rNT.J. REMOTEsENsrNc, 1992, vor. 13,No. 4, 581-585

Cover

Lake Eyre in flood

R. P. CECHET and A. J. PRATA CSIRO,Division of AtmosphericResearch, Aspendale, Victoria 3195,

The NOAA AVHRR falsecolour satelliteimage displayed in figure I and on the front cover showsan areaof the southerncentral part of the Australiancontinent after the occurrenceof unusuallylarge amountsof rainfall over this region during March and April 1989.The rainscaused severe flooding over much of the areawhich resultedin the filling of the largesalt lakesclearly visible in the image.The imagewas obtainedfrom the NOAA-ll satelliteon 16 May 1989at 14.30Local Standard Time(LST) and rectifiedto a transverseMercator projection. A sketchmap of thearea coveredby the imageand its positionon theAustralian continent is shownin figure2. The area,which can be classifiedas arid to semi-arid,is one of the driestparts of the Australiancontinent, with an annualaverage rainfall of 10G-150mm in the north, and 150-250mmin the south.To the immediatenorth of EyreNorth liesthe Simpsondesert, a region of Australia which has the highestvariability in annual rainfall.,Lake Frome,Lake Torrens and Lake Gairdnerare ephemeral salt lakesand usuallydry (i.e.they do not containwater on a regularor seasonalbasis and are only filled by episodicalinundations in their catchmentareas). Figure 3 showsa similar false colour enhancementof the samearea obtained from the NOAA-Il satellitebefore the inundation(12 January1989 at 13.43LST). Lake Eyre, Lake Torrensand Lake Gairdnerare all dry. Lake Fromecontains some water from severe thunderstormactivity during early December. Lake Eyre South and North have a combinedsurface area of some 10000km2 whenfilled and arejoined by the Goyderchannel, l3 km long and up to 2 km wide.In July 1964 Donald Campbell drove his gas turbine car Bluebird 2 to a record 848'6kmh-r acrossthe hard salt crust in the southernpart of Lake Eyre North, wherea crust of up to 0.5m coverslarge areas The falsecolour enhancementshave been produced by assigningAVHRR channel I to blue, channel2 to greenand channel4 to red. This procedurehighlighted the floodedareas and gavethe most visuallyacceptable result by displayingthe greening of the areassurrounding the .AVHRR channel2 allowsvegetation assessment asit is highly sensitiveto thepresence of chlorophyll.Patchy low stratocumuluscloud and shadowingcan be seentowards the westernside of figure l. The Flindersranges, rising to about 1000m,lie to the westof Lake Fromeand appearreddish-green. The small whitish featuresin the south-easterncorner of Lake Frome are islands,a few squarekilometres in area.Lake Frome is fed by small creeksdraining the ,and apart from direct inundation the floodwatersmust have beenderived mainly from this source.Floodwater also enters Lake Frome at its northernmosttip via a systemof smallersalt lakesleading up to Lake Eyre. To the north and westof Lake Frome the country is coveredwith sandysoils and sanddunes of high porosity.

0143-l16l/92 $3.00 O 1992Taylor & FrancisLtd ; 4t } l{ f ffi;?1;7.:'t i*J-,y],.,.: *r''- *. qt 5

Figurel. NOAA-l I AVHRR falsecolour cnhancement. Orbit number3296, t'6 May 1989' 14.30LST.

1547,12 January Figure3. NOAA-I I AVHRR falsecolour enhltncc-ntent.Orbit nunrber 1989.l-r.-t-1 LST.

Figurre-s 1 & 3 sitotrld be roLated I S0 degr.ees . tt*f <: Couer

v rrens \s.S \ t

L Cairdner

Figure2. Sketchmap of the areacorresponding to the satelliteimage showing the salt lakes, major creeksand other geographicalinformation referredto in the text.

