The influence of spatial arrangement and site conditions on the fate of infiltrated stormwater

Urbanization inevitably involves the replacement of native soils with impervious areas. Doing so reduces infiltration and thus the contribution of groundwater to stream baseflows. The use of infiltration-based Stormwater Control Measures (SCMs) is increasingly common to restore lost baseflows in urban areas, although there remains considerable uncertainty regarding the optimal arrangement of such measures in the landscape, along with the influence of site conditions on the path and fate of infiltrated stormwater. This study aimed to investigate how site conditions and arrangement of infiltration-based SCMs influence the water cycle and stream baseflows. To undertake this study, we used a modelling approach which combined the outputs from a stormwater model (MUSIC) with a 3D groundwater flow model (MIKE SHE). We simulated a range of plausible site conditions and spatial arrangements of SCMs for a hypothetical hillslope situated in two different locations in Australia-Gold Coast (sub-tropical climate) and Melbourne (temperate climate). The modelling results predicted that placing the SCMs in a centralized way at the bottom of the hillslope (End of the pipe approach) could deliver more stream baseflows compared with a distributed network of systems. Placing the SCMs in a distributed manner throughout the landscape was predicted to deliver local benefits such as enhanced evapotranspiration and augmented soil moisture. In addition, we found that soil hydraulic conductivity played a major role in the fate of infiltrated stormwater for all scenarios. Our results highlight the importance of placing infiltration-based SCMs appropriately in the landscape if the goal is baseflow restoration. They also point to the importance of understanding local site conditions before designing and siting stormwater infiltration systems. A mix of different SCM strategies (e.g. rainwater tanks with a dedicated baseflow outlet) will be required for sites with very low hydraulic conductivities.