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Infiltration Best Management Practices (BMPs) are becoming more readily acceptable as a means of reducing post-development runoff volumes and peak flow rates to pre-construction levels, while simultaneously increasing recharge. Sizing BMPs to hold and store a predetermined volume of runoff, typically called the Water Quality Volume, has become a widely accepted practice. This method of sizing BMPs does not account for the infiltration that is occurring in the BMP during the storm event; which could result in significantly oversized BMPs. The objective of this study was to develop a methodology to simulate varying infiltration rates observed from a large scale rock infiltration basin BMP. The system consists of three infiltration beds filled with coarse aggregate, lined with geotextile filter fabric, overlain with pervious concrete and underlain by undisturbed silty sand. Recorded data indicates a wide variation of linear infiltration rates for smaller storm events. A model was developed using the Green-Ampt formula to illustrate the infiltration occurring in the basin for small storm events characterized by an accumulated depth of water of less than 10 cm. The effectiveness and accuracy of the model were determined by comparing the model outputs with observed bed water elevation data recorded from instrumentation on site. Results show that hydraulic conductivity is the most sensitive parameter, and that the storm event measured infiltration rate is substantially less than the measured saturated hydraulic conductivity of the soil. The governing factor affecting hydraulic conductivity and, subsequently, infiltration rate is water temperature.