Lake Eyre North is fed by creeksmainly around its northern perimeter.On the westernside there are severalcreeks (Neales, Alberga and Hamilton) which appearto be swollenwith floodwaterand draininginto the lake.Lake EyreSouth appears to be flooding due to water entering from the west via the Warriner and Margaret creeks. Lake Eyre drains the EromangaBasin, in total an areawhich coversone-seventh of the Australian continent; a region as large as France,Spain and Portugal combined. Lake Torrens drains a much smaller basin, an area the size of the Mediteranean islandsof Sardinia and CorSica.The variability in annual rainfall in the Eromanga Basinis the highestof all basinson the Australian continent.In addition, Lake Eyre is the lowest region of the Aushalian continent;the shorelineis 9 m below seaJeveland the bed of Lake Eyre North is l5'2m below sea-level. The large variability in seasonaland annual runoffare causedby tropical cyclonic and summermonsoonal rainfall and also extendedperiods of drought, somelasting for many yeani. The Australian monsoon is erratic both in space and tim6, as 584 Couer northern Australia barely lies in the peripheralregion of the planetarymonsoonal system" Floodsalternate with major long-lastingdroughts. During the earlydevelopment of the continent,the isolation,harshness and ruggednessof this region meant that before 1949only two explorershad seenwater in Lake Eyre; Ross in 1869and Halligan in 1922.Historical rainfall recordsin the Lake Eyrecatchment area suggest that floodingwas possibleon at leastanother l0 occasions,with smallerinflows possibleon anothei 5 occasions(Kotwicki 1986).The first documentedflooding of Lake Eyre occurredin March and April of 1949.The following 10years saw the lake nevercompletely dry in any calendar year. ln 1974 the largestknown flood filled Lake Eyre,with waterremaining in thelake until 1978.The 1984flood eventdiffered from pieviousdocumented fillings in that it resultedfrom heavyrainfall directly over the iake region and in the immediateenvirons. It was only the secondtime this century that LakeEyre South had filled(Williams and Kokkinn 1988),the previousoccasion beingin l974whenwater from Lake Eyre North flowedthrough the Goyderchannel. Otherminor floodshave occurred in 1963,1967, 1977 and 1987. On March 12 1989a well-developedand deepeningactive monsoon depression was locatedoff the north coastof the continent,near the town of Derby in Western Australia,and moving to the south-east.The depressionwas associated with a s,-rriace trough which extendedfrom the centreof the depressionthrough northern South Austialiaand into Victoria.During the next few days this tropical airmass, which was virtually saturatedat all levelsand relativelywarm, moved over centraland southern Australiacausing widespread rain and flooding.Flooding was most pronouncedin the catchmentareas of Lake Fromeand LakeTorrens. Mopena, a smalltown 20km westof Lake Torrens,recorded a rainfallof 273mmin 24 hourson 14 March, and 340mm in total for the event.This exceededthe previousextreme 24 hour rainfall for South Australiaof 222mm in 1946,which was alsoexceeded during this eventby Balcanoona,30 km westof Lake Frome (246mm),and Beltana,30 km north-eastof Lake Torrens(236mm). The flooding in the relativelysmall catchment area of Lake Torrensproduced the first completefilling of the lake since 1878,when rainfall of tropicalorigin alsooccurred in March. This wasthe only previousoccasion that Lake Torrenshad beenseen full sinceEuropean settlement' Aircraft and local observationsof streamflow and surfacewater over this region during the month of March indicatethat the rainfall in the Lake Eyrecatchment area appeaiedto havebeen considerably less than further south.The creekflows and large amountsof standingwater to the westof Lake Eyre and north-westof Lake Torrens suggestthat this areahad receivedvery heavyrainfalls. Further significantrainfalls *..* ,..o.ded over this region of in late March. Rainfall in the area at the northern end of the Flinders Rangestotalled 400mm for March, spreadover 4 events(a record March rainfall). During April and May an activesummer monsoon pattern caused a numberof occurrencesof heavyrain in the channelcountry of south-westernQueensland. This regionis part of the Lake Eyre catchmentand dependingon the severityof the flood event,water may finally reach Lake Eyresome fOO-OOOlm to the south-westvia Cooper'sCreek after a few months. It is possibleto estimatethe maximumwater depth in theSelakes using a satellite remoteiensing technique. Prata (1990) developed an algorithmbased on data from the NOAA nVHnn instrument,to determinewater depth for shallowwater bodies 'ground-truth' using data collectedfrom the 1984Lake Eyre flooding event.This atgorittrm usesthe ratio of the near infra-red channeland the visible channelto Couer 58s distinguishbetween flooded and dry land.The pixelsclassified as flooded are counted to determinethe total areaof standingwater, and the known relationsbetween the depth and area for Lake Eyre South and North are usedto calculatethe maximum depth of water. For the imagein figure I the levelof water in Lake Eyre South was 10.4+0'l m belowsea-level. This implies a maximumdepth of about2.8 +0'l m. For Lake Eyre North the levelof water was 12'3*0'lm below sea-level,indicating a maximumdepth of 2'9*0'l m. The heightof the sill at the northernend of Goyder channelis about l0'4 m belowsea-level, and thereforethis indicatesthat waterwould be flowing from Lake Eyre Southto Lake EyreNorth. Independentcorroboration of this has been obtainedfrom the South Australian Engineeringand Water Supply Department(Kotwicki, privatecommunication) who notedthe flow from Lake Eyre Southto Lake EyreNorth on the 27 Apnl1989 and obtaineda levelof waterin Lake Eyre North of l2'7 m belowsea-level. The levelof Lake EyreNorth determinedfrom this imageis consistent,as at the time the imagewas obtainedthe lakeswere still filling. During the 1984-85filling svent, Prata (1990)analysed a seriesof NOAA satelliteimagery and determinedfrom the depth-arearelationship that the evapo- ration rate for Lake Eyre was in excessof 2m per year.The water retentionperiod and maximum water depths for the previousdocumented fillings (major) are M months and 4.7m (1949-52),51months and 5.9m (197+78),and 18 monthsand 3.0m (198+1985).The currentwater depth is similarto themaximum depth for the 198+85 eventand thereforewater can be expectedto be visiblein Lake Eyre North until at leastlate 1990.

References KorwtcKt, V., 1986,Floods of Lake Eyre.Engineering and WaterSupply Department, GovernmentPrinter, South Austrata. PR.rrrA,A. J., 1990,Satellite derived evaporation from LakeEyre, South Australia. Inter- nationalJournal of RemoteSensing,llr 205l-2069. WrrLLe,MS,W. D., and KorrnN, M. J., 1988,The biogeographicalaffinities of the fauna in episodicallyfilled salt lakes: A studyof Lake EyreSouth, Australia. Hydrobiologia,158, 227-236.