A wireless sensors architecture for efficient irrigation watermanagement

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Agricultural Water Management

j o ur na l h o me pa g e : w w w . e l s e v i e r . c o m / l o c a t e / a g w a t

A wireless sensors architecture for efﬁcient irrigation water management

H. Navarro-Hellín

^a,∗

, R. Torres-Sánchez

^b

, F. Soto-Valles

^c

, C. Albaladejo-Pérez

^a

, J.A. López-Riquelme

^c

, R. Domingo-Miguel

^d

aWidhocSmartSolutionsS.L.,ParqueTecnológicodeFuenteÁlamo,CEDIT,CarreteradelEstrecho-LobosilloKm2,30320FuenteÁlamo,Murcia,Spain

bIngenieríadeSistemasyAutomáticaDepartment,UniversidadPolitécnicadeCartagena,CampusMuralladelMar,DoctorFleming,s/n30202,Cartagena, Murcia,Spain

cTecnologíaElectrónicaDepartment,UniversidadPolitécnicadeCartagena,CampusMuralladelMar,DoctorFleming,s/n30202,Cartagena,Murcia,Spain

dProducciónVegetalDepartment,UniversidadPolitécnicadeCartagena,PaseoAlfonsoXIII,48,30203Cartagena,Murcia,Spain

a r t i c l e i n f o

Articlehistory:

Availableonlinexxx

Keywords:

Remotemonitoring Wirelesssensors GPRScommunication Wateroptimization Irrigationmanagement

a b s t r a c t

Waterisanessentialresourceforthedevelopmentofagriculture.Inseverallocationslikethesouth- eastofSpainwaterisscarceanditscostishigh,sooptimalmanagementofthisimportantresourceis essential.Therefore,theapplicationofirrigationstrategiestoimprovethewateringprocess,affectsthe proﬁtabilityofcropsquitesigniﬁcantly.Itisnecessarytocarryouttheinstrumentationofthevariables thataffectthegrowingprocessofthecrop(soil,waterandplant)andusethetechniquesassociated withthisinstrumentationtotakeactionstooptimizetheproduction.Thesystemproposedinthispaper usesinformationandcommunicationtechnologies,allowingtheusertoconsultandanalyzetheinfor- mationobtainedbydifferentsensorsfromanydevice(computer,mobilephoneortablet)inaneasyand comfortableway.TheproposedarchitectureisbasedondifferentwirelessnodesequippedwithGPRS connectivity.Eachwirelessnodeiscompletelyautonomousandmakesuseofsolarenergy,givingitvir- tuallyunlimitedautonomy.Differentcommercialsensorsformeasuringthewiderangeofparameters ofthesoil,plantandatmospherecanbeconnectedtothenodes.Thedataissentandprocessedona remoteserver,whichstorestheinformationofthesensorsinadatabase,allowingfurtherconsultation andanalysisofdatainasimpleandversatileway.

1. Introduction

Irrigatedagricultureisthebiggestconsumeroffreshwaterin aridandsemi-aridzones,withashareofaround70–80%ofthe totalvolumeofusedwater.

However,increased demand for water by other sectors and weather-associatedlimitations (increasing aridity asa result of climatechange), suggeststhatthewaterresourcesavailablefor agricultureinforthcomingdecadeswillbelowerinbothquantity andquality.Atthesametime,newgrowerswithlowerproduc- tioncoststhanthetraditionalonesintheseareasareforcingthem toreducecostsbyloweringtheirmaininputs.Itisinthisareaof costsreductionwheretheefﬁcientuseofthewaterisbecomingan increasinglyimportantconsideration,sometimesattheexpenseof

∗ Correspondingauthor.Tel.:+34968197583.

E-mailaddress:[email protected](H.Navarro-Hellín).

cropquality.Howeversuchissuesmustbeaddressedifsuchcrops aretoremaincompetitive.Itisaprovenrealitythatinsemiarid zonesthelimitedavailabilityofwaterhascontributedtocreat- ingincreasinginterestinwaterconservation,particularlyamong practitionersofirrigatedagriculture.Fortheseandotherreasons, thescientiﬁc-andtechnical-basedirrigationschedulingofwater tomaintainandevenimproveyieldandqualityhasbeenandwill remainamajorchallengeforirrigated agriculture(Puertoetal., 2013).

Inrecentyears,theincorporationofsensorsinthecontextof agriculturalproductionforwatermanagementandconservation hasreceivedanincreasinginterestforestablishingirrigationman- agement strategies,such asregulateddeﬁcit irrigation(RDI)or partialroot drying(PRD). Bothstrategies allowvery signiﬁcant increasesinirrigationwaterproductivity(yieldproducedperunit ofirrigationwaterapplied),especiallyinwoodycrops(Egeaetal., 2009;Jones,2004;Puertoetal.,2013).However,thesestrategies needseveralsensorstoestimatetheplatwaterstatus.

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Mostcommonsensorstoestimateplantwaterstatusaresoil sensors,sincetheyprovideimportantinformationthatisfamil- iartotheuser.Thesesensorsprovidedataofthematricpotential (m,soilwaterstatus)orvolumetricsoilwatercontent(v,soil moisture).Overtheyears,thesesensorshaveevolvedsigniﬁcantly sincetheﬁrstmanualgaugesthatmeasuredthepressureofthesoil watertobecomeevermoreeffectiveandpreciseinmeasuringthe soil’swaterstatusandmoisturecontent,integratingcommunica- tionwithdataloggers(FereresandGoldhamer,1990;Hansonand Edwards,2000).

