Assessing the impact of evapotranspiration from green roofs on reducing surface temperatures

Abstract

Evapotranspiration is an important process in green roofs to reduce surface temperature; however, it is still not well understood. This study aims to assess the influence of irrigation and vegetation parameters in the evapotranspiration of green roofs and surface temperature. Green roofs with almost no vegetation, short Sedum and tall Sedum with 0, 3 and 6 mm irrigation were considered in Lisbon (Portugal) and Florianópolis (Brazil) climates. The outside roof surface temperature was experimentally assessed in an extensive green roof prototype and used to validate the computer simulation. The results showed that although irrigation can enhance evapotranspiration, mainly in drier regions, denser vegetation has a higher impact on this process. The annual latent heat losses of the tall Sedum green roof overcome those of roofs with almost no vegetation by 695.9 kWh/m2 in Lisbon and by 471.8 kWh/m2 in Florianópolis. Green roofs with tall Sedum also resulted in lower substrate temperatures, even without irrigation. The hourly substrate temperature of green roofs with tall Sedum can be lower than the outside surface temperature of traditional roofs by up to 33.4 °C in Lisbon and up to 24.0 °C in Florianópolis. In these roofs, the latent heat fluxes correspond to 40 % and 48 % of the total losses in Lisbon and Florianópolis, respectively. It was concluded that latent heat losses accounted for a higher percentage of the energy balance of the green roofs whose reductions in surface temperature were more significant. This research provides valuable insights regarding the relationship between evapotranspiration and surface temperature and the role of evapotranspiration on the energy balance of green roofs with crassulacean acid metabolism (CAM) plants.