Groundwater–surface water interaction and the impact of a multi-year drought on lakes conditions in South-East Australia

Abstract

South-East Australia is currently experiencing a severe multi-year drought (not, vert, similar11 years of below long-term average rainfall between 1997 and 2008). To refine our understanding of the impacts of such climate stresses on lake systems, and to overcome the limitations of insufficient field based measurements, lake water levels and lake salinity values are combined with remote sensing data (Landsat-5TM) to analyse water and solute budgets for two large lakes in the Corangamite catchment. For these two lakes, data during pre-drought (1992–1996) and drought (1997–2006) periods are used to investigate the impact of the drought on lake and groundwater interaction and possible changes in mechanisms controlling the salinity of the lakes. For 26 other lakes in the Corangamite catchment with limited or no on-ground data, water and evaporite mapping using remote sensing data (Landsat-5TM) were used as proxies for drought impacts on lake quantity and salinity. Within the study area, this drought has resulted in a 60% loss of lakes area inundated (220 × 106 m2), and for the two largest lakes the water loss equates to an 80% decrease in volume (714 × 106 m3). Although the lakes are groundwater-fed, the rapid declines in lake water levels have not resulted in systematic increases in groundwater discharge to lakes across the study region. The lakes that are most sensitive to changes in the hydrologic budget within the catchment (10 of the 28 monitored lakes) have changed from groundwater throughflow or discharge lakes to intermittent recharge lakes. The decrease in lake levels is accompanied by an increase in lake salinity, however the dominant controls on increases in salinity varies for different lakes between just evaporation, and evaporation plus episodes of increased net groundwater discharge. The drought has exacerbated pre-existing high lake salinity levels; by 2006 one quarter of the lakes have evaporites where previously (before 1997) these lakes were evaporite free. These observations on changes in lakes quantity, interaction with groundwater, and salinity highlights the spatial variability in the lakes response to regional drought impacts.