Inadditiontothesesensors,measurestocontrolthevolume or concentration of soluble salts in the irrigation water have often beenused in irrigated agriculture, especially in arid and semi-aridareas.Waterelectricalconductivity(EC)isimportant, andirrigationwatercanbeclassiﬁedasrunningfromexcellent (EC≤250␮Scm⁻¹)tounsuitable(EC≥3000␮Scm⁻¹)(Jamesetal., 1982).Moreover,ithasbeendemonstratedthatthecropproduc- tiondecreasesastheECoftheirrigationwaterincreases(Maasand Hoffman,1977).TheoverallEC(orquality)oftheusedwatercan beadjustedbyusingwaterfromdifferentsources,whichexplains theuseofsensorsthatprovidemeasurementsofthevolumeand qualityofwaterheldinreservoirs.

Thistypeofsensorprovidesusefuldataforregulatingthecondi- tionsrequiredbyeachoftheseirrigationstrategies.Thesesensors havebeenusedinconjunctionwithinstrumentationsystemsto controlthe irrigationprocess, traditionallywithwired datalog- gers.Inrecent years, thequickdevelopmentof wireless sensor networks(WSNs) hasledtothe useofsensor equipmentwith very littlewiring, and great improvements in theirinstallation andmaintenance (Hussain, 2012;Khanet al.,2014;Nolz etal., 2013;Ruiz-Garciaetal.,2009; Yuetal.,2013).Wirelesssensor networksarecomposedofdifferentmeasuringpoints,callednodes, abletocommunicatewirelessly.Thedataobtainedfromeachone of them, is stored in a sink nodeand processed for managing irrigation.

Theultimateaimistousethedatafromﬁeldsensorstowardsa fullyautomaticirrigationsystem.Althoughafullyautomaticsys- temhasnotyetbeenachieved,remoteirrigationmanagementis possiblebymeansofroutersandInternetconnectionsthataccess thedata collectedbydataloggers (wired) orby the sinknodes (WSNs)(Puertoetal.,2013).Thecommonpointofthesesystems isthattheyhaveacentralizedcommunicationstructure,thatis, theydependonadevice(gateway)tocommunicatewiththeuser, whichcanleadtoconnectionproblemsforthewholesystemifsuch adevicehaslimitedconnectivity.

AdvancesinmobilenetworksandcheaperInternetcommuni- cations(GSM/GPRS,3G)hasledinrecentyearstotheincorporation ofmodulesthatprovideWSNswithaccesstomobilenetworksby usingmachine-to-machine(M2M)modules.Thisfeatureprovides moreﬂexibilityintheinstallation,asitisnotlimitedbytheconnec- tivityofthesinknodeordatalogger.Eachnodecontainsamobile communicationmodule(GSM/GPRS,3G)changingthetopologyof thenetworkfromcentralizedtodistributed.Oneoftheimportant advantageofthisapproachisthataconnectionfailureatanode doesnotaffectthenormal operationofthenetwork. However, thehigher energy requirementsof themobilecommunications networkrepresentacomplexchallengeforthedesignanddimen- sioningoftheinstrumentationofthenodes(SindujaandSowmya, 2013).

Thisarticledescribesthedesignanddimensioningofawire- lessnodetofulﬁllthespeciﬁedrequirementsof autonomyand reliability,allowinganeasyinstallationandtheabilitytousethe widerangeofelectricalinterfacesinagronomicsensors.Theresults obtainedindifferentagriculturalscenariosarethenusedtoverify theoperationoftheequipmentandthegoodnessoftheagronomic data.

2. Materialsandmethods 2.1. Hardwaredescription

Oneofthemostimportantaspectstoconsiderwhendesigning adeviceformonitoringanykindofvariableistheabilitytocover awiderangeofsigniﬁcantparametersforthemonitoredactivity thatwillprovidethedevicewiththeversatilitynecessaryforusein abroadvarietyofrealsituations.Suchmeasurementsareprovided byspeciﬁcsensorsthatareconnectedtothedevicebymeansofa widerangeofpossibleconnectioninterfaces.

Inthepresentarticle,acompletelyautonomousdeviceforesti- matingtheplantwaterstatusinseveralscenariosispresented.The device,whichmustbecompatiblewithawidevarietyofsensors thatprovideplant-relateddata,isbasedonamodularstructure thatconsistsofseveralelectronicinterconnectedboards:

1.Anelectronicboard,theMain-Board,which hasalreadybeen usedinotherapplications,includingoceanographicmonitoring bymeansofaZigBeenetwork(Albaladejoetal.,2012).Thisboard isresponsibleforsensors’data.Itisalsoinchargeofcommuni- cationwiththerestoftheboardsandensuresthatthewhole systemisworkingproperly.

2.The Sensor-Board providesthe interfacewiththe connected sensors.Thedesignoftheboardisoptimizedfor agricultural monitoring,allowingthesimultaneousconnectionofacommon precisionagriculturesensors.

3.TheGPRS-Boardisinchargeofestablishingthecommunication withthemobilenetworkby meansofa GSM/GPRSprotocol, sendingthedatatotheremotestorageserverforfurthercon- sultation.

