(Sub-) surface hydrology of photovoltaic green roofs revisited under extreme rainfall conditions

Green roofs are particularly effective means of nature-based solutions (NBS) in urban areas: They increase evapotranspiration at the expense of surface runoff and thus effectively reduce heat islands. Thus, they provide important ecosystem services, similar to other NBS. This way, green roofs are a key feature in transforming grey to green cities in the light of climate change adaptation. However, there is an urgent need for climate change mitigation efforts, requesting a transformation of energy production, which also suggests a massive increase of rooftop photovoltaic installations. The last decade has seen numerous studies demonstrating the successful combination of both green roofs and rooftop photovoltaic installations. However, in many cases preference is given to either green roofs or rooftop photovoltaic installations. One question that has been little studied so far, is how the runoff coefficient changes due to solar panels, as they spatially redistribute rainfall. Therefore, we scrutinize the impact of solar panels on (sub-) surface hydrology under extreme rainfall conditions in laboratory experiments and a subsequent numerical modelling study. Finally, some recommendations are given on suitable green roof geometries – including rooftop photovoltaic installations – that still have particularly high retention effects.