Lake Eyre Basics
Australia has a dubious distinction of being presently the most lakeless and waterless continent. Salt lakes ('dry lakes' would be probably a more accurate designation) are, however, plentiful and in recent years they have received considerable attention. The largest of them, Lake Eyre - in fact the largest ephemeral lake in the world - which has its own enigmatic fascination to limnologists the world over, was considered permanently dry since its discovery in 1840 until its first recorded filling in 1949. In the decades that followed, fillings of Lake Eyre were still considered as isolated, unique and independent events. Only recently have they been looked at as a predictable manifestation of the world-wide atmospheric and oceanic circulation, of which the El Nino-Southern Oscillation (ENSO) is one of the most significant phenomena. It is suggested that Lake Eyre floods are a physical manifestation of strong and positive Southern Oscillation phases and are usually out of phase with the Pacific Dry Zone rainfall and El Nino events. Links have been found between ENSO and large-scale precipitation patterns and 17 global core regions that appear to have a clear ENSO-precipitation relationship have been identified. Five of those are located in Australia and two of them, eastern Australia and central Australia, cover the entire Lake Eyre basin.
LAKE EYRE BASIN
Lake Eyre is the terminal point of the great continental drainage system which spreads over 1.14 x 106 km² of arid central Australia. The Lake Eyre basin is mostly flat with extensive areas of sandy and stony deserts. There are regions with some relief near its boundaries, but only 30% of the area is elevated more than 250 m a.s.l. Catchment area to lake surface area ratio AC/AL = 118, significantly more than for most major closed-drainage systems in the world, and virtually identical to that of Lake Chad (AC/AL = 120) in Africa. Some hydrological parallels could be drawn between both lakes: Lake Chad has a basin of 2.5 x 106 km², about double the size of the Lake Eyre basin, its surface area is 20 830 km² and its volume 72 km³, about twice the size of Lake Eyre in 1974. Both lakes are subject to similar evaporation rates (2000 mm year-1) but the Lake Chad basin receives some 500 mm year-1 of rainfall, about twice that of the Lake Eyre basin. In effect, the Chari River, Lake Chad's main tributary, carries on average 40 (19 to 54) km³ year-1, compared to 2.4 (0 to 25) km³ year-1 of the Diamantina River, the main tributary of Lake Eyre. As a result, Lake Eyre dries up after periodical floods and Lake Chad remains permanent, despite significant water-level fluctuations.
The Lake Eyre basin lies in the arid zone which covers some 60% of Australia. However, the aridity of this region does not equal that of the Sahara or Namib and even a slight increase in rainfall would transform it into a semi-arid environment. It may be helpful to visualise here what a deficiency of water is. The Lake Eyre basin, which covers 0.8% of the Earth's land area or 4.3% of the areas without outflow to the sea, receives some three times less rainfall than the world average and is subject to some three times more intense evaporation. Although these figures may appear at the first glance not very significant, they entail far-reaching consequences. If the amount of the fresh liquid water on Earth were distributed uniformly by areal proportions, then Lake Eyre basin would contain some 50 times more water flowing in rivers and 200 times more water stored in lakes than it currently does.
The average annual temperature varies from 21oC in the south of the basin to 24oC in the north, and the average maximum temperatures are 18oC and 24oC respectively in July, and 36oC and 39oC in January. The annual hours of sunshine vary from 3250 to over 3500, and the average global radiation is 600 mWh cm-2 day-1.
Large parts of the Lake Eyre basin are poorly instrumented, but it is estimated that an area of some 5 x 105 km² receives less than 150 mm of rainfall per year on average. The highest rainfalls, with annual averages of about 400 mm, occur in the northern and eastern margins, where rainfall is received from the southern edges of the summer monsoon. However, this region lies barely on the periphery of the planetary monsoon system, and the Australian monsoon is erratic both in space and time.
The mean annual evaporation as measured by Class A evaporation pans ranges from 3 600 mm for the central and south western part of the basin to 2 400 mm for its eastern edge. The pan coefficient which converts pan data into evaporation from large water surfaces is of the order of 0.6 in the Lake Eyre area.
The annual evaporation rate for the filled Lake Eyre ranges from 1800 to 2000 mm. The evaporation rate from the dry lake floor (excluding the salt crust) is of the order of 170 mm year-1 and the net evaporation rate from the salt-covered surface of Lake Eyre at between 9 and 28 mm year-1.
Most of the Lake Eyre basin overlies the Great Artesian Basin which covers an area of 1700 000 km² and is one of the largest in the world. Aquifers are formed from porous sandstones of the Triassic, Jurassic and Cretaceous, which crop out along the Great Dividing Range and are up to 3000 m deep in its central part. From the mountainous recharge zone, water percolates very slowly towards the terminal base level of Lake Eyre, reaching this area after a calculated travel time of up to three million years. Intakes along the arid western rim make only a minor contribution.
