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1/53Olympic Dam Expansion Supplementary Environmental Impact Statement 20101
Salt balance of Northern Spencer Gulf
APPENDIX H8
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2/53Olympic Dam Expansion Supplementary Environmental Impact Statement 20101
Long-term effects of desalination and climate change
APPENDIX H8.1
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
TheEffectsofDesalinationandClimateChange
ontheLongTermSaltBalanceofNorthernSpencerGulfRICHARD.A.NUNES-VAZ
EXECUTIVE SUMMARY
Thisreportexaminesthepotential impactsofdesalinationandclimatechangeon
thelargescale,longtermsaltbalanceofnorthernSpencerGulf(NSG),addressing
thefollowingthreeissues:
how rapidly Spencer Gulf responds to influences that disturb its
salinity,andhowlongittakesto recover followingadisturbance
the degree or extent to which salinity is expected to change in
responsetoadisturbance,and
whetheranticipatedperturbationsassociatedwithdesalinationand
climate change will be large enough to shift the Gulfs salinity,
irreversibly, toanewequilibriumstate.
SpencerGulfexperienceshighersalinitiesthantheadjacentoceanduetoanetloss
offreshwaterfromevaporation.Thisfluidlossisreplacedbyaninfluxofseawater
atameanvelocity~103ms
1 (Nunes&Lennon,1986)whichbringsadditionalsalt
thatmustbeejectedbyoceanographicmechanisms.
Theannualand interannualvariationsofsalinityofthewholebodyofNSGwere
determined by spatial interpolation of field data collected on twelve occasions
duringthe1980s (Nunes,1985)and fromtwonewsurveys in2008and2009.All
salinitydatawerecollectedandcrossreferencedby thesame investigator,using
thesamecalibration instrumentsandstandards,therebyproducingan internally
consistentdatasetspanning27years.
Assumingthat themeansalinityofNSG isgovernedbythecumulativeeffectsof
net evaporation, a correlation between NSG salinity in the 1980s, and net
evaporationmeasuredbytheBureauofMeteorologyatPrice(~250kmfromNSG)
wasdetermined. The correlationwas greatest (with a coefficient of 0.89)when
dailynetevaporationwasaveragedover the sixmonths (192days)prior toany
salinityobservation.ContinuingtheaveragingofPricenetevaporationthroughto
2009, it was shown that themore recent observations of NSG salinity remain
consistentwiththe1980srelationship.
Thus,despite large interannualanddecadaltrends inthe records, it isapparent
that NSG mean salinity on any particular day is primarily determined by the
cumulative effectsofnet evaporationduring the prior sixmonths, and that the
Gulfhasessentiallynomemoryof longer(e.g.,decadal)influences.
Regression betweenmeanNSG salinity, and 192day averaged net evaporation,
indicated thata sustainedchangeof1x103mday
1ofnetevaporation induceda
salinitychangeof0.23gl1.Withaveragednetevaporationasaproxy indicatorof
NSGsalinity,themeansalinityofNSGduringtheperiod19822009wasinferredto
be41.56gl1;themeanpeaktotroughannualvariationwas1.38gl
1;meansalinity
shiftedupwardsby0.77gl1duringthetenyearperiodto2007/8and,onaverage,
theannualsalinitymaximumwasreachedon12thApril,andtheminimumon14th
October.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Assumingthatdesalination(byremovingfreshwater)isequivalenttoevaporation,
theregressionresultwasusedtoestimatepotentialdesalination impactsonNSG
salinities.Theworstcaseassumptioninwhichallreturnwaterassociatedwithan
averagedesalinationrateof251megalitresday1,isabsorbedentirelywithinNSG,
impliesanincreaseofNSGsalinitybyupto0.11gl1,toanewmeanof41.67gl
1.
TheeffectsofclimatechangeonNSGsalinityweresimilarlyassessedbyestimating
changesofnetevaporationfromtheprojectionsofSuppiahetal(2006), implying
increases ofmean NSG salinity of up to 0.07gl1 in 2030, and 0.22gl
1 in 2070.
Combining theseestimatesof climate changeanddesalination impactswith the
highest inferredsalinityof thepast27years (obtained from thenetevaporation
proxy), implies an upward shift ofmean NSG salinity of 0.61gl1 in 2030, and
0.76gl1in2070comparedwiththe19822009mean.
Finally,theoceanographicmechanismsofsaltdischargewereexaminedinorderto
assesswhethertheymaysufferirreversibledisruptionbyanticipatedperturbations
associatedwithdesalinationandclimatechange.Regardlessofwhichmechanism
is
primarily
responsible
for
maintaining
the
long
term
salt
balance
of
NSG
(that
is,
gravitational circulation or turbulent dispersion) each is expected to adapt
smoothly,modestlyandreversiblytoanticipateddisturbances.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
1 Introduction
On 1May 2009 BHP Billiton released the Olympic Dam Expansion Draft Environmental
ImpactStatement (BHPBilliton,2009)detailingtheproposaltoexpand itsmining facilities
and related infrastructure. Included within the planned development was a coastal
desalinationplantatPointLowlyinUpperSpencerGulfwhichisrequiredtoyieldanaverage
of 251 megalitres day1 of freshwater for the Olympic Dam expansion. In community
discussionabouttheproposal,questionshavebeenraisedaboutthepotentialforsignificant
longterm change of salinity in Upper Spencer Gulf, particularly when combined with
anticipatedeffectsofclimatechange.
ThisreportaddressesquestionsrelatingspecificallytothelongtermsaltbalanceofSpencer
Gulf. It does not address localscale issues associatedwith the dilution and discharge of
returnwater inthevicinityofthedesalinationplant itself.Thepurposeofthisreport isto
examine the factors thatareprimarily responsible fordetermining the largescalebalance
and distribution of salt in the Gulf, and to assess the potential for both natural and
anthropogenicinfluencestoperturbtheirextantcharacter.
Basedprimarilyon longtermmeasurementof salinityand environmental conditions, this
reportquantifiestherelationshipbetweennaturalevaporativeforcingandsalinitychangein
northernSpencerGulf.Assuming(asdiscussedbelow)thatdesalinationandclimatechange
may be treated as perturbations of evaporation (freshwater removal) and precipitation,
quantitative assessment of their separate and combined effects on the northern Gulfs
salinityarederived.
1.1 Background and Context
Figure1.TheSouthAustralianGulfs
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
SpencerGulf(Figure1)isaninverseestuary(seeforexample,Bye,1981;Nunes&Lennon,
1986,hereafter calledNL86;andNunesVazetal,1990,hereaftercalledNV90), that is,a
semienclosedseainwhichseawaterismeasurablyconcentrated(ratherthandiluted,which
isthecaseinaclassicalestuary)bytheremovaloffreshwaterduetoevaporation.Annual
meansalinity rises fromvaluesofapproximately36gramsper litre1 (gl
-1)on theadjacent
continental
shelf,
to
more
than
45gl1
near
the
head
of
the
Gulf,
at
Port
Augusta.
To
avoid
confusionwithotherstudies,theareaofUpperSpencerGulfnorthof33Swillbereferred
toasnorthernSpencerGulf(NSG),asidentifiedinFigure2.
Figure2.Theareaofinterest,termednorthernSpencerGulf
Figure3showsasimpleschematicrepresentationofSpencerGulfand itsonedimensional
fluxesof salt.Annualmeannetevaporation2over theentire surface, removes freshwater
whichmustbereplacedbyanetinfluxofseawater.Thisatmosphericallyinducedlandward
salt flux is representedbySa inFigure3. It is theoriginoftheGulfselevatedsalinitiesor
inverse
estuarine
character
because
the
freshwater
loss
can
only
be
replaced
by
seawater.
Increasingly remotepartsof theGulf (equivalent to increasingdistance from theocean in
this onedimensional representation) are characterised by higher salinities, because the
additionofsalt iscumulativeonmovingnorthwiththe landwardflow.Theneteffect isto
createaonedimensionalsalinitygradientalongtheGulfwithhighestsalinitiesatthehead,
asindicatedinFigure3.
1 Despitetheoceanographicconventionwhichhasnowremovedallreferencetounits,salinitywillbe
expressedinunitsofgramsperlitrefromhereonwards,inkeepingwithparallelmodellingand
ecology
reports
associated
with
the
BHPB
desalination
proposal. 2 Thecycleofnetevaporationisnegativeatvarioustimesinthewinterwhenprecipitationexceeds
evaporationbut,averagedthroughtheannualcycle,netevaporationispositive.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure3.Schematicinverseestuaryshowingonedimensionalsaltfluxes
Iftherewerenootherprocessesoperating,thesalinityofSpencerGulfwouldcontinueto
rise year after year (almost) without bounds. However, observations (and common
experience)indicatethatsalinitiesdonotcontinuallyrise,andthisisbecausethereisanet3
seaward fluxof saltofsimilarmagnitude to the landward flux.The seawardsalt flux isof
oceanographic origin, and is indicated as So in Figure 3. Preciselywhich oceanographic
mechanismsgenerate the seaward salt flux isactuallyof little importance in thisanalysis,
butanumberof studies (forexample,Lennonetal,1987;NV90)have indicated that it is
associatedwiththecombinedeffectsofturbulentdispersion,frominternalshearandwind
andtideinducedstirringatthesurfaceandseabed,andgravitationalcirculationdrivenby
thehorizontalgradientsofdensity (largelydue to salinity).Both turbulentdispersionand
gravitational circulation always act tomove salt down the salinity gradient, that is, fromhighersalinitiestolowersalinities.Thus,theoceanographicsaltfluxwillalwaysalignitselfto
counter theeffectsof the landward fluxdue tonetevaporation. It isalsoassumed4,and
examinedindetailinsection6.6,thattheseawardoceanographicsaltfluxisapproximately
constantallyearroundtothesameextentthatthesalinitygradientcanalsobeconsidered
approximatelyconstant.
