south west coast - bureau of meteorology · the south west coast region covers approximately...

South West Coast10 South West Coast ..................................................... 2

10.1 Introduction ........................................................ 2

10.2 Key information .................................................. 3

10.3 Description of region ......................................... 4

10.3.1 Physiographic characteristics.................. 6

10.3.2 Elevation ................................................. 7

10.3.3 Slopes .................................................... 8

10.3.4 Soil types ............................................... 9

10.3.5 Land use ............................................. 11

10.3.6 Population distribution .......................... 13

10.3.7 Rainfall zones ....................................... 14

10.3.8 Rainfall deficit ....................................... 15

10.4 Landscape water flows ................................... 16

10.4.1 Rainfall ................................................. 17

10.4.2 Evapotranspiration ............................... 20

10.4.3 Landscape water yield ......................... 23

10.5 Surface water and groundwater ....................... 26

10.5.1 Rivers ................................................... 26

10.5.2 Streamflow volumes ............................. 28

10.5.3 Streamflow salinity ............................... 28

10.5.4 Flooding ............................................... 31

10.5.5 Storage systems ................................... 31

10.5.6 Wetlands ............................................. 31

10.5.7 Hydrogeology ....................................... 35

10.5.8 Watertable salinity ................................. 35

10.5.9 Groundwater management units ........... 35

10.6 Water for cities and towns ................................ 39

10.6.1 Urban centres ....................................... 39

10.6.2 Sources of water supply ...................... 41

10.6.3 Perth .................................................... 41

10.7 Water for agriculture ......................................... 48

10.7.1 Soil moisture ........................................ 48

10.7.2 Irrigation water ...................................... 49

10.7.3 Irrigation areas ...................................... 49

10.7.4 Harvey Water Irrigation Area ................. 52

South West Coast

2 Australian Water Resources Assessment 2012

10 South West Coast10.1 Introduction

ThischapterexamineswaterresourcesintheSouthWestCoastregionin2011–12andoverrecentdecades.Itstartswithsummaryinformationonthestatusofwaterflows,storesanduse.Thisisfollowedbydescriptiveinformationfortheregionincludingthephysiographiccharacteristics,soiltypes,population,landuseandclimate.

Spatialandtemporalpatternsinlandscapewaterflowsarepresentedaswellasanexaminationofthesurfaceandgroundwaterresources.Thechapterconcludeswithareviewofthewatersituationforurbancentresandirrigationareas.ThedatasourcesandmethodsusedindevelopingthediagramsandmapsarelistedintheTechnicalSupplement.

3Australian Water Resources Assessment 2012

10.2 Keyinformation

Table10.1givesanoverviewofthekeycomponentsofthedataandinformationinthischapter.

Table 10.1 Key information on water flows, stores and use in the South West Coast region

Landscape water flows

Evapo-transpiration

Landscapewater yield

Rainfall

Regionaverage Differencefrom1911–2012long-termannualmean

Decilerankingwithrespecttothe1911–2012record

499mm +14% 8th—aboveaverage

455mm +15% 10th—verymuchaboveaverage

28mm -24% 4th—average

Streamflow (at selected gauges)

Annualtotalflow:

Betweenaverageandverymuchbelowaverageflowthroughouttheregion

Salinity:Annualmedianelectricalconductivitypredominantlyabove2,000μS/cminlandandlesssalineinthecoastalriversinthewest

Flooding: Sporadicminorandmoderatefloodinginthesouthwestriverbasins

Surface water storage (comprising about 92% of the region’s total capacity of all major storages)

Totalaccessiblecapacity

30June2012 30June2011 Change

accessiblevolume

%oftotalcapacity

accessiblevolume

%oftotalcapacity

accessiblevolume

%oftotalcapacity

951GL 309GL 32% 210GL 22% +99GL +10%

Groundwater (in selected aquifers)

Salinity: Non-salinegroundwater(<3000mg/L)onlyalongthesouthwestcoastlineandinscatteredareasinthenorth

Urban water use (Perth)

Totalsourcedin2011–12 Totalsourcedin2010–11

Change Restrictions

248GL 247GL +1GL EasedtoStage5inSeptember,backtoStage7

inJune

Annual mean soil moisture (model estimates)

Spatialpatterns: Predominantlyaveragetobelowaverageannualmeansoilmoisturewithlargeareasofverymuchbelowaveragesoilmoisturetotheeastandinthesouthwest

Temporalpatternsinregionalaverage:

Belowaveragesoilmoisturethroughouttheyear,onlyreachingaveragelevelsinsummer

South West Coast


10.3 Descriptionoftheregion

TheSouthWestCoastregioncoversapproximately326,000km²ofland.ItisboundedtothewestbytheIndianOceanandtothesouthbytheGreatAustralianBight.ThenorthernboundaryisthePilbara–GascoyneregionandtheeasternboundaryistheSouthWesternPlateauregion.

Thelandscapeoftheregionisgenerallyflatandsandyininlandareas.HigheraltitudetopographicfeaturesincludetheDarlingScarpthatformsanancientgeologicalboundarytothe30-kmwideSwanCoastalPlainalongthewestcoastandtheStirlingRangenearAlbany.TheSwanCoastalPlainconsistsofrelativelyinfertilesandysoilsandcoastaldunes;anumberofestuariesandwetlandsareseparatedfromtheseabythesedunes.Subsections10.3.1–10.3.4givemoredetailonphysicalcharacteristicsoftheregion.

Theregionhasapopulationjustover2millionpeople,whichaccountsforjustover9%ofthenation’stotalpopulation(AustralianBureauofStatistics[ABS]2011b).Figure10.1highlightsthemajorpopulationcentresintheregionthatincludesPerth,Mandurah,BunburyandAlbany.Furtherdiscussionoftheregion’spopulationdistributionandurbancentrescanbefoundinsubsection10.3.6andsection10.6respectively.

Thenorthoftheregionisdominatedbypasture,becomingamorecomplexpatternofpastureanddrylandcropstowardsthecentralandsouthernpartsoftheregion.Forestryisimportantinthewetterhigheraltitudewesternslopes,withextensiveconservationreserveareasinthispartoftheregion.Conservationshrublandandheathlandreservesarewellrepresentedinthemorearideast(Figure10.1).

Drylandandirrigatedagricultureaccountforaverysmallproportionofthelanduseinthearea.Intensivelandusessuchasurbanareasalsoaccountforsmallproportionsoftheregion.ThelargestareasofirrigatedagriculturearelocatedinthePeel–Harveyirrigationdistrict,withaconsiderableproportionofthewaterusedtoirrigatepasturefordairyandforvegetableandfruitcrops.Section10.7hasmoreinformationonagriculturalactivitiesintheregion.

Theregionhasatemperateclimatewithwarmdrysummersandcoolwinters.Mostrainfalloccursinthewestwithreducedrainfallfurtherinland.TheStirlingRangereceivesoccasionalsnowfall.Subsections10.3.7and10.3.8providemoreinformationontherainfalltypesanddeficitsacrosstheregion.

Thelongestriversarelocatedinthesouthwest,withtheAvonandBlackwoodrivershavingthelargestcatchmentareas.DuetotheraisedtopographyandorientationoftheDarlingScarp,mostofthemajorriversdraintowardstheIndianOceanbutthehighsoilpermeabilityresultsingenerallylowsurfaceflows.

