Understanding the Role of Evapotranspiration in Bioretention: Mesocosm Study

Abstract

Many regions are turning toward sustainable green infrastructure practices, such as bioretention to mitigate urban stormwater runoff. Currently, the designed volume control in bioretention stormwater control measures (SCMs) is attributed to infiltration, although evapotranspiration (ET) can be considered a viable mechanism. From a water cycle perspective, reduced ET in urban spaces is a primary source of the increased stormwater runoff volume. This research demonstrates that ET is a substantial water balance component of two differently configured bioretention mesocosm systems. Over a two-year span of time, a freely draining bioretention mesocosm (lysimeter) converted 50% of the direct rain falling on the mesocosm to ET, contrasted by a bioretention mesocosm (lysimeter) with an internal water storage (IWS) layer that converted 78% of the direct rainfall to ET. The measured daily average ET was 6.1 mm/d for the IWS lysimeter and 3.1 mm/d for the freely draining lysimeter from April to November in 2010 and 2011. A primary factor in this study is the availability of water held within the lysimeters between storm events. The IWS system has a reserve of water inherently that promotes more and prolonged amounts of ET over the freely draining system. The measured ET is compared to several potential ET equations in common use, and no one equation is a fit.