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

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.