Modelling water and salt balances in a deep, groundwater-throughflow lake-Lake Purrumbete, southeastern Australia

Lake Purrumbete is a deep volcanic freshwater lake, cylindrical in shape, located within an extensive basalt plain in southeastern Australia. A modified difference water budget method for lakes, that estimates net groundwater flux through the difference between the level of the lake and the water table, along with the specific yield and area of the aquifer, successfully modelled the lake level fluctuations. The major influences are evaporation and direct precipitation; however groundwater fluxes are significant (similar to 17%). The salt balance modelling shows that the salinity in Lake Purrumbete is largely controlled by groundwater and surface water fluxes. Lake Purrumbete has a greater interaction with the surrounding groundwater system than other nearby volcanic lakes due to its significant depth and the presence of highly permeable basalts along the upgradient shoreline. Its low salinity reflects the substantial salt export in the groundwater and surface outflow, as well as the lake's relatively small evaporation due to the low surface area/volume ratio. The fluctuations in lake level and salinity for Lake Purrumbete are much less than for other lakes in the region, even during the 1997-2010 drought, due partly to the large, relatively constant groundwater flux. Low salinity, deep water and estimated depth of stratification from this study, which leads to a distinct limnology, implies that the creatures need to adapt to a different habitat to survive in the vicinity of Lake Purrumbete. Thus, Lake Purrumbete will be an important aquatic ecosystem refugium if climate change causes frequent seasonal drying of other lakes in the plain, and it is important to develop management strategies to maintain its water resources and quality.