Quantitative Analysis of the Water Budget of a Rain Garden in Pennsylvania

Stormwater control measures (SCMs), such as rain gardens, aim to restore the hydrologic cycle impacted by development. Development increases runoff and reduces infiltration, groundwater replenishment, and evapotranspiration (ET). Unlined rain gardens designed to infiltrate are one commonly used type of SCM. Water exits unlined rain gardens via three possible mechanisms: an overflow pipe, infiltration, or evaporation. The infiltrated water can replenish the groundwater table (recharge), transpire through the vegetation, or become baseflow in our streams. The percentage of water going to each of these mechanisms is not measured easily, and often is estimated. In addition, evaporation and transpiration often are combined into one term, evapotranspiration. This study analyzed the water budget of an unlined rain garden on Villanova University’s campus in southeastern Pennsylvania. At this site, the inflow was estimated using a calibrated mathematical model, the overflow was measured, and the water reaching the groundwater table was calculated using data from monitoring wells surrounding the rain garden. These values were used to estimate the percentage of water removed by evapotranspiration, which was the only unknown in the water budget. This analysis revealed that for storms less than 18 mm (0.71 in.) almost all the water that entered the rain garden was evapotranspired, and for storms larger than 18 mm (0.71 in.) and less than 57.2 mm (2.25 in.) that did not overflow, approximately 60% of the inflow was evapotranspired. These findings align with previous studies of evapotranspiration in this region, and indicate that ET is a significant portion of the hydrologic budget.