Convective Groundwater Flow Induced by Climatic Gradients inthe Great Basin: A Computational Approach to System Dynamics

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Present-day environmental fluid flow problems are routinely concerned with making predictions over large space and time scales in systems with sparse field data. The classical approach identifies characteristic length and time scales as a means of resolving the basic forces or fluxes driving fluid circulation. In a study of the hydrology of closed basins, Duffy and Al-Hassan (1988) applied the concept of scale invariant topography to construct an empirical topographic distribution for the range bordering the Pilot Valley Playa in western Utah. Regional correlation of mean precipitation with altitude was used to construct scaled inputs for modeling mountain-front recharge. At the playa or ephemeral lake, high evaporation rates were observed to produce concentrated brines in the shallow groundwater, resulting in unstable stratification and the potential for free convection. These observations guided numerical experiments conducted over a range of closed -basin geometries, and led to the development of a force or flux balance for subsurface circulation driven by mountain precipitation and playa-lake evaporation. The results suggest that the playa margin becomes a critical hyhdrologic zone where upwelling groundwaters from mountain-front recharge (forced convection) and brine recirculation from the central playa (free convection) converge, producing fresh and/or brackish springs. The premise of this work is that the position of the freshwater-saltwater interface represents a balance of hydro-climatic forces in the basin. Using characteristic lengths and velocities of the closed basin, the Rayleigh number for salinity was shown to be linearly related to a dimensionless group representing the steady-state position of the interface. The proposed research will attempt to extend previous numerical experiments to include complexities of space and time variability, namely: (1) the dynamic response of subsurface convection resulting from transient fluctuations in mountain recharge and playa evaporation; (2) the implications of deterministic and stochastic heterogeneity to the estimation of effective basin parameters; (3) regional flow from multiple- cascading basins of varying base level; (4) the role of surficial hydrologic processes (runoff); and (5) the significance of unsaturated flow and the atmosphere-land surface boundary condition.