Evaluating the infiltration capacity of degraded vs. rehabilitated urban greenspaces: Lessons learnt from a real-world Italian case study

Climate change (higher frequency and intensity of precipitation events) and land use change (urbanization reducing soil drainage capacity) are increasingly causing stormwater problems globally, especially in cities. Nature-based solutions such as urban greenspace rehabilitation programs are gaining considerable attention for restoring soil retention capacity and protecting cities against increasing flood risk. However, a better understanding of how effective such measures are in practice is needed to enable and promote their adoption across urban settings. To this end, in this study, we assess the effect of soil rehabilitation measures in terms of soil compaction, vegetation cover and unsaturated hydraulic conductivity by analysing the results of an infiltration measurement campaign conducted across a wide range of real-world greenspaces (from recently rehabilitated, to poorly maintained, down to highly degraded) in the Italian city of Milan, one of the most urbanized areas in Europe. Our results show that the unsaturated hydraulic conductivity varies significantly across the examined greenspaces, due to differences in time from rehabilitation, soil compaction and vegetation cover. Specifically, we find that the highest unsaturated hydraulic conductivity is obtained after approximately 5 years since implementation of soil rehabilitation measures, which can be explained by the time needed (i) by the introduced vegetation to develop root systems, and (ii) by the soil matrix to develop a coherent structure that allows stable connections between pores and thus the strengthening of preferential water pathways. Finally, our study shows that, in absence of soil and vegetation maintenance, unsaturated hydraulic conductivity may decrease rapidly after about 9–12 years. These findings provide useful information for supporting the planning of nature-based solutions in practice, which will become increasingly important to protect our cities from climate change impacts and widespread urbanization going forwards.