The Great Basin is a region of the Basin and Range Physiographic Province, which is completely isolated hydrologically from the sea. All precipitation that falls within the Great Basin is lost from the land surface or from the surf ace of closed inland lakes through evapotranspiration. Playas are often found at the base of these undrained basins. This study focuses on subsurface groundwater flow patterns in closed basins. Because all discharge from the basins occurs via evapotranspiration on and near the playa, the water table of the aquifer beneath the playa is often just below the ground surface. Fluctuations of the water table due to climatic events cause the water table to rise and dissolve the salts on the playa surface. This mass transfer can produce density gradients that in turn cause flow. This study is an extension of the work performed by Duffy and Al-Hassan (Duffy and Al-Hassan 1988) in which numerical experiments were used to show that the free convection, caused by the variation in density of the bulk fluid propenies, appears to play an important role in determining the patterns of groundwater flow beneath the playa. Their work considered only homogeneous, isotropic porous media in symmetric basins. The effects of anisotropy, periodic stratification, and asymmetric basins on the groundwater flow and transport patterns was studied here. Dimensionless parameters, the Rayleigh number and the salt nose length, L0*, were defined for each of the systems incorporated and were shown to be linearly related. The implication of this relationship is that the Rayleigh number can be used to predict basin-scale circulation patterns in the hypothetical closed basins studied. It was also determined that an equivalent anisotropic system could be defined for a horizontally stratified aquifer in order to predict basin-scale circulation patterns. An equivalent isotropic system was defined for each anisotropic system in a similar manner.

Focused groundwater discharge in endorheic basins provides opportunities to investigate mechanisms for closing hydrologic budgets in arid regions. The Salar de Atacama (SdA), a closed basin in northern Chile, has accumulated over 1800 km3 of halite and a lithium-rich brine since the late Miocene primarily through evapotranspiration of groundwater. The hydrologic balance of SdA and sources of water and solutes required to explain this deposit are not well constrained. An adapted chloride mass balance method drawing on a database of over 200 water sample sites is applied to a remotely-sensed precipitation dataset to estimate spatially-distributed modern groundwater recharge. Comparing groundwater recharge to evapotranspiration in a steady-state water budget constrains potential regional-scale watersheds on the Altiplano-Puna Plateau. The sodium mass balance of the deposit is used to predict long-term water discharge from the basin and place modern fluxes in a paleo-hydrologic context. A 2D groundwater model informed by published paleoclimate reconstructions evaluates whether draining groundwater storage contributes to the modern hydrologic system. Modern recharge from precipitation in the topographic watershed is extremely small compared to evapotranspiration. The missing water is sourced from precipitation in an area over 4 times larger than the topographic watershed, and groundwater recharged during wetter periods in the late Pleistocene is still actively draining and discharging from storage without a corresponding input into the system. These results have implications for lake-level based paleoclimate reconstructions, conceptualizations of watershed boundaries and water resource management.