbeneededeveryfewdays.Centrepivotandlinearmoveirrigatorsareidealinthissituationastheamountofwaterappliedcanbeeasilycontrolled,lowratescanbeappliedandreturntimescanbeshort.

14 Tools for Monitoring Soil Moisture14

5. Tools for Monitoring Soil Moisture• Soilwaterbudget

• Soilmoisturemeters

15Tools for Monitoring Soil Moisture 15

Therearetwomainmethodsforschedulingirrigation:thesoilwaterbudgetandsoilmoisturemeters.

Soil water budgetSofarwehavecoveredhowsoilandcropfactorswillinfluenceavailablewatercapacity,triggerpointandrefillamount.Thisinformationcanthenbeusedforirrigationscheduling.Themosteffectiveirrigationschedulingwillmeasureorcalculatesoilwatercontent(SWC)eachdaytodeterminewhenSWCwillreachthetriggerpoint.Irrigationwillthenbeappliedonthisdayattherefillamount.ThispracticewillkeeptheSWCabovetriggerpointsoyieldismaximisedbutmaintainSWCbelowfieldcapacitytominimisetheriskofdrainageandsubsequentleaching.

ClearlytheAWCofapaddockwillchangeasrootdepthincreases.Autumnsowncropswillhaverootsclosetothemaximumdepthbythetimetheirrigationseasonstartsinthespringsothereisnoneedtoconsiderchangingrootdepthforsuchcrops.However,laterspringandsummersowncropswillstillbeextendingtheirrootsduringtheirrigationseasonsoTPandRAwillincreaseastheseasonprogresses.Suchcropsmayneedirrigatingmorefrequentlyandinsmalleramountsearlierintheirgrowthcycletoensuretheyarenotwaterstressed.Adetailedirrigationscheduleshouldtakethisintoaccount.

Forecasting soil water content (SWC) using a water budget is based on the following equation:SWCtomorrow=SWCtoday+irrigation+rainfall-evapotranspiration-drainage

ETgivesagoodestimateoftranspirationwhenthecropiswellwateredandfullycoveringtheground.WhenthecrophasincompletegroundcoverasimplecorrectionisachievedbyhalvingETifcropcoverislessthan50%andusingthefullrateofETifcropcoverisgreaterthan50%.Evaporationismoredifficulttoestimateasitchangessubstantiallydependingontimesinceawettingevent.Ageneralruleisthatforacropwith50%groundcover,evaporationwillbe50%ofETfollowingawettingevent,thendecreaseto35%onday2,20%onday3andthenbe15%onallfollowingdays.Cropcalculatorsaremoreaccurate.Theypredicttranspirationandevaporationfromdetailedcalculationsconsideringgroundcoverandsoilwatercontent.

IftheSWCisabovetheTP,theeffectivecropETwillbeequaltoET.WhenSWCisbelowtheTP,effectivecropETwillbethelesserofET(demand)orsoilwatersupply.Soilwatersupplyistheamountofwaterthatthecropcanextractfromthesoileachdayandiswellrepresentedas10%oftheavailablewaterinthesoil.Forexample,iftheavailablewatercontentis20mm,soilwatersupplywillbe2mmonthatday(seeSection3.Maximumdailyrateofwateruptake).

Rainfallandirrigationneedtoberecordedforthewaterbudgettobeaccurate.Rainfallvariessubstantiallyovershortdistancessoitisbesttouserecordsfromarain-gaugeonthefarm.Theamountofirrigationappliedmayvaryfromthemanufacturer’sspecifications.Itisagoodideatochecktheactualdepthofwaterapplied.Therearealsoanumberofinefficienciesassociatedwithirrigationsystemsandthesemayneedtobeaccountedforinawaterbudget.Therearecommercialservicesavailabletoassesstheapplicationratesandefficienciesofirrigatorsandprovideadviceonpossibleimprovements.Therearesomeeasychecksgrowerscandothemselves(seeSection6.Distributionuniformity).Intheabsenceofsuchanassessmentitcanbeassumedthatanirrigatoris80%efficient,sotheapplicationamountdeterminedfromrain-gaugesorflowmetersshouldbemultipliedby0.8forentryintothewaterbudget.