Inadditiontothemeansalinitygradient,observationsduringtheearly1980s(NL86)showed
that there isanannualsalinitycyclewithanamplitude that increases fromapproximately
0.3gl1on the shelf tomore than5gl
1near thehead.Theannual salinity cycle isdelayed
relative to the annual cycle of net evaporation. Salinity was seen to reach its annual
maximum
in
early
to
mid
March
in
northern
Spencer
Gulf
(and
not
until
early
June
at
the
mouth)comparedwith thenetevaporationcyclewhichgenerally reaches itsmaximum in
January.The lagof salinitiesbehind thenet evaporation cyclemaybe seenasa formof
halineinertia,akintothethermalinertiawhichcausesthelagofGulftemperaturesbehind
theinsolationcycle.Thetemperaturelagduetothermalinertiaalsoincreasesfromlessthan
3 annualmean 4 NunesVaz&Lennon(1987),forexample,establishedthatseawardsaltfluxinSpencerGulfcanbe
episodic,thatisunsteady,showingaseasonalbiaswithmaximumsaltdischargetendingtooccur
aroundautumn.Thetendencytostratifyandgeneratedensityoutflows,andtheirresistanceto
turbulentdestruction,however,wereshowntobeinverselyrelatedtowaterdepth.Thus,theupper
(shallower)
parts
of
Spencer
Gulf
are
expected
to
display
a
tendency
to
more
frequent
(daily
to
weekly,ratherthanseasonal)shortlived(oftheorderof1hour)dischargeevents,andthereforea
relativelysteadyseawardsaltfluxcomparedwithmoresoutherlyregionsoftheGulf.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
amonthinthenorthernreachesofNSGtonearlythreemonthsatthemouth(seeNL86for
details)5.
Inanysingleyear(seeFigure4)therewillbetimeswhenthelandwardsaltflux(represented
by theblue curve inFigure4) is larger than itsannualmean (during the summermonths
whennetevaporationisrelativelyhigh)sothatthemagnitudeofthelandwardfluxexceeds
thatoftheseawardflux(representedbythestraightblue line inFigure4).Atthesetimes,
theGulfaccumulatessalt,andsalinities(red)showarisingtrend.Atothertimesoftheyear
(particularlyinwinter),whenthelandwardfluxislessthanitsannualaverage,theseaward
flux exceeds the landward fluxand salinities fall. Thenet salt flux (magenta),due to the
combinationofthetwoinfluencesshowsanannualcyclewithameanclosetozero.
Figure4.RepresentationofonedimensionalsaltfluxesinSpencerGulf.Thefluxofsalt
inducedbyevaporation(bluecurve)hasapositive(landward)meanwhichisbalancedby
anequalnegative(assumedconstant)seawardflux(blueline)causedbyvarious
oceanographicexchangeprocessesdiscussedinthetext.Thenetfluxofsalt(purple),
whichispositiveinsummerandnegativeinwinter(withameanofzero),causesasalinity
cycle(red)thatlagsthesaltfluxcycle,causingmaximumsalinitytooccurlaterthanthe
peakofevaporation.
On a longterm basis, the landward atmosphericallyinduced salt flux is approximately
balancedbytheseawardoceanographicsaltfluxsothatthemeansalinityofSpencerGulf
varies only modestly according to the slight, interannual imbalance between the two
influences. Why is this so why are the two influences approximately balanced? The
strengthoftheseawardsaltflux,whethergeneratedbyturbulentdispersionorgravitational
circulation, is related to themouthtohead salinity gradient. If net evaporationwere to
increaseforasustainedperiod,thiswouldenhancethelandwardsaltflux,Sa,causingboth
5 Itisinterestingtonote(NV90;Petrusevics,1993;andWhitehead,1998)thatthenaturaldeclineof
thelongitudinal salinitygradientfromtheheadtothemouth,permitssummertimetemperature
gradientstoswitchtheGulffromitsinverseestuarinecharactertoclassicalestuarinecharacterinthe
vicinityofthemouth.Itseffectistoblockthedischargeofaccumulatedsaltforamonthortwoduring
summer,
but
this
arrangement
is
necessarily
broken
during
the
cooler
months
when
longitudinal
gradientsoftemperaturereinforcethoseofsalinityallowingtheGulftofreelyexchangesaltwiththe
adjacentshelf.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
thesalinityattheheadandthealongGulfsalinitygradient,toincreasewhich,inturn,would
increase the seaward (counter) flux.Thisnegative feedback systemproducesa longterm
stableequilibriuminwhichthemeanmagnitudeofthesalinitygradient,drivingtheseaward
flux,adaptstothemeanstrengthofthelandwardflux.
However,threequestionsremainwithregardtotheGulfsequilibriumdynamics,whichthis
reportaimstoanswer,namely:
1. how rapidlySpencerGulf responds to influences thatdisturb its salinity,andhow
longittakestorecoverfollowingadisturbance?
2. the degree or extent to which salinity is expected to change in response to a
disturbance,and
3. whetheranticipatedperturbationsassociatedwithdesalinationandclimatechange
willbe largeenough to shift theGulfs salinity, irreversibly, toanewequilibrium
state.
In answering these questions, the recent history (1982 to 2009) of precipitation and
evaporation is firstexamined tounderstandmeanatmospheric forcing,and theextentofanysignificanttrends. ThesalinityresponseofnorthernSpencerGulfisthenexaminedover
thesame27yearperiod,todeterminethenatureoftherelationshipbetweenforcingand
response.Oncecalibratedinthisway, expectedperturbationsofmeanNSGsalinitydueto
desalinationand climate changearederived, followedbya theoreticalassessmentof the
dynamicsof the seaward salt fluxandwhether itmay beoverstressedby (andpossibly
unstableto)anticipatedchanges.
2 Long-Term Data Trends and Relationships
2.1 Atmospheric Influences
The Bureau of Meteorology operates many data collection stations in South Australia.
However,theregularityand longevityoftheserecordsvarysubstantiallywhichmeansthat
very fewarepotentiallyuseful to this study.Figure5 shows the locationsof four stations
(Ceduna,Spalding,PriceandAdelaideAirport), closest toSpencerGulf, forwhich records
span the period 1982 to 2009, and for which both precipitation and evaporation were
measured.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure5.AdelaideAirport,Price,SpaldingandCedunameteorologicalstationsproviding
longtermrecordsofprecipitationandevaporation.
Evaporation measurements reported by the Australian Bureau of Meteorology are
unadjustedUSClassAPanEvaporimetermeasurements, representing themeasureddaily
lossofwaterfromametalpanofstandarddimensions.Discussionoftheinterpretationand
adjustment of pan evaporation data is held over until section 2.1.1 below, but for the
purposesofthisreport,panevaporationmultipliedbyafactor(calledthepanfactororpan
coefficient,seeHounam,1973)of0.7calledadjustedevaporation isusedasalongterm
proxyindicatorofenvironmentalevaporation.
Net evaporation is evaluated as adjusted evaporationminus precipitation (measured in
millimetres day1). Figure 6 shows the daily, net evaporation data for the four stations,
plottedonequivalentverticalscales.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure6.DailynetevaporationatSpalding,Price,AdelaideAirportandCedunastations
From approximately 2000 onwards, data from Spalding show anomalous behaviour,
suggestingthatSpaldingmaynotprovidelongtermdataofsuitablequality.
Figure7.DailynetevaporationatSpalding,Price,AdelaideAirportandCedunastations
Figure
7
shows
the
same
data
plotted
for
the
shorter
period
1982
to
1986,
exposing
the
fact
thatthePricerecordhasagreatmanymissingdatapoints,whichisthecasethroughoutthe
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
record. In fact themissingdata reflectadifferentdata collection regimewhereby several
days(usuallyincludingweekends)mayelapsebeforeanotherreadingistaken.Replacingall
missingdatawithzerosprovidesadataset that isuseful forassessing longtermaveraged
behaviour,smoothingoutthelargedaytodayfluctuations.
Arunningmeanforanydatasetmaybecalculatedasfollows.Therunningmeanhasafilter
orwindow lengthwhichdescribes thespanof thedata itassesses.Fora1year (365day)
runningmean,365dailyvaluesarereadin,themeaniscalculatedandthevalueisassigned
toaparticularday.Foracentral runningmean, thealgorithm takesadatapoint together
with182pointsfromeitherside,andassignsthemeanofall365valuestothecentralpoint.
Itthenmovesto thenextpointand repeats theprocess.A365daycentral runningmean
cannotbeevaluateduntilthe183rddatapointfromthestartofthedataset,andsimilarlyit
yieldsnoresultsfordatacloserthan183datapointsfromtheend.Thusthefilterproducesa
resultwhichishalfthefilterlengthshortateachend(foracentralisedfilter).
Figure8.OneyearrunningmeansofnetevaporationatSpalding,Price,AdelaideAirport
andCeduna.
Missing
data
points
in
the
Spalding
and
Price
records
shown
in
Fig
7,
were
filledwithzeros.
Figure8 shows the resultof thisprocessapplied to the fournetevaporationdatasets.All
filtered results are visually correlated (moreeasily seen in figure 9)which isencouraging
withregardtodataquality.Notethatallrecords indicate increasednetevaporation inthe
late2000s,butPrice showsaverymarked changenotduplicatedatSpalding (the closest
stationtoNSG).
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure
9.
One
year
running
means
of
net
evaporation
at
Spalding,
Price,
Adelaide
Airport
andCedunastations
Figure9 indicates that therewas significantagreementbetweennet evaporationdata at
AdelaideAirport,PriceandSpalding,while theCeduna record showed similar interannual
structurearoundasubstantiallyhighermean.Duetothehighfrequencyincompletenessof
the Spalding and Price records, their filtered results show significantlymore noise. The
following analysiswill focus on Spalding, Price and Adelaide Airport records, as Ceduna
appearstoshowdisparatebehaviourscomparedwiththethreestationsclosesttonorthern
SpencerGulf.
Figure10showsthelongtermtrendsofatmosphericforcing,thatis,precipitation,adjusted
evaporationandnetevaporationforAdelaideAirportfortheentire1982to2010period,as
1year,3yearand5yearcentralrunningmeans.Notethattheyaxisscalerangeisthesame
forprecipitation(shownasnegative)andevaporation,butthesedifferfromthescalingused
fornetevaporation(whichhasalargerrange).