2.2. Control,managementanddatacollectionsubsystem

Fig.1showstheflowdiagramoftheMain-board.Whenitispow- eredup,themicrocontrollerobtainstheconfigurationinformation fromtheSD-Card.Thiscardcontainsinformationaboutthesensors connectedtotheSensor-Board,thesampleperiodandtransmission rate,theGPRS andremote serverconfigurations, etc.Bysimply modifyinga fewfiles,theuserisabletocompletelychangethe behaviourofthesystem,providinggreatflexibility.Assoonasthe configurationisinitialized,thesystementersinasleepmodein whichthepowerconsumptionkeptbelow600␮A.Whenthesam- pleratetimehasexpired,themicrocontrolleris“wokenup”by aninterruptsignalandtheprocessofretrievingtheinformation fromthesensorsstarts.Thisprocessiscarriedoutbytheinterface board.Then,themicrocontrollerobtainsthetimefromtheRTCand storesalltheinformationintheSD-Card.Next,thesystementers thesleepmodeagainandwaitsforanotherinterrupt.Whenthe sendingratetimehasexpired,theMain-Boardestablishescom- municationwiththeGPRS-Boardtosendtheinformationobtained fromthesensorstotheremoteserver.Thisboardalsoperformsa chargingalgorithmtooptimizebatterylife,activatinganddeacti- vatingacircuitintheSensor-Boardwiththepurposeofconnecting thesolarpanelonlywhenthebatteryisoutsideaspecifiedvoltage range.

2.3. Sensorinterfacesubsystem

TheSensor-Boardisoneofthemostimportantpartsofthewhole systemsinceitprovidestheinterfacesthatwilldeterminecompat- ibilitywithexternalsensors.GiventhattheSDI-12interfaceisone ofthemostimportantandstandardizedinterfacesintheﬁeldof precisionagriculture(Lópezetal.,2009),itprovidesgreatﬂexibil- itybecauseawiderangeofcommercialsensorsusedinagriculture arecompatiblewithit.Accordingtothetechnicalcharacteristics

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Fig.1. FlowdiagramoftheMain-Board.

of the SDI-12 protocol (SDI-12, 2005). The sensors that use SDI-12arepoweredat12V.Asthebatteryoperatesintherange of3.7–4.2V,a12VDC/DCconverterwasincludedtopowerevery sensorconnectedtotheSensor-Board,whilea5VDC/DCwasused topowerthetri-statebufferneededfortheSDI-12interface.This powersourcecouldalsobeusedtopowerupsensors.Therestof thesensorinterfacesincludedinthisboardwere:

• One4–20mAinterface.

• Threevoltageinterface.

• Onedigitalpulseinterface.

Fig.2presentstheﬂowdiagramoftheSensor-Board.Intheini- tialstate,thepowersuppliesofthesensorsareinactive.Assoonas theMain-Boardentersthereadingstate,theSensor-Boardwarms thesensorsfor2s,thenstartsreadingalltheSDI-12sensors,after whichit beginsreadingtheanaloguesensors.FinallytheMain- BoardstoresthedataintheSD-CardandtheSensor-Boardenters theInitialStateagain.

2.4. Communicationsubsystem

ThecommunicationmoduleisbasedontheSIM900GSM/GPRS chipfromSimcomLtd.TheSIM900modulecommunicateswiththe Main-BoardbymeanofAT-Commandsthroughaserialinterface.

ThedecisiontouseGSM/GPRStechnologyratherthanthemore modern3Goreven4Gisbasedontwoimportantpoints:

1.GSM/GPRSismoreestablishedthananyothermobiletechnol- ogy,soitscoverageisgreaterthanthatofitscounterparts.Also, thevastmajorityoftheareasthatneedtobemonitoredforpreci- sionagriculturewillbelocatedinthecountryside,where3G/4G isnotalwaysavailable.

2.AlthoughGPRS/GSMtechnologyismuchslowerthan3G/4G,the amountofdatacollectedateachsamplingissosmallthatspeed isnotanissue.Moreoverthepowerconsumptionofdevicescom- patiblewith3G/4GismuchgreaterthaninthecaseofGPRS/GSM –anaspectthatismuchmoreimportantthanspeedinscenarios wheretheautonomyofthedeviceisofkeyimportance.

Thismoduleisoneofthemostimportantpartsofthedevice andallowsachangeinthetopologycommonlyusedinWSN,from centralizedtodistributed.Asinkoragatewaynodeisnotneeded

aseverydeviceinthenetworkhasitsowncapacitytotransmitdata byInternet.Fig.3showstheﬂowdiagramofthecommunication subsystem.AstheMain-BoardenterstheSendingstate,theGPRS moduleispoweredupandtriestoestablishconnectionwiththe remoteserverthroughthemobilenetwork.Ifthecommunication withtheremoteserverisestablishedsuccessfully,theGPRS-Board enterstheSendingstate,inwhichthedataisprocessedandsent totheserverseparatelyinburstsof1kB.Ifanyproblemoccurs duringthisprocess,theGPRSwillentertheinactivemodetosave thebattery.Inthiscase,theGPRS-Boardwilltrytotransmitallthe dataatthenextsendingtime.