DrainageshouldbeminimalunderwellmanagedirrigationbecauseSWCwillbemaintainedbelowfieldcapacity.However,drainagecanstilloccurwhenrainfalleventsfillthesoiltoabovefieldcapacity.Whiledrainageisincludedinthesoilwaterbudget,theneedtocalculateitcanbeavoidedbyassumingthatSWCcannotexceedfieldcapacity.

Forautumnsowncrops(whereirrigationisusuallynotneededintheautumn)itisappropriatetoassumetheSWCwillbeatfieldcapacityon1Julyandstartthewaterbudgetfromthere.Forspringandsummersowncropsstartthewaterbudgetonthedayofplanting.Inthiscase,itwillbenecessarytomakeanassumptionaboutSWConthedayofplanting.Ifthesoiliswet,SWCcouldbeassumedtobeatfieldcapacity,ifitismoderate,assume80%offieldcapacityandifitisdry,assume60%.

Example of a soil water budgetAnexampleofasoilwaterbudgetispresentedinFigure4.Awheatcropwasplantedon1August.Thesoilhas0.6mofsiltloamoverlyinggravelsandarootdepthof1.0mgivinganAWCof120mm,aTPof60mm,andarefillamountof50mm.

• SWCremainsclosetofieldcapacityduringJulyandAugustwhileETis lowandanumberoflargerainfalleventsoccur.

• SWCbeginstodropinSeptemberwhenETincreasesandrainfallislow.

• SWCreachestheTPforthefirsttimeinearlyOctoberand50mmof irrigationisapplied.Thisshowsupas40mminthewaterbudget becauseoftheirrigationefficiencyof80%andreturnstheSWC to100mm.

• AsecondirrigationistriggeredinlateOctober,followedsoonafterby arainfallevent.Becausetherefillamountallowssomecapacityto absorbrainfallinthesoilthisdoesnotcauseanydrainage.

• IrrigationisfrequentthroughNovemberasETishighandrainfallislow. SubstantialrainfallinDecembermeansirrigationisnotneededagain untilJanuary.

• ThiscropwasexpectedtofinishgrainfillattheendofJanuaryand irrigationiswithheldinlateJanuary,aimingtohaveSWCattrigger pointsoonaftergrainfillingfinishesandthecropisnolongersensitive tomoisturestress.

Figure 4: An example of a soil water budget

Wat

eran

drain

falla

pplie

d(m

m)

0

20

40

60

80

100

120

140

160

180

200

Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May

Drainage

Field CapacitySoil water deficitTrigger deficitIrrigation

Rain

16 Tools for Monitoring Soil Moisture16

ToapplysuchasoilwaterbudgetthroughouttheseasonitisnecessarytoenteractualET,rainfallandirrigationinformationuptothecurrentdatetocalculatethecurrentSWC.Topredictwhenthenextirrigationisneeded(whenfutureSWCreachestriggerdeficit)itisalsonecessarytohaveanestimationoftheupcomingweatherevents.UpcomingETcanberepresentedbylong-termaveragevaluesfortheareaand,forthepurposeofirrigationscheduling,assumeupcomingrainfalliszero.IfETsubstantiallydiffersfromlongtermaverages,orifrainfalloccursbetweenthecurrentdateandthenextscheduledirrigationdate,itmaybenecessarytoupdatethewaterbudgettodetermineifirrigationisneededsooner(inhotconditions)orlater(ifrainfalloccurs)thantheinitialdate.