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure10.1year,3yearand5yearrunningmeansofmeteorologicaldatafromAdelaide
Airport
From the early 1980snet evaporation atAdelaideAirport,which shows significant inter
annualvariability,experiencedadownwardtrenduntilthemid1990sand,sincethattime
has risento levelsabovethoseof theearly1980s.Thedownwardtrend isconsistentwith
theaveragedbehaviourofallAustralianClassApans from theearly1980s (Giffordetal,
2005;Rodericketal,2009).Giffordetal(2005)areambiguousindescribingtrendsafterthe
mid1990s,althoughtheydostatethat from1994thedata forAustraliahaveshowna
slowdown and reversal of the downward trend especially in the eastern part [of
Australia].This isconsistentwiththedataforAdelaideAirport(andPriceandSpalding),
wherenetevaporationhasrisenconsistentlyformorethanadecade,andmaystillberising
(accordingtothe5yearrunningmean).
The trends of Adelaide Airport net evaporation are evident in both precipitation and
evaporation data (precipitation is plotted on a negative scale tomake the additionwith
evaporation
visually
simpler).
The
long
term
trend
of
increasing
evaporation
was
accompanied by decreasing precipitation (and vice versa). This correlation, which is
commonly seen in such data (Gifford et al, 2005), extends well into shorter period
fluctuations.
These conclusions are also consistent with the behaviour of meteorological data from
SpaldingandPricestations.
2.1.1 Extrapolating Pan Evaporation to Evaporation from Northern SpencerGulf
Muchhasbeenwrittenabout thevalidityorotherwiseofusingdata fromUSClassAPan
Evaporimeters to infer environmental evaporation rates (see forexample, Linacre, 2005).
Evaporationpanssufferfrommanysourcesoferror(Linacre,2005) includingalgalgrowth,
shadeorwindshelterduetotreesandbuildings,thechangingalbedoofsediment,animalor
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
birddrinking(althoughmeshguardsaregenerallyusednowadays),heatappliedtothesides
ofthepanfromdirectanddiffuseradiation,andbothshortwavereflectionandlongwave
radiationfromthechangingsurrounds.
However, despite these difficulties, pan evaporation provides data which is generally
consistentwiththePenmanequation,whichpredictspotentialevaporationbasedondaily
mean temperature, wind speed, relative humidity and solar radiation, and with
measurementsoflakeevaporation,determinedbywaterbalancemethodswherethelake
is small and shallow enough to have no horizontal or vertical currents nor to alter
appreciably the atmosphere above it (Linacre, 2005). Such correlations improve when
considering longerterm averaged data, such that Johnson & Sharma (2007) obtained a
correlationof0.97betweenmonthlymeansofpanevaporationandevaporation inferred
fromthePenmanequation.
ComparisonbetweenevaporationmeasuredusingClassApans,andestimates frommass
balancecalculationsinlakes,andfromthePenmanequationindicatethatClassApansover
estimateevaporation,ingeneralbecausepansaresubjecttoadditionalheateffectsthrough
their
sides.
For
this
reason,
a
correction
factor,
called
the
pan
factor
or
pan
coefficient,
is
commonlyappliedtopanevaporationmeasurements inordertoestimatetheevaporation
which is expected to occur from small water bodies such as lakes. Estimates of pan
coefficientsvaryfrom0.7to0.8accordingtocontextanddataset,however,Linacre(1994),
in Australian conditions, found that lake evaporation is approximately 0.7 times the
evaporationfromaClassApan.
Evaporation from the oceans is notwell represented by evaporation from Class A pans
(Farquhar&Roderick,2005),firstlybecausetheirthermalinertiasareverydifferent,thatis,
theoceansabilitytoabsorbheatwithoutappreciablychangingitstemperaturemeansthat
the pan is not a goodmodel of oceanic heat budgets, and secondly because amarine
boundarylayerformsoverlargebodiesofwater,whichgreatlymodifiesproperties,suchas
humidity, thatplay a large role indriving evaporation.Thuspan evaporation from South
Australianmeteorological stations is not expected to provide a valid proxy indicator of
evaporationfrom,forexample,SouthAustralianshelfwaters.
However,theupperpartofSpencerGulf,northof33S isarelativelynarrowandshallow
water body. A significant fraction of its evaporation is anticipated to occur around its
margins where thermal inertia is relatively small and diurnal temperature changes are
significant.Furthermore,amarineboundarylayermayremainundevelopedforallbutwinds
withpredominantly southerlyornortherly components,although theseare thedominant
windsinsummer.
Thus,althoughevaporationinferredfromClassApansremainsanimperfectproxymeasure
of
evaporation
from
northern
Spencer
Gulf,
it
is
expected
that
it
should
provide
useful
insightsintopropertiesthataresignificantlyinfluencedbyevaporation.Onthebasisofsuch
caveats, the analysisproceedsusing evaporationmeasuredbyClassApanevaporimeter,
adjustedusingapan factorof0.7,asan indicatorofevaporation fromnorthern Spencer
Gulf: a value at the low end of the normal range of pan coefficients because this will
exaggerate or lead to upperbound estimates of the potential environmental (and
desalination)effectsonNSG.
2.2 The Salinity Response of Northern Spencer Gulf
NunesVaz (1985)andNL86reportedthe resultsof fourteen largescalethreedimensional
surveys of temperature and salinity in SpencerGulf frommid1982 to early 1985. These
surveys (using an internallyrecording Applied Microsystems electronic conductivity
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
temperaturedepth,orCTD,probe)profiledconditionsatmanystationsbetweenthehead
andpointsasfarsouthas3420S.
In NSG, where salinities often rise above the measurable limit (42gl-1) of typical
oceanographicinstrumentation,aperspexsleevewasinsertedintotheprobesinductivecell
to reduce the cells sample volume and thus reduce its sensitivity, to bring the highest
salinitieswithinmeasurable range. Thispracticedestroyed themanufacturers calibration
betweenactualandmeasuredsalinityand,forthisreason,bothsurfaceandbottomsamples
werecollected (usingNansenorNiskinbottles)ateverystation, formeasurementback in
thelaboratoryusinganaccuratebenchsalinometer.
When measuring salinity using the (Yeokal) bench salinometer (actually measuring
temperature,andconductivityratiorelativetoapreciselyknownseawaterstandard),itwas
necessary to dilute the samples using a precisely known amount of deionisedwater, in
ordertolowerthesalinitybelow42gl-1,againintomeasurablerange.Oncedetermined,the
measurementwas adjustedupwards according to theamountofdilution.These samples
consequently provided a separatemulti (typically 50plus) point calibration of each CTD
survey.
Twelve
such
surveys
provided
substantial
data
in
NSG
during
the
1980s.
TwofurthersurveysofNSGwereconductedon6August2008and2March2009(thedata
areprovidedinAppendixA).However,aprofilingCTDprobewasunavailable,andsosalinity
at each station was sampled using only nearsurface and nearbottom water samples
collected using a Nansen bottle. Nevertheless, the samples were analysed at Flinders
Universityusingthesamebenchsalinometerastheanalysesconducted inthe1980s.New
salinitystandardswereshipped6fromtheUKandacrosscheckwasperformedbetweenthe
newstandards,andthreeoftheglasssealedsalinitystandardsshippedfromtheUK inthe
1970s:thesamebatchofstandardsusedtoreferencetheanalysesofNSGsalinityatFlinders
Universityduringthe1980s.Thisintercalibrationshowedthatthe1970sstandardshadnot
measurablydrifted,andpermittedthenewsurveystobepreciselyreferencedtothe1980s
data(towithin
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Figure11.NorthernSpencerGulfwithNovember1982surveystations(redcircles)and a
2kmby2kminterpolationgrid(bluedots).
Figure 11 shows the location of stations in NSG from a typical survey, in this case 24th
November1982.Foranyparticularsurveysuchasthis,thedetailedverticalprofileofsalinity
ateachstationyieldedadepthaveragedsalinity,andthusallstationsinthesurveyallowed
the region tobe contoured forhorizontaldepthaveraged salinity.Using these contoured
distributions, salinitieswere then interpolated or extrapolated onto the regular (2km by
2km)grid,showninFigure11asbluedots.Ateachgridpoint,themassofsalt(thatis,the
interpolatedsalinitymultipliedbythedepth,thenmultipliedby4km2)wasdetermined,and
summedforall(130)gridpoints.Thissaltmasswasthendividedbythemeandepthofall
gridpoints(andalso4km2)togivethemeansalinityofNSG.Thismethodwasthenrepeated
forallothersurveysofNSG.MeansalinitiescalculatedinthiswayarecollectedinTable1.
# SURVEYDATE
MEAN
SALINITY
1 28 Jul 1982 42.04gl-1
0.05gl-1
2 24 Nov 1982 41.67gl-1
0.05gl-1
3 15 Jan 1983 41.92gl-1
0.05gl-1
4 9 Feb 1983 42.21gl-1
0.05gl-1
5 20 Apr 1983 42.57gl-1
0.05gl-1
6 22 Jun 1983 41.68gl-1
0.05gl-1
7 11 Aug 1983 40.90gl-1
0.05gl-1
8 1 Nov 1983 40.99gl-1 0.05gl -1
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9 6 Mar 1984 42.33gl-1
0.05gl-1
10 26 Jun 1984 42.04gl-1
0.05gl-1
11 10 Oct 1984 41.04gl-1
0.05gl-1
12 23 Jan 1985 41.57gl-1
0.05gl-1
23 year gap
13 6 Aug 2008 41.97gl-1 0.08gl -1
14 2 Mar 2009 42.70gl-1
0.08gl-1
Table1.MeansalinitiesofNSGdeterminedaccordingtothemethoddescribedinthetext.
Minimumandmaximumvaluesfromthe1980ssurveysarehighlightedinboldfacetype.
Estimation of the error associated with the mean salinity from 1980s surveys was
determined by adjusting the arrangements of salinity contours to the highest salinity
(contoursmovedasfarsouthaspossible)and lowestsalinity(contoursmovedasfarnorth
aspossible)distributionsusingnonlinearinterpolation,whileremainingconsistentwiththe
measureddata.Themeansalinitywasthencalculatedfromeachextremeintheusualway,
givinghighestandlowestestimatesforeachsurvey.