Fig.4ashowsthecompletelyassembleddevice.Inordertoguar- anteethestabilityofthenetwork,aninstallationprotocolmust befollowed.Firstly,ananalysisofthesignalpoweratthepoint of installationisperformedfor each mobileoperatorby means ofanAnritsuMS2713Espectrumanalyzer.Theoperatorwiththe strongestsignalwillbethemostsuitable.Usingthemostsuitable GPRSantennaisveryimportant,sincethesensitivityoftheSim900 moduleis around−109dBm (SIM900HardwareDesign Guide, 2010).Ifthethresholdcannotbereachedwithaninternalantenna of2dB,a5dBexternalantennashouldbeusedinstead.Thisproto- colensureshighqualityGSM/GPRScommunicationsundernormal workingconditions.

TheelectronicsarelocatedinsideanIP-67,a boxofreduced dimensions (120mm×122mm×75mm),whosecover is trans- parentsothatthesolarpanelinsideisprotectedfromexposure totheweather(Fig.4b).

2.5. Experimentalsites

Four experimental tests were carried out to analyze the behaviourofthedeviceinrealscenarios.Thedatawascollected fromJanuary2014onwardsinfourcommercialplantationslocated in the south-east of Spain. In order to manage the irrigation, soilmoisturesensorsratherthanplantbasedsensorswereused becausesoilmoistureinformation is generallymore familiarto the farmers. The control variables were matric potential (m) and volumetric soil water content (v) which are common in theirrigationmanagement.Thecriteriaforschedulingirrigation usedinthescenarios1,2and3(seebelow)weredecidedbythe farmer.Inscenarios1and2theaimwasmaintainsoilmoisture conditionsequivalentorclosetoﬁeldcapacitytosatisfythemaxi- mumcropwaterrequirements.Inthethirdscenariosoilmoisture

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Fig.2. FlowdiagramoftheSensor-Board.

Fig.3.Flowdiagramofthecommunicationsubsystem.

Fig.4. (a)Assembleddeviceand(b)installeddevice.

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Table1

Summaryofthesensorsusedandtheirtechnicalspeciﬁcations.

Sensor Measureddata Output Range Resolution Accuracy Supplyvoltage range

Power URL

10HS Soilmoisture 0.3–1.25V 0–57%VWC 0.08%VWC ±3%VWC 3–15VDC 15mA http://www.decagon.com/

HPII Moisture, conductivityand temperature

SDI-12 0–100%VWC 0.01–1.5S/m

−10^◦to+65^◦C

±0.3%VWC

±0.0014S/m

±0.6^◦C

±3%VWC

±0.0014S/m

±0.6^◦C

9–20VDC 30mA http://www.stevenswater.com/

MPS-2 Soilmatric potentialand temperature

SDI-12 −10to−500kPa

−40^◦to+50^◦C 0.1kPa 0.1^◦C

±25%(from

−10kPato

−100kPa)

6–15VDC 10mA http://www.decagon.com/

GS3 Moisture, conductivityand temperature

SDI-12 0–100%VWC 0–23dS/m

−40^◦to+50^◦C

0.1%VWC 0.001dS/m 0.1^◦C

±3%VWC

±1dS/m

±1^◦C

3.6–15VDC 30mA http://www.decagon.com/

LMK Pressure 4–20mA 0...10mH2 0.003mH2 ±0.05mH2 8–32VDC 20mA http://www.sensotec-instruments.com/

ES-2 Waterelectrical conductivityand temperature

SDI-12 0–120dS/m

−40^◦to+50^◦C

0.001dS/m 0.1^◦C

±0.01dS/m

±0.1^◦C

3.6–15VDC 0.5mA http://www.decagon.com/

conditionsdependedonthecropphenologicalstage.In general,

mwasusedtodecidetheirrigationfrequencyandvtoadjust thegrossirrigationdoses.Inscenario3itwasdecidedtouseaGS3 sensorinsteadofthe10HSbecauseofitsrobustness,accuracyand easierinstallationforittobeinclosecontactwiththesubstrate.

Table1summarizesthemaincharacteristicsofthesensorsused.

Thedifferentscenariosaredescribedbelow.

Scenario1(woodycrop).Finolemontrees(CitruslimonL.Burm.

ﬁlcv.49.)onC.macrophyllaWester,growinginasoilwithalow waterretentioncapacity,wereirrigatedinordertooptimizethe useoftheirrigationwaterbasedontheinformationprovidedby thesensors.Thesoilhadasandyclayloamtextureandtheirrigation water had an electrical conductivity (EC)of 2200␮Scm⁻¹. The orchardconsistedof10year oldlemontrees. Treespacingwas 7.0m×5.5m,withanaveragegroundcoverageofabout47%.Two dripirrigationlines(0.8mapart)wereusedforeachtreerow.There were4emitters(4Lh⁻¹)onbothsidesofeachtree.Twosensor nodeswereinstalledinthe5.5haorchard,eachonewithasoil matricpotentialsensor(MPS-2,Decagondevices,Inc.,Pullman,WA 99163,USA)atadepthof30cmandthreesoilmoisturesensorsat adepthof20,40and80cm(10-HS,Decagondevices,Inc.,Pull- man,WA99163,USA)located25cmfromarepresentativedripper and2.25mfromthetrunk.Thedeviceautomaticallytookread- ingsofthesoilmatricpotentialandvolumetricwatercontentevery 15min.

Scenario2(vegetablecrop).Endiveplantswereplantedinbeds with 1.0m centres. There were two rows per bed, and plants werespaced35cm apartwithintherow(triangularplanting).A singlelateraltubewith0.2mbetweenemittersandadischarge rateof5Lh⁻¹ wasused tomeettheirwaterrequirements. The irrigationwaterhadanelectricalconductivity(EC)of800␮Scm⁻¹. Onesensor nodewas installed inthe 2.5ha plotwithtwo soil matricpotentialsensors(MPS-2,Decagondevices,Inc.,Pullman, WA99163,USA)atdepthsof17and35cm,andoneFDRsensorat adepthof20cm(HydraProbeII,StevensWaterMonitoringSys- tems,Inc.,Portland,OR,USA)located20cmfromarepresentative dripperand20cmfromtheplant.

Scenario3(greenhousesoillessculture).Thetestwascarried outinaplasticgreenhouse(800gauge)usinghydrangeasgrowing in12Lplasticpotscontainingasubstratemixture(2partspeat, 1partcoconutﬁbreand1partperlitebyvolume).Thepotswere spaced50cmapartwithintherowandwith60cmbetweentwo contiguousrows.Theirrigationwassuppliedusingasinglelat- erallinethat had two compensatingdrippers (2Lh⁻¹) perpot.

TheirrigationwaterhadanECof800␮Scm⁻¹.Onesensornode withthreeGS3sensors(Decagondevices,Inc.,Pullman,WA99163,

Table2

Summaryofthesensorsusedoneachnode.

Nodetype Sensors

10HS HP-2 MPS-2 GS3 LMK807 ES-2

Woodycrop 3 0 1 0 0 0

Vegetablecrop 0 1 2 0 0 0

Greenhousesoillessculture 0 0 0 3 0 0

Waterreservoir 0 0 0 0 1 1

USA)wasinstalledinthegreenhousewithoneGS3sensorlocated mid-heightineachpot.

Scenario4 (waterreservoir).The waterreservoir monitored, witha500mperimeterandatotalcapacityof52,000m³,isused toirrigatedifferentﬁeldslocatednearby.Thereservoir receives waterfromadesalinationplantandseveralwells.Tokeeptrack ofthewaterlevelandconductivity,apressuresensor(LMK807, SensotecIncorporated,Columbus,OH)andaconductivitysensor (ES-2,Decagondevices,Inc.,Pullman,WA99163,USA)wereused.

Thepressuresensorwaslocatedatthebottomofthereservoirand theconductivitysensor1mbelowthewatersurface.

Table2summarizesthenumberandtypeofsensorsconnected toeachnode.

2.6. Remotesoftwarestoragemanagement

Theinformationfromthesensorsconnectedtothedeviceis periodicallysenttoadataserver,whereitisprocessedandstored inarelationaldatabase.

Thesoftwarearchitectureofthedataserverisbasedaround three main components: (1) an application developed in Java, whichisinchargeofcommunicationwiththenodes,(2)arela- tionaldatabasethatstoresthedata,and(3)aWebapplicationto checktheinformationofthesensors.

Fig.5showsthearchitectureofthesystemfromafunctional point of view.Thenode, after samplingtheconnected sensors, establishesaTCPcommunicationthroughtheGSM/GPRSmodule withtheJavaapplicationthatisbeingexecutedintheserver.Once thesocketofthecommunicationisestablished,thenodesendsthe datainaspeciﬁcformat.TheJavaapplicationmanageseachcon- nectionandevenincasesofsimultaneity,itreceivesandsendsthe informationofeachsensortoaMySQLdatabase.TheWebappli- cationisusedtoquerythedatabasetoshowselecteddatatothe user.

TheflowoftheJavaapplicationisasfollows:first,theheaderof themessage,whichcontainsinformationabouttheidentification

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Fig.5.Architectureofthesystem.

ofthenode,itssamplerate,thelocation,thesensorsconnected, etc.,isanalyzed.Then,thesensordataischeckedandprocessed and,iftheinformationisbeloworabovethethresholdestablished bytheuser,anemailissenttowarnofapossibleincident.The dataandtheexacttimestampsentbythenodearestoredinthe database,whiletheinformationgatheredbythesensorsismade availableontheInternetthroughawebsite.ThisWebapplication isdevelopedinHTML5,JavascriptandPHP,asshowninFig.6.The topoftheﬁgureshowsthegeographiclocationofthedevicesand theconnectedsensors.Thebottomoftheﬁgureshows,ingraphic form,theinformationobtainedfromthesensorsconnectedtothe device.Thisallowsthedesireddatatobevisualizedandtherange ofmeasurementsanddatestobedownloadedandexpressedinxml format.

3. Resultsanddiscussion

3.1. Agronomicresults

Fig.7a–cpresentsthedataconcerningthesoilwatercontent andwaterstatusofthecropsrecordedinthethree commercial plantationsandawaterreservoirrespectively.Theﬁrstthreeﬁg- uresclearlyrepresenttheevolutionofvandm,whileFig.7d plotstheevolutionofthewaterconductivityandvolumeofthe irrigationreservoir.

Fig.7ashowshow,astheseasonprogresses,themoistureinthe rootzoneofthelemontree(top50cm)increases,whileat80cm itdecreasesslightly.Themoisturecontentfrom20to40cmdepth reﬂectsthehighavailabilityofwaterforthecrops,basedonm

Fig.6. Webapplicationinterface.

