Impacts of underground structures on the hydrