How does root oriented preferential flow impact rain garden hydrology?

With increasing pressures from climate change and urban expansion, the development of resilient “sponge cities” is essential to mitigate flooding and reduce pollution. Rain gardens represent a key green infrastructure intervention and have the potential to be implemented far more widely in new developments or retrofitted into existing ones. Rain gardens are particularly appealing to urban planners because they can deliver multiple co-benefits by enhancing biodiversity and amenity while achieving water management objectives. However, gaps in the understanding of rain garden hydrology remain a barrier to widespread adoption. In contrast to grey infrastructure, which is supported by extensive empirical research, confidence in the hydraulic performance of vegetated systems remains limited. To embed rain gardens more effectively in urban design, their hydrological functioning must be quantified more accurately and design parameters refined. 

A major source of uncertainty lies in the behaviour of rooted soils. Recent studies highlight that root-oriented preferential flow can substantially increase soil hydraulic conductivity, reduce surface runoff and prevent sediment from clogging drainage structures. Plant roots may also improve soil water retention, enhance rainfall interception, attenuate peak flow and support pollutant removal. Yet despite this growing awareness, these mechanisms remain poorly quantified and are rarely represented in models of green infrastructure. As a result, current engineering design typically relies only on physical soil parameters, without accounting for dynamic plant–soil interactions. 

This study investigates the influence of root-oriented preferential flow on rain garden hydrology through a mixed-methods approach combining laboratory experimentation, field observation and mathematical modelling. The first phase involves single-plant mesocosms in a three-year longitudinal laboratory study of rooted soil hydrology, complemented by regular MRI imaging to capture root architecture development. This study presents initial findings from this longitudinal experiment, demonstrating how high-resolution MRI scanning can be integrated with continuous hydrological monitoring to reveal emerging flow pathways in rooted soils. These data will inform a mechanistic model that quantifies the effects of preferential flow across different root types and depths, providing new parameterisations for use in rain garden performance models.