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Fig.7.Evolutionsofvolumetricwatercontentandmatricpotentialatdifferentdepthsduringtheindicatedperiod(76–102DOY).Sceneries(a)woodycrop,(b)vegetable cropand(c)greenhousesoillessculture.Evolutionofvolumeandelectricalconductivityofirrigationwaterduringtheindicatedperiod(76–102DOY).Scenery(c)water reservoir.

valuesat30cm(Domingoetal.,1996).Theirrigationstrategyis basedonmaintainingahighlevelofavailablewaterinthezone ofhighestrootactivityduringthecriticalperiodsofﬂoweringand fruit-setting,whileoptimizingtheuseoftheirrigationwaterby minimizingdeeppercolationlosses.Thehighfrequencyofstrong winds(>5ms⁻¹)intheareaweretakenintoconsiderationdur- ingtheﬂoweringperiods.Toensuretheavailabilityofwaterinthe top50cmofsoil,twodailyirrigations(insteadofone)wereapplied afterday95(Fig.7a).Thetotalamountofwaterappliedinagrowing seasonwas5775m³ha⁻¹incontrasttothe6000m³ha⁻¹applied byDomingoetal.(1996).Theseauthorsmanagedtheirrigation accordingtotheFAOmethodologyusingaClassAevaporationpan.

Theclimaticconditions, irrigationsystemandtotal yieldswere similar(≈50tha⁻¹).Thedifferencerepresentsawatersavingof 225m³ha⁻¹ withtheirrigationcontrolledbysensors.However, theirrigationwaterqualityandannualrainfallpresentedinthe scenario1werelowerthantheonespresentedbytheprevious authors(2200␮Scm⁻¹vs1250␮Scm⁻¹and100mmvs315mm fortheECandrainfall,respectively).Consideringthatcitrusfruits aresaltsensitive(StoreyandWalker,1998)andbearinginmindthe contributionfromrainwater,thesavingscouldhavebeenhigher.

Nevertheless,in order tochoose oneirrigation methods orthe other,itwouldbenecessarytokeepregisteringtheinformation duringatleasttwomoreseasons.

Endiveplantrequiresconsistentlevelsofwater(nearreference demandlevels)throughoutitsrapidgrowthandfinaldevelopment periods.Reductionsin theavailablewater duringthese growth periodscanresultinreducedleafdevelopment(Maynardetal., 1999),sothatitisimportanttomaintainthemoisturecontentofthe soilnearfieldcapacityatdepthswheretherootsystemislocated practicallythroughoutthegrowthcycle.Fig.7bplotsthelastdaysof thecropcycle(DOY76–103),illustratingthehighmvaluesand theevolution dynamicsofthev.Soilmoisture(HydraProbeII) andsoilmatricpotentialsensors(MPS-2)showthehighirrigation frequencyandthelittlevariationintheminimumhumidityval- uesreached,withaslightincreasingtrendclosetoharvesting.The informationprovidedbythesesensorswasofgreathelpformain- tainingthemoisturecontentofthesoilatfieldcapacityvaluesat thedepthsexploredbytherootsystemofthecrops.

Fig.7cshowstheevolutionofthevolumetricwatercontentof thesubstratemeasuredinthreedifferentpotsusingaGS3 sensor inserted at mid-depth. As can be seen, there is no clearly deﬁnedpattern asthewatercontentchanges constantly.In the period76–103DOY,vgraduallyfellover8consecutivedaysbefore sharplyincreasing.Thiswasfollowed byanotherreductionand thenapermanentrecovery.Regardlessofthegrower’sirrigation procedure, Fig.7cshows:(i)similarpatterns inthewater con- tentevolutioninthethreepotslocatedindifferentrows,(ii)rapid

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Fig.8. Woodycropnodepowerconsumption.

drainageduringtheﬁnalperiodand(iii)noise-freerepresentation inasubstratewhereotherssensorsmighthavecontactproblems.

Itmustbetakenintoaccountthatgoodsoildrainageinthiskindof cultivationpracticeisessentialtoavoidrootrotting.

Fig.7dplotsthewaterconductivityandvolumeoftheirrigation reservoirusedtoirrigateseveralnearbyﬁelds.TheES-2sensorused tomeasuretheconductivityofthewaterisofkeyimportanceto maintainthewaterqualityinthedesiredrange.AsshowninFig.7d, thevalueoftheconductivityduringthemeasurementperiodwas below2200␮Scm⁻¹.TheLMK807measuredthevolumeduringthe sameperiod.ThevariationintheECwasevidentwhentakingwater forirrigationoraddingwaterfromthewellsandthedesalination plant.TheinformationshowninFig.7disofgreathelpformix- ingthewaterfromthedifferentsourcestoattaintheconductivity requiredlevels.

3.2. Powerandenergyconsumptionresults

Inordertoanalyzethepowerconsumptionofthedevice,an exhaustiveenergeticanalysisofeachscenariowascarriedoutto determinethemostsuitable battery andsolar panelneededto makethedeviceenergeticallyautonomous.

Adetailed explanationabout thepowerconsumption ofthe woodycropnodeintheworst-casescenario,thatis,asamplerate of10minandasendingrateof1h,isgivenbelow.

Thenodehaseightfunctionalstates,numberedasfollows:(1) initializationandconﬁguration,(2)standby,(3)interfacewarm-up, (4)digitalsensordataacquisition,(5)analoguesensordataacqui- sition,(6)SD-Carddatastorage,(7)GPRSstandbyand (8)GPRS moduletransmission.

