This paper outlines the development of a conceptual hydrological flux model for the long term continuous simulation of runoff and drought risk for green roof systems. A green roof s retention capacity depends upon its physical configuration, but it is also strongly influenced by local climatic controls, including the rainfall characteristics and the restoration of retention capacity associated with evapotranspiration during dry weather periods. The model includes a function that links evapotranspiration rates to substrate moisture content, and is validated against observed runoff data.to typical extensive green roof configurations is demonstrated with reference to four UK locations characterised by contrasting climatic regimes, using 30-year rainfall time-series inputs at hourly simulation time steps. It is shown that retention performance is dependent upon local climatic conditions. Volumetric retention ranges from 0.19 (cool, wet climate) to 0.59 (warm, dry climate). Per event retention is also considered, and it is demonstrated that retention performance decreases significantly when high return period events are considered in isolation. For example, in Sheffield the median per-event retention is 1.00 (many small events), but the median retention for events exceeding a 1 in 1 yr return period threshold is only 0.10. The simulation tool also provides useful information about the likelihood of drought periods, for which irrigation may be required. A sensitivity study suggests that green roofs with reduced moisture-holding capacity and/or low evapotranspiration rates will tend to offer reduced levels of retention, whilst high moisture-holding capacity and low evapotranspiration rates offer the strongest drought resistance.

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Evapotranspiration (ET) is a key parameter that influences the stormwater retention capacity, and thus the hydrological performance, of green roofs. This paper investigates how the moisture content in extensive green roofs varies during dry periods due to evapotranspiration. The study is supported by 29 months continuous field monitoring of the moisture content within four green roof test beds. The beds incorporated three different substrates, with three being vegetated with sedum and one left unvegetated. Water content reflectometers were located at three different soil depths to measure the soil moisture profile and to record temporal changes in moisture content at a five-minute resolution. The moisture content vertical profiles varied consistently, with slightly elevated moisture content levels being recorded at the deepest substrate layer in the vegetated systems. Daily moisture loss rates were influenced by both temperature and moisture content, with reduced moisture loss/evapotranspiration when the soil moisture was restricted. The presence of vegetation

12The extent to which the finite hydrological capacity of a green roof is available for retention of 13 a storm event largely determines the scale of its contribution as a Sustainable Drainage System 14 (SuDS). Evapotranspiration (ET) regenerates the retention capacity at a rate that is variably 15 influenced by climate, vegetation treatment, soil and residual moisture content. Experimental 16 studies have been undertaken to monitor the drying cycle behaviour of 9 different extensive 17 green roof configurations with 80 mm substrate depth. A climate-controlled chamber at the 18 University of Sheffield replicated typical UK spring and summer diurnal cycles. The mass of 19 each microcosm, initially at field capacity, was continuously recorded, with changes inferred 20 2 to be moisture loss/gain (or ET/dew). The ranges of cumulative ET following a 28 day dry 1 weather period (ADWP) were 0.6-1.0 mm/day in spring and 0.7-1.25 mm/day in summer. 2These ranges reflect the influence of configuration on ET. Cumulative ET was highest from 3 substrates with the greatest storage capacity. Significant differences in ET existed between 4 vegetated and non-vegetated configurations. Initially, seasonal mean ET was affected by 5 climate. Losses were 2.0 mm/day in spring and 3.4 mm/day in summer. However, moisture 6 availability constrained ET, which fell to 1.4 mm/day then 1.0 mm/day (with an ADWP of 7 7 and 14 days) in spring; compared to 1.0 mm/day and 0.5 mm/day in summer. A modelling 8 approach, which factors potential evapotranspiration (PET) according to stored moisture 9 content, predicts daily ET with very good accuracy (PBIAS = 2.0% [spring]; -0.8% [summer]). 10