EXTENDING TOTH'S EXAMINATION OF REGIONAL AND LOCAL GROUND-WATER FLOW TO RUGGED 3-D MOUNTAINOUS TERRAIN

Toth's seminal 1963 paper highlighted the importance of topography in defining the distribution of regional versus local ground-water flow and discharge. The potential for 3-D flow in rugged mountain drainage basins was not examined, however, due to the limits of analytical solutions. We examine steady-state regional and local ground-water flow and discharge patterns in an idealized mountain range using HydroGeoSphere, a 3-D numerical model describing fully-integrated subsurface and surface flow. The idealized mountain range is comprised of 1st, 2nd and 3rd order drainages with topography defined by sinusoidal functions and stream gradients. Typical values for stream gradient, valley wavelength, amplitude, flank slope, hydraulic conductivity, and recharge are taken from hydrologic and geomatic literature. Three types of mountain ranges with differing topographic and stream gradients are modeled: extremely dissected (e.g., Himalaya); highly dissected (e.g., Rocky Mountains); and moderately dissected (e.g., Appalachians), comparable to the topography in Toth's model. Additionally, using the highly dissected range as a base case, a sensitivity analysis of topographic and hydrgeologic variables was completed. Preliminary model runs suggest that discharge is focused in the headwaters of the 1st and 2nd order streams and the outlet of the 3rd order stream where the topographic gradient is highest. Locally, ground-water flow perpendicular to the regional topographic gradient (local flow) can be 10-100 times greater than flow parallel to the regional gradient (regional flow), suggesting that mountain systems must be examined as 3-D ground-water systems. It is hoped that the modeling will reveal critical topographic and hydrogeologic characteristics that control and partition ground-water flow between local and regional regimes in mountain settings.