Kirono, DGC, Jones, Roger ORCID: 0000-0001-6970-2797, Kent, D and Leahy, P (2009) Modelling lake levels under climate change conditions: three closed lakes in Western Victoria. In: 18th World International Association for Mathematics and Computers in Simulation (IMACS) Congress and -- International Congress on Modelling and Simulation (MODSIM09) - Interfacing Modelling and Simulation with Mathematical and Computational Sciences, 13 - 17 July 2009, Cairns, Queensland.

Abstract

Lakes Keilambete, Gnotuk and Bullenmerri are lakes in Western Victoria that are highly regarded for their ecological, social and scientific values. These are maar lakes (lakes in shallow craters of volcanic tuff) and are recognized as being of National (Gnotuk) and International Significance (Keilambete, Bullenmerri). As closed lakes, they have no streams coming in or out, and are predominantly controlled by rainfall and evaporation at the water's surface. These crater lakes are very sensitive to climate change and have been used to diagnose past changes in climate. Previous research has shown that the lakes have been falling since the mid 1800s and that the likely cause was a decrease in rainfall and increase in evaporation of natural origin. Recent rainfall decreases have increased the rate of lake level decline. With the prospect of future climate change further altering regional rainfall and evaporation, this study assesses the potential impact of climate change due to enhanced greenhouse gas on future lake levels at these lakes. Lake levels (~1880-2100) are constructed using the lake water balance model of Jones et al. (2001). This model is quite robust in modelling historical lake levels, and has been used for simulating climatic conditions reproducing lake levels for the past 16,000 years. Time series of monthly climate data were obtained from 14 global climate model simulations contained in the database used in the IPCC's Fourth Assessment Report (IPCC, 2007). For the future, these GCM simulations were forced by the A1B and A2 greenhouse gas and sulphate aerosol emission scenarios. The use of multiple GCMs and different future emission scenarios, result in scenarios that represent the range of likely outcomes of regional climate under enhanced greenhouse conditions to 2100. The results suggest that all lake levels will continue to fall, with the declines for Bullenmerri expected to be larger than those for the other two lakes. The decline was initiated with the historical change in climate from around the mid 19th century. Recent reductions in the precipitation/evaporation ratio show that the rate of decline is accelerating. For Bullenmerri, the median model simulation projects a rainfall/evaporation (P/E) ratio of around 0.73, 0.69, and 0.67 in the 2030s, 2050s and 2070s, respectively. These are similar to the median values for Lake Keilambete and Lake Gnotuk (0.71, 0.68 and 0.64, respectively). The median estimates of the rate of change for Keilambete is -7.8, -5.8, and -4.2 cm/year in the 30 year periods centred on 2030, 2050 and 2070, respectively. For Gnotuk these are -14.5, -15 and -10.9 cm/year and for Bullenmerri they are -24.4, -26.1 and -30.4 cm/year respectively. In addition to the drying trend, proposed land subdivision and urbanization within the Gnotuk catchment, for example, threatens the visual amenity of the lake and will add to the nutrient and sediment load to the lake. The deeper crater lakes, particularly Lake Bullenmerri, are becoming increasingly important as a resource for recreation and amenity for the regional community. This is placing added pressure on the scientific and environmental qualities of the lakes. Lakes Keilambete and Gnotuk will become increasingly hypersaline, and Lake Bullenmerri will also become more saline. This may reduce the invertebrate diversity in the lakes. In Bullenmerri, the rising salinity could cause a shift in the algal plankton and so influence the whole food web, recreational fish species included, putting its place as an important recreational fishery at risk. This suggests that a regional plan to manage the quality and amenity of lakes in Western Victoria under a drying climate and other threats is urgently needed.

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