Influence of vegetation type and climatological conditions on evapotranspiration from extensive green roofs

Evapotranspiration (ET) is the key hydrologic process governing the capacity of a green roof to retain rainfall as it regenerates available water storage space in the green roof substrate (soil) between rainfall events. In this field study, the impact of climatological conditions, vegetation type, and substrate depth on ET rates were evaluated from experimental, modular extensive green roofs installed in three climate regions in Canada: Calgary AB (Prairies), London ON (Great Lakes/St. Lawrence), and Halifax NS (Atlantic/ Maritime). Daily ET rates and cumulative ET over two field seasons were quantified for two substrate depths (10 and 15 cm), substrate only treatments (no vegetation), and four vegetation treatments (monoculture treatments of Sedum spurium, Spor-obolus heterolepis, and Aquilegia canadensis, and a mixed species treatment consisting of the three aforementioned species). 73 %, 67 % and 33 % of the cumulative rainfall was returned to the atmosphere via ET over the two field seasons in Calgary, London and Halifax, respectively. Importantly, ET rates were shown to be a strong function of substrate moisture content as well as atmospheric forcing with the relative importance of each varying between the different cities. ET rates were typically greater in the mixed species treatment in comparison to the three monoculture treatments (Sedum spurium, Sporobolus heterolepis, and Aquilegia canadensis) and for deeper substrate depths, although ET rate enhancement with substrate depth was comparatively small. This study provides valuable insights on the impact of climatological conditions and green roof design parameters (i. e., vegetation type and substrate depth) on ET rates.