More than half of the world’s population lives in cities, and the trend of urban migration is expected to continue for the foreseeable future. As city populations swell, the “built environment” expands; paved surfaces, buildings, and other urban infrastructure replace forests, grasslands, wetlands, and agriculture. From a hydrologic perspective, this transformation results in a large-scale transition from a water-penetrable (pervious) surface to water-impenetrable (impervious) cover.
The loss of natural land cover leads to a host of urban water issues, including increased risks of flooding and pollution and reduced groundwater recharge. As these problems have grown, many cities have attempted to improve urban hydrology through green infrastructure initiatives that aim to improve stormwater management. Rain gardens, green roofs, disconnected downspouts, permeable pavement, and soil amendments have all been implemented to improve water capture and soil infiltration at the scale of individual parcels. Despite their proven effectiveness, however, adoption rates remain low in many cities, in part because homeowners lack the necessary incentive to renovate their properties but also because of uncertainty about which practices best address surface runoff and subsurface hydrology.
Here Voter and Loheide explore green practices to identify which strategies most effectively alter surface runoff, deep drainage, and evapotranspiration at the scale of a residential parcel. The authors developed 96 different simulations that compared the effects of different hydrologic interventions. The simulated conditions were based on on-the-ground conditions in Madison, Wis.
They categorized the interventions in three ways. “Impervious-centric” strategies aim to disconnect nonporous surfaces on the basis of the understanding that parcel runoff is highest when water flows from a roof to a driveway to the street, for example. Downspouts that empty into a yard and driveways that slope toward the yard (instead of only toward the street) are examples of impervious-centric practices. “Pervious-centric” modifications focus on improving the absorption of vegetated land cover in cities, with the understanding that urban vegetation is not as effective as its natural counterparts at reducing runoff. Amending yard soils and adding in microtopographic variations are both models of a pervious-centric approach. A “holistic approach” combines elements from both impervious- and pervious-centric strategies.
The results suggest that the holistic approach best reduces surface runoff and increases deep drainage and evapotranspiration. The combination of strategies yielded synergistic impacts, meaning the effects were greater than just summing up the impacts of pervious and impervious approaches. The benefits of the holistic approach were particularly noticeable at the interface between pervious and impervious surfaces. The authors also found that soil compaction plays a significant role in urban hydrology and that impervious-centric approaches achieved greater impacts when paired with soil decompaction.
The study indicates that improvements on individual parcels can cumulatively improve hydrology across a metropolitan area. Incentives and local policies are two mechanisms through which cities can drive these improvements. However, the authors note that the effectiveness of these hydrologic outcomes varies from year to year because of weather. (Water Resources Research, https://doi.org/10.1029/2018WR022534, 2018)
—Aaron Sidder, Freelance Writer