When to stop irrigatingThescenariooutlinedaboveassumesthatcropsrequireoptimalwatersupplyatalltimes.Somecropsbenefitfromaperiodofmoisturestresswhenirrigationshouldnotbeapplied(e.g.whitecloverseedcrops).Tocalculatewhenirrigationshouldbestoppedtoproduceastressperiod,identifywhentheperiodofstressiswanted.UsethewaterbudgettoworkoutwhenirrigationshouldbestoppedtoensuretheSWCwillbebelowTPatthattime.Thewaterbudgetcanbeusedinasimilarmannertodeterminewhenirrigationshouldbestoppedattheendoftheseason.Estimatewhenthecropwillcompletegrainfill,identifythispointonthewaterbudgetandstopapplyingirrigationsoSWCisreachingTPataboutthistime.Thewheatcalculatorforecaststhedategrainfilliscomplete.

AquaTRAC™(irrigationschedulingsoftware)isavailablefromFAR,itincludes:

• Multi-paddocks

• Importedweatherdata

• Theabilitytoinputactualsoilmoisturemeasurements

• Dateofnextscheduledirrigation

• Prioritiseswhichcropstoirrigateifwaterislimited

Soil Water MeasurementAsoilwaterbudgetisinexpensiveandreasonablyaccurateifitisdonecarefully.However,errorscanaccumulateovertimesosoilmoisturemeasurementsareagoodwayofgettingthebudgetbackontrack.Therearearangeofdevicesthatcanbeusedincluding:

• Spade(growerjudgement) • Tensiometer • Gypsumblock • Aquaflexcables • Neutronprobe • Capacitancemeters • Timedomainreflectometry

Mostsoilmoisturemetersneedinterpretationforeffectiveirrigationscheduling.Noneofthesedevicesmeasuresoilwatercontentinmm,rathersomemeasureofsoilwetness;eitherthesoilwatercontentasa%orthesoilwaterpotential.

• Soilwatercontentisameasureofthepercentagevolumewater presentinthesoil.Howeverameasureofthesoilwatercontentdoes nottellyouhowmuchofthatwaterisavailabletotheplant. KnowledgeofthesoilandcropmuststillbeusedtodeterminetheTP.

• Soilwaterpotentialisameasureofhowtightlythesoilholdswater andconsequentlyhowavailablethewateristotheplant.Soilwater potentialindicateshowfreelywaterisavailabletotheplantand thereforewhenthetriggerpointisreached,butnothowmuchRAW isstoredinthesoil.ThiscouldbecalculatedbyrecordingtheETormm ofwaterusedfromFCtotheTP.

Measurementsfromneutronprobesareeasilyconvertedtommofsoilwater.Measurementsfromotherdevicesaremoredifficulttointerpretbecausethetranslationfromtheunitstheymeasuretoactualamountsofavailablewaterinthecrop’srootzonearegreatlyinfluencedbysoiltexture,thepresenceofstones,themethodofinstallation,thetimesinceinstallation,thepositionofinstallation(relativetosoilvariabilitywithintherestofthepaddock)andthedepthofinstallation(relativetothecrop’srootdepth).Allofthesethingsmustbetakenintoaccountindeterminingwhensoilwatermeasurementsmeanirrigationisrequiredornot.

Soil Moisture MetersTensiometer Tensiometershaveaporouscupattheirbase,aninterconnectingplastictubefullofwaterandavacuumgaugeatthetop.Thedeviceisinsertedintothesoilmakingsurethereisgoodcontactbetweentheporouscupandthesoil.Tensiometersmeasuresoilwaterpotential.Asthesoildries,waterissuckedfromthetubeintothesoilcausingasuctiongradientwhichisdetectedbythegauge.Ifthesoiliswetthenthesoilhasagreaterwaterpressurethanthatoftheplastictubewhichcauseswatertopassivelydiffuseintothecup.Consequentlythevacuumintheplastictube isdecreased.

Ifonetensiometerisplacedinthemiddleoftheactiverootsitcanbeusedfordecidingwhentoturntheirrigationon.Atensiometerplacedtowardthebottomoftheactiverootscanbeusedtogaugewhentherootzoneisfull.Caremustbetakenwhenshiftingthetensiometerthatthegaugeisnotbumped,thereforelosingitsaccuracy.

Positive attributes:• Itisrelativelycheap. • Measuressoilwaterpotentialwhichismorerelevanttoplantstressand easytointerpret.