TheerrorassociatedwithestimatesofmeanNSGsalinityfromthe2008and2009surveys
was larger than that of the 1980s because the data lacked information about the depth
variationof salinityateach station. For this reason,oneextremearrangementof salinity
contourswasderivedbyassuming that thehigherof the surfaceandbottom salinitiesat
eachstationappliedtothewholedepth,andcontourswerepushedasfarsouthaspossible
consistentwiththestationdata, leadingtothehighestestimateofNSGmeansalinity.The
otherextremeinvolvedassumingthatthelowerofsurfaceandbottomsalinitiesappliedto
the
whole
depth
at
each
station,
and
contours
were
pushed
as
far
north
as
possible
consistentwithdata,givingthelowestmeansalinityestimate.
Figure12showsthe30monthvariationofNSGmeansalinitywithacubicsplinefittothe
data.Figure13enablesmoredirectcomparison,givingasenseoftheinterannualvariation
of the seasonal salinity response of NSG during 1982/83 (blue), 1983/84 (green) and
1984/85 (magenta).The latesummermaxima inApril1983andMarch1984weresimilar,
even though theywere preceded by very differentwinter/springminima. Theminimum
observedin1982wassubstantiallyhigherthanthatof1983,withtheminimumof1984lying
betweenthetwo.
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Figure12.ThevariationofmeanNSGsalinitythroughoutthe30monthperiodofdetailed
measurement
Figure13.
The
mean
salinity
of
NSG
as
separate
annual
cycles
overlain
for
ease
of
comparison
Themean salinityofNSGbasedon2March2009data (42.70gl1)was thehighestof the
fourteensurvey, 27year dataset, although it was only 7.8% outside the salinity range
(40.90gl1 to 42.57gl
1)seenintheearly1980s.Themeansalinityof6August2008laywithin
thevariationsseenatsimilarseasonaltimingsinthe1980s.Thus,therecentdatacouldnot
beinterpretedasinanywayanomalouswhenreferencedagainst1980sdata,althoughthis
interpretation is subject topotential revisionawaitingassessmentofatmospheric forcing,
anditsinfluenceoverequivalentperiods.Thisisthesubjectofthenextsection.
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3 The Relationship Between Atmospheric Forcing and the Salin ityResponse of Northern Spencer Gulf
Theargumentsof section1 implyahypothesiswhichmaynowbe tested.Thehypothesis
canbestatedasfollows:
the mean salinity of northern Spencer Gulf reflects, in a consistentmeasurableway, the integrated influencesofnetevaporationduringa
precedingperiodwhoselengthmaybedetermined.
In other words, anomalously high and sustained net evaporation is expected to raise
salinities inNSG relative tomeanbehaviour.Similarly, sustained lowernetevaporation is
expectedtoleadtolowerNSGmeansalinities,notingthatthemeaningofsustainedmust
alsobeexamined.
The hypothesis implies that the salinity on any particular day (say day-n) reflects the
cumulative effects of net evaporation for x days preceding day-n. Unlike the central
running mean filter, a lagged filter is now required: one that averages xdays of net
evaporationdata,andassignstheresulttothefinaldayofthedatawindow.Thefilterthen
movesforwardonedayandrepeatstheprocess,continuingtotheendofthedataset.
Figure 14 shows raw (daily)net evaporationdata fromAdelaideAirportduring the early
1980s (light blue, highly variable dataset), together with data obtained using a 90day
centralrunningmeanfilter(darkblue),a90daylaggedmeanfilter(magenta)anda180day
laggedmean filter (red).Notice that the90daycentral runningmean,and90day lagged
filterseriesareidentical(astheyshouldbe)exceptfora90dayrelativeshiftalongthetime
axis.Notealsothatthelongerthefilter,thesmootherthedata,andthesmallertherange.
Forlaggedfilters,allofthedatalossassociatedwiththefilterlengthoccursatthebeginning
ofthedataset,whichiswhyeachfilteredresultstartsonadifferentday.
Figure14.NSGproxynetevaporation,withvariousfilteredderivatives
In order to compare lagged mean net evaporation (LMNE) with mean NSG salinity
(MNSGS),bothdatasetsmustbenormalised(rescaledtoarangefromzerotoone)sothat
they can be plotted on the same axes. The assessment of the relationship between net
evaporationandsalinityresponseinitiallyfocusesonthe1980sdataalone,sothatdatafrom
themorerecentsurveysmaybereservedasindependenttestsofanyinferredrelationship.Mean NSG salinities were normalised using the minimum (survey #7 of Table 1) and
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maximum(survey#5ofTable1)measuredvalues.Similarly,after filteringnetevaporation
data, the resultwasnormalizedusing theminimumandmaximumLMNEobservedwithin
thesameperiod.
Figure15 showsnormalised laggedmeannetevaporationusing sixdifferent filter lengths
(90daysupto208days)plottedwithnormalisedmeanNSGsalinity.A filter lengthof208
dayswasthemaximumthatcouldbeappliedwithoutlosingtheabilitytocomparewiththe
firstsalinitydatapointwhichwasobtained209daysinto1982.
Figure15.LMNE(blackline)atAdelaideAirportversusMNSGS(reddatapoints:seetext)
forvariousdifferentfilterlengths
Thevariance(sumofthesquareddifferencesbetweenthediscretesalinitydatapointsand
thespecificvaluesofLMNEonequivalentdays)isalsoshownineachoftheplotsofFigure
15.Byvarying the lagperiodonedayata time, thevariancewas seen to followabroad
minimumforlaggedfilterlengthsbetween190daysand200days,withtheoptimumlagat
190days.
Figure16showsthefilteredresultsusingoptimumlagsfromeachofAdelaideAirport(lagof
190days),Price(192days)andSpalding(192days),andtheirvariances,fittedtothe1980s
NSG salinity results. The resultwith the smallest variance (0.16)was obtained using net
evaporationfromPrice,althoughallthreeresultsaresimilar.
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Figure16.LMNE(solidlines)fromAdelaideAirport,PriceandSpaldingstations,forlags
thatachievethesmallestvariancewhenfittedtoMNSGSdata(discretepoints).The
optimumfilterlag,indays,isindicatedoneachpanel,togetherwiththecorresponding
variance.
The message from these results is significant: that, at least during the 30months of
observations in the early 1980s, themean salinity of NSG on any particular day closely
reflectedtheaccumulatedeffectsofnetevaporationduringthepreceding190or192days
(approximately six months). This goes some way to answering the question about the
responsetimescaleofNSG: itsuggeststhattheGulf largelyrespondedtovariationsof less
thanannualperiod,althoughthislimited(30month)datasetisnotdefinitivewithrespectto
longerterm(decadal)influences.
Figure17providesadifferentviewofthesamedatashowninFigure16.Notethattheinter
annual differences between themaxima of 1983 and 1984 have the same sense as the
changesinobservedsalinityinthesameyears.Similarly,theminimaofLMNEin1982,1983
and1984mirror thechanges in thedepthof the salinityminima (other thanSpalding for
1983and1984).
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Figure17.LMNEfromAdelaideAirport(black),Price(red)andSpalding(blue),forlagsthat
achievethesmallestvariancewhenfittedtoMNSGSdata(discretepoints).
3.1 The Long-Term Behaviour of Northern Spencer Gulf
ResultsdiscussedabovesuggestthatNSGsalinityonanyparticulardaywasdetermined,toa
significantdegree, by the cumulative effectsof net evaporation during thepreceding six
months.Thequestionremains,however,astowhetherthereisalongertermcomponentof
theresponsethataccumulatestheeffectsofmuch lowerfrequencies intheforcing,which
mightdrivesignificantshiftsofmeanNSGsalinity.
Given
that
net
evaporation
sustained
a
consistent
increasing
trend
for
more
than
a
decade
(Figure10), ifNSGsalinity issensitivetothe longertermrisingtrendthenextensionofthe
simple1980s sixmonthmemory relationship, should revealwhetherashiftof themean
salinitydatumhasoccurred.
Figure 18 shows this result using net evaporation from Price. It was constructed by
continuing the 192day lagged mean filter for net evaporation, and maintaining the
normalisationusedforthe1980sdata,that is,bothMNSGSandLMNEwerenormalisedto
theearly1983summermaximumandlate1983winterminimum.
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Figure18.LMNEforPrice(solid),using192dlagandthesame1980snormalisation
establishedabove,extendedthroughto2009,comparedwiththefullMNSGSdataset
(discretepointswitherrorbars).
Figure 19 shows the results of Figure 18 replotted to include only those years when
observationsof salinityweremade.Again, theFigurewasnot rescaledor renormalised,
and thus theupwardshiftofobservedNSG salinitybetween1980sand late2000sdata is
mirroredintheupwardshiftoflaggednetevaporationmeasuredatPrice.
Figure19.DatafromFigure18replottedwithyears1986to2007removed,toshow
detail.
Haddecadalshiftsofnetevaporationcausedirreversible(ratcheted)responsesofsalinity,
thenthenetevaporationshifts thathaveoccurredbetweenthe1980sandthe late2000s
wouldhavedestroyedtheearlierrelationship,andthefittorecentdatawouldbepoor.Itis
apparent in these results that themean salinityofnorthernSpencerGulfhas tracked the
1980scalibrationquitecloselyacrossaperiodofnearlythreedecades,despitethepresence
ofsubstantialintra andinterdecadaltrendsevidentinthenetevaporationdata(Figure18).
FromthisitisdeducedthatmeanNSGsalinityatanyparticulartimeisprimarilydetermined
bythecumulativeeffectsofnetevaporationduringtheprecedingsixmonths,andthatthe
Gulfisnotsignificantlyinfluencedbyenvironmentalforcingonlongertimescales.Thisresult
maybeusedtodetermineacalibrationofchangesinNSGsalinityrelativetochangesofnet
evaporation,inthenextsection.