Thehydrogeologyoftheregionisdominatedbyalargeareaofoutcroppingfracturedbasementrock.Thegroundwatersystemsinfracturedrocktypicallyofferarestrictedlowvolumegroundwaterresource.Significantgroundwaterresourcesare,however,availableonpartsofthecoastalplain.Approximately35–50%ofthewatersuppliedtothecityofPerthissourcedfromgroundwater.Amoredetaileddescriptionoftheriversandgroundwaterstatusintheregionisgiveninsection10.5.


Figure 10.1 Major rivers and urban centres in the South West Coast region

South West Coast


Figure 10.2 Physiographic provinces of the South West Coast region

10.3.1Physiographiccharacteristics

ThephysiographicmapinFigure10.2showsareaswithsimilarlandformevolutionaryhistories(Painetal.2011).Thesecanberelatedbacktosimilargeologyandclimaticimpactswhichdefinetheextentoferosionprocesses.Theareashavedistinctphysicalcharacteristicsthatcaninfluencehydrologicalprocesses.TheSouthWestCoastregionhastwophysiographicprovinces,namelytheWesternCoastlandsandYilgarnPlateauprovinces.

TheWesternCoastlandsprovincestretchesalongthewesternsideoftheregion,occupying9%ofthearea.Inthenorthithasdissectedferruginous

plateausandhillsonsedimentaryrockswithareasofextensivecoastaldunesystems.Thecentralportionoftheregionhasduneridges(onlimestonealongthecoast)andinneralluvialplains.Inthesoutheasttherearelowmoderatelydissectedferruginousplateausondownfaultedsedimentaryrocksandaswampysouthcoastalplain.Inthesouthwestthereisanarrowgraniticridgewithcalcareousdunesanddissectedferruginousplateaus.

TheYilgarnPlateauprovincecovers91%oftheareaandislargelyflattoundulating,withsaltlakesandoccasionallowhillsofigneousandmetamorphicrock.Someareashaveferruginousbreakawaysanddunes.


10.3.2Elevation

Figure10.3presentsgroundsurfaceelevationsintheSouthWestCoastregion.InformationwasobtainedfromtheGeoscienceAustraliawebsite(www.ga.gov.au/topographic-mapping/digital-elevation-data.html).Thelandscapeoftheregionisgenerallyflatininlandareas.HigheraltitudetopographicfeaturesincludetheDarlingRange,reachingaltitudesexceeding400mabovesealevel,andtheStirlingRange,withitshighestpeaksreachingjustover1,000mabovesealevel(Figure10.3).

TheDarlingRangeformsanancientgeologicalboundarytothe30-kmwideSwanCoastalPlainalongthewestcoast.

TheSwanCoastalPlainconsistsofrelativelyinfertilesandysoilsandcoastaldunes.Anumberofestuariesandwetlandsareseparatedfromtheseabythesedunes.

Movingfromthewesttotheeastintothevastinlandarea,altitudesgraduallyrisefromaround200mabovesealevelinthewesttoaltitudesexceeding400mabovesealevelattheeasternborderoftheregion.

Figure 10.3 Ground surface elevations in the South West Coast region

http://www.ga.gov.au/topographic-mapping/digital-elevation-data.html

http://www.ga.gov.au/topographic-mapping/digital-elevation-data.html

South West Coast


10.3.3Slopes

Areaswithsteepslopesprovidehigherrun-offgeneratingpotentialthanflatareas.TheSouthAustralianGulfregionhashardlyanyareaswithsteepslopes.Mostoftheareaisratherflat(Table10.2andFigure10.4).Theslopeswerederivedfromtheelevationinformationusedintheprevioussection.

Table 10.2 Proportions of slope classes for the region

Slope class (%) 0–0.5 0.5–1 1–5 > 5

Proportionofregion(%) 20.6 28.0 48.0 3.4

ThesteepslopesinFigure10.4inparticularhighlighttheStirlingRangeinthesouthandsomedeepervalleysintheDarlingRangewhichhavebeencutoutbyrivers.Thewesterncoastalplainsrunalongthecoastforabout400km,althoughthenorthernpartisseparatedfromtheseabyahighandwideridgeofsanddunes.

TheinlandoftheregionisnotasflatasotherinlandpartsofAustralia.Thegentleslopestheredonotinterferewithadvancedcommercialuseofland,suchasdrylandagriculture.OnthesteeperslopesoftheDarlingRange,however,forestryisadominantlanduse.

Figure 10.4 Surface slopes in the South West Coast region


Figure 10.5 Soil types in the South West Coast region

10.3.4Soiltypes

Soilsplayanimportantroleinthehydrologicalcyclebydistributingwaterthatreachestheground.Watercanbetransportedtoriversandlakesviathesoilsurfaceasrun-offorenterthesoilandprovidewaterforplantgrowthaswellascontributingtogroundwaterrecharge.

Thenatureofthesehydrologicalpathwaysandthesuitabilityofsoilsforagriculturalpurposesareinfluencedbysoiltypesandtheircharacteristics.SoiltypeinformationwasobtainedfromtheAustralianSoilResourceInformationSystemwebsite(www.asris.csiro.au).

About86%oftheSouthWestCoastregioniscoveredbyfoursoiltypes,namelysodosols,tenosols,kandosolsandchromosols(Figure10.5andFigure10.6).

Sodosolsarethemostcommonsoilsinthisregion.Theyaredominantinthecentral-southtosoutheastoftheregion.Thesesoilshaveastronglycontrastingtexture,withimpermeablesodicsubsoilsarisingfromelevatedsodiumconcentrationsandclay.Theyaresusceptibletodrylandsalinityaswellaserosion,ifvegetationisremoved.Sodosolsareusuallylowinnutrientstatusbut,inthisregion,arepresentinareasusedfordrylandcropsandpastures.

Similarlychromosolshaveastronglycontrastingtexture,buthavepermeablesubsoilswhicharenothighinsodium.Theyhavemoderatechemicalfertility,water-holdingcapacityandagriculturalpotential.Soilacidificationandsoilstructuraldeclinemayalsooccur.Theyarecommoninthewesternpartoftheregionandaremostlypresentinpasturesandareasusedforforestryornatureconservation(Figure10.6).

Tenosolsandkandosolsaresoilswithweakandminimaldevelopmentthroughouttheprofile(tenosols)andarelessstructured(kandosols).Thesesoilsarelowinfertilityandwater-holdingcapacity,havingaweakprofiledevelopment.Theyareoftenshalloworstony.Tenosolsarecommoninthenorthernaswellasinthewesternpartoftheregionandarescatteredinthewesternpart.Kandosolsarescatteredallovertheregion.Thesesoilsareusedforforestry,pasturesandnatureconservation.

TheothersoiltypesthathaveminimalrepresentationintheSouthWestCoastregionarehydrosols,calcarosols,podosols,kurosolsandrudosols(1–5%ofthetotalarea).

www.asris.csiro.au

South West Coast


Figure 10.6 Soil type distribution in the South West Coast region


Figure 10.7 Land use in the South West Coast region

10.3.5Landuse

Figure10.7presentslanduseintheSouthWestCoastregion(datafromdata.daff.gov.au/anrdl/metadata_files/pa_luav4g9abl07811a00.xml).Muchoftheregion'snorthisdominatedbypasture,becomingamixofpastureanddrylandcropstowardsthecentralandsouthernpartsoftheregion(Figure10.8).

Forestryisamajorlandcoverinthewetter,higheraltitudesouthwesternslopes,withextensiveconservationreserveareasinthispartoftheregion.

