Modelling historical lake levels and recent climate change at three closed lakes, Western Victoria, Australia (c.1840–1990)

Lake levels of three hydrologically closed maar lakes, Lakes Keilambete, Gnotuk and Bullenmerri in Western Victoria, Australia, have declined since the first recorded observations in 1841. High levels were previously sustained for several centuries with the lakes in climatic equilibrium. Historical survey and field investigations provide a detailed picture of both historical land-use changes and of the geological and hydrogeological influences on the water budget. Groundwater components include baseflow from deep percolation within the catchment and discharge from a surrounding low-yield aquifer. A perched watertable at Lake Keilambete helped maintain high lake levels. Climate records back to 1859 were reconstructed; inhomogeneities from 1863 were removed creating a high quality instrumental record. A water balance model simulating the historical decline demonstrates important features. (1) Regional climate expressed as a lake precipitation/evaporation (P/E) ratio remains the over-riding influence on lake levels. (2) The lakes fell in response to a change in climate. (3) This climate change pre-dated instrumental records. (4) Land-use change did not contribute to declining water levels. The fall in water levels was initiated by a decrease in P/E ratio from a pre-1840 value of 0.94–0.96 to a historical value of 0.79. This change probably involved a decrease in rainfall, possibly associated with increases in solar radiation and decreases in cloud cover. Temperature (T) may also have increased but the likelihood of an altered temperature–evaporation relationship means that a quantified estimate is not possible. The ability of the lakes to reflect climate independently of land-use change is highly unusual, both in Australia and elsewhere. These lakes provide a rare opportunity to discriminate between human impact on regional hydrology and climate change.