The measurements of the current reported in this paper were taken with a YOKOGAWA WT210 digital wattmeter and WTVIEWERsoftware.Fig.8 showsan examplesequence ofthe powerconsumptionofthenodewhenthesystemisfunctioning.

Afterinitializationandconfiguration,thedataarecollectedfrom thesensors,firstfromthedigitalsensorsandfinallyfromtheana- loguesensors.Afterthat,thedatais storedintheSD-Card.The sequenceofstates(3),(4),(5)and (6),isrepeatedevery10min (samplerate).After1h(sendingrate),allthesensordatacollected beforeandavailableinthecardaresenttotheserver.Afterthat,the

Table3

Currentcomponentsandtheirdescription.

Typeofcurrent Description

¯IStandby Currentconsumedinstandbymode

¯IInitialization CurrentconsumedintheprocessofinitializationtheGPRS node

¯IInterfacewarm-up Currentconsumedwiththecurrentconvertersinactive mode

¯IDigitalsensor Currentconsumedbydigitalsensors.OneMSP-2for woodycrop

¯IAnalogsensor Currentconsumedbyanaloguesensors.ThreeHS10for woodycrop

¯IStorememory Currentconsumedintheprocessofstoringdatainthe ﬂashmemory

¯IGPRSwarm-up Currentconsumedintheprocessofestablishingthe connectionwiththephonecell

¯IGPRSsend Currentconsumedinsendingofdatatotheserver

systemreturnstostandbymode,untilthenextscheduledreading process.

Theultimateaimofthisstudywastodeterminehowmuchcur- rentthedeviceconsumesonaverage,soastorelatetheresulting ﬁguredirectlywiththecapacityofthebatteriesandhencedeter- minethedevice’sautonomy.Theaveragecurrentconsumptionof theGPRSnodemaybedeterminedthus:

¯ITotal = ¯IStandby+ ¯IInitialization+ ¯IInterfacewarm-up+ ¯IDigitalsensor

+ ¯IAnalogsensor+ ¯IStorememory+ ¯IGPRSstandby+ ¯IGPRSsend (1)

Each ofthe current components is explained in Table3.To calculatetheautonomyof thenode, worst-casescenarioswere assumedforeachmeasurement.Thus,expression(2)showssen- sorsystemconsumptioninstandbymode.Eq.(3)expressesthe estimatedaverage currentconsumed by thedevicein thepro- cessofinitialization.Expressions(4)–(7)arecalculatedinthesame way. The interval between samplings considered in this study was10min, which isthe worst-casescenarioregarding energy consumption,althoughtheaveragesamplingperiodinrealdeploy- mentsis15min.Expressions(8)and(9)arecalculatedinthesame way.

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Fig.9.Woodycropnodebatteryvoltage.

Consideringanaveragecurrentconsumptionof5.93mA(1),the nextstepistochooseabatteryandasolarpanelthatenablesthe nodetobecompletelyautonomous.Inordertoachievethis,sev- eraltestswerecarriedoutwithawidespectrumofcommercial batteriesbeforethedecisionwasmadetouseaLI103450Abattery, a1900mAhLi-ionrechargeablebatterywithanominalvoltageof 3.7V,equippedwithaprotectioncircuitthatpreventsovercharg- ingaswellasdeepdischarging.Withthisbatteryandconsidering thescenariopresented,expression(10)estimatesautonomyofthe woodycropnodewithoutsolarpanels,whichisaround13.35days.

TheSensor-Boardhasacircuitthatmanagestheloadbasedonthe switchingofanexternalsolarpanel.Usingthisbatteryinconjunc- tionwithasolarpanelof0.8W(witharatedvoltageof5Vand aratedcurrentof160mA)willallowthenodetobecompletely autonomousevenunderprolongedpoorlightcondition.

¯IStandby=0.6mA (2)

¯IInitialization≈ (12−0.6)mA×1s

24×3600s =0.132␮A (3)

¯I_DA/DC≈ (215−0.6)mA×4s

600s =1.429mA (4)

¯IDigitalsensor≈ (235−0.6)mA×1s

600s =0.390mA (5)

¯IAnalogsensor≈(310−0.6)mA×6s

600s =0.394mA (6)

¯IStorememory≈(7−0.6)mA×0.5s

600s =5.333␮A (7)

¯IGPRSwarm-up≈(70−0.6)mA×10s

3600s =0.192mA (8)

¯IGPRSsend≈(400−0.6)mA×2s

3600s =0.222mA (9)

Numberofdays≈ Batterycapacity(mAh)

¯ITotal(mA)×24h = 1900mA×h 5.93mA×24h

=13.35days (10)

Totesttheselectedbatteryandsolarpanel,a validationtest wasconductedoveraperiodoffourmonths(fromJanuarytoApril

2014).AwoodycropnodewassetupinMiranda(Cartagena)ina 4hamandarinorchard.Fig.9showsindetailthevoltagerecorded overa periodof28 daysinwhichthebatteryvoltagewaskept betweenthedesiredvaluesthankstothesolarpanelandthecharg- ingalgorithm.

TheconsumptionofallthenodeswasalsoanalyzedinTable4.