Apart from natural vertical leakage through the confining beds over the entire basin, natural discharge occurs from some 600 springs located in 11 groups along the western and southern boundaries of the basin, many of which appear to be fault controlled. Flows from the mound springs amount to 0.03 km³ year-1 and are thought to be only a minor discharge component. Artificial withdrawal from the basin is around 0.5 km³ year-1, diminishing steadily from around 0.7 km³ in 1915. Measured flows from springs range from 0.0001 to 0.23 m³ s-1 totalling around 1 m³ s-1. Salinities range from 700 to 1 400 g m-3, pH from 7.1 to 8.0 and water temperature from 30 to 40oC.
TRIBUTARIES OF LAKE EYRE
The drainage system of Lake Eyre is exceptional for the persistence of an extensive network, despite its inclusion in the driest parts of the continent. This is a result of the favourable structural disposition of more effectively watered peripheral uplands combined with centripetal lowland slopes on weak and generally impervious rocks, leading to the downfaulted terminal basin. Much of this drainage is disconnected as a relict from linked river systems which developed under higher rainfall conditions and which became disorganised, under the present arid climate.
The lake is fed mainly by its eastern tributaries, the Cooper Creek and the Diamantina and Georgina rivers system. Significant runoff originates in the desert west of the basin, which is drained by the Neales and the Macumba. Under the current climatic conditions only some 68% of the basin contributes water to the lake -although in pre-Quaternary time, under more moderate and wetter conditions, drainage from the Simpson Desert and Finke River catchments would have contributed also.
All streams are characterised by extreme variation in discharge and flow duration. Mean annual runoff of the basin is 4 km³ or 3.5 mm in depth, the lowest of any major drainage system in the world. This is significantly less than the 57 mm for the whole of Australia and only 1.5% of the mean annual runoff of all the land areas of the world, estimated as 247 mm. The aridity of the basin is more graphically demonstrated by its specific yield of only 10 m³ km-2 day-1 in comparison to the Nile or the Amazon which have specific yields of 115 and 2200 m³ km-2 day-1 respectively.
In arid central Australia 15-20 mm of rain of moderate intensity can cause a flow in minor streams that lasts only for an hour or two. Such a flow may occur as often as five times each year. However, about 50 mm is needed for a full channel flow and such falls can be expected less frequently than once a year. Major floods in the Lake Eyre basin result from annual rainfalls which exceed 500 mm: the runoff coefficient in such years approaches 0.05.
Eastern tributaries of Lake Eyre originate in more humid areas and flow quite frequently, however, only a small proportion of these flows reach Lake Eyre, due to very high transmission losses. The Cooper Creek for example can lose up to 107 m³ km-1 in its central reach, filling innumerable salt lakes and interdune corridors of the Tirari Desert. Usually the Cooper Creek terminates at the Coongie Lakes, the only permanent wetlands within the 5 x 106 km² of arid central Australia. The Cooper Creek only reaches Lake Eyre on average, once in six years. The transmission losses of the Diamantina River are usually much lower. The mean velocity of floods is 12.5 km day-1 for the Diamantina and 3.1 km day-1 for the Cooper.
Water quality analyses in the central reaches of the main tributaries show that the mean annual inflow of 4 km³ transfers some 2.5 x 105 t of salt into the lake, some 0.2 t km-2 year-1 in comparison to the world average of 26 t km-2 year_1. On this basis, the period of accumulation represented by existing salt crust appears to be as short as 1600 years. The load of suspended solids is of the order of 1.5 x 106 t year-1, the equivalent of a 0.15 mm layer over the lake. The specific suspended sediment load is 1.5 t km-2 year-1, very low in comparison to the world average, of 90 t km-2 year_1.
The hydrology of the western tributaries is largely unknown. Several major rivers carry water infrequently, possibly only once in ten years. Flood volumes can be, however, enormous: for example in 1984 and in 1989 Lake Eyre received some 8 km³ from western tributaries in three days, which implies discharges of the order of 30 000 m³ s-1, or one-sixth that of the Amazon.
Lake Eyre, whose lowest parts lie 15.2 m below sea level, consists of two sections: Lake Eyre North and Lake Eyre South, joined by the narrow Goyder Channel. Lake Eyre North was a permanent saline lake from 5000 to 10 000 years ago and entered a playa phase between 5000 and 3000 years ago.
A salt crust up to 460 mm thick covers the southern part of Lake Eyre North. The 4 x 108 t of salt deposited in the lake dissolve completely at times of major inflows. The origin of this salt is a matter of considerable discussion, as the possible sources are very diverse ranging from marine aerosol transport ("cyclic salt") to rock weathering and groundwater discharge from aquifers deposited during the Cretaceous marine transgression that inundated central Australia.
The presence of several old beach lines, 0.7, 1.6 and 2.8 m above the 1974 water level, indicate the occurrence of previous unrecorded major inflows. The term "full" should therefore be used cautiously in relation to Lake Eyre. The above levels would represent approximate storages of 35, 48 and 67 km³ respectively and the potential available storage to sea-level is more than 200 km³, i.e. almost seven times greater than the 1974 storage.
FILLINGS OF LAKE EYRE
Lake Eyre lies in a remote and desolate area. Despite the current level of interest in the lake and its hydrology, very little data on its fillings have been collected, and many minor floodings escape attention.
