Water-bearing rocks within the 7 sq mi of Storage Unit I of the Santa Barbara Groundwater Basin, consist of unconsolidated deposits that range in thickness from < 300 ft along the north perimeter of the unit to > 1,000 ft near the Pacific Ocean. The groundwater system was simulated as two horizontal layers separated by a confining bed. The model boundaries coincide with mapped faults on all sides. The faults were considered no-flow boundaries except for the offshore fault that forms the south boundary. This boundary was simulated as a general-head boundary , which allows water to move into and out of the modeled area. The model was calibrated by simulating both steady-state conditions (approximated by July 1978 and February 1983 water levels) and transient-state conditions (represented by May 1978 through December 1979 water level changes). The calibrated model was then used to simulate the period from January 1980 through December 1983 in order to verify the model. Model results generally closely matched measured data throughout Storage Unit I. During the transient and verification simulations, 9,980 acre-ft of groundwater was pumped from Storage Unit I for municipal use. Results of the model indicate that 42% (4,190 acre-ft) of the water pumped from the system was withdrawn from storage, 33% (3,290 acre-ft) was derived from changes in underflow across the offshore fault, and 25% (2,500 acre-ft) was derived from decreased groundwater discharge to drains. The model simulated that municipal pumpage induced about 1,380 acre-ft of water to move across the offshore fault toward Storage Unit I. Several model simulations were used to estimate aquifer response to different municipal pumpage patterns that could be used as management alternatives. Results of the simulations indicate that spreading municipal pumpage more evenly throughout Storage Unit I, by increasing the number of wells while reducing the pumping rate at the individual wells to maintain the same total pumpage, significantly reduces the inflow of groundwater across the offshore fault. (Author 's abstract)