Water and thermal regime of extensive green roof test beds planted with sedum cuttings and sedum carpets

Implementation of green roofs could help to reduce rapid runoff and help cities to mitigate heat islands. The aim of the study is to assess the water and temperature regimes of four experimental green roof test beds having different growing media and plant coverage during the vegetation season 2018. Experiments were conducted in four test beds (1 × 1 m) established on a flat roof. Two types of growing media were used. The first (A) was a substrate composed of crushed spongolite, crushed expanded clay, and peat. The second (B) was a coarser substrate, composed of crushed expanded clay, crushed bricks, peat, and compost. Two test beds, hereafter designated ACu and BCu, were filled with substrates A and B respectively and planted with a mixture of Sedum spp. cuttings with approximately 10% coverage. The substrate thickness was 6 cm. Two other test beds, designated ACa and BCa, were filled to a depth of 4 cm with A and B growing media, respectively, and planted with a carpet of Sedum spp. with approximate coverage of 100%. The experiment was conducted over one growing season. Continuous monitoring of substrate temperature, water content, and outflow was conducted on each test bed. The lowest runoff coefficient was observed in test bed ACu, while the highest runoff occurred in test bed BCu, with twice the amount of outflow as ACu. The total runoff coefficient of ACa was more than one-third higher than that of ACu. The lowest maximum substrate temperature on the hottest day of the season was observed in bed ACa with a temperature of 40.6 °C, while the highest temperature was seen in bed BCu, 7.9 °C higher. The analysis of the rainfall-runoff relationship calculated for individual rainfall events demonstrated that runoff coefficients depended on initial water content, rainfall intensity, rainfall depth, substrate type, and vegetation cover. Beds planted with sedum carpets and having more extensive vegetation coverage were superior at moderating extremes of temperature.