Media Moisture Content to Determine Evapotranspiration from Swales and Bioretention Cells

Average 24-hr dry weather ET was determined for a bioretention cell and a grassed swale near Auckland, New Zealand in the winter, spring, and early summer. As both systems were lined and monitored for outflow, soil moisture content measured during dry periods could be used to cost-effectively determine average 24-hr ET. Grassed swale ET ranged from < 0.5 mm/24-hr in winter to ∼ 1 mm/24-hr in early summer. Bioretention cell ET was greater, at 1–3 mm/24-hr in winter and 2–3 mm/24-hr in summer. Published climate statistics significantly overestimate grassed swale ET, but are relatively comparable for the bioretention cell. Continuous simulation using a water balance approach would not be suitable for the grassed swale using published data. Grassed swale ET is likely affected by its microclimate more substantially than a bioretention cell, as the former is also influenced by limited moisture storage capacity. Additional work is required to expand the limited data set and investigate interactions of climate parameters' influence on ET from LID devices.