Conservationshrublandsandheathlandreservesoccupymostofthearideastoftheregion.Irrigatedagricultureaccountsforaverysmallproportionofthelanduseofthearea.Intensivelandusessuchasurbanareasalsoaccountforsmallproportionsoftheregion.

ThelargestareasofirrigatedagricultureintheregionarelocatedinthePeel–Harveyirrigationdistrict,withaconsiderableproportionofthewaterusedtoirrigatepasturefordairyandforvegetableandfruitcrops.

http://data.daff.gov.au/anrdl/metadata_files/pa_luav4g9abl07811a00.xml

http://data.daff.gov.au/anrdl/metadata_files/pa_luav4g9abl07811a00.xml

South West Coast


Figure 10.8 Land use distribution in the South West Coast region


10.3.6Populationdistribution

WhiletheSouthWestCoastregionoccupiesthemorepopuloussouthwesterncornerofWesternAustralia,itspopulationdensity,incontrasttotheNorthEastCoastandSouthEastCoastregions,isrelativelylow.

WiththemajorityofthepopulationresidinginPerth,theregion’spopulationcentreislocatedonitswesterncoastalfringe.ThepopulationsofPerthandsurroundingdistrictshavebeenbolsteredinrecentyearswhileprovidingsupportingservicestotherenewedandincreasedminingactivitiesintheState.

OutsideofPerth,agriculture,miningandforestryhaveplayedimportantrolesinthedistributionoftheregion’sremainingmajorpopulationcentres.ManyofthesecentresarelocatedinthesouthwesterncoastaldistrictsoftheregionandincludetheurbancentresofBunburyandAlbany.Theeasternextentsoftheregionencompassmuchofthewheatbeltdistrict,wherefarmingactivitiesplayamajorroleintheexistenceofmanytowns.

Figure10.9showsthespatialdistributionofpopulationdensityfortheregionandisbasedondatafromtheABS(2011b).

Figure 10.9 Population density and distribution of the South West Coast region

South West Coast


10.3.7Rainfallzones

TheSouthWestCoastregionhasatemperateclimate.Mostrainfalloccursinthesouthwestduringwinterwithreducedrainfallfurtherinland.Medianrainfallisrathervariablethroughouttheregion.

Figure10.10showsthatthenortheasterninlandpartoftheregionissemi-aridtoaridandmedianannualrainfalltotalsdonotexceed500mm.

Movingtothesouthwest,therainfallbecomesmorewinterdominantandmedianannualtotalsincreasetolevelsexceeding800mm.Especiallyinthemostsouthernareas,annualrainfalltotalsoftenexceed1,200mm.

Formoreinformationonthisandotherclimateclassifications,visittheBureauofMeteorology's(theBureau's)climatewebsite:www.bom.gov.au/jsp/ncc/climate_averages/climate-classifications/index.jsp

Figure 10.10 Rainfall zones in the South West Coast region

http://www.bom.gov.au/jsp/ncc/climate_averages/climate-classifications/index.jsp

http://www.bom.gov.au/jsp/ncc/climate_averages/climate-classifications/index.jsp


10.3.8Rainfalldeficit

Therainfallminuspotentialevapotranspirationindicatorgivesageneralimpressionaboutwhereintheregionmoisturedeficitsarelikelytooccurovertheperiodofayear.TheSouthWestCoastregionhasaratheruniformpatternofsubstantialpotentialdeficitsovertheentireregion(Figure10.11).

Seriousdeficitscanbeexpectedininlandareas.Duetotheseasonalityoftherainfall,thisareaisstillwellsuitedfordrylandagriculture,whichoccursatanextensivescale.

Alongthesouthwestcoast,wherethedeficitismarginal,forestryisamajorlandusecomponent.Thefarsouthwesthassomeareasofmoistureabundancewhicharepredominantlyinusefornatureconservation,containingsomeofthetallesttreesinAustralia,includingtheKarriforests.

Formoreinformationontherainfallandevapotranspirationdata,seetheBureau'smapsofaverageconditions:www.bom.gov.au/climate/averages/maps.shtml

Figure 10.11 Rainfall deficits in the South West Coast region

http://www.bom.gov.au/climate/averages/maps.shtml

http://www.bom.gov.au/climate/averages/maps.shtml

South West Coast


Figure 10.12 Landscape water flows in 2011–12 compared with the long-term record (July 1911–June 2012) for the South West Coast region

10.4 Landscapewaterflows

Thissectionpresentsanalysesofthespatialandtemporalvariationoflandscapewaterflows(rainfall,evapotranspirationandlandscapewateryield)acrosstheSouthWestCoastregionin2011–12.Nationalrainfallgridsweregeneratedusingdatafromanetworkofpersistent,high-qualityrainfallstationsmanagedbytheBureau.EvapotranspirationandlandscapewateryieldswerederivedusingthelandscapewaterbalancecomponentoftheAustralianWaterResourcesAssessmentSystem(VanDijk2010).ThesemethodsandassociatedoutputuncertaintiesarediscussedintheIntroductionandaddressedinmoredetailintheTechnicalSupplement.

Figure10.12showsthattheregionhasahighlyseasonalrainfallpatternwithawetwinterandadrysummerperiod.Evapotranspirationfollowsthispattern,albeitwithaboutamonthdelay,therebygenerallyexceedingrainfallduringspringandsummer.Themonthlylandscapewateryieldhistoryfortheregionshowsastablepatternofverylowyieldinthedryperiod.Itmarginallyincreasesduringwintermonths.

The2011–12yearwasarelativelywetyear,withsomehighrainfallbetweenOctoberandDecember.TheDecemberrainfallwasthehighestonrecordoverthe1911–2012period.Ontheotherhand,somebelowaveragerainfalloccurredduringAprilandMay2012.

Withaboveaveragesoilmoistureconditions,duetoaboveaveragerainfallduringtheOctober–Decemberperiod,evapotranspirationwasalsorelativelyhighforthisperiod.December2011hadthehighestevapotranspirationonrecord,whereasNovemberhadthesecondhighestevapotranspirationonrecord.

Thelandscapewateryieldfor2011–12showedsomedistinctpatterns,withDecemberhavingthesecondhighestlandscapewateryieldonrecord.Ontheotherhand,theApril–Mayperiodhadthefifthlowestlandscapewateryieldonrecord;however,withverylowabsolutelandscapewateryieldoccurringintheseperiods,theabsolutedifferencefromthemeanisnothigh.


Figure 10.13 Spatial distribution of (a) annual rainfall in 2011–12 and (b) their decile rankings over the 1911–2012 period for the South West Coast region

10.4.1Rainfall

RainfallfortheSouthWestCoastregionfor2011–12isestimatedtobe499mm.Thisis14%abovetheregion’slong-termaverage(July1911–June2012)of438mm.Figure10.13ashowsthatthehighestrainfalloccurredalongthesoutherncoastalareaswithannualtotalslocallyexceeding900mm.Therestoftheregionmostlyhadrainfallrangingbetween300and600mmfor2011–12.

Rainfalldecilesfor2011–12indicateaboveaveragerainfallformostoftheregionoverthecourseoftheyear(Figure10.13b).Mostoftheinlandpartsoftheregionreceivedaboveaveragerainfallwithsomepartsintheeastreceivingverymuchaboveaveragerainfall.ThewesterncoastalareasouthofPerthlocallyhadbelowaveragerainfall.

