south west coast - bureau of meteorology · the south west coast region covers approximately...
TRANSCRIPT
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
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10 South West Coast10.1 Introduction
ThischapterexamineswaterresourcesintheSouthWestCoastregionin2011–12andoverrecentdecades.Itstartswithsummaryinformationonthestatusofwaterflows,storesanduse.Thisisfollowedbydescriptiveinformationfortheregionincludingthephysiographiccharacteristics,soiltypes,population,landuseandclimate.
Spatialandtemporalpatternsinlandscapewaterflowsarepresentedaswellasanexaminationofthesurfaceandgroundwaterresources.Thechapterconcludeswithareviewofthewatersituationforurbancentresandirrigationareas.ThedatasourcesandmethodsusedindevelopingthediagramsandmapsarelistedintheTechnicalSupplement.
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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
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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.
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Figure 10.1 Major rivers and urban centres in the South West Coast region
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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.
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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
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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
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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).
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Figure 10.6 Soil type distribution in the South West Coast region
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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.
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Figure 10.8 Land use distribution in the South West Coast region
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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
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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
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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
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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.
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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.
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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.
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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).
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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
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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).
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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
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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
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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.
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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
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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.
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Figure 10.22 Rivers and catchments in the South West Coast region
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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.
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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
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Figure 10.24 Salinity as electrical conductivity (μS/cm) and its associated coefficient of variation for 2011–12 in the South West Coast region
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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%
South West Coast
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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
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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
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Figure 10.27 Location of important wetlands in the South West Coast region
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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
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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)
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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
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Figure 10.30 Groundwater management units in the South West Coast region; data extracted from the National Groundwater Information System (Bureau of Meteorology 2013)
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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
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Figure 10.31. Population range of urban centres in the South West Coast region
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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
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Figure 10.32 Water supply schematic for Perth and surrounds
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Figure 10.33 Urban supply storages in the South West Coast region
South West Coast
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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
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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
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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
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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
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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
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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
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Figure 10.41 Annual irrigation water use (GL) per natural resource management region for 2010–11 in the South West Coast region (ABS 2011a)
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Figure 10.42 Irrigation areas in the South West Coast region
South West Coast
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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
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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