The evaporation efficiency of a passive capillary irrigated green roof system: a case study in Amsterdam, the Netherlands

Abstract

Green roofs are promoted as a climate adaptation measure to lower air temperatures and improve comfort in urban areas, especially during intensive dry and warm spells. However, there is much debate on the effectiveness of this measure. The cooling effect of a green roof is directly related to evapotranspiration and therefore to the availability of water in the system. When water runs short, evapotranspiration will decrease and the cooling effect might become negligible. To gain insight in the effect of water availability on evapotranspiration and the energy balance, we compared the actual evaporation rate and latent heat flux of a conventional green roof with that of so called blue-green roofs equipped with a passive capillary irrigation system. With on-site climatic measurements, the potential evaporation was determined by parameterizing the Penman-Monteith equation. The potential evaporation was afterwards compared with the actual evaporation of weighing lysimeters to evaluate the efficiency of the different roofs. Also, climatic data from the “The Royal Netherlands Meteorological Institute” (KNMI) was used to determine the hydrological behaviour of the green roof for an extended period. The blue-green roofs have a larger evaporation efficiency during dry periods. The storage of water and capillary irrigation of these systems maintain high ratios of the latent heat flux during dry periods, which contribute to the cooling effect. In comparison, conventional green roofs were found to contribute more to an increase of the air temperature, due to a larger sensible heat flux than the blue-green roofs. It is found that the latent heat flux from the conventional green roof consumes only 46% of the net radiation energy during daytime, while the blue-green roof consumes almost 80%. The blue-green roof, with storage and capillary irrigation, is more efficient than the conventional green roof and changes the microclimate of the green roof significantly. We therefore conclude that water availability is crucial for green roofs to be an effective climate adaptation measure.