Negative attributes:• Dataneedstobemanuallycollected. • Operatingrangedoesnotextendaswelltodrysoils.

Gypsum BlockThegypsumblockalsomeasuressoilwaterpotential.Asthesoildriesthesoilwaterpotentialdecreasesandtheplantneedstoexertmoreenergytoaccessthatwater.Thereadingfromthemetergivesdirectlywhetherthecrophasadequatesoilmoisturetomaximiseyield.Incomparisonmetersthatmeasuresoilmoisturecontentneedinterpretation.

TheGypsumblockiscomposedofaporousblock(thegypsum)whichcontainstwoelectrodes.Theporousstructureoftheblockallowswatermovementinandoutwiththeimmediatesoilenvironment.Thewaterinthesoilwillreachanequilibriumwiththewaterinthegypsumblockandtheelectricalresistanceisthendeterminedandrelatedtosoilwaterpotential.

17Tools for Monitoring Soil Moisture 17

Positive attributes:• Itisrelativelycheap. • Measuresoilwaterpotentialwhichismorerelevanttoplantstress andeasytointerpret. • Aloggingsystemisavailableandthesoilmoisturetrendovertimecan bedisplayed.

Negative attributes:• Notthataccurateinwettersoils. • Blocksdissolveovertime.

AquaflexAquaflexusestimedelaytransmission(TDT)technologytomeasuresoilwatercontentandsoiltemperature.Thesensorconsistsofa3mlong,dual-corewirewithaflexibleplasticcoating.Anelectricalpulseissentalongthestrip.Theelectriccurrentchangesshapeandspeedaccordingtotheamountofmoisturepresent.Thetapeisburiedinatrench.Itcanbeplacedonanangledownthroughtherootzone.Anothertapecouldbeplacedatthebaseoftherootzonetodetectifoverwateringisoccurring.

Itdoesrequiresomeworktoinstallcomparedwithsoilmoistureprobes.Soildisturbancecancauseunrepresentativeconditionssothecableshouldbeinstalledearlytoallowtimeforthesoiltosettlearoundthetape.

Positive attributes:• Allowsforcontinuousmonitoringsothatthechangeinsoilmoisture andtemperatureovertimecanbecaptured. • Datacanbedownloadeddirectlyontoalaptopcomputeror transmittedtoacomputer. • Measuringsoilwatercontentacrossalargervolumegivesamore representativereadingthanprobetypesensors.

Negative attributes:• Relativelyexpensive. • Soildisturbanceduringinstallation. • Needtoremovebeforecultivation.

Figure 5. Installing aquaflex

Neutron probeTheneutronprobemeasuressoilwatercontentfromsequentialdepths.Itisarobustpieceofequipmentusedforresearchandcommercialirrigationschedulingservices.

Itconsistsofanuclearsource/detectorsuspendedfromacableandahousingwhichcontainsthecount/storageelectronics.Aluminiumaccesstubesareinstalledatthesite.

Thesource/detectorislowereddownthetubetotherequireddepthandareadingtaken.

Theneutronprobecontainsasmallamountofradioactivematerialwhichwhenactivated,releasesaknownnumberoffastneutrons.Thetechniqueisbasedonmeasurementoffastmovingneutronsthatareslowedbycollisionwithhydrogeninthesoil.Theneutronprobecanthencalculatetheamountofhydrogenpresentintheimmediatesoil.Itismainlythehydrogenatomscontainedinwatermoleculeswhichareresponsibleforslowingtheneutronsandtherefore,themeasurementcanberelatedtotheamountofsoilwater.

Positive attributes:• Themostrobust,accuratemethodofsoilwatercontentmeasurement. • Measuresalargevolumeofsoil. • Notaffectedbyaccesstubeairgaps.

Negative attributes:• Expensivetopurchase. • Ownerrequiresalicence. • Needstobecalibratedtospecificsoiltype. • Manualreading.

