The impact of vegetation and media on evapotranspiration in bioretention systems

Bioretention systems have known benefits for managing urban stormwater, but there remain knowledge gaps about evapotranspiration (ET) and its role in these systems. This paper investigated how design parameters including growing media and vegetation, as well as climatic variables, can influence ET in bioretention systems. To this end, twenty-four bioretention mesocosms constructed using three media types (i.e., two sandy media types and clay loam mixed with wood chips) and planted with three vegetation types (i.e., herbaceous mixture, woody mixture, and turfgrass as control) in Okotoks, Alberta, Canada were monitored during the growing seasons (from May to October) between 2018 and 2020. The media moisture in the mesocosms was monitored at the depths of 20 and 40 cm to study how the design parameters and their interactions could influence ET. The results confirmed the roles of design and climatic variables on ET, while their effects were more prominent at the surface layer. The sandy media with the low organic matter (SD1) and the woody vegetation appeared to outperform the other media and vegetation types in promoting ET. The findings demonstrated the non-stationary nature of the ET function in the mesocosms. The effects of the design variables, in particular the vegetation, became more prominent over time. In addition, the impact of media-vegetation interactions on ET was identified. The results suggest the need for optimizing bioretention systems with consideration to the design variables, whose roles on ET are time- and depth-variant, to promote ET and, in turn, bioretention performance.