4 Calibration of Salin ity Change in Northern Spencer Gulf to Net
Evaporation
4.1 Behaviour from 1982 to the present
Theresultsabove implythatachangeofnetevaporation (measuredataSouthAustralian
land station) is correlatedwith a change ofmean salinity in northern SpencerGulf. The
relationship can be deduced by carefully comparing the variations of both properties
without normalising either. However, some consideration of the effects of filtering a
periodictimeseriesisrequired.
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Figure20.Theeffectsoffilteringaperiodictimeseries,asdiscussedinthetext.
Figure20showsaperiodictimeseries(inblack)withanamplitudeofoneunitandanotional
periodof365timestheintervalbetweenindividualvalues,representinganannuallyvarying
quantitysampledonadailybasis.Theseries (inblue)withanamplitudeofapproximately
0.6
units
is
the
result
of
applying
a
(central)
filter
of
190
samples
(or
days)
in
length,
showing
thattheamplitudeofthefilteredresultissubstantiallysmallerthanitsunfilteredequivalent.
Thereductiondependsonthefilterlengthanditsrelationshipwiththeperiodoftheseries.
Foranannualperiodvariable,filteredusinga190dfilter,theamplitudeisreducedto0.61of
the unfiltered amplitude. A filter length of 192d, produces a series with an amplitude
reducedbythefactor0.60.
Figure21shows theresultofthisprocessapplied tonetevaporationmeasurements from
AdelaideAirport7. Individual points representdailymeasured values (with ameanof 2.4
millimetres perday). The 190d filtered result is shown as a solid red line. Increasing the
amplitudeofthefilteredresultbydividingbythefactor0.61,alsoensuringthatthemeans
coincide,produces the result shown as theblue solid line in Figure 20. The filtered and
adjustedresultrepresentsthesmootheddailyvariationofnetevaporation.
Figure21.NetEvaporationatAdelaideAirport,with190dfilterapplied(redsolidline)and
thefilteredresultadjustedtocompensateforamplitudereductionduetofiltering(blue
solidline).
Figure22showsthemeansalinityofNSGasmeasuredonfourteenoccasionsbetweenJuly
1982andMarch2009,withfilteredandadjustedvaluesofnetevaporationfromPriceon
7 Adelaide Airport was chosen for illustration because Price and Spalding data suffer from many
missing daily values.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
thesamedays.Theleastsquaresregressionresult(withacorrelationcoefficientof0.89) is
alsoplottedontheFigure.Theresultindicatesthatasustainedchangeofonemillimetreper
day of net evaporation corresponded to a change ofmeanNSG salinity of 0.230gl1. For
completeness, the regressionresult fittedto1980sdataalonegaveagradientof0.228gl1
andacorrelationcoefficientof0.87.
Figure22.MeanSalinityofNSGagainstcorrespondingvaluesoffilteredandadjustednet
evaporationfromPrice(usinga192dfilter).Theleastsquaresregressionresultisplotted
togetherwith95%confidenceintervals.
Based on this relationship, by using net evaporation as a proxy (hindcast) indicator, the
historical behaviour of salinity inNSG during the period 1982 to 2009may be inferred.
Figure23 shows theNSGmean salinity time series inferred from its relationshipwithnet
evaporation,togetherwith3yearand5yearcentralrunningmeans.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure23.InferredbehaviourofNSGsalinityduringthepast27years,with3yearand5
yearcentralrunningmeans.DiscreteobservationsofmeanNSGsalinityarealsoshown
togetherwitherrorbars.
Results inferred from thisproxy suggest that,during theperiod 1982 to 2009, themean
salinityofNSGwas41.56gl1; theannualmeanpeaktotroughvariationwas1.38gl
1,with
themean annual peak being 42.25gl1; the lowest annual average salinity occurred from
1July1997 to 30June1998 with a value of 41.25gl1; the highest annualmean salinity
occurredintheyear1July2007to30June2008withameanof42.02gl1;thustheannual
mean salinity ofNSG rose by 0.77gl1 during the ten year period to 2007/8. The annual
salinitymaximumwasreached,onaverage,on12thApril,andtheannualminimumon14
th
October.
5 Natural & Anthropogenic Influences on Northern Spencer GulfSalinity
5.1 Potential Desalination Impacts
Theprocessofdesalination involves the intakeofacontinuous flowofambient seawater
fromamarinezone,theremovalofsaltfromafraction(typically40%to45%)ofthe flow
whichistappedoffasfreshwater,anddischargeoftheremainingflow,whichcarriesaway
the sameamountof saltas theoriginal inflow,atanelevated (typicallyninefifthsof the
original)salinity.Thus,theneteffectofdesalination istheremovaloffreshwater,which is
alsotheneteffectofevaporation,althoughtheformeractsatapointwhilethe latteracts
(approximately)uniformlyoveralargearea.
This distinction in theway desalination and evaporation act is ofparamount importance
whenconsideringpotentialeffectsoflocalisedbrinedischarge.However,theaimhereisto
assessitseffectsonalargevolumeofSpencerGulf,thatisnorthernSpencerGulf,overthe
long
term.
Figure24.Schematicgulfsubjecttoevaporationanddesalination.Notethatthereisno
intentiontoprovidemeaningfulindicationofaregionofinfluenceassociatedwith
withdrawalfrom,anddischargeoffluidsintotheGulf.Itisalsoimportanttonotethatthe
schematiclocationofthedesalinationplantputsitentirelywithintheconfinesofNSG,
whichdiffers
from
its
proposed
location
at
the
intersection
of
NSG
and
the
mid
Gulf.
For
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thisreason,thediscussionsurroundingthisarrangement,inthetext,maybeconsideredto
overestimatepotentialimpactsonNSG.
Figure24showsaschematicgulf(thesameasinFigure3)withtheadditionofadesalination
plant on its shore. The plant draws in seawater from an undefined zone and discharges
brines, as shown here, entirely to northern Spencer Gulf. Thus, freshwater removal by
desalination increases the total lossof freshwater fromNSG (representedby the vertical
arrowsintheFigure),whichenhancesthecompensatinginflow, Sa,andinturn,raisesthemeansalinityofNSG(asdescribedearlier).
RegardlessofthedegreeofmixingofdesalinationreturnwaterintoNSG,themeansalinity
will reflect the gross removal of freshwater (whether localised or not) which drives a
compensating landward salt flux, so that desalination acts as a small but equivalent
mechanism to evaporation. This report explicitly excludes consideration of any localised
effects focusing, instead,on the longterm (measured inmonths)grossbehaviourofNSG,
whichmust respond todesalination in the sameway that it responds toevaporation,by
elevatingmeansalinity.
ItisalsoassumedinthefollowinganalysisthatthedesalinationplantissitedalongsideNSG,
anditseffectsareentirelyappliedtoNSG.Thisassumptionexaggeratespotentialimpactson
NSGbecausethe intake isproposedtobesitedveryclose (oftheorderof100m)toPoint
Lowly(seeFigure2)withthedischargeonthePointitselfwhichmeans,giventhegenerally
northsouth orientation of tidal flows and themean effects of the deeper gravitational
circulation (discussed in section 6.2), that some of the effectswill actually be applied to
watersoutsideNSG.
5.1.1 Quantitative Estimate of Desalination Impact
ReferringagaintoFigure24,weusethefollowingnumerics:
Proposeddesalination(orfreshwaterremoval)rate: 251megalitresday1
=2.51x108litresday
1
=2.51x105m
3day
1
NorthernSpencerGulf:
Volume 4.4x109m
3
SurfaceArea 0.52x109m
2
MeanDepth 8.4m
WholeofSpencerGulf
SurfaceArea 2.2x1010m
2
Assumingthatdesalination isgrosslyequivalenttotheremovalofa freshwater layer from
theentiresurfaceareaofNSG,thelayerthicknessisdeterminedas:
(totalvolumeremovedbydesalinationperunittime) / (surfaceareaofNSG)
= (2.51 x 105m3day
1) / (0.52 x 10
9m2)
= 0.00048 m day1
= 0.48 x 10-3
m day-1
= 1.92 x 10-6x DR m day-1
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
whereDRisthedesalinationrateinmegalitresperday.Thus,theevaporationequivalentof
desalination is a loss of approximately one half of amillimetre per day from the entire
surface of NSG, representing about 15% of the (adjusted) mean daily evaporative loss
measuredatSpaldingandPricebetween1982and2009.For comparison, the best case
alternativescenarioinwhichthefreshwaterlossisconsideredtobedrawnfromtheentire
surface
area
of
Spencer
Gulf
implies
that
desalination
effectively
removes
a
surface
film
of
1.1x 10-5
m day-1,orapproximately0.3%ofnaturalevaporation.
Asdeducedinsection4.1(andFigure22),afreshwaterlossof1x103mday
1(inadjusted
LMNE at Price) corresponded to a change ofMNSGS of 0.23gl1 implying that the gross
effects of desalination, if assumed to impact entirely onNSG, are expected to cause an
increaseofmeanNSGsalinityof(0.48x0.23gl1 or)~0.11gl
1 (or4.4x104xDRgl1).This
representsariseequivalenttoapproximately8%ofthemeanannualsalinityrange(1.38gl1)
observed in NSG from 1982 to 2009, implying a rise of annualmean NSG salinity from
41.56gl1 to 41.67gl
1.Italsorepresentsachangeequivalentto14%oftheobservedinter
decadalshift (0.77gl1)ofmeanannualNSGsalinitybetween1997and2007, inferredfrom
thenetevaporationproxy.
Forcomparison,usingtheNSGcalibrationofsalinitywithnetevaporation,whichisexpected
tooverestimate the salinity responseof thewholeof SpencerGulf, the gross impact of
desalinationonthewholeGulfisinferredtobeanincreaseofmeansalinityof~0.003gl1(or
1x105xDRgl1).
5.2 Potential Climate Change Impacts
5.2.1 Past Trends in Climate Variables
Suppiahetal(2006,hereaftercalledS06)reportobservedtrendsandmodelledpredictions
of climate change effects on South Australian atmospheric andmarine conditions. This
report
does
not
question
the
veracity
or
validity
of
these
predictions
in
the
context
of
debateaboutclimate change,but simplyaccepts the figures inorder todeducepotential
impacts.
