Green roof performance monitoring: Insights on physical properties of 4 extensive green roof types after 2 years of microclimatic measurements

Green infrastructure elements like green roofs provide a range of ecosystem services to urban areas. Although shallow extensive green roofs contribute less to stormwater retention and microclimatic improvement than other green roof types, they remain the most widely used system in European market. This study aims to establish a basis for optimizing green roof designs by thoroughly examining the differences in the physical characteristics of the roofs as well as their environmental impacts and performances in relation to their properties. Two years of microclimatic monitoring on four green roofs revealed distinctly different behaviors regarding evapotranspiration, substrate moisture, substrate temperature, ground-level air temperature, and heat flux. The performances in evapotranspiration and cooling of ground-level air temperature in summer were clearly related to substrate thickness and water storage capacity of the roofs. Furthermore, differences in rainwater interception and stomatal conductance were quantified among various plant species. The results highlight that green roofs with thick substrates or large water storage capacities, combined with highly transpiring and densely growing plant species are most likely to deliver the ecosystem services needed in cities. The collected data also proofs to be essential for high-precision modeling of green roofs’ microclimatic effects in the context of urban planning.