Recorded fillings

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, whenLake 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, LakeTorrens, 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 CooperCreek reached Lake Eyre for the first time since 1974.

Modelled fillings

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 km ³ year^_1coinciding 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 km³year^-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 CooperCreek and from other tributaries was 0.63 and 0.72 km³ year^-1respectively.

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 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.

More about this trip in the Blog