THE USE OF PILOT POINT REGULARIZATION AND PREDICTIVE ANALYSIS IN SIMULATIONS OF GROUND WATER FLOW IN THE LOWER BOISE RIVER BASIN

A ground water flow model of the lower Boise River aquifer system was developed to evaluate the potential effects of increased withdrawals and altered recharge rates on regional ground water levels. The model was constructed using the three-dimensional, finite difference MODFLOW code (Harbaugh et al., 2000), and was calibrated using the PEST parameter estimation code (Doherty, 2000). The use of pilot point regularization enabled the use a larger number of parameters than would otherwise have been possible. Scenarios were simulated using PEST’s predictive analysis tools.

Calibrated parameter values reflected basin stratigraphy, with higher estimated conductivity values associated with coarser-grained sediments in upper and eastern aquifer strata, and lower conductivity values in lower and western portions of the model domain. Based on simulation results, most recharge occurring in shallow aquifer zones does not reach lower aquifers. Results of the predictive analyses indicated the possibility of ground water level declines in some areas. These simulated water level declines and/or changes in mass balance components reflect a combination of parameter uncertainty and response to different hydraulic stresses.