South West Coast


Figure 10.14 Time-series of (a) annual rainfall, and (b) five-year retrospective moving averages for the summer (November–April) and winter (May–October) periods for the South West Coast region

Rainfall variability in the recent past

Figure10.14ashowsannualrainfallfortheregionfromJuly1980onwards.Overthis32-yearperiodtheannualaveragewas430mm,varyingfrom340mm(2006–07)to563mm(1998–99).Temporalvariabilityandseasonalpatterns(overthesummerandwinterperiods)since1980arepresentedinFigure10.14b.

Thegraphsshowaparticularlystablepatternofannualrainfallincomparisontootherregions.Rainfallinthewinterperiodisconsistentlyhigherthaninthesummerperiod,althoughadecreaseinwinterperiodrainfallhasoccurredafter2001.Sincethen,onlythelastyearofthisperiodreachedanannualrainfalltotalthatwassubstantiallyhigherthanthe32-yearperiodaverage.


Figure 10.15 Spatial distribution of (a) trends in annual rainfall from 1980–2012, and (b) their statistical significance at 90% (weak) and 95% (strong) confidence levels for the South West Coast region

Recent trends in rainfall

Figure10.15apresentsthespatialdistributionofthetrendsinannualrainfallforJuly1980–June2012.Thesearederivedfromlinearregressionanalysesonthetime-seriesofeachmodelgridcell.ThestatisticalsignificanceofthetrendsisprovidedinFigure10.15b.

Figure10.15ashowsthatsince1980astrongdecreaseinrainfallhasoccurredgenerallyoverthewholeofthewesterncoastalzone.

Thefallingtrendsarestronglysignificantinlargepartsofthisarea(Figure10.15b),withthepeakdecreasearoundPerth.ThishasseriouslyimpactedthesurfacewatersupplyforPerthandsurroundings(seesection10.6formoreinformationonurbanwatersupplyintheregion).

South West Coast


10.4.2Evapotranspiration

ModelledannualevapotranspirationfortheSouthWestCoastregionfor2011–12isestimatedtobe455mm.Thisis15%abovetheregion’slong-term(July1911–June2012)averageof397mm.Thespatialdistributionofannualevapotranspirationin2011–12(Figure10.16a)issimilartothatofrainfall(Figure10.13a).Inabsoluteterms,evapotranspirationwaspracticallyequaltorainfallinthemajorinlandpartoftheregion.

Evapotranspirationdecilesfor2011–12indicateaboveaverageorverymuchaboveaveragetotalsacrossmostoftheregion(Figure10.16b).ThiscoincideswiththespatialpatternofrainfalldecilesinFigure10.13b.Withlimitedwateravailabilityinthesouthwest,duetobelowaveragerainfall,evapotranspirationalsoremainedbelowaverageinthisarea.

Figure 10.16 Spatial distribution of (a) modelled annual evapotranspiration in 2011–12, and (b) their decile rankings over the 1911–2012 period for the South West Coast region


Figure 10.17 Time-series of (a) annual evapotranspiration, and (b) five-year retrospective moving averages for the summer (November–April) and winter (May–October) periods for the South West Coast region

Evapotranspiration variability in the recent past

Figure10.17ashowsannualevapotranspirationfortheregionfromJuly1980onwards.Overthis32-yearperiodtheannualevapotranspirationaveragewas395mm,varyingfrom317mm(2010–11)to535mm(1999–2000).Temporalvariabilityandseasonalpatterns(overthesummerandwinterperiods)since1980arepresentedinFigure10.17b.

Despitethelowertemperaturesinthewinterperiod,evapotranspirationisconsistentlyhigherthaninthesummerperiod,duetolowerwateravailabilityduringthesummerperiod.Comparedwiththeseasonalrainfall(Figure10.14b),thetemporalpatternsofevapotranspirationarehighlysimilarandalsodisplaytheminordecreaseinevapotranspirationinthewinterperiodsince2001(Figure10.17b).

South West Coast


Recent trends in evapotranspiration

Figure10.18apresentsthespatialdistributionofthetrendsinmodelledannualevapotranspirationfor1980–2012.Thesearederivedfromlinearregressionanalysesonthetime-seriesofeachmodelgridcell.ThestatisticalsignificanceofthetrendsisprovidedinFigure10.18b.

Figure10.18ashowsthat,since1980,fallingtrendsareidentifiedinthewesterncoastalzone.Intheeast,largelymarginallyrisingtrendsarecalculated.

AsshowninFigure10.18b,thetrendsaregenerallyonlystatisticallysignificantinthosecoastalareaswherethetrendexceeds–2mm/year.Thespatialdistributionofthetrendsissimilartothatofrainfall.Asevapotranspirationisdrivenbytheavailabilityofmoisturecomingfromrainfall,thespatialdistributionofthetrendsandthetrendsignificancearesimilartorainfall,asshowninFigure10.15.

Figure 10.18 Spatial distribution of (a) trends in annual evapotranspiration from 1980–2012, and (b) their statistical significance at 90% (weak) and 95% (strong) confidence levels for the South West Coast region


10.4.3Landscapewateryield

ModelledlandscapewateryieldfortheSouthWestCoastregionfor2011–12isestimatedtobe28mm.Thisis24%belowtheregion’slong-term(July1911–June2012)averageof37mm.Figure10.19ashowsthespatialdistributionoflandscapewateryieldfor2011–12,whichissimilartotheannualrainfalldistribution(Figure10.13a;notethedifferenceinthescalesbetweenthetwofigures).

Thedecile-rankingmapofFigure10.19b,however,showsaspatialpatternthatisdissimilartothosepatternsinrainfallofFigure10.13b.Themapshowsmostlyaveragetobelowaveragelandscapewateryieldsformuchlargerareasalongthecoastaswellasfurtherinland.Ithastobeacknowledgedthatmodelverificationassessmentsforthisregionidentifiedmorequestionableperformancesthaninotherregionsandresultsshouldbeinterpretedwithcare.

Figure 10.19 Spatial distribution of (a) modelled annual landscape water yield in 2011–12, and (b) their decile rankings over the 1911–2012 period for the South West Coast region

South West Coast


Figure 10.20 Time-series of (a) annual landscape water yield, and (b) five-year retrospective moving averages for the summer (November–April) and winter (May–October) periods for the South West Coast region

Landscape water yield variability in the recent past

Figure10.20ashowsannuallandscapewateryieldfortheSouthWestCoastregionfromJuly1980onwards.Overthis32-yearperiod,annuallandscapewateryieldwas30mm,varyingfrom11mm(2010–11)to52mm(1999–2000).Temporalvariabilityandseasonalpatterns(overthesummerandwinterperiods)since1980arepresentedinFigure10.20b.

Landscapewateryieldisconsistentlyhigherduringthewinterperiodandparticularlylowduringthesummerperiod.Thefallingtrendinlandscapewateryieldsince2001forthewinterperiodisconsistentwiththatofrainfallandevapotranspiration.


Recent trends in landscape water yield

Figure10.21ashowsthespatialdistributionofthetrendsinmodelledannuallandscapewateryieldfor1980–2012.Thesearederivedfromlinearregressionanalysesonthetime-seriesofeachmodelgridcell.ThestatisticalsignificanceofthetrendsisprovidedinFigure10.21b.

AsshowninFigure10.21a,trendsarenegativeinthecoastalzoneandoverabandcoveringalargepartofthesouthoftheregion.Inthisband,trendsarelocallystatisticallysignificant,reachingvaluesof–0.5mm/year.Thefarnorthandfarwesthavesomeareasofsignificantrisingtrends,withtrendlineslopesupto0.5mm/year.

