Evapotranspiration in Rain Gardens Using Weighing Lysimeters

Quantification of evapotranspiration (ET) and infiltration from vegetated stormwater control measures (SCMs), such as rain gardens, is necessary to properly assess their volume reduction potential. Weighing lysimeters at Villanova University mimic three rain garden designs and measure water budget parameters to determine how design elements impact ET. The designs compare two soil media with the same unconstricted drainage systems: a sandy loam and sand and two drainage systems with the same media (sand): an unconstricted valve (UV) outflow and internal water storage (IWS) outflow. A custom system distributed water over the lysimeters to mimic the runoff typically delivered to a rain garden during natural storm events. Runoff simulation trials were performed in summer and fall and compared to direct rainfall to the lysimeters during those seasons. Evapotranspiration rates were unaffected in the sandy loam lysimeter and increased in the sand lysimeters by the simulated rainfall in each system. Average ET over the entire study period was 2.9, 2.7, and $4.3\mathrm{mm}/\mathrm{d}$ for the sandy loam, sand, and sand internal water storage, respectively. Evapotranspiration made up 47, 43, and 70% of the water budget for the sandy loam, sand, and sand IWS systems, respectively. The native soil (sandy loam) did not negatively affect the rate of ET. The IWS drainage system enhanced ET over the unconstricted valve drainage system. In an effort to promote inclusion of ET into rain garden design, seasonal and monthly ET rates are provided as baseline values. The ASCE Penman-Monteith ET and Hargreaves ET equations adequately represent the unconstricted valve lysimeter systems and underrepresent the IWS lysimeter system.