AbstractQuantifying evapotranspiration (ET) and infiltration from vegetated stormwater control measures (SCMs), such as rain gardens, is necessary to physically represent their volume reduction potential. Most states and regulatory entities utilize a design storm for rain garden design in which the rain garden’s capacity is considered using a static volume contribution. The static storage volume approach excludes the dynamic functions of infiltration during an event and ET between events. This work seeks to provide a method to incorporate the function of ET during interevent times into a design storm approach. The suggested method to do this is to estimate the void space recovery due to both ET and gravity drainage. An example is used to demonstrate the method for rain gardens in Pennsylvania where the void space recovery was estimated for 6 and 12 days between events. The void space recovery was estimated using a mathematical model based upon the 1D Richards equation coupled with the ASCE Penman-Monteith model. The mathematical model was validated using data from eight rain garden lysimeters in Villanova, Pennsylvania. This location is in the mid-Atlantic region with a Cfa climate in the Koppen-Geiger classification system. This void space recovery ranged from 15% to 40% (for both ET and gravity drainage) depending on soil type, drainage, rooting depth, crop coefficient and days between events. This void space recovery can be used to calculate a semidynamic recovery based on expected performance using the commonly employed design storm approach.