TOM WINTER STUDIES AND EVOLUTION OF HYDRODYNAMIC MODELS OF GRAVITY-DRIVEN FLUXES IN LAKE-AQUIFER SYSTEMS

The explorations of two- and three-dimensional hydrodynamics of lake-aquifer interactions by T. Winter (1976, 1978, 1983) set benchmarks for the latest studies of lake hydrogeology. Winter expanded on a concept postulated by Toth and Freeze; namely, the topography of the water table controls the interaction between local and regional flow systems. Winter investigated seepage fluxes in lakes occupying varying topographic positions with respect to the undulating water table, Winter's (1986) field study in the Nebraska Sand Hills, USA clearly demonstrated water table effects on the gravity-driven lake-aquifer exchange by using available piezometric methods for system characterization.

These results stimulated studies by L. Townley and colleagues in 1994-2006 who proposed a hydrodynamics-based approach to lake classification. They explored a three-dimensional box-shaped groundwater watershed with a single embedded lake. The model required complete knowledge of velocities (fluxes) on watershed boundaries and groundwater recharge instead of topography of an undulating water table. Although this approach allowed discriminating among discharge, recharge, and flow-through lake regimes, the velocities and other required data could be obtained only in rare occasions. These models were germane in interpreting the flow regimes using natural tracers as shown by Turner and Townley (2006) for lakes in Western Australia.

Our approach extends Winter's studies of water table topography effects to lake classification. We developed a criterion (gradient ratio) that compares regional and local fluxes for discriminating lake regimes, based on piezometeric data only. However, the accuracy of input data may be the limiting factor for model applications. Therefore, piezometeric studies of lake fluxes must be supplemented by inference from natural tracers and geophysical techniques. We illustrate this lake regime classification in the area of Winter's classic field studies, the Nebraska Sand Hills. Contrasts in lake water-groundwater chemistry, geolectric properties, and temperature permit confident discrimination of lake regimes in addition to piezometeric studies and will be a growing area of research.