The existence of a large lake in central Australia is traceable to the middle Jurassic (150 million years before present (BP)) when freshwater Lake Walloon occupied the central eastern part of the continent: in the late Cretaceous (85-75 million years BP) it shrunk to a smaller, 5 x 105 km² Lake Winton, which in the early Miocene (21 million years BP) assumed a position closely resembling the recent location of Lake Eyre. Lake Dieri, a Pleistocene "greater Lake Eyre" underwent large scale climatic variations, entering into the Recent epoch as a dry salt pan subject to occasional flooding.
A longer period of constant filling of Lake Eyre occurred probably in the late Pleistocene, during a wetter climate in southern Australia, between about 45 000 and 25 000 BP. This was followed by a phase of lake contraction, dune building and desiccation. Since European occupation, the climate has been almost equivalent to that of the driest phases of the last 10 000 years.
The existence of water in Lake Eyre was first reported by Ross in 1869 and then by Halligan in 1922, however, their reports were dismissed as observation errors. Madigan who explored the area after a long drought in 1929 was convinced that the lake is permanently dry. The first reliable record of filling was in 1949-50, when Lake Eyre North reached a peak storage of 21 km³. This was followed by a series of minor floodings in 1953, 1955, 1956, 1957, 1958, 1959, 1963, 1967 and 1971, leading to a most significant flood event which began in 1973, reached its peak in 1974 and persisted until 1977. Lake Eyre North reached its highest recorded level of -9.09 m AHD in June 1974, and the equilibrium level of -9.5 m AHD between both lakes was achieved in October 1974. The peak combined storage was 32.5 km³.
The last decade brought two unexpected events. The filling of 1984 with a total volume of 10 km³ was a relatively minor one, but if proved that the western tributaries can fill Lake Eyre in a matter of days. Lake Eyre South this time filled first - an event never previously recorded and considered to be extremely unlikely - and overflowed to Lake Eyre North. In 1989 this event was repeated, coinciding with the filling of the second largest Australian playa, Lake Torrens, which filled for the first time since 1878.
The wet spell continued into 1990 when after some of the most devastating floods in Australian history, water from the Cooper Creek reached Lake Eyre for the first time since 1974.
The inflows to Lake Eyre in the period 1885-1984 were reconstructed using a rainfall-runoff model on the short streamflow records available, and then extending modelling for the period in which only rainfall data were available.
The modelling clearly showed that the inflows to Lake Eyre North were relatively frequent and occurred on average every alternate years in the period investigated. The mean annual inflow was 3.8 km³, with a standard deviation of 6.2 km³. Annual inflows shown in Figure 1 have a slight downward trend of 0.005 km³ year_1 coinciding with the decrease in lake water levels in the Northern Hemisphere estimated as 30 km³ year_1 since 1940. Global warming need not necessarily be the cause, as in the period 1910-1940 the said decrease was of the order of 50 km³ year-1. For more illustration, the Caspian Sea dropped its level by 3 m in 1860-1970, the Dead Sea by 4.5 m in 1885-1960 and the Great Salt Lake by 3.5 m in 1850-1960, but the Great Lakes in turn, are at their highest in decades. The overall mechanism causing lake level variations throughout the world is clearly not recognisable at this stage.
The Diamantina and Georgina rivers system, which covers only 32% of the Lake Eyre basin, contributed 65% of the total water entering the lake. The mean annual inflow from this source was 2.4 km³, with a standard deviation of 3.8 km³. The input from Cooper Creek and from other tributaries was 0.63 and 0.72 km³ year-1 respectively.
Rainfall-runoff modelling undertaken and a series of recent fillings show that Lake Eyre North fills much more frequently than was previously thought. In fact a return period of a 10 km³ inflow is 8 years. Such a volume of water covers almost the entire surface area of Lake Eyre North and evaporates during the following year.
"The Greenhouse" effect
The behaviour of such a delicate system under the influence of enhanced "greenhouse" processes is a subject of justifiable curiosity. Although future fillings of Lake Eyre cannot be presently predicted with any degree of confidence, the effects of the possible climatic changes can be evaluated.
The most frequently quoted forecasts today suggest that in Australia by the year 2030, the temperature will have increased by 2 to 4EC, summer rainfall will increase by 50%, winter rainfall will decrease by 20% and overall wind speed decrease by 10 to 20%. As more than 95% of total inflows to Lake Eyre are generated during the Australian summer (December to March), the change in winter rainfalls will have relatively little effect on the frequency of filling of the lake. The effect of other factors on runoff tend to cancel each other.
The 50% increase in the summer rainfall would have by itself a substantial effect on streamflow, increasing it possibly two- or threefold as it is known that an increase of 25% in precipitation and a decrease of 1EC in temperature can increase the simulated runoff by 250%. The central reaches of all tributaries would be inundated more frequently which would significantly intensify the evaporation transmission losses, where a change of 1EC corresponds to a 4% change in evapotranspiration. The larger areal extent of evaporating water surface and ground would, however, be partially compensated by the lower wind speed which would reduce evaporation. Although it appears that major tributaries will probably be flooded more often in their upper and central reaches, it is unclear whether their waters will be reaching Lake Eyre more often due to the increased transmission losses.