Semi-intensive green roofs - a solution for negative impacts of climate change in urban landscapes

Green roofs are often proposed as climate change adaptation and environmental protection measures in urban environments. The goals are to cool down the environment, to handle the extreme precipitation events, to increase the CO2-sequestration and to enhance the biodiversity. However, the most widely used extensive roof greening is not very efficient for climate cooling, CO₂ sequestration and biodiversity. Intensive green roofs are much more efficient, but often are resource- and energy-consuming. The aim of our study was, therefore, to investigate the microclimatic effects of semi-intensive rainwater-irrigated green roof, which could be implemented with little technical and financial effort. For this purpose, an extensive green roof was equipped with solar-powered rainwater irrigation system and planted with selected plant species. The substrate used (10 cm layer) has maximum water storage capacity of vol. 40%. The roof is irrigated during the summer months with 2 l m^-2 day^-1. The effects on microclimate, water balance, particulate matter binding and biodiversity are measured continuously starting in 2020. The microclimatic effects are measured with automatic weather stations on the semi-intensive model roof and a control area (parking lot) at the heights of 2m and 0.5 m above surface. In addition, manual albedo measurements as well as the measurements with manual and drone-borne thermal cameras were carried out. The green roof albedo values vary between 0.147-0.174 and are higher than parking lot (0.139-0.145). The measurements show that the effects of semi-intensive roof on the microclimate are quite variable. The reduction of temperature extremes (up to 2°C) and especially warm nights is observed. The roof surface is particularly cooler under calm conditions, with minimal external influences. There are, however, also the well-expressed warming effects of the roof depending on the weather conditions. The effects of different plant species and substrate to total cooling or warming of green roof were also quantified.