Figure 6. Neutron probe

Capacitance meters Theseareeitherburiedsensors,usuallyconnectedtoadatalogger(e.g.Enviroscan),ormoreportablemethodswithaccesstubes(e.g.Gopher),orsteelspikes(e.g.Thetaprobe).Thesedevicesmeasuresoilwatercontent.Theirhighsensitivity,lackofrobustnessandcalibrationproblemshavelimitedtheiruseinthefieldtodate.

HeretheEnvironscanisdiscussedinmoredetail.ThisdevicehasanumberofcapacitancesensorsinstalledatdifferentdepthsinaPVCaccesstube.Thisallowsthegrowertoseewheremoistureisbeinglostfromthesoilprofile.TheEnviroSCANtakesreadingsasfrequentlyaseveryminute.

Positive attributes:• Continuousrecording. • Candisplaytrendsinsoilwateronthecomputerscreen. • Infiltrationrate,rootactivityandcropwaterusemaybeinferred. • Canmonitormultidepthsatonce.

18 Tools for Monitoring Soil Moisture18

Negative attributes:• Relativelyexpensive. • Notportable. • Measurementverysensitivetogoodaccesstubeinstallationwhichis difficultonstonysoils.

Time Domain Reflectometry (TDR)TheTDRmethodinvolvesconnectingasmallelectronicdevicetoapairofmetalrodswhichareinsertedintothesoil.Therodsareusuallyabout5mmindiameterandnormallynolongerthan70cm.Goodcontactwiththesoiliscritical.Theelectronicdeviceemitsapulseofelectromagneticradiation,similartoaradarpulsewhichtravelsdowntherods.TheTDRdevicethenmeasuresthetimetakenforthepulsetobereflectedbackfromtheendoftherodsandthisisrelatedtothesoilwatercontent.Usedinresearch,especiallyforclose-to-surfacesoilmeasurements.

Positive attributes:• Caneasilydomanualreadingsatmultiplesites. • Goodaccuracy. • Largemeasurementvolume.

Negative attributes:• Needdifferentlengthrodstogetmeasurementsfordifferentdepths.

Sources and further readingMcLaren,R.G.andCameronK.C.(1997)SoilScience:Sustainableproductionandenvironmentalprotection.OxfordUniversityPress.

Charlesworth,P.(2005)IrrigationinsightsNo.1.Soilwatermonitoring:Aninformationpackage.LandandWaterAustralia.

19Improving Water Use on Farms 19

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.

Forunirrigated,orinadequatelyirrigatedcrops,earliersowingwillreducetheriskofyieldlossfromwaterstress.Thisisbecausethecrop’srootscangrowdeeperandmaximisetheuseofthesoilwater.Also,thecropwillbethroughmoreofitsgrowthcyclebeforethehighETratesofsummerreducesSWCandcauseswaterstress.

Irrigator managementi. Sources of lossWecanuseourknowledgeofcrops,soilsandsoilwatertodeterminewhentoirrigateandhowmuch.Thefinalpartofthepackageistoensuretheirrigatorisapplyingtherightamountofwaterandthatnoneofthewaterbeingappliedislost.

Therearemanypathwaysforwaterlossinanirrigationsytem.Manypeoplethinkthemostwaterlossiscausedbystrongwindsordaytimewatering.Inreality,themainwaterlossisfrompoordistributionuniformity(Table7).Distributionuniformity(DU)describeshowevenlytheirrigatorapplieswatertothepaddock.Asystemwillneverreach100%uniformityandwillcreateregionsofunderandoverirrigation.

Table 7. Type and amount of water loss from irrigation systems.

Losscomponent Range Typical

Leakingpipes 0-10% 0-1%

Evaporationintheair 0-10% <3%

Windblowingwaterofftarget(drift) 0-20% <5%

Interception(canopylosses) 0-10% <5%

Surfacerunoff(sprayirrigation) 0-10% <2%

Unevenapplicationand/orexcessiveapplicationdepthsandrates

5-80% 5-30%

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.