Figure 10.21 Spatial distribution of (a) trends in annual landscape water yield from 1980–2012, and (b) their statistical significance at 90% (weak) and 95% (strong) confidence levels for the South West Coast region

South West Coast


Swan River bank, Perth | BockyTandiono (iStockphoto)

10.5 Surfacewaterand groundwater

ThissectionexaminessurfacewaterandgroundwaterresourcesintheSouthWestCoastregionin2011–12.Rivers,wetlandsandstoragesarediscussedtoillustratethestateoftheregion’ssurfacewaterresources.Theregion’swatertableaquifersandsalinityaredescribed.NodatawasavailableattheBureauinasuitableformatforadetailedanalysisonindividualaquifers.

10.5.1Rivers

Thereare14riverbasinsintheSouthWestCoastregion,varyinginsizefromabout1,700–121,000km2(Figure10.22).

DuetotheelevatedtopographyandorientationoftheDarlingScarp,mostofthemajorriversdraintowardstheIndianOcean;however,thehighsoilpermeabilityintheSwanCoastalPlainresultsinsubstantiallossesofsurfacewatertogroundwater.Thenorth–southcoastaldunepatterncausesmanywatercoursestoturnabruptlyandfloweitherinasouthoranorthdirectionalongtheextensiveduneswales,oftenjoiningwithotherwatercoursesbeforedischargingtothesea.TherearealsoinlandendorheicriverbasinstotheeastoftheDarlingScarp.

Thelongestriversarelocatedinthesouthwest,withtheAvonandBlackwoodrivershavingthelargestcatchmentsareas.TheSwanRiverbecomestheAvonRiverattheDarlingScarp.Mostoftheriversareperennial,butinthenorthandwestmanyareephemeral.


Figure 10.22 Rivers and catchments in the South West Coast region

South West Coast


10.5.2Streamflowvolumes

Figure10.23presentsananalysisofflowsat30monitoringsitesduring2011–12relativetoannualflowsfortheperiodfromJuly1980–July2012.Monitoringsiteswithrelativelylongrecordsacross16geographicallyrepresentativeriverswereselected(seeTechnicalSupplementfordetails).Theannualflowsfor2011–12arecolour-codedaccordingtothedecilerankateachsiteoverthe1980–2012period.

TheflowsgenerallyreflectthemostlyaveragetobelowaveragemodelledlandscapewateryieldresultsshowninFigure10.19b.AboveaverageannualflowswereobservedattwositeslocatedontheArthurandBlackwoodriversinthecentralsouthwestoftheSouthWestCoastregion.Averagetotalannualflowswereobservedat12monitoringsites,mainlylocatedontheriversinthecentralsouth,centralwest,theLockhartRiverinthecentreandtheMooreRiverinthenorthofPerth.

Belowaverageflowsoccurredat11sitesintheregionwhichwerelocatedontheriversinthewestoftheregion,especiallysurroundingPerth,andintheDeepandKalganriversinthecentralsouthoftheregion.Ofthe30monitoringsites,therewereonlyfivesitesacrosstheregionwhichrecordedverymuchbelowaverageflowsrecordedacrossthein2011–12.ThesefivemonitoringsitesweremainlylocatedontheriversinthefarsouthwestandintheHarveyRiverinthecentralwest.

Flowdecilesforthesummerperiod(November2011–April2012)aredifferenttothosefortotalannualflowsasshowninFigure10.23.Thedifferenceisthatrelativelyhigherflowsoccurredthroughouttheregioninthesummerperiod.

10.5.3Streamflowsalinity

Figure10.24presentsananalysisofstreamflowsalinityfor2011–12at28monitoringsitesthroughouttheSouthWestCoastregion.Monitoringsiteswithatleastafive-yeardatarecordwereselectedforanalysis.Theresultsarepresentedaselectricalconductivity(EC,μS/cmat25°C).ThisisacommonlyusedsurrogateforthemeasurementofwatersalinityinAustralia.StandardEClevelsfordifferentapplications,suchasfordrinkingwaterortypesofirrigationareprovidedintheTechnicalSupplement.ThemedianannualECvaluesareshownascolouredcircles.ThesizeofthecircledepictsthevariabilityinannualEC,shownasthecoefficientofvariation(CV),beingthestandarddeviationdividedbythemean.

ThemedianECvaluesforfiveofthemonitoringsitesinthemainriversfallintherange0–1,000μS/cm,suitableformostirrigationuses.MostoftheselectedmonitoringsitesintheriversandcreeksoftheregionhavemedianECvaluesoutsidethisrange(seeFigure10.24).Thisisunsuitableformostirrigationusesandformanyaquaticecosystems.Ofthe28monitoringsites,14%hadmedianECvaluesbelow500μS/cmandatonesite(4%)themedianfellintherange500–1,000μS/cm.Fortheremainderofthesites(82%),ECvalueswereabove1,500μS/cm.Mediansalinitiesvaluesaregenerallyhighertothewestoftheregion.

Streamflowsalinitywasabove2,000μS/cmat21ofthe28monitoringsites.Thesearelocatedontheriversinthesouthwest,centreandsomesitesinthecentralwestoftheregion.Highsalinityinthemonitoringsitesinthisareamaybeinfluencedbyextensivelandclearing,largenaturalstoresofsaltinthelandscape,poorirrigationpractices,salinebaseflowandevaporationduringlowflow.Naturallyoccurringsalinesoilsarealsoakeyfactorinfluencingin-streamsalinityintheregionandareassociatedwithlacustrinesedimentsofsaltlakesystems(McArthur1991).

TheCVistheexpectedvariabilityofameasurementofECvalue,relativetotheannualmeanofEC.TheCVishighatsomesitesinthecentralwestoftheregionwhereasvariabilityisrelativelylowfortheriversinthesouthwestoftheregion.TheCVinECishighlyrelatedtothevariabilityinannualflowatthemonitoringsites.Ofthe28monitoringsites,7%ofthemhadaCVbelow20%;57%ofthesiteshadaCVbetween20%and60%;and36%wereabove60%.Theseweremainlylocatedontheriversinthecentralwestandsouthwestoftheregion.TheCVwasabove80%atfiveofthe28monitoringsites.ThesewerelocatedontheriversinthecentralwestandatMobrupCreekinthecentralsouthwestoftheregion.


Figure 10.23 Average annual and summer period flow volumes of selected sites for 2011–12 and their decile rankings over the 1980–2012 period in the South West Coast region

South West Coast


Figure 10.24 Salinity as electrical conductivity (μS/cm) and its associated coefficient of variation for 2011–12 in the South West Coast region


10.5.4Flooding

TherewerenomajorfloodsintheSouthWestCoastregionduringthe2011–12year.ThelocationsofminorandmoderatefloodsareshowninFigure10.25.

10.5.5Storagesystems

Thereare21majorpubliclyownedstoragesintheSouthWestCoastregion,withatotalstoragecapacityinexcessof1,000GL.TheBureau'swaterstoragewebsiteincludesinformationonapproximately95%oftheregion'spubliclyownedstoragecapacities(asatAugust2012).Moststoragesintheregionsupplytwomajorsystems;theHarveyIrrigationAreaandtheIntegratedWaterSupplySchemewhichsupplieswatertothePerthurbanarea.

Table10.3givesasummaryofthemajorstoragesystemsintheregiontogetherwithanoverviewofthestoragelevelsattheendof2010–11and2011–12.ThelocationofallthesystemsandassociatedstoragesareshowninFigure10.26.

