The Effects Of Vegetation Type And Substrate Depth On Rain Retention Performance Of Green Roof

Green roofs have been reported to significantly contribute to alleviation of heat island effect, increase of urban green space, energy saving from heat isolation by plant covers, pollution abatement and noise reduction. Moreover, green roofs capability to manage stormwater runoff, make it a hot issue in the research circles. To explore the effects of vegetation type and substrate depth on performance of rain retention by green roof, simulated expeimrnt was conducted in Athens, Greece. Festuca arundinacea Shreb, Origanum onites L. and Sedum sediforme (Lacq.) Pau. coupling with 8cm and 16cm substrate depths were selected as treatment groups. The results are concluded as follows:(1) With irrigation in early summer, vegetation types and the interaction effects from vegetation type and substrate depth have significant influence on water absorption rate. Treatments vegetated with Origanum and Festuca absorbed 92.3%,95.1% water, which was greater than Sedum group and control. Among which, Origanum*:16cm and Festuca* 16cm absobed as much water as 98.5% and 95.8%, respectively. On the contrary, Sedum*16cm only absorbed 54.8% water, even lower than Sedum*8cm group.(2) The effects of vegetation types on water absorption were affected by season. In summer, substrate water consumed by Origanum and Festuca were much quicker than by Sedum, which lead to greater absorption. However, when temperature dropped down, the absorption performance of Festuca group sharply dropped. While the Sedum group remained its absorption capacity, only slightly lower than Origanum group.(3) With non-irrigation in winter and spring, vegetation types, substrate depths and their interaction effects have significant influence on rain retention percentage. Origanum group had 77.3% rain retention, which was the greatest. Sedum group retained 66.2% rain water, which was greater than Festuca group and the control. In addition, deeper substrate resulted in greater rain retention performance.16cm group retained 65.3% rain water, which was 8.7% greater than 8cm group. Among different treatment, the retention percentage of Origanum**6cm reached 87.2%, which was significantly greater than Origanum*8cm (67.4%), and the retention percentage of Sedum*16cm reached 74.7%, which was significantly greater than Sedum*8cm (57.8%). However, the situation for Festuca was the exact opposite.54.8% retained by Festuca*8cm was greater than 44.9% of Festuca* 16cm.(4) In the 25.2mm individual rain event in spring, green roof vegetated with plant greatly reduced the amount of runoff. Origanum*16cm and Sedum*16cm had best retention performance, only produced 5.72mm and 8.18mm runoff and delayed initial runoff 491mins and 465mins. Besides, deeper substrate lead to longer delay of initial runoff,16cm group delayed initial runoff for 433.7min, which was 73.0min greater than 8cm group.(5) Amount of precipitation had significant influence on the rain retention performance of green roof treatments. Vegetation types, substrate depths and their interaction effects have significant influence on individual rain retention percentage. As precipitation increased from 15.5mm to 25.2mm, the difference of rain retention percentage of Origanum group and Sedum group only had 34.5% and 38.7%, which means their great management on runoff. Among different treatments, the difference of rain retention percentage of Origanum*16cm and Sedum*16cm only had 22.7% and 31.9%, which were the best.Conclusively, Sedum sediforme (Lacq.) Pau., which has great drought tolerance and can retain a great percent of rainfall during winter time, is an ideal choice for Mediterranean area, where summer is hot but dry, winter is mild and humid. Origanum onites L. has outstanding rainfall retention performance during the whole year, however, its excessive high water consumption during summer limits it be used only in site where irrigation is available or in area which is rainy all year round. Festuca arundinacea Shreb has high evaportranspiration which leads to fast water consumption during summer, but low rainfall retention in winter due to its weak plant vigor. In addition, deeper substrate resulted in greater rain retention performance, however, there is no linear correlation between these two variants.