Green roofs reduce runoff from impervious surfaces in urban development. This paper reviews the technical literature on green roof hydrology. Laboratory experiments and field measurements have shown that green roofs can reduce stormwater runoff volume by 30 to 86%, reduce peak flow rate by 22 to 93% and delay the peak flow by 0 to 30 min and thereby decrease pollution, flooding and erosion during precipitation events. However, the effectiveness can vary substantially due to design characteristics making performance predictions difficult. Evaluation of the most recently published study findings indicates that the major factors affecting green roof hydrology are precipitation volume, precipitation dynamics, antecedent conditions, growth medium, plant species, and roof slope. This paper also evaluates the computer models commonly used to simulate hydrologic processes for green roofs, including stormwater management model, soil water atmosphere and plant, SWMS-2D, HYDRUS, and other models that are shown to be effective for predicting precipitation response and economic benefits. The review findings indicate that green roofs are effective for reduction of runoff volume and peak flow, and delay of peak flow, however, no tool or model is available to predict expected performance for any given anticipated system based on design parameters that directly affect green roof hydrology.

International audienceGreen roofs recover green spaces in urban areas and benefit the public, farmers, and wildlife by providing many environmental, ecological, and economic advantages. Green roofs reduce stormwater runoff, mitigate urban heat island effects, absorb dust and smog, sequester carbon dioxide, produce oxygen, create space for food production, and provide natural habitat for animals and plants. Here, we studied the environmental impact of green roofs in terms of runoff quality and greenhouse gas CO2 sequestration. We screened more than 650 scientific papers and we reviewed detailed findings from 52 publications. There are two major points: (1) Concerning pollution, the concentrations of minor pollutants, such as heavy metals, biochemical oxygen demand (BOD), total suspended solids (TSS), and turbidity, are small and thus do not pose an immediate threat to the environment. However, the concentrations of major pollutants, such as nitrogen of 0.49–9.01 mg/l and phosphorus of 0.04–25 mg/l, vary highly for different green roofs and can adversely affect runoff quality. Nutrient leaching may be controllable through proper mitigation measures including better design and system management which require further research. According to both laboratory experiments and field monitoring data, the main factors affecting runoff quality are precipitation properties, growth media composition and depth, plant species, and maintenance protocols. Research gaps exist in quantifying how these factors affect leachate pollutant load. Systematic studies are needed for improving green roof designs to reduce adverse impacts. (2) Concerning CO2 sequestration, studies reveal that green roofs directly sequester substantial amounts of carbon in plants and soils through photosynthesis. Green roofs reduce ambient CO2 concentrations in the vicinities. Green roofs also indirectly reduce CO2 releases from power plants and furnaces by reducing demand for heating and cooling, suggesting long-term economic and environmental benefits of green roofs

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A relationship developed in a companion paper to estimate emissions from surface-aerated reactors that accounts for both gas-and liquid-phase masstransfer resistances is extended to reactors with diffused-aeration systems. The method accurately predicts the observed stripping rate of 20 volatile organic compounds (VOCs) with a wide range of Henry's coefficients over a wide range of hydrodynamic conditions. The degree of gas saturation and the liquid-and gasphase mass-transfer coefficients are estimated.

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