Water levels in the aquifers of the Denver Basin Aquifer System have declined significantly in recent years and have created much debate about the sustainability of groundwater resources in the Denver metropolitan region. The sustainability of groundwater resources can only be measured if both supply and demand can be quantified and used in long-term water planning. Water demand or water use is difficult to quantify in the Denver area because groundwater production is not measured and reported consistently from the numerous municipal, industrial, and domestic users. However, the effects of pumping are recorded in groundwater levels measured during annual groundwater monitoring events; therefore, it is possible that production rates of the primary water users could be inversely modeled from changes in aquifer water levels. This study defines a methodology for modeling production rates by implementing the Theis (1935) solution in a Geographic Information System (GIS) and calibrating the production model against measured water levels. Production modeling of the Denver Basin Aquifer System was completed for Douglas County, Colorado using public-domain data. Modeling indicates that the production rate from the confined Arapahoe aquifer, by the 15 primary Douglas County water users, averaged 30,225 acre-ft per year between 1998 and 2003. The unique GIS-based modeling approach developed during this study could be applied to other aquifers with limited hydrogeologic and production data.