The Impact of Biofiltration Systems, Rainwater Tanks and Urban Irrigation on Urban Canopy Evapotranspiration

Urban vegetation is generally considered as a key tool to modify the urban energy balance through enhanced evapotranspiration. Yet, vegetation is most effective when its healthy so that one should consider water sensitive urban design (WSUD) strategies to retain water in the urban landscape through stormwater harvesting and fit-for-purpose reuse. This study presents the implementation of a vegetated lined biofiltration pit combined with a rainwater tank and urban irrigation system in the single-layer urban canopy model Community Land Model - Urban (CLM-U). Runoff from roof and impervious road surface fractions is harvested and used to support an adequate soil moisture level for vegetation in the biofiltration pits.

In a first stage, modelled soil moisture dynamics are evaluated and found reliable based on observed soil moisture levels from biofiltration pits in Smith Street, Melbourne (Australia). Secondly, changes in total urban canyon evapotranspiration are assessed for anumber of scenarios and compared to a base-case scenario without biofiltration system and urban irrigation. From an evapotranspiration (ET) point of view, soil texture of the filter media and cover fraction of the biofiltration system significantly increase ET up to 17 mm compared to a total of 11.7 mm for the base-case scenario. When the irrigation system is enabled ET further increases up to 22.6 mm, which is almost a doubling compared to the 100 % impervious base-case scenario. In case sufficient households contribute to a large enough rainwater tank, the water volume stored in the latter is able to sustain an optimal soil moisture content throughout the period considered here. Overall, the methodology presented in this study can effectuate future research with state-of-the-art urban climate models to further explore the benefits of vegetated biofiltration systems as a water sensitive ur29 ban design tool optimised with an urban irrigation system to maintain a healthy vegetation.