Surface energy balance of an extensive green roof as quantified by full year eddy-covariance measurements

Green roofs are discussed as a promising type of green infrastructure to lower heat stress in cities. In order to enhance evaporative cooling, green roofs should ideally have similar Bowen ratio (beta = sensible heat flux/latent heat flux) characteristics such as rural sites, especially during summer periods with high air temperatures. We use the eddy-covariance (EC) method to quantify the energy balance of an 8600 m(2) extensive, non-irrigated green roof at the Berlin Brandenburg Airport, Germany over a full annual cycle. To understand the influence of water availability on green roof-atmosphere energy exchange, we studied dry and wet periods and looked into functional relationships between leaf area, volumetric water content (VWC) of the substrate, shortwave radiation and beta. The surface energy balance was dominated by turbulent heat fluxes in comparison to conductive substrate heat fluxes. The Bowen ratio was slightly below unity on average but highly variable due to ambient meteorology and substrate water availability, i.e. (beta increased to 2 in the summer season. During dry periods mean daytime 13 was beta, which is comparable to typical values of urban instead of rural sites. In contrast, mean daytime (beta was 0.3 during wet periods. Following a summer wet period the green roof maximum daily evapotranspiration (ET) was 33 mm, which is a threefold increase with respect to the mean summer ET. A multiple regression model indicated that the substrate VWC at the present site has to be >0.11 m(3) m(-3) during summer high insolation periods (>500 W m(-2)) in order to maintain favourable green roof energy partitioning, i.e. mid-day beta < 1. The microclimate benefit of urban green roofs can be significantly optimised by using sustainable irrigation approaches. (C) 2016 Elsevier B.V. All rights reserved.