Since the 1950s, South Australias average air temperature has increased by 0.20C per
decade(theaveragemaximumhasincreasedby0.21Candtheaverageminimumby0.18C
per decade). Sea surface temperatures (SST) have also risen, but by smaller amounts.
Specificallybetween1900and2005,theSSTofwatersnearthemouthofSpencerGulfhas
increasedby0.05Cperdecade,oratahigherrateof0.11Cperdecadesince1950(S06).
Over the whole State, rainfall trends were generally weak, showing a small tendency
towardswetter conditions in thenorthof theState,anddrier conditions in the southern
coastalareas,changingbyapproximately5x103mdecade
1from1900to2005.Theperiod
1950 to2005 showeda slightly strongerdrying trend for coastal regionsofup to102m
decade1,althoughthedetailintheseobservationssuggeststhattheregionofNSGmayhave
experienced no significant longterm trend (see Figure 6 of S06). Longterm changes of
rainfall were very small, although there were very large interannual and interdecadal
variationsbetween1956and2009froma lowof234.6millimetresannualrainfall in2006,
up to730.8millimetres in1992,aroundameanof441.7millimetres (atAdelaideAirport,
BureauofMeteorologyonlineclimatestatistics8).
Evaporationmeasurementswereassessed fromUSClassApanevaporationdata,which is
acknowledgedtobepronetosignificanterrors(S06)atthreesites,Woomera,Cedunaand
8
http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=139&p_display_type=dataFile&p_startYear=&p_stn_num=023034
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http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=139&p_display_type=dataFile&p_startYear=&p_stn_num=023034http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=139&p_display_type=dataFile&p_startYear=&p_stn_num=023034http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=139&p_display_type=dataFile&p_startYear=&p_stn_num=023034http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=139&p_display_type=dataFile&p_startYear=&p_stn_num=023034 -
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Mount Gambier. Only Woomera showed a statistically significant trend, rising by
11.5x103myear
1,eachyeararoundameanof3.21myear
1.
5.2.2 Projected 2030 and 2070 Air Temperatures
S06wenton toassess the resultsof25climatemodels frommanydifferent international
sources.Thesemodelsshowsubstantialinconsistenciesintheirprojectionsofchangewhich,
whencombinedwiththeIntergovernmentalPanelonClimateChangealternativescenarios,
lead topredictionsofchange that fall intobroad ranges.With regard toair temperature,
SouthAustraliaisassessedinthreeregions:lessthan200kmfromthecoast(whichwerefer
toasthe coastalband);200to600kmfromthecoast;andgreaterthan600kmfromthe
coast. We will assume that Spencer Gulf belongs to the coastal band, as continental
conditionsmayoverwhelmthesmallmaritimecontributionofNSG inthesecondband.By
2030, air temperaturewithin the coastalband isprojected to riseby0.2C to 1.6C,and
2070projectionsindicateariseof0.5Cto4.7C.
S06alsoprovidedprojectionsfortheNationalResourceManagement(NRM)regions.The
two NRM regions thatmost closely straddle NSG are those of the Northern and Yorke
Region and Eyre Peninsula. Summaries are provided for these regions and are largely
consistentbetweenthetwo.By2030airtemperature isexpectedtoriseby0.4Cto1.2C,
and by 2070 this is projected to be 1.0C to 3.8C, so the NRM forecasts are more
conservativethanthoseofthecoastalband.
5.2.3 Projected 2030 and 2070 Precipi tation
Developingforecastsforrainfallissaid(S06)tobemorecomplexandlessconsistentthanfor
temperatures.Using the30yearannualaverage rainfall centredon1990as thebaseline,
projectionsforthecoastalbandshowachangeof 15%to0%by2030,and 45%to0%by
2070.ProjectionsfortheNRMregionsindicatechangesof 10%to0%by2030,and 30%to
1%
by
2070
which
are,
again,
more
conservative
than
those
of
the
coastal
band.
5.2.4 Projected 2030 and 2070 Evaporation
S06 did not report evaporation trends, primarily because direct measurement of
evaporation is difficult, and evaporation is usually assessed as the residual term in heat
budgetcalculations.
5.2.5 Estimating 2030 and 2070 Climate Change Impacts on the Salinity ofNorthern Spencer Gulf
InordertodeduceclimatechangeeffectsonNSGsalinity, it isnecessarytotranslatetheir
effectsintoanoverallassessmentofthechangetonetevaporation.Estimatingevaporation
astheresidualterminaheatbudgetcalculationwillnotsufficeherebecausesuchestimates
wouldinherittheuncertaintiespresentinallotherterms.
Twoalternativemethods toassessevaporationareofferedbelow.Firstly,asnoted in the
discussion of Figure 10 and section 2.1, evaporation and precipitation appear to be
negatively correlated and thus, projections of changing rainfallmay be used to deduce
evaporationprojections.Alternatively,projectedchangesofairandseatemperature (S06)
allowevaporationtobeestimatedusingbulkaerodynamicformulae.Bothmethodswillbe
usedtoestimateevaporationin2030and2070which,combinedwiththeS06projectionsof
precipitation, yield estimates of net evaporation fromwhich changes toNSG salinity are
deduced.
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5.2.5.1 EstimatingImpactsfromProjectionsofPrecipitation
The3year runningmeansofevaporationandprecipitationatAdelaideAirport (shown in
Figure10) implya relationshipbetweenevaporationandprecipitation trendswhichmight
bededucedthroughsimpleregression. Suchaprocedure,usingannualmeans,showsthat
thetwoparametersarecorrelatedaccordingto:
(Evaporation)365d mean = -0.75 x (Precipitation) 365d mean + 4.6
wherebothevaporationandprecipitationaremeasured inmillimetresperday.Thisresult
supports translation of the climate projections of precipitation into estimates of the
correspondingevaporationtrends.
2030: Suppiah et al (2006) projected precipitation changeswithin the coastal band for
2030 and 2070 from the 30year baseline centred on 1990, indicating that a
reductionofbetween0%and15%isanticipatedin2030(amoreextremeworstcase
thanwasobtainedbyconsideringthetwoNRMregions).
SinceAdelaideAirport showsausefulprecipitationevaporationcorrelation,and it
belongswithinthecoastalband,theresultisusedasfollows.Forthe30yearperiodcentredon1990,theannualaverageprecipitationatAdelaideAirportwasmeasured
tobe448x103myear
1(BureauofMeteorology
9).
The2030annualaverage rainfall is consequentlyprojected (S06) to liewithin the
range381 x103myear
1 to448x10
3myear
1.From the regression relationship
above, the 2030 annual evaporation is therefore inferred to lie between
1.38myear1and1.33myear
1.Thus,theworstcasenetevaporation(evaporation
minus precipitation) is inferred to be 1.00m year1, representing an increase of
0.12myear1 or0.32x10
3mday
1over1990centredaverageconditions
2070: S06projecteda reductionofprecipitation in thecoastalbandofbetween0%and
45%
by
2070,
thus
precipitation
at
Adelaide
Airport
is
expected
to
lie
within
the
range246x103myear
1and448x10
3myear
1in2070(notingthatprojectionsfor
theNRMregionswerelessextremethanthis).
Evaporation is consequently projected to lie between 1.48 m year1 and
1.33 m year1, leading to a worst case increase of net evaporation in 2070 of
0.35myear1,or0.97x10
3mday
1over1990centredvalues.
5.2.5.2 EstimatingNetEvaporationChangesfromProjectionsofAirTemperatureand
Precipitation
Estimates of evaporation may be obtained using bulk aerodynamic formula (see, for
example,Brutsaert,1982;andHolloway,1980),as follows.Thebulkaerodynamic formula
forthelatentheatflux(He)associatedwithevaporationisgivenas:
He= aL C Eu 10(q 0 q 10)
whereaisthedensityofair,Listhelatentheatofvapourisationofwater,CE isthebulktransfer coefficient for moisture (~0.0019), u10 is the wind speed at the standard
meteorologicalheightoftenmetres,andq isthespecifichumidityatthesurface(suffixof
zero)andattenmetres(suffixof10).
Thespecifichumidityisgivenby:
q=(0.622e)/(P0.378 e ) 0.622e/P
9 http://www.bom.gov.au/climate/averages/tables/cw_023034.shtml
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where P is atmospheric pressure, and e is the vapour pressure, which is temperature
dependentaccordingtothefollowingpolynomial:
es(T) = a0+ a 1.T + a2.T2+ a 3.T
3+ a 4.T
4+ a 5.T
5
inwhicha0=6.11,a1=4.44x101 ,a2=1.42x10
2,a3=2.71x104,a4=2.74x10
6 and
a5
=
2.73
x
10
8
.
Evaporation isdrivenby thegradientof specifichumiditybetween thewater surfaceand
10mheight,whichisgivenby:
q0 q 10 [e (T0) e (T10)] / (1.61 P) = [es(T 0) R.es (T10)] / (1.61 P)
where thevapourpressureat10mheight isequal tothesaturatedvapourpressure times
therelativehumidity(R)at10m, leadingtoestimationofevaporationchangesaccordingto
projectedchangesinairandseasurfacetemperatures.
2030: Projectionsofmeanatmospherictemperatureriseinthecoastalbandforthe2030
timeframe (S06)werebetween0.2Cand1.6C.Basedonhistoricdataforthe last
halfcenturyS06alsonoted thatseasurface temperature (SST)near themouthof
SpencerGulfhadrisenatapproximatelyhalftherateofairtemperature.Itmightbe
argued thatSSTwithinSpencerGulf,particularlyNSG, shouldmore closelymatch
the fullmagnitude of the rise in air temperature.However, using different rates
produces a larger atmospheric vapour pressure gradient and hence greater
evaporation,representingaworstcasescenario.Onthisbasis,itisinferredthatSST
inNSGin2030willhaverisenby0.1Cto0.8C.
