Infrastructure Engineering - Theses
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ItemModelling hydrologic and climatic controls of closed lakes, western Victoria( 1995)To quantify changes in water level and climate, a water balance model, calibrated using historical climate and lake levels, is used to construct a detailed 16,000 year record of precipitation/evaporation (P/E) ratio for three closed lakes in western Victoria. Lakes Keilambete, Gnotuk and Bullenmerri are maar lakes situated in western Victoria, 200 km W of Melbourne. With high lake/catchment ratios, these lakes are sensitive to atmospheric conditions affecting water balance. Their levels are controlled by direct precipitation and evaporation; groundwater (baseflow) is a minor contributor to the water budget. Previous studies have constructed detailed records of lake levels and palaeosalinities, providing a continuous hydrologic record from 16,000 years ybp to the present. The lakes were observed at high levels by early European settlers but since 1841, waterlevel has undergone the most rapid and sustained decline recorded for the Holocene. Regional meteorological records are used to construct climate series representative of the study area from 1859-1990. Regional homogeneity is demonstrated between the study area and climate stations elsewhere in southern Victoria, South Australia and Tasmania. Climate change recorded at the lakes is therefore indicative of regional climate. No evidence of climate change is found early in the instrumental record, but a statistically significant increase in of 0.3 °C in temperature occurs after 1960. Evaporation is calculated using Morton's Complementary Relationship Lake Evaporation model, utilising long records of air and dewpoint temperature and sunshine ratio. The model calculates water budgets from rainfall, evaporation and baseflow. Baseflow is calculated from the infiltration rate which is derived from modelled soil moisture. Baseflow is optimised for each lake to reproduce the historical fall in lake level. The equilibrium climate for the pre-European lake level is modelled as a P/E ratio of 0.94-0.96, compared to the current ratio of 0.79-0.80. The historical fall in all three lakes preceded the period of instrumental climate measurement and major land-use changes. It is therefore ascribed to a natural climate change. Equilibrium and non-equilibrium modelling of lake water levels have produced detailed estimates of P/E ratios, demonstrating wide climatic fluctuations during the Late Pleistocene and Holocene. Holocene P/E ratios vary from 0.77-0.80 in the early and late Holocene to > 1.10-1.20 from 7,000-5,500 ybp. Late Pleistocene P/E ratio varied between 0.70-0.80. The resultant record enables precise estimates of P and E to be determined if independent estimates of either variable can be supplied. It provides a detailed view of climate throughout the Holocene and is of special relevance to continental and hemispherical climate records. Further information on palaeolake levels from the study lakes and other related maar lakes will permit the construction of an even more detailed record of P/E ratio for the Holocene. With other long records from the region, it will provide a highly detailed record of climate change in south-eastern Australia, dating from the late glacial period. The study lakes also show great potential for monitoring future changes in P/E ratios, particularly precipitation change.