Moisture content behaviour in extensive green roofs during dry periods: the influence of vegetation and substrate characteristics

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 resulted in higher daily moisture loss. Finally, it is demonstrated that the observed moisture content data can be accurately simulated using a hydrologic model based on water balance and two conventional Potential ET models (Hargreaves and FAO56 Penman–Monteith) combined with a soil moisture extraction function. Configuration-specific correction factors have been proposed to account for differences between green roof systems and standard reference crops

Effects of recycled aggregate growth substrate on green roof vegetation development: a six year experiment

Green roofs have the potential to address several of the environmental problems associated with urbanisation, and can be used as mitigation for habitats lost at ground level. Brown roofs (a type of green roof) can be used to mitigate for the loss of brownfield habitat, but the best way of designing these habitats remains unclear. This paper reports an experiment to test the effects of different types of recycled aggregate on the development of vegetation assemblages on brown roof mesocosms. Five recycled aggregates were tested: (1) crushed brick, (2) crushed demolition aggregate, (3) solid municipal waste incinerator bottom ash aggregate, (4) a 1:1 mix of 1 and 2, and (5) a 1:1 mix of 3 and 2. Each was seeded with a wildflower mix that also included some Sedum acre and vegetation development was studied over a six-year period. Species richness, assemblage character, number of plants able to seed, and plant biomass were measured. Drought disturbance was the key factor controlling changes in plant assemblage, but effects varied with substrate treatment. All treatments supported a similar plant biomass, but treatments with a high proportion of crushed brick in the growth substrate supported richer assemblages, with more species able to seed, and a smaller amount of Sedum acre. Crushed brick, or recycled aggregates with a high proportion of crushed brick, are recommended as good growth substrate materials for encouraging brown roof plant diversity. This investigation demonstrates the importance of multi-year studies of green roof development for the generation of robust findings

The Soil-Root Strength Performance of Alternanthera Ficoidea

The rise of awareness on environmentalism has demanded that all parties involved in built environment to implement green technology in their construction projects. Great care must be taken when designing a green roof system including the selection of plants and appropriate substrates. This study was performed to investigate the soil-root composite strength of two types of green roof vegetation (A. Ficoidea and Z. Japonica) at different growth periods for up to 6 months. Both plants were planted in six plastic plots (45 cm × 29 cm × 13 cm) containing a mixture of perlite, vermiculite and organic soil. Every two months, a series of direct shear tests were conducted on a sample from each species to determine the root-soil shear strength. The tests continued until the 6th month. The average results showed that Z. Japonica had higher soil-root shear strength (49.1 kPa) compared to A. Ficoidea after two months of growth. In the 4th month however, A. Ficoidea managed to surpass Z. Japonica (28.7 kPa versus 18.5 kPa) in terms of shear strength. However, their average peak shear strength decreased sharply compared to the previous month. Lastly, in six months, A. Ficoidea sustained a higher average peak soil shear strength (56.5 kPa) compared to Z. Japonica (14.3 kPa). Therefore, it can be concluded that A. Ficoidea may offer a better soil reinforcement than Z. japonica and thus it could potentially be a good choice of green roof vegetation

Strawberry fields forever? Urban agriculture in developed countries: a review

International audienceFood production in cities has long been a tradition in many countries around the world and a mainstream activity for many developed countries. While urban agriculture plays an important role in increasing food security and social well-being, it comes with significant costs and constraints. Here, we review the growth of urban agriculture throughout the developed world in order to clarify the different benefits, risks, and hindrances associated with the practice. Through this analysis, we identify the need for better understanding of the following five aspects if urban agriculture is to make a meaningful contribution to food security and social well-being in the future: (1) the impacts of continued urban sprawl and loss of peri-urban agricultural land; (2) appropriate government and institutional support at local, regional, and country levels; (3) the role of urban agriculture in self-sufficiency of cities; (4) the risks posed by pollutants from agriculture to urban ecosystems and from urban ecosystems to agriculture; and (5) the carbon footprint of urban agriculture and use of “food miles.” If urban agriculture is to have a legitimate place in resolving the global food crisis as advocates claim, then it is time to take urban agriculture seriously and assess more rigorously both the positive and negative impacts, especially carbon emissions. Only then can the world’s limited resources be properly allocated to the development of urban agriculture