Thestoragesintheregioncurrentlyhaveparticularlylowvolumes,especiallyinrelationtothestoragesintheeasternpartofAustralia.Thelowerrainfalltotalsduringthewinterperiod(asfoundinFigure10.14b)haveresultedinthecatchmentupstreamofthedamstobeinaverydrystate.

Newrainfallcominginhastopartlyfillupthesesoilsfirstbeforerun-offisgenerated.Withtheaveragetoaboveaveragerainfallof2011–12intheriversupstreamofthestorages,thishaspartlybeenachieved.Togetherwiththecommissioningofaseconddesalinationplant,whichreducedthedemandsonthestoragesupply,waterlevelswereabletoriseby10%ofaccessiblecapacity.

FurtherinformationonthepastandpresentvolumesofthestoragesystemsandtheindividualstoragescanbefoundontheBureau’swaterstoragewebsite:water.bom.gov.au/waterstorage

10.5.6Wetlands

ThereareanumberofRamsar-listed,internationallyimportantwetlandsintheSouthWestCoastregionaswellasanumberofwetlandsofnationalimportancementionedintheAustralian Directory of Important Wetlands (www.environment.gov.au/water/topics/wetlands/database/diwa.html).Thewetlandsvaryfromcoastaltidalflatstoinlandephemerallakesandlargesaltlakes(Figure10.27).

Nodetailedassessmentontheinflowsofselectedwetlandshasbeenperformedforthisregion.

Table 10.3 Major public storage systems in the region as identified in the Bureau’s water storage website (August 2012), with ‘non-allocated’ accounting for the storages not allocated to a particular system

Systemname Systemtype SystemcapacityAccessiblevolumeat30June2011

Accessiblevolumeat30June2012

Perth urban 593GL 107GL—18% 139GL—23%

Harvey rural 279GL 72GL—26% 135GL—48%

Non-allocated — 79GL 30GL—38% 35GL—44%

Total 951GL 210GL—22% 309GL—32%

http://water.bom.gov.au/waterstorage

http://www.environment.gov.au/water/topics/wetlands/database/diwa.html

http://www.environment.gov.au/water/topics/wetlands/database/diwa.html

South West Coast


Figure 10.25 Flood occurrence in 2011–12 for the South West Coast region, with each dot representing one gauging station and the colour of the dot representing the highest flood class measured


Figure 10.26 Storage systems in the South West Coast region (information extracted from the Bureau’s water information website in August 2012)

South West Coast


Figure 10.27 Location of important wetlands in the South West Coast region


10.5.7Hydrogeology

Figure10.28showsthemajoraquifergroupspresentatthewatertable.Theregionisdominatedbyfracturedrockgroundwatersystemsthatmayprovidealowvolumegroundwaterresource.Groundwatersystemsthatprovidemorepotentialforextractionarelabelledas:

• Surficialsedimentaquifer(porousmedia—unconsolidated);

• Mesozoicsedimentaquifer(porousmedia—consolidated);and

• Uppertertiary/quaternaryaquifer(porousmedia—unconsolidated).

Confinedaquifersystems,underlyingsomeofthewatertableaquifersshowninthefigure,provideanimportantresourcefortheregion.MostnotablearetheLeedervilleandYarragadeeaquifersbeneaththeSwanCoastalPlain.

10.5.8Watertablesalinity

Figure10.29showstheclassificationofwatertableaquifersasfresh(totaldissolvedsolids[TDS]<3,000mg/L)orsaline(TDS≥3,000mg/L)wateraccordingtowatertablesalinity.Mostpartsoftheregionareconsideredtohavesalinegroundwater.Thecoastalregionswithusablegroundwaterresourcesarethoseidentifiedasnon-saline.

10.5.9Groundwatermanagementunits

Thehydrogeologyoftheregionisdominatedbyalargeareaofoutcroppingfracturedbasementrockwhichtypicallyoffersarestrictedlowvolumegroundwaterresource.Significantgroundwaterresourcesare,however,availableonpartsofthecoastalplain.Approximately35%to50%ofthewatersuppliedtothecityofPerthissourcedfromgroundwater.

ThegroundwatermanagementunitswithintheregionarepresentedinFigure10.30.Mostofthesmallerunitsarelocatednearthewesternboundaryoftheregion,whilethelargerunitsarelocatedintheeastandsouth.Thesizeisinverselycorrelatedtotheyieldandqualityofgroundwaterresources,thatis,largeunitstypicallyhavelowvolumeandlowquality.Infact,themajoraquiferswithintheregionincludesurficialaquifersoftheGnangaraMoundandtheunderlyingconfinedLeedervilleandYarragadeeaquifers.

South West Coast


Figure 10.28 Watertable aquifers of the South West Coast region; data extracted from the Groundwater Cartography of the Australian Hydrological Geospatial Fabric (Bureau of Meteorology 2012)


Figure 10.29 Watertable salinity classes of the South West Coast region; data extracted from the Groundwater Cartography of the Australian Hydrological Geospatial Fabric (Bureau of Meteorology 2012)

South West Coast


Figure 10.30 Groundwater management units in the South West Coast region; data extracted from the National Groundwater Information System (Bureau of Meteorology 2013)


10.6 Waterforcitiesandtowns

ThischapterexaminesurbanwatersupplyintheSouthWestCoastregionin2011–12andoverrecentdecades.Themainurbancentresintheregionarediscussedbriefly.Themainurbanisedportion,thePerthwatersupplyarea,ispresentedindetailincludingtheirwatersupplysystems,storageposition,historicalandcurrentwaterrestrictions,watersourcesandsupplyinformation.ThemainsourceofdataforthissectionistheNationalPerformanceReport(NationalWaterCommission[NWC]2013).

10.6.1Urbancentres

PerthisthelargesturbancentreintheSouthWestCoastregionwithapopulationofmorethan1.6millionpeople.ItincludesthesuburbancentresofJoondalupinthenorth,ArmadaleinthesoutheastandFremantle,RockinghamandMandurahalongthesouthcoast.About80%oftheregion’spopulationlivesinPerth.

OutsideofPerththeregionhastwomajorurbancentres(populationsover25,000people).TheseareBunburyandAlbany.Table10.4providesasummaryoftheirpopulation,surroundingriverbasinandsignificantwaterstorages.

Bunbury,situated175kilometressouthofPerth,hasapopulationofabout64,000.Itisaportcity,servicingthefarming,miningandtimberindustriesofthesouthwest.

ThepopulationofAlbany,locatedabout400kmsoutheastofPerth,isjustunder27,000anditisalsoaportcity.ThemainindustriesofAlbanyconsistoftourism,fishingandagriculture.

Inadditiontoitsmajorurbancentrestheregionhasanumberofsmalltowns,inparticularthroughitssouthwest.Figure10.31showstheseregionscitiesandtownsalongwiththeirpopulationranges.

Table 10.4 Cities and their water supply sources in the South West Coast region

City Population1 Riverbasin Majorsupplysource

Perth 1,630,000 SwanCoastalGroundwaterCanning,SerpentineandSouthDandalupstorages,desalination

Bunbury 64,000 PrestonRiver Groundwater

Albany 26,500 KalganRiver Groundwater

1 Australian Bureau of Statistics (2011b)

South West Coast


Figure 10.31. Population range of urban centres in the South West Coast region


10.6.2Sourcesofwatersupply

UnlikemuchofAustraliatheregionasawholereliesheavilyongroundwaterfortheprovisionofitsurbanwatersupply.Whilesurfacewaterstoragesareanimportantcomponentofsupplysystems,theregion’shighlyvariablerainfallandshiftingclimaticpatternshaveseenaneedtoaugmentsupplieswithwaterfromadditionalsources.AsaresultdesalinationhasbecomeanimportantsourceofwaterforPerth.

