Three-Dimensional Convection in Dry Salt Lakes

Dry salt lakes are striking geological features, displaying patterns consisting of flat polygons bounded by raised ridges. They are found in semi-arid regions of the world, with the patterns typically being one to a few metres across, a characteristic of dry lakes that can be observed worldwide. The groundwater in the porous soil beneath dry lakes collects salts as it flows towards these terminal valleys, and despite the intense environment, the water table can often be found close to the surface. As the water evaporates, salt is left behind, forming a crust, allowing the ridges to grow and develop into an intricate network.

To balance the evaporation of water from the surface, a throughflow is present in the porous soil, which transports the dissolved salts to the surface. As the groundwater evaporates, heavier saltier water overlies lighter fresher water and the competition between the upward advection and the downward diffusion of salt gives rise to a natural steady-state. This state may become unstable, resulting in buoyancy-driven convection within the porous soil beneath the lake. This results in spatial variation of the salt transport to the surface, which may aid the growth of the crust in some places and hinder it in others. This provides a possible explanation for the emergence of the surface pattern observed at dry lakes: the polygonal pattern is a surface expression of the subsurface fluid dynamics.

In this thesis, the linear instability resulting from the natural steady-state is analysed, showing that an instability can only occur when the throughflow is sufficiently weaker than the opposing buoyancy effects. The convection resulting from this initial instability is investigated through numerical simulation. Patterns in the salinity transport to the surface are observed, providing evidence that this may be a significant contributor to the crust pattern observed at dry lakes. The net rate of salinity transport and the overall pattern scale are analysed as the lake conditions are changed, showing agreement with observations taken from the field.