MeansummertimedailymaximumairtemperaturesatPortAugustaPowerStation
(the top of NSG) between 1962 and 1997 were approximately 31C (Bureau of
Meteorology),whileSSTsinNSGweremeasured(NL86)atapproximately24C.Daily
peak summertime latent heat fluxes using these temperatures (and assuming a
relative
humidity
of
75%,
the
approximate
average
summertime
value
coincident
withpeakdailyairtemperatures,recordedatYarravilleShoal,3317S13735.6E,
approximately35kmsouthsouthwestofPointLowly,NunesVazdata)givesapeak
evaporationrateof2.7x108ms
1.Assumingthat,in2030,meandailymaximumair
temperatureswill have risen to 32.6C,withNSG SSTs at 24.8C leads to a peak
evaporationrateof4.0x108ms
1,representinga45%increaseinpeakevaporation.
Making the two furtherassumptions thatdailyevaporation is sinusoidalbetween
zeroand thepeakdaily value,and that theannualevaporation isalso sinusoidal,
withanannualmeanofapproximatelyhalfthesummertimepeak(bothofwhichare
approximately true from observations), implies that the 2030 estimate of
evaporation isapproximately11% largerthan1990values, reaching1.47myear1.
Combiningthiswiththeprojectedchangeofprecipitationimpliesthatmeanannual
net evaporation in 2030 will reach 1.09 m year1, an increase of
0.21myear1.
2070: A similar procedure, assuming that air temperature rises to 35.7C and the sea
temperature rises to 26.3C implies that annual evaporation increases to
1.83myear1and meanannualnetevaporationin2070willreach1.59myear1,an
increaseof0.71myear1.
5.2.5.3 ImpliedEffectsonNorthernSpencerGulfSalinity
Withtheseestimatedchanges,itisnowpossibletoprovideinferredimpactsonNSGsalinity,
based
on
the
result
of
section
4.1.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
2030: Projections of precipitation changes for 2030 (S06) indicate an increase in net
evaporation of 0.32 x 103m day
1,which implies an increase in NSG salinity of
0.07gl1.
Air and sea temperature projections, combined with projections of precipitation
changes,indicateanincreaseinnetevaporationof0.21x103mday
1,whichimplies
anincreaseinNSGsalinityof0.05gl1.
2070: Projections of precipitation changes for 2070 (S06) indicate an increase in net
evaporation of 0.97 x 103m day
1,which implies an increase in NSG salinity of
0.22gl1.
Air and sea temperature projections, combined with projections of precipitation
changes for 2070, indicate an increase innet evaporationof 0.71 x 103mday
1,
whichimpliesanincreaseinNSGsalinityof0.16gl1.
These results are collected, togetherwith the estimated impacts of desalination and the
observednaturalvariationofsalinitybetween1982and2009,inTable2.
Influence Basedon Changeof
Net
Evaporation
(x103mday
1)
Changeof
NSGmean
salinity
g l-1
Inferred
Mean
Salinity
g l-1
Impacts,
Relativeto
Natural
Variation
Annual
Variation
Mean1982to2009 6.0 1.38 41.56 100%
Desalination 180x106litresday
1
-
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Figure25.MeanannualsalinityvariationinNSGinferredfromthePriceLMNEproxy(blue)
togetherwithinferredsalinitycyclesthatincludetheeffectsof:desalination(green);
climatechangein2030(orange);andclimatechangein2070(red).TheinferredNSG
salinitiesduring199798and200708(inblack)areincludedtoindicatetheextentofinter
annualvariability,notingthatthesetracesincludetheeffectsoflargeprecipitation
variabilityandirregulardigitisationassociatedwiththecollectionofmeteorologicaldata.
Figure26.Fourpanelsshowingcomparativeestimatesofbothclimatechangeand
desalinationimpactsonNSGsalinity.Theupperleftpanelcompares2070climatechange
alone,andcombinedwithdesalination,onthemeanannualsalinitycycleinferred
between1982and2009.Theupperrightpanelshowstheseperturbationsasadditionsto
themostextremeannualsalinitycycleinferredbetween1982and2009.Thelowerleftand
lowerrightpanelsareequivalenttotheupperpanels,wherepredictionsoftheimpactof
2030climatechangehavebeensubstitutedforthoseof2070.
ThestrongbluelinesinFigure26areallequivalentandshowtheannualmeansalinitycycle
inferredfromnetevaporationbetween1982and2009(mean41.56gl-1,amplitude0.69gl
-1).
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
Irregular (black) traces (and their smoothed interpretation) show the inferred cycle for
200708(mean42.02gl-1)whichwasthehighestmeansalinitycycleinferredfromtheproxy
between1982and2009.Theimpactsofclimatechange(red),andthenincombinationwith
desalination(theadditionalgreyband)arerepresentedasperturbationstothemeansalinity
(leftpanels)and to thehighestextreme salinity (rightpanels) forboth2070 (upper)and
2030
(lower)
climate
change
predictions.
With respect toworst case scenarios, combining the impactsofdesalination and climate
changewiththemeanofthehighestannualcycleofsalinityinferredfromthepast27years
(in200708),themeansalinityofNSG isexpected to riseto42.20gl-1 in2030 (lowerright
panelofFig25),and42.35gl-1in2070(upperrightpanel).Theseresultsrepresentincreases
of 46% and 57%, respectively,of themean annual salinity variation (of 1.38gl-1) inferred
between 1982 and 2009. The peak salinities for the extreme salinity cycles shown in
Figure26wouldbe42.92gl1and43.07gl
1,for2030and2070respectively,comparedwith
theannualmeanpeakof42.25gl1(asshownbythebluelineinFigure26).
Atthispoint,thefirsttwodrivingquestionshavebeenresolved,thatis:
1. howSpencerGulf responds to influences thatdisturb its salinity,andhow long it
takestorecoverfollowingadisturbance,and
2. the degree or extent to which salinity is expected to change in response to a
disturbance.
Thenextsectionaddressesthelastofthethreequestions,namely:
3. whetheranticipatedperturbationsassociatedwithdesalinationandclimatechange
willbe large enough to shift theGulfs salinity, irreversibly, to anewequilibrium
state.
6 Equilibrium Dynamics of Oceanographic Salt Exchange in North
Spencer Gulf
Toanswer the thirdquestionabove requiresexaminationof thenatureofoceanographic
saltexchangeprocessesoperatinginSpencerGulf.Asbrieflydiscussedinsection1.1,there
are twoprocesses thatcontribute to the transportof saltdown theGulfsnaturalsalinity
gradient, namely gravitational circulation, and turbulent diffusion and dispersion. Both
contributetothemaintenanceofalongtermequilibriumandeachdominatestheother in
different(wind,tideandcurrent)conditions,locationsandtimes(forexample,seasons).
However, the assessment may be made relatively straightforward by examining each
mechanisminturn,toaskwhethertheremightbeanaturallimitationontheexchangethat
eachcanachieveindividually.
6.1 Horizontal Gravitational Circulation
NV90usedasimpletwodimensional(flat)representationofUpperSpencerGulf(seeFigure
27) characterised by two contributions to flow, namely: a net inflow, at speed uE, re
supplyingthefluidlostbynetevaporation(whichwasalsoshownschematicallyinFigure3);
andahorizontalcirculationat speeduC,carrying lowersalinity fluid (si)northwardon the
westernside,andhighersalinityfluid(so)southwardontheeasternside,toaccountforthe
observed twodimensional orientation of isohalines. In this arrangement, NV90 showed
(theirequation3)thatthecirculationspeedisgivenby:
)(.
).(2
io
i
c ss
s
bh
APEu
= (1)
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
where(E P)isthenetevaporationfromsurfaceareaA,bisthehorizontalwidthandhis
the depth of the southern section through which the flows occur. Assuming that all
quantitiesare fixed, including the salinityof the inflow (si), the salinityof theoutflowwill
increase as the speed of the circulation decreases. Intuitively, thismeans that amore
sluggish circulation is more affected by evaporation causing the outflow salinity to be
correspondingly
higher.
Figure27.SchematicTwoDimensionalSaltExchangesinSpencerGulf
(reproducedfromNunesVazetal,1990)
Figure28.Therelationshipbetweencirculationspeedandinflowoutflowsalinitycontrast
definedbyequation(1)
Takingthesouthernboundarytobeat33S(thesouthernlimitofNSGasdefinedhere),and
using thedataofNV90 (i.e., (E P) 4.4 x 10-8
ms-1, A ~ 6 x 10
8 m
2, b ~ 1.6 x 10
4 m,
si~ 41gl-1
) with h ~ 8.4m Figure 28 showstherelationshipbetweencirculationspeedand
the inflowoutflow salinity contrast. Assuming, as in NV90, that the outflow salinity
so~ 42gl-1impliesthatuC isapproximately0.02ms
-1
.Thus,arathermodestnetcirculationisrequiredtoaccountfortheflushingofexcesssaltfromNSGproducedbynetevaporation.
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
As these flowsarehorizontallydistinct, there isonlyoneconceptual limitationon thenet
fluxofsalttheyareabletoachieve,which issetbythehydraulicsofthechannel inwhich
the flows takeplace.Thehydraulic limit is representedbyaFroudenumber,Fr, (seeany
hydraulicstext):
gh
u
Fr= (2)
whereg istheaccelerationofgravity,u isavelocityandh isadepth,whichexpressesthe
notion that the gravitational potential energy of the system (represented by the
denominator)cansustainaflowatthecontrolsection(whichwetaketobe33S)toproduce
aFroudenumberno greater thanone.This limit isachievedwhereu =gh implyinganupperlimitonthecirculationspeedofapproximately8ms-1. Clearlythisdoesnotconstitute
asignificantconstraintonhorizontallydistinctexchangeflows.
However, there isnodynamicmechanismable to sustain the complete lateral separation
between inflowandoutflow in thepresenceof gravity. Ignoring the Earths rotation, the
flows
would
instead
be
vertically
differentiated
(with
a
low
density
inflow
above
a
high
densityoutflow),whichisconsiderednext.
