Laboratory Tests of Substrate Physical Properties May Not Represent the Retention Capacity of Green Roof Substrates In Situ

Abstract

Green roofs can be used to reduce the volume of polluted stormwater that is generated by cities. Modelling rainfall retention is critical, but green roof water balance models often rely on the physical properties of substrates. In these models, substrate water holding capacity (WHC) determines the depth of water which can be stored before runoff is generated; whereas, the permanent wilting point (PWP) limits evapotranspiration. The WHC and PWP, as well as plant available water (PAW; where PAW = WHC − PWP), as determined from laboratory tests, may not truly reflect how substrates perform on green roofs. We therefore ran a simulated rainfall experiment on green roof modules to (i) compare the rainfall retention of vegetated and non-vegetated substrates with different WHC and PAW, and (ii) relate retention to substrate storage capacity, as calculated from laboratory measures of WHC and PAW. We found that the PAW of a substrate is a better indicator of evapotranspiration and retention when compared with WHC. However, we also found that substrates always retained less water than their calculated storage capacity would suggest, most likely being due to their high permeability. Our results indicate that using laboratory-derived measures of WHC and PAW in green roof models may be over-estimating both evapotranspiration and rainfall retention.

Water permeability, water retention capacity, and thermal resistance of green roof layers made with recycled and artificial aggregates

“…As a sustainable ecosystem system, the green roof is known for its ability to provide thermal resistance for rooftops in some cases and buffer the surface stormwater runoff in urban areas [11][12][13][14][15][16][17][18]. The shape and type of materials used in green roof drainage and substrate layers significantly impact energy efficiency and rooftop water evacuation [19][20][21][22].…”

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“…[49] Other parameters that play a determining role in the hydrological performance of green roofs are the vegetation characteristics and the physical properties of the layers' components. [50,51] The remaining water that escapes green roofs during intensive precipitation events can lead to leakage of nutrients and metals and thus may adversely affect the quality of downstream water bodies, especially in the case of recently applied constructions. [52] This can be mainly attributed to the high organic matter content of the green roof's soil substrate, which leads to a more brownish colour, dissolved salts and higher nutrient concentrations like phosphorous in the removed water.…”

Soluções baseadas na Natureza para gestão de águas pluviais em São Paulo: desempenho hidrológico e dinâmica da plantação em tetos vegetados com diferentes profundidades de substrato.

“…The plant layer on a vegetated roof can have great influence on its rainfall-runoff management properties as indicated by the international literature (SOULIS et al, 2017;SZOTA et al, 2017). Previous studies analyse this influence and point out that interception can be a great cause of these contributions (STOVIN; VESUVIANO; KASMIN, 2012) and also the evapotranspiration rates that will be further discussed.…”