ThisTable4showsthemeanpowerconsumption ofthedevice for the differentscenarios, considering samplingrate values of between10and60minandsendingratesfrom1to12h.Logically, thehigherthesamplingrate,themorepowerthedeviceneedsto operateandsothepowerconsumptionisincreased.Increasingthe samplerateprovidesmoreinformationinlesstime.Ifthesend- ingrateisreduced,theinformationwillbeavailablesoonerbut thedevicewillbelessefﬁcientenergeticallyasveryfewsamples willbesentduringeachGSM/GPRSconnection.Afteranalyzingthe table,itwasconcludedthatthesendingratehasagreatereffect inscenarioswherethepowerconsumptionofthesensorsislow (Irrigationreservoirnode,vegetablenode);however,inscenarios wherethesensorshaveahigherpowerconsumption(woodycrop node),thesendingrateisoflittleimportancefortheoverallpower consumptionofthedevice.

Fig.10.Realtimeaveragepowerconsumption.

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Table4

Averagepowerconsumptionofthedevice(mA).

Samplerate(min)

10 20 30 40 50 60

Woodycrop GPRScommunicationrate(h) 1 5.93 3.47 2.65 2.24 2.00 1.83

2 5.73 3.27 2.45 2.04 1.79 1.63

4 5.62 3.16 2.34 1.93 1.69 1.52

8 5.57 3.11 2.29 1.88 1.64 1.47

12 5.55 3.09 2.27 1.86 1.62 1.45

24 5.54 3.08 2.26 1.85 1.60 1.44

Vegetablecrop GPRScommunicationrate(h) 1 2.06 1.54 1.36 1.28 1.22 1.19

2 1.86 1.33 1.16 1.07 1.02 0.98

4 1.75 1.23 1.05 0.97 0.91 0.88

8 1.70 1.18 1.00 0.91 0.86 0.83

12 1.68 1.16 0.98 0.90 0.84 0.81

24 1.67 1.14 0.97 0.88 0.83 0.79

Greenhousesoillessculture GPRScommunicationrate(h) 1 2.61 1.81 1.55 1.41 1.33 1.28

2 2.41 1.61 1.34 1.21 1.13 1.07

4 2.30 1.50 1.24 1.10 1.02 0.97

8 2.25 1.45 1.18 1.05 0.97 0.92

12 2.23 1.43 1.17 1.03 0.95 0.90

24 2.22 1.42 1.15 1.02 0.94 0.88

Waterreservoir GPRScommunicationrate(h) 1 2.20 1.61 1.41 1.31 1.25 1.21

2 1.99 1.40 1.20 1.10 1.04 1.00

4 1.89 1.30 1.10 1.00 0.94 0.90

8 1.84 1.24 1.05 0.95 0.89 0.85

12 1.82 1.23 1.03 0.93 0.87 0.83

24 1.80 1.21 1.01 0.91 0.85 0.81

InTable4,thedevicesentmultiplesamplesduringeachconnec- tioninordertooptimizethebatteryinscenarioswhererealtime isnotnecessary.Fig.10showsthecurrentconsumptionofallthe nodesforasamplingrateof15minandabove,andincasethatreal timeinformationisneeded;thatis,assoonasthesensorinforma- tionisread,thedataissenttotheserver,sotheinformationfor theuserisavailableinrealtime.Inthesescenarios,thepowercon- sumptionincreasedineverycasebutwasstillacceptablegiventhe speciﬁcationsofthedevice.

4. Conclusions

Thispaperdescribesthedesign,optimizationanddevelopment ofapracticalapplicationtooptimizewaterresourcesinirrigated agriculturebymonitoringsoilwaterstatusandtheirrigationwater.

Thearticleshowsthetechnicalchallengesaddressedintermsof WSNsandGSM/GPRSnetworkcommunications,thesensorsused andtheirconﬁgurationandenergyoptimizationinasystembased onadistributedarchitecture.

Tohighlighttheeffectivenessand accuracyofthedeveloped system,severalcasestudieshavebeenpresentedwiththeaimof analyzingthebehaviourofthedeviceinrealscenarios.Itiscon- cludedthatthesensornodesareofgreathelptoreachthedesired moistureconditionsatthedifferentdepthsandtomaintainwater electricalconductivityleveloftheirrigationreservoiratthedesired value,thuscontributingtotheabilitytoadapttheirrigationstrat- egyaccordingtoeachsituation.

Thedevelopmentofasmall,compactandautonomousunitis ofkeyimportanceforconvenientuseinagriculturalfacilities,soas nottointerferewithﬁeldoperations,avoidthetheftofmaterialand allowinstallation.Tomakethesystemcompatiblewithcommercial sensors,ithasbeennecessarytoadaptthedevicetothedifferent interfacesadoptedbysensors’vendors.

ThesamplingrateofthesensorsandsendingrateusingaGPRS connectionisimportantconsiderationswhenestimatingthepower consumptionofthedevice.Intheperformedtests,thesampling ratewasadjustedto15minandthesendingrateto30min.These

ratesareconsideredagoodchoiceforcorrectagronomicdecisions andformaintainingthecompleteautonomyofthedevicewithan adequatebatteryandsolarpanelpower.

Acknowledgments

The developmentof this workwas supported bythe Span- ishMinistryof Scienceand Innovation(Ministeriode Ciencia e Innovaci ´on) (MICINN) through the projects MICINN, AGL2010- 19201-C04-04and MINECO,AGL2013-49047-C2-1-R.We would liketothankWidhocSmartSolutionsS.L.forlettingususetheir facilitiestocarryoutthetests.

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