Inthemoreremotetownstothenorthandeastoftheregion,groundwaterisoftenamajorsourceofwatersupply.TheWesternAustralianWaterCorporationhasalsoconnectedmanytownstotheirpipelinesupplynetwork,particularlyinthesoutheast.

Recycledwaterandharvestedstormandrainwatersourcesarealsousedtosupplytheurbanwaterdemandsthroughouttheregion.

10.6.3Perth

ThewatersupplysystemofPerthiscontrolledbytheWaterCorporationofWesternAustralia.Itistheprincipalsupplierofwater,wastewateranddrainageservicestohomesandbusinessesinthePerthwatersupplyarea,aswellasprovidingbulkwatertofarmsforirrigation.TheWaterCorporationmanagesPerth’sIntegratedWaterSupplyScheme(IWSS).

TheWaterCorporationalsorecycleswater,mainlyforsupplytosportingovalsandgolfcourses,butalsoforindustryandagriculture.Varioustrialsfortoppingupgroundwatersupplieswithrecycledwastewaterarealsobeingconducted,includingtheBeenyupReplenishmentTrial.

Withgrowingurbanwaterdemand,sourcesofwater,inparticularthegroundwatersystems,arebeingsubjecttosignificantstress.Tomanagegroundwaterabstractionsustainably,ecologicalwaterrequirementsandenvironmentalwaterprovisionsaredeterminedaspartofwaterallocationprocesses(WaterCorporation2008).

EnvironmentalwaterprovisionsaredeterminedbytheWesternAustralianDepartmentofWaterinaccordancewiththeprinciplesandprocessessetoutinitsEnvironmentalWaterProvisionPolicy(2000).

Theenvironmentalwaterprovisionsarethewaterregimesprovidedasaresultofthewaterallocationdecision-makingprocesses.Thisisacomprehensiveassessmentthattakesintoaccountecological,socialandeconomicimpacts.

Supply system

Perthobtainsitswatersupplyfromacombinationofgroundwater,surfacewater,desalinationandrecycledwater.TogetherthesesourcesformtheIWSS.AschematicofthisschemeisprovidedinFigure10.32.

UpuntiltwodecadesagosurfacewaterwasthemajorsourceofwaterforPerth.However,consistentlybelowaveragerainfallinthelastthreedecadeshasseengroundwaterbecomethemajorsourceofwatersupplyforurbanuse.

Withitsrelianceonthegroundwater,drawnfromtheGnangaraandJandakotmounds,anddecreasedrechargearesultofchangingclimaticconditions,Perth’swatersupplyaquiferlevelshavedeclined.Inresponsetothis,theStateGovernmenthasbegunconstructionofdesalinationplantstoprovidereliableandclimate-independentwatersupplytothePerthregion.

Thefirstdesalinationplant,inKwinana,wascompletedin2006withthecapacitytoproduce45GLperyear.Theseconddesalinationplant,inBinningup,becameoperationalinSeptember2011.TogetherthesedesalinationsourcesprovideabouthalfofthewaterneededforthePerthmetropolitanarea.

SurfacewatersuppliesforPertharedrawnfromtenmajorwaterstorageslocatedinthecatchmentstotheeastofPerth.ThesestoragesareshowninFigure10.33.Togethertheyhaveacombinedtotalaccessiblestoragecapacityof580GL.

South West Coast


Figure 10.32 Water supply schematic for Perth and surrounds


Figure 10.33 Urban supply storages in the South West Coast region

South West Coast


Storage volumes

ThehistoricaltimeseriesoftheaccessiblevolumeofPerth’sthreemajorstorages(Serpentine,CanningandSouthDandalup)areshowninFigure10.34.Togetherthesestoragescomprisealmost60%ofPerth’stotalsurfacewaterstoragecapacity.

Thesignificantdeclineinstoragevolumesthathasresultedfromshiftingclimaticpatternsandtheresultingdecreaseinsurfacewaterrun-offcanbeclearlyobservedacrossallstorages.Inparticular,reducedwinterrainfallsanddryingcatchmentshaveplayedanimportantroleinthedeclineofthesestorages.

Figure 10.34 Variation in the amount of water held in storage for the combined area over recent years (light blue) and over 2011–12 (dark blue), as well as total accessible storage capacity (dashed line)

Serpentine

Canning

South Dandalup


Water restrictions

WaterrestrictionsinPertharesetbytheStateGovernmentandenforcedbytheWaterCorporation.Waterrestrictionpolicy,includingrestrictionlevels,isenactedthroughWaterAgencies(WaterUse)by-lawsandlegislatedundertheWesternAustralianWater Agencies (Powers) Act 1984.

AhistoricaltimeseriesofwaterrestrictionsforPerthispresentedinFigure10.35.ThefigurehighlightsthedeclineinsurfacewaterresourcesandthesignificantrolewaterrestrictionshaveplayedinthelivesofPerthresidentsaswellasitscommercialandindustrialsectorsoverthelastdecade.

InSeptember2011,followinganincreaseinwinterrainfallandthecommissioningofPerth’sseconddesalinationplant,aneasingofrestrictionstoStage5wasallowed.However,inJune2012restrictionlevelswerebacktoStage7.

Figure 10.35 Urban water restriction levels for Perth since 1990 shown against the combined accessible water volume of Serpentine, Canning and South Dandalup storages

South West Coast


Sources of water obtained

GroundwatercontinuestobethemainsourceofwatersuppliedtoPerth;however,inthefuturethismaynotbethecaseduetothelowrechargeofgroundwateraquifers.Figure10.36illustratesthat,onaverage,morethan50%ofthebulkwatersourcedbytheWaterCorporationisfromgroundwater(NationalWaterCommission2011a).

Anincreaseindesalinationcapacity,asaresultoftheBinningupdesalinationplantbecomingoperationalin2011,hasseenwatersuppliedfromthissourcegrowfrom18GLin2006–07tojustover50GLin2011–12.

RecycledwatercontinuestoplayacomparativelyminorroleinPerth’surbanwatersupply.Watersuppliedfromthissourcehasexhibitedmodestgrowth,reaching6GLin2011–12,upfrom4GLin2006–07.

Despiterecordgrowth,thetotalwatersourcedforPerthwatersupplyhasremainedrelativelyconsistentovertheanalysisperiod.ThisisdirectlyattributabletodemandmanagementandwaterconservationmeasuresimplementedthroughoutPerthaswellasthebroaderarea.

Onaverage,about5GLofwaterhasbeensourcedfromrecyclingonanannualbasissince2006–07.Thiswaterissuppliedtoconsumersforarangeofactivitiesincludingtheirrigationofagriculture,vineyardsandmarketgardens.

Figure 10.36 Total urban water sourced for the Perth water supply area from 2006–07 to 2011–12


Categories of water delivered

Figure10.37showsthetotalvolumeofwaterdeliveredtoresidential,commercial,municipalandindustrialconsumersinthePerthwatersupplyareabetween2006–07and2011–12(NWC2013).Thetotalwatersuppliedincreasedby8GLoverthisperiodfrom240–48GLduetoanincreasingpopulation.