6.2 Vertical Gravitational Circulation
Spatially differentiated exchange flows influenced by gravity in a nonrotating frame of
reference are verticallyjuxtaposed, separated by a horizontal interface with inflow and
outflowlayerseachoccupyingapproximatelyhalfthedepth.Thetwo layers(withdensities
iando,wherethesubscriptsrepresentthelessdenseinflow,i,andmoredenseoutflow,
o)remainstableinthepresenceofshearacrosstheinterfaceuntilthegradientRichardson
number,Ri, decreases to approximately 0.25 (Turner, 1973). The gradient Richardson
numberisgivenby:
2)(
'
io
iuu
hgR
= (3)
inwhich h is the vertical scale of the interface, and g is the reduced gravity (given by
oiog ).( whichassumesthattheBoussinesqapproximation isvalid,that is,densitydifferences are small). Large Richardson numbers represent stable conditions. As the
Richardsonnumberapproaches0.25,theenergyimpartedbytheshearacrosstheinterface
becomessufficienttoovercomethestabilityofthestratification,andtheinterfacetypically
forms large rolling (KelvinHelmholtz)billows that rapidlyachieve verticalmixingof scalar
properties(suchassalinityordensity)andmomentum.
Thus, an organised vertically differentiated inflow/outflow is able to support the net
exchangeof saltuntileither thedensitycontrast falls,or the shear increases such thatRi
approaches0.25.Whensuchan interfacebecomesunstable,boththedensitycontrastand
the shear are suddenly reduced in a catastrophic breakdown of the exchange flow
arrangement.This is the limit thatmustbeunderstood because it representsapotential
limittothemaintenanceofanequilibriumbetweentheprocessesthatgenerateanexcess
ofsaltinNSG,andthosethatremoveit.
An alternative stability criterion is represented by the internal or densimetric Froude
number,Fri,givenby
hg
u
Fri '=
(4)
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
noting that reduced gravity replaces normal gravity in equation (1).While the use of a
Richardsonnumberstabilitycriterionwouldbepreferred,itisnotpossibletoestimatepre
and postmixing interface thicknesses. The internal Froude number uses a characteristic
verticalscaleof,h,thelayerdepthratherthantheinterfacethickness,andisthereforemore
easilyapplied.AstratifiedshearflowlosesstabilityastheinternalFroudenumberincreases
and
approaches
unity.
Reinterpreting the horizontal net circulation discussed in section 6.1 as a vertically
differentiated exchange flow with each layer occupying half the depth, equation (1)
describestherelationshipbetweentheverticalshear(twicethecirculationvelocity)andthe
verticalsalinitycontrast.Thus,forgivenenvironmentalconditions,equation(4)canbeused
toquantifytheinternalFroudenumber.
Figure29.Theoreticalspeedofthesouthwardcirculationcomponent,asafunctionofits
salinity,requiredtomaintainthelongtermsaltbalanceofNSG(assumingthatthesalinity
ofthenorthwardcomponentis41.0gl1).TheinternalFroudenumberofthestratified
exchangeflowisalsoshown.
Assuming that the balance expressed by equation (1) applies to a vertically separated
exchange circulation, and that the inflow salinity (si) remains at 41.0gl1 (anchored by
conditions furthersouth) therelationshipbetween theoutflowsalinityandthecirculation
speed(Figure28)impliesthatasthesalinitycontrastbetweeninflowandoutflowdecreases,
boththecirculationspeedand,throughequation(4),theinternalFroudenumber,increase.
Figure
29
indicates
that
internally
critical
flow
(Fri~ 1 )
is
reached
when
the
outflow
salinity
falls to 41.35gl-1 which implies that, if the exchange circulationwere entirely vertically
separatedthenthedischargeofexcesssaltfromNSGcouldnotbemaintainediftheaverage
salinitycontrastbetweeninflowandoutflowfellbelow0.35gl-1.Limiteddataobtainedfrom
NSGdonothaveadequatespatialandtemporalresolutionto indicatewhether in factthe
sectionmaintains stratification to this degree, but this appears to be quite a stringent
conditionwithregardtothedischargeofexcesssalt.
Continuingthislineofthought,assumingthatthiscrosssectionremainsacontrolsectionfor
the internal hydraulics as additional stresses associated with desalination and climate
changeareapplied,manipulationofequations (1)and (4) implies that theverticalsalinity
stratificationmustincreasewith .Thus,ifnetevaporationincreasesby10%,the
salinitystratificationat33Smustriseby6.5%implyingthatthedeepoutflowsalinitymust
3/2)( PE
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
rise from 41.35gl-1 to 41.37gl
-1, a verymodest adjustment, tomaintain the equilibrium
exchange.
6.3 Gravitational Circulation Including Earths Rotation
In fact neither the horizontallydifferentiated nor verticallydifferentiated flows are
physicallyrealisticsolutionsbecausetheEarthsrotational(Coriolis)effectsareimportantat
thisscale.Theeffectofrotationistotilttheinterfaceawayfromthehorizontal,moresoas
the rotational effects increase, so that the horizontal differentiation between the flows
increases.Ou(1984)showedthatthedistancebetweentheinterfaceintersectionswiththe
surfaceand theseabed isapproximately twicethe internalRossbyradiusofdeformation,
Roi,givenby:
f
hgRoi
'= (5)
wherefistheCoriolisparameter2..sin,inwhichistherotationrateoftheEarth(7.29x
10
5
rad
s
1
)
and
is
the
latitude.
Thus
as
f
increases
the
Rossby
radius
decreases
and
the
interfacebecomesmorevertical.
Thestratificationrangeshown inFigure29 impliesan internalRossbyradiusofbetweena
fewhundredmetresandapproximatelythreekilometres,comparedtothewidthofNSGat
33Swhich ismorethan7kilometres.Thus,the interfacebetweeninflowandoutflow(see
Figure 30) is inferred to bemore vertical than horizontal implying that the horizontally
separatedexchangecirculation(discussedinsection6.1)isabettermodelthanthevertically
separatedcase(discussedinsection6.2).Consequently,theexchangecirculationisunlikely
tobelimitedtoanysignificantextentbyaninterfacialstabilitycriterion.
Figure 30. (a) Smoothed version of the steadystate density field after geostrophic
adjustmentfrom
an
initial
vertically
uniform
non
linear
horizontal
gradient.
The
horizontal
scale, y, is measured in units of the internal Rossby radius of deformation. (b) The
correspondinggeostrophicvelocityfieldperpendiculartotheplaneofthediagraminthe
caseofSpencerGulf,anegativevelocity shouldbe interpretedasa southwardoutflow.
ReproducedfromOu(1984).
6.4 The Role of Temperature in Driving Gravitational Circulation
Given that the effects of climate change are expected to include elevation of Gulf
temperatures, it is important toassess the roleof temperature in theGulfsgravitational
exchangeprocesses,andwhetherhighertemperaturesmayhaveanadverseeffectonthe
exchange mechanisms. The gravitational exchange is driven by alongGulf density
differences,andbothtemperatureandsalinityaffectdensity,althoughbydifferingamounts.
Temperature affects density through the thermal expansion coefficient, which is
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Long-Term Salt Balance of Northern Spencer Gulf R. A. Nunes-Vaz
approximately1.7x104perC,that is,thedensitydecreasesbyapproximately0.017%for
each degree of temperature rise. Similarly, the haline contraction coefficient is
approximately7.8x104pergl
1ofsalinitychange,ordensity increasesby0.078%foreach
gl1of salinity increase.Thus theeffectof salinity (ingl
1)ondensity isapproximately4.6
timesthatoftemperature(inC)10.
NL86reportedtemperatureaswellassalinityfromeachofthetwelve1980sGulfsurveys.
Theyshowed(NL86,Figure4)thatthetemperaturedifferencefromtheheadoftheGulfto
33S(i.e.,alongNSG)reachesamaximuminJanuaryofapproximately2C.Equivalently,the
maximumsalinitydifferencereachesapproximately7gl-1,inautumn.Thesetemperatureand
salinitygradients(bothwithhighervaluesatthehead)actondensityintheoppositesense,
such that the temperaturecontribution reduces thecontributionof salinity to thedensity
gradient, by approximately 6%. At other times of the year,when the temperatures are
relatively low at the head, the temperature gradient reinforces the salinity gradient and
strengthensthedensitygradient.
Thus,climatechangemustproducearather largedifferencebetweenthetemperaturesat
Port
Augusta
and
those
at
Point
Lowly
if
the
temperature
gradient
is
to
play
a
significant
role
inreducingthedensityinducedforcingofgravitationalexchange.
6.5 Turbulent Diffusion
The alternative to an organised gravitational exchange circulation involves turbulent
diffusionanddispersion(Fischeretal,1979),represented(seeNL86)by:
2
2
.)(
x
sKs
h
PE
t
sx
+
=
(5)
wherevariablesretaintheirformermeanings,xisanalongGulfcoordinatewithitsoriginat
the head, and Kx is the alongGulf eddy diffusion coefficient associated with boundary
induced and internal sources of turbulence. Using the observed annual mean salinity
gradient, NL86 (see their Figure 7) deduced that the diffusion coefficient required to
maintain a southward diffusive salt flux equal to the northward flux associated with
evaporation,ischaracterisedbyvaluesof~10 m2s
-1nearthehead,increasingto~100 m
2s
-1
atthesouthernlimitofNSG(at33S).
AssumingthatNSGmaintainsasteady(repeatableannualmean)salinitygradientalong its
axisremovesthetimedependency,andleadsto:
2
21.)(
x
s
sKhPE x
= (6)
whichshowsthatachangeofnetevaporationrequiresanadjustmentofthecurvatureofthealonggulfsalinitygradient,assumingthatlevelsofturbulence(generatedbywinds,for
example)donotalsochange.
Using a simple inverse logarithmic representation of the alonggulf salinity variation
indicatesthata10% increase innetevaporationrequiresan increaseofNSGmeansalinity
from41.56gl1to41.74gl
1tomaintaintheequilibriumsouthwarddiffusivefluxofsalt.Thus,
again,ifthisweretheonlymechanismresponsibleformaintainingthesaltbalanceofNSG,
thenitisinferredthatrelativelymodestadjustmentofthealonggulfsalinitygradientwould
besufficienttorespondtoprojectedenvironmentalchan