Flow capture efficiency and average detention time are the performance measures commonly used in assessing the long-term pollutant removal effectiveness of storm water detention ponds. A statistical formulation is presented for estimating these two performance measures for typical detention ponds where outflow is controlled by an orifice or weir type structure. The flow capture efficiency is determined with the estimation of the total spill volume. The total spill volume is calculated as the combination of the event spill volume and the carryover spill volume. Thus the carryover effect of consecutive runoff events is quantified. A closed-form analytical expression is derived for estimating the average volume-weighted detention time, taking into account the variable inflow and outflow rates and the random spacing between runoff events. Analytical determinations of the average detention time are confirmed by continuous simulation modeling. Statistical solutions of flow capture efficiency closely resemble those obtained from continuous simulation models. The statistical models presented, and the insights gained from their use, can be applied in the design or evaluation of detention ponds for storm water quality control. ¿ 1999 American Geophysical Union