Green roofs (GRs) are becoming a trend in urban areas, favouring thermal performance of buildings, promoting removal of atmospheric pollutants, and acting as possible water collection spots. Rainwater harvesting systems in buildings can also contribute to the management of stormwater runoff reducing flood peaks. These technologies should be enhanced in Mediterranean countries where water scarcity is increasing and the occurrence of extreme events is becoming very significant, as a result of climate change. An extensive pilot GR with three aromatic plant species, Satureja montana, Thymus caespititius and Thymus pseudolanuginosus, designed to study several parameters affecting rainwater runoff, has been in operation for 12 months. Physico-chemical analyses of roof water runoff (turbidity, pH, conductivity, NH4(+), NO3(-), PO4(3-), chemical oxygen demand) have shown that water was of sufficient quality for non-potable uses in buildings, such as toilet flushing. An innovative approach allowed for the development of an expression to predict a 'monthly runoff coefficient' of the GR system. This parameter is essential when planning and designing GRs combined with rainwater harvesting systems in a Mediterranean climate. This study is a contribution to improving the basis for the design of rainwater harvesting systems in buildings with extensive GRs under a Mediterranean climate.

In addition to the possible contributions of buildings to mitigating CO 2 emissions, increased attention is being paid to the potential impacts of climate change on urban environments. According to the United Nations, about 54% of the planet's population currently lives in cities, but this percentage is expected to rise to 66% in 2050, which reveals the scale of this issue. This paper develops a reflection on the possible contributions of water-related building installations to mitigate emissions and increase urban area adaptation to the effects of climate change. One of the most promising solutions to facing climate change, which is analysed in detail in this paper, is combining rainwater harvesting systems with green roofs. However, in view of developing the necessary engineering projects, there are insufficient existing studies to estimate the parameters to be used in each location given their climate characteristics, particularly the monthly runoff coefficients, which constitute the key parameter for designing these installations in some regions. Some recent standards present generic theoretical values for designing these combined installations, but they are far from reality in some regions, such as the Mediterranean basin. Therefore, based on the data available in Portugal, this paper reports some of the results obtained from research on the values of the monthly runoff coefficients.

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