Hydrological and thermal regime of a thin green roof system evaluated by physically-based model

Abstract

Green roofs, as an element of the green infrastructure, contribute to the urban heat island effect mitigation and the urban drainage outflow reduction. To achieve the desired functions, it is essential to understand the role of the individual roof layers and ensure their proper design.A physically-based model was used to assess the hydrological and thermal regime of two experimental green roof test beds containing distinct soil substrates (a local Technosol and a more permeable commercial substrate “Optigreen”). The test beds together with a meteorological station were built on the building green roof. Each test bed has an effective area of one square meter and is equipped with a soil temperature sensor and an outflow gauge; one of the test beds is continuously weighed. The observed conditions were simulated using one-dimensional numerical model describing the water flow in variably saturated porous medium by Richards’ equation and the heat transport by the advection-conduction equation.The model was able to satisfactorily reproduce the measured outflow and soil temperature. The water-potential-gradient based root water uptake module effectively captured the water storage depletion between the rainfall events. The difference between the two soil substrates tested is demonstrated by the contrasting ability of the soil layers to retain water. Model representation of the thermal conditions within the green roof soils was achieved using independently evaluated thermal properties of the soils and drainage board. The model was also used to analyze the effects of the substrate depth and type of vegetation cover on the transpiration and soil water regime of the green roofs. Increasing the substrate depth causes a rise of root water uptake and induces a significant reduction of the maximal temperature. The thinner soil profiles are more sensitive to the plant species selection.