Systemtype Amountapplied(mm)

DU% Minimumamounttowaterwholepaddockwithappliedamount(mm)

%extrawater

Extracost($/ha/

irrigation)*

Centrepivot 9 85 10.6 18 3

Centrepivot 11.6 67 17.3 49 11

Lateralmove 23 84 27 17 8

Rotorainer 50 70 71 42 42

Rotorainer 34 57 60 76 52

Gun 50 45 111 122 122 *Assuming irrigation costs of $2/mm/ha

DUisrelativelyeasilycheckedandallitneedsisatleast20plasticbucketsandareasonablyaccurateliquidmeasuringdevice.Forcentrepivotandlateralmoveirrigators,thebucketsarelaidoutevenlyacrossthelineoftravel,andtheirrigatorisrunoverthem.Fortravellinggunandboomirrigatorsthebucketsareleftinplacewhiletheadjacentrunsareirrigatedtoaccountforoverlappingirrigation.Theamountineachbucketismeasuredandconvertedtommofwaterbydividingthewatervolumebytheareaofbucket.

DU(%)isthetotalamountinthe25%ofbucketscontainingtheleastamountofwater,dividedbythetotalamountinallthebuckets.

Theirrigationrate(mm/hr)iscalculatedfromtheaveragedepthofwaterfromallthebucketsdividedbythetimetakenfortheirrigatortopassoverthebuckets.

Guidelinesandcalculationsheetsdesignedforgrowerstodotheirownassessmentareavailableatwww.pagebloomer.co.nz/resources/tools/irrigation-calibration.

Figure 7. Measuring distribution uniformity of a travelling boom irrigator

21Improving Water Use on Farms 21

Toensurecompletecoverageofapaddockwithaplannedamountofwatermeansthatyouhavetoapplytheplannedamount(say,50mmofirrigation)dividedbytheuniformity.SoiftheDUis50%,theplannedapplicationof50mmisdividedby0.5.Thistellsthefarmerthatsomeareasofthepaddockwouldactuallyonlyreceive25mmoftheplanned50mm,whichmeansthatthefarmerwouldneedtodoubletheplannedapplicationamounttoensurethatenoughwaterisappliedtothoseareasthatgetthelowestamount.Howeverdoublingtheapplicationamountisnotdesirable,sincethewell-wateredpartsofthepaddockwouldthengetfartoomuchwater(inthiscase,upto100mm),leadingtodrainageandrunoff.Tominimisetheriskofwaterloss,ensurethatyoursystemisperformingtoitsmaximumuniformity.Thiswillalsoimproveoverallcropproductionandminimiseyourwateruse(andcosts!).Itwillalsominimiserisksofnutrientleachingwhichhasafinancialcostandenvironmentalimpact.

CommoncausesoflowDUarerelatedtopoorirrigationset-upand/ormaintenancesuchasmismatchedpumppressuredeliveryandirrigationpressurerequirement,poorlysetvariablespeeddrives,blockedlinesandsprinklers,andpoorlymatchedsprinklersfittedalongtheirrigatorarm.SprinklersareactivecomponentsofyourirrigationsystemandsufferwearandtearresultinginlossesinDU.Mostofthesimplerproblemsareeasilyfixed.Havingtheperformanceofyourirrigatorevaluatedbyqualifiedconsultantswillpointoutanysourcesofreducedperformance,andrectifyingtheproblemswillsoonsaveyouthecostsoftheevaluation.

Evenwithawellset-upandmaintainedirrigationsystem,significantlossescanoccurthroughpoorirrigationmanagement,whichincludesoverandunder-watering,irrigatingtooearlyortoolate,andapplicationrateexceedingsoilinfiltrationrate.Applyingmorewaterthantheexistingdeficitwillresultinmorewaterbeingappliedthanthesoilcanhold,thereforetheexcesswillbelostthroughdrainageandrunoff.Thiswasteswaterthroughdrainageandincreasespumpingcosts,fornoadditional cropgrowth.

