The cooling potential of Rain Gardens and Green Roofs - The influence on energy partitioning in Kvillebäcken, Gothenburg

Abstract: Blue-green infrastructure (BGI) has become a popular way of combating the effects of climate change, both regarding heat mitigation and runoff from precipitation. This thesis aims to understand the potential of two BGI types, namely rain gardens and green roofs, of increasing the latent heat fraction in an urban area and thereby also understand the possible cooling effect from these measures. Rain gardens and green roofs are further compared to the effect on energy partitioning of high albedo roofs and an increased surface fraction of grass in the Kvillebäcken area in Gothenburg. The thesis was performed using an energy and water balance model called SUEWS in order to investigate heat and water fluxes. Scenarios of increased rain garden and green roof surface coverage was used to model the influence on latent and sensible heat fluxes. Results show that both rain gardens and green roofs have potential to increase latent heat and thereby reduce temperatures. However, both BGI-solutions increase latent heat during periods of frequent precipitation and not when it is needed the most, that is during heatwaves and periods of strong Urban Heat Island (UHI). Increasing grass areas have a larger effect than other BGI but is commonly outcompeted in urban areas due to lack of space, making it a less attractive option in densely built areas. During dry and hot periods, it was found that high albedo roofs have the largest potential to decrease temperatures, however through a reduction of sensible heat. The study concludes that water availability is crucial for BGI to increase latent heat, as water needs to be frequently added for maximum efficiency. To combine the varied benefits of BGI with high albedo surfaces should therefore give the largest effect on cooling the urban climate.