Parameters influencing the regeneration of a green roof’s retention capacity via evapotranspiration

Abstract

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

Observation and Estimation of Evapotranspiration from an Irrigated Green Roof in a Rain-Scarce Environment

“…The maximums of the observed monthly ET averages of the sedum (4.07 mm d −1 ) and grass (4.93 mm d −1 ) are higher than the maximum daily rate of 3.4 mm/d from the grass and sedum covers after a 7-day dry period measured in a climate-controlled chamber [29]. Yet, the observed ET rates during the summer with irrigation applied are not always significantly larger than the reported values in the rain-rich environments.…”

“…For the grass cover, the VIP scores for air temperature, solar radiation, surface moisture, and relative humidity are 1.48, 1.33, 1.07, and 1.01, respectively. dry period measured in a climate-controlled chamber [29] [86]. There is no consistent relationship between the observed summer ET rates and the reported values.…”

“…Compared to the nonvegetated roofs, vegetation could elevate ET rates by over 25% on average. The observed ET rates of sedums and grass tend to be slightly higher than most studies conducted in rain-rich environments [23,29,33,[85][86][87]. For example, Rezaei [85] observed ET rates of 1.68 mm/d for sedums and 1.06 mm/d for the non-vegetated medium with unlimited water supply in an indoor environment.…”

“…The field capacity (0.35) was determined via lab tests by oven-drying soil samples. There is no established way to measure the wilting point directly other than visual observation, which, however, is difficult to determine and could be inconsistent between observers and cases [29,33]. As no visual wilting was clearly observed, the wilting point of this study was approximated based on the following empirical equations [67,68]:…”

Detention and Retention Behavior of Four Extensive Green Roofs in Three Nordic Climate Zones

“…Observed retention differences between the locations were most likely explained by variations in precipitation amounts, precipitation pattern and AET [38], but also local conditions like roof configuration and vegetation development might have influenced the results. AET in green roofs was found to increase with increasing soil moisture [54]. Higher precipitation in BER would lead to a generally higher soil moisture content, which would increase the observed AET and could be part of the explanation for higher absolute retention in BER.…”