ImprovingtheDUby5%resultsinsavingsofwaterof19mmandcostsavingsof$38/haat$2/mmassumingapplying300mm.Ifapplying 300mminanaverageirrigationseasonwithanirrigatorcovering100haa5%savingequatesto$3800ortheabilitytoirrigateafurther5ha/year.Usingtheexampleforwheatonpage13at$380/hanet=$1900netontopofthesavings.

WheninstallinganewirrigationsystemitisessentialtospecifytheDUthesystemhastomeetinthesupplycontract.AgoodDUforacentrepivotorlateralmoveirrigatoris85to90.

Sources and Further readingMartin,R.J.,Thomas,S.M.,Zyskowski,R.F.,andStevens,D.R.Irrigationefficiencies.ProvidedbySIDE-SouthIslandDairyEvent,NewZealand.

CostsApplyingwatertocropswillnotalwaysprovideaneconomicreturntogrowersandthedecisiontoinstallanirrigationsystemortoirrigateacropshouldonlybemadewithgoodknowledgeofthecosts,andpotentialreturnsfromirrigation.

InthisFARFocusawatercostof$2/mmappliedhasbeenused(basedonastudyoffiveirrigatedarablefarms).Thiscostismadeupofoperatingcosts-pumping,labour,R&Mandsupplychargesandownershipcostsincludingdepreciation,insuranceandinterest.

GrowerscanreducethecostsofirrigationbyimprovingDU,installingenergyefficientpumpingsystemsandapplyingthewatertomeetcropdemand.

InthisFARFocusirrigatingdidnotincreaseyieldsinbarleyinallyears.SimilarlyinFARArableExtra90CostofProdutionthegrossmarginforirrigatedcropswasnotalwaysbetterthandrylandcrops.

22 Resources22

7. Glossary and Acknowledgements

23Resources 23

GlossaryAvailable water capacity (AWC)Theamountofwaterasoilcanstoreforplantgrowth.Alsocalledplantavailablewater(PAW).

Distribution uniformity (DU)Distributionuniformitydescribeshowevenlyanirrigatorapplieswatertoapaddock.

Evapotranspiration (ET)Therateaplantuseswaterfortranspirationplusevaporationfromthesoilsurface.

Field capacity (FC)Afterrapiddrainagehasceasedandthesoilwatercontenthasbecomerelativelystable.Alsocalledthedrainedupperlimit.

Permanent wilting point (PWP)Amountofwaterinthesoilatwhichplantsarepermanentlywilted.Alsocalledthelowerlimit.

Plant available water (PAW)Theamountofwaterasoilcanstoreforplantgrowth.

Refill amount (RA)80%oftriggerpoint.

Readily available water (RAW)Theamountofwaterbetweenfieldcapacityandtriggerpoint.

Relative yield decline (RYD)Asthesoildriesfromthetriggerpointtowardwiltingpointpotentialyielddecreasesinastraightline.

Rooting depth (RD)

Soil water content (SWC)Thevolumeofwaterinthesoil.

Soil water deficit (SWD)Thedifferencebetweentheamountofwaterinthesoilatanytimeandtheamountatfieldcapacity.

Soil water potential (SWP)Ameasureofhowtightthesoilholdswaterandthereforethedegreeofdifficultyaplantwillhaveinremovingwater.

Trigger point (TP)Thepointwhereplantscannolongerextractenoughwaterformaximumgrowth.Alsocalledthecriticaldeficit,stresspointorrefillpoint.

24 Appendix24

AcknowledgementsFundersFARwouldliketoacknowledgefundingsupportfromtheMAFSustainableFarmingFund(MAFSFF)andHorticultureNZforthispublication

AuthorsHamishBrownandDickMartin(PlantandFoodResearch) RobCraigie(FAR)

Graphic designMelissaHillmer,BNSDesign&Print.

PhotosRichardChynoweth,RobCraigie,HydroServices,Aquaflex

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: far@far.org.nz Web: www.far.org.nz

FAR FocusISSN 1175-5504

Irrigation Management for CroppingA Growers Guide

ISBN 978-0-9864533-4-2