Australian scientists are investigating a mysterious decline in water levels in the internationally significant crater lakes of the state of Victoria.
Local Aboriginal people are recorded as saying 'drought came with the white man', but researchers have concluded that land-use change is probably not a factor.
"These changes are definitely pre-greenhouse," says Dr Roger Jones, from CSIRO Atmospheric Research, a division of Australia's national research organization, CSIRO.
"However there are signs that recent warming is affecting evaporation rates from the lakes."
Dr Jones compares the crater lakes to giant rain gauges.
"They occupy up to a half of the crater area, and have no streams coming in or out, so they are dominated by rainfall and evaporation at the water's surface."
In 1841, Lake Bullenmerri, Victoria's deepest natural lake, was recorded as overflowing into its twin crater, Lake Gnotuk. This was the last time it did so.
Since then, the crater lakes have continued to fall. Some are now dry lake beds, and three - Lakes Keilambete, Gnotuk and Bullenmerri - are still falling.
"These lakes are internationally significant. Nowhere else in the world have we seen such a close relationship between changing climate and water levels in lakes. Their ability to reveal climate change without being affected by past land-use change is unsurpassed," says Professor Jim Bowler, from the University of Melbourne
In the 1960s Professor Bowler surveyed these lakes, unlocking a history of past lake level changes dating back thousands of years. Now, a team of scientists including Prof. Bowler has shown that a climate change early in the 1800s initiated this fall.
"If the climate changes through a change in rainfall or evaporation, the lakes will rise or fall for several hundred years until they either come into water balance with the crater, dry up or overflow," says Professor Bowler.
The researchers reconstructed the historical lake fall from survey records and a long-term record of climate from 1859. Before Europeans arrived, the lakes had been at high levels for almost 2,000 years.
In a paper just published in the Journal of Hydrology, the researchers show that modern rainfall on the lakes is only about 80% of lake evaporation. To maintain the pre-European lake levels, rainfall would have to have been about 95% of lake evaporation.
"A climate change is the only explanation for the fall in water level. Rainfall and cloud cover probably decreased, and temperature probably increased but the exact combination is still unknown," says Dr Jones.
Melbourne University's Centre for Environmental and Applied Hydrology hosted this study, which is working to unravel historical relationships between the land, water and climate. Understanding these relationships will help Australia deal with salinity and water supply issues.
Journal
Journal of Hydrology