Development of a Stormwater Control Measure Microcosm to Measure Evapotranspiration

The use of stormwater control measures (SCMs) that seek to restore the natural hydrologic cycle has increased dramatically in recent years. These SCMs are designed and implemented based upon, and LEED credits are determined by, the potential for volume and peak flow reductions. With the increasing cost of land development, engineers often choose the most parsimonious SCM design to meet the regulatory goals and maximize profitable development. In doing so, engineers tend to use only a few of the many possible SCMs to meet stormwater goals because some measures may infiltrate more stormwater per unit area than others. The amount of infiltrated water is critical because presently volume reduction is based only on infiltration. Evapotranspiration (ET) is neglected in water balance calculations and in the crediting of SCMs; however, ET has the potential to be a substantial portion of the water budget. The research described in this paper describes the development of a methodology to quantify ET and a predictive tool for expected volume reduction from ET for SCM crediting. To quantify ET, SCM microcosms were developed for four different types of SCMs: a bioinfiltration rain garden, a bioretention rain garden, a constructed stormwater wetland, and a green roof. Each microcosm was designed to mimic an existing system that is studied on Villanova University's campus in terms of soil type, plant type, and root depth. Each microcosm is a lysimeter in which the inflow, outflow, and infiltration are carefully monitored. Once these data are known, the ET can be quantified by applying a mass balance on the system.