Onaverageabout70%ofthewatersuppliedeachyearwasusedforresidentialpurposes.

Commercial,municipalandindustrialwaterusecomprisedaround20%ofthewatersuppliedwiththeremainderaccountedforbyotherwateruses.

TheaveragewatersuppliedbytheWaterCorporationperpropertyforresidentialusebetween2006–07and2011–12wasestimatedtobe269kL.Themaximumresidentialwateruseperpropertywas281kLin2006–07andtheminimumwas250kLin2011–12.

Figure 10.37 Total urban water supplied to Perth from 2006–07 to 2011–12

South West Coast


10.7 Waterforagriculture

ThissectiondescribesthewatersituationforagricultureintheSouthWestCoastregionin2011–12.Soilmoistureconditionsarepresentedandimportantirrigationareasareidentified.TheHarveyIrrigationAreaisdescribedinmoredetailandinformationisprovidedregardingsurfacestorageandgroundwater.

10.7.1 Soilmoisture

Sincemodelestimatesofsoilmoisturestoragevolumesarebasedonasimpleconceptualrepresentationofsoilwaterstorageandtransferprocessesaveragedovera5kmx5kmgridcell,theyarenotsuitableforcomparisonwithlocallymeasuredsoilmoisturevolumes.Thisanalysisthereforepresentsarelativecomparisononly,identifyinghowmodelledsoilmoisturevolumesof2011–12relatetomodelledsoilmoisturevolumesofthe1911–2012period,expressedindecilerankings.

Soilmoisturedistributiondecilesfortheregionindicateverydryconditionsduringtheyearforagreatportionoftheregion,asshownbythebelowaveragetoverymuchbelowaveragesoilconditions(Figure10.38).Althoughtheamountofrainfallinthe

southwestwashigherthanaverageduringspringandearlysummer,evapotranspirationthatwasaboveorverymuchabovethehistoricaverageintheregionexceededtherainfallinputs(seerecentpatternsinlandscapewaterflowssection,Figure10.12).

Thedecilerankingofchangesinsoilmoistureduringthe2011–12showsthattheregionexperiencedbelowaverageconditionsthroughoutthewholeyear.Amarginalrisewasobservedduetohighrainfallperiodsinspringandearlysummer(Figure10.39).

Figure 10.38 Deciles ranking of annual average soil moisture for 2011–12 with respect to the 1911–2012 period for the South West Coast region

Figure 10.39 Decile ranking of the monthly soil moisture conditions during the 2011–12 period in the South West Coast region


10.7.2 Irrigationwater

Comparisonofannualirrigationwaterusebetween2005–06and2010–11acrosstheSouthWestCoastregionbynaturalresourcemanagementregions(NRM)isshowninFigure10.40.Figure10.41showsthemapofannualwateruseofthesameNRMregionsin2010–11.

Dataforthe2011–12yearperiodwasnotavailableatthetimeofpreparationofthisreport.

TheHarveyWaterIrrigationAreaisdescribedinsubsection10.7.4asanexampleofwaterusebyirrigatedagricultureintheSouthWestCoastregion.

10.7.3 Irrigationareas

MuchoftheSouthWestCoastregionisclearedforgrazinganddrylandcropping.Principallandusesincludeannualdrylandcropping(mainlywheat)andgrazing(mainlysheep).

Theregionhasatemperateclimatewithcool,wetwintersandhot,drysummers.Rainfalldeclinesrapidlyfurtherinland.Theregion’sstreamflowandgroundwaterrechargearegeneratedmostlyfromrainfallduringthecooler,wettermonthsofMay–October.

Irrigatedagricultureiscarriedoutinthemetropolitanareaandpartsofthesouthwestoftheregion.Highsalinitylevelslimitthesuitabilityofwaterresourcesforagriculturalproduction.

ThemainirrigationschemeprovidingwatertoirrigationareasinthemetropolitanandsouthwesternpartsoftheregionistheHarveyWaterIrrigationArea(Figure10.42)whichisoneofthemainirrigationschemesinWesternAustraliaaftertheOrdIrrigationScheme,bothconstitutingmorethan60%ofwaterallocationsintheState.IrrigationintheHarveyWaterIrrigationAreaismostlyforpasture,followedbyvegetablesandhorticulture.

ThePrestonValleyIrrigationCo-operativeisanotherschemeandisoperationalintheDonnybrookShirenearBunbury.

Figure 10.40 Total annual irrigation water use for 2005–06 to 2010–11 for natural resource management regions in the South West Coast region (ABS 2006–2010; 2011a)

South West Coast


Figure 10.41 Annual irrigation water use (GL) per natural resource management region for 2010–11 in the South West Coast region (ABS 2011a)


Figure 10.42 Irrigation areas in the South West Coast region

South West Coast


10.7.4 HarveyWaterIrrigationArea

TheHarveyWaterIrrigationAreaislocatedtothewestoftheDarlingScarpontheSwanCoastalPlain(Figure10.43).ItlieswithinthePeel–HarveycatchmentthatincludestheHarvey,CollieandPrestonriverbasinscoveringanareaof3,000km2.

TheHarveyWaterIrrigationAreacoversanareaof1,000km2inthreeirrigationzones:Harvey,WaroonaandCollie.Themajorityoflandisusedfordairyfarmingandbeefproduction.Currentlyaround100km2oflandisunderpermanentirrigationfordairyfarming,beefgrazingandhorticulture,withatotalirrigableareaofapproximately300km2(HarveyWater2011).ItisWesternAustralia’sprimeirrigateddairyarea,supplyingPerthandthesouthwestwithmorethan40%ofitsmilk.

HarveyWaterisresponsibleforthewaterdeliveryinfrastructureandislicensedtodrawwaterfromDrakesBrook(LakeMoyanup),Harvey,LogueBrook(LakeBrockman),SamsonBrook(LakeKabbamup),Stirling,Waroona(LakeNavarino)andWellingtonstorages.Waterissuppliedbygravityflowfromstoragestofarmsviaanetworkofopenchannelsandpipes.Thetransfersystemisprogressivelybeingmademoreefficientbytheuseofhighdensitypiping.

Figure 10.43 Harvey Water Irrigation Area


Surface water storage volumes

Totalvolumeforthesevenwaterstoragesintheirrigationareabetween1990and2012areshowninFigure10.44.Thetotalstoragevolumewasdrawndowntolowlevels,approximately20%ofcapacity,duringthe2002–03drought.Thedataindicatestoragetotalvolumeduring2011–12periodrecoveredfromsimilarlylowlevelsinspring2011tobeinexcessof200GLinsummer2011–12.Thisrecoverydidnotextendintothefollowingautumnduetoreducedinflowtothestorages.

TheirrigationwateruseinthethreedistrictsofCollie,HarveyandWaroonaintheHarveyirrigationareahasdroppedfromanaverageof70GLperyearsince1996tojust48GLperyearbetween2010and2012(Figure10.45).Thereductioninwateruseinrecentyearshasbeenrelatedtodeteriorationofwaterqualityandreductionindemand.

Figure 10.44 Variation in the amount of water held in seven storages supplying the Harvey Water Irrigation Area over recent years (light blue) and over 2011–12 (dark blue), as well as total accessible storage capacity (dashed line)

Figure 10.45 Water use in the three irrigation districts of the South West Coast region between 1996 and 2012

south west coast - bureau of meteorology · the south west coast region covers approximately...

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