GEOLOGIC AND CLIMATIC HETEROGENEITY IN A REGIONAL GROUNDWATER SYSTEM
The tectonic extension, compression, and intrusions in the Great Basin create geologic and climatic heterogeneity. The mountains cause discontinuities that do not occur in more homogeneous regional systems. Flowpaths cannot be inferred directly from water levels, but are convoluted and flow may take preferential courses around mountains or through small areas with relatively high transmissivity.
A numerical groundwater flow model is being constructed to test the concept of an interconnected groundwater system in consolidated rock and basin fill, the occurrence and effect of recharge mounds, and the effects of areas with low transmissivity. Problems in developing the numerical model have included lack of water-level data in the mountains, large areas with little data, lack of long-term stream and spring discharge data, and the inability to define small-scale heterogeneities that affect the system. To augment the limited data, altitudes of perennial mountain streams and springs were used as estimates of groundwater levels in the mountains, assuming that single or limited-time measurements represented steady-state conditions, and allowing more geologic heterogeneity in the model than is shown by the geologic data.
Preliminary model results indicate that geologic heterogeneity is required to match spring and stream discharges. The model provides estimates of the different areas and values of hydraulic conductivity, but more detailed geologic data and discharge measurements should be obtained to better understand the effects of current and future development on natural discharge. Preliminary model results also indicate less interbasin flow than previously estimated at some basin boundaries where recharge on relatively low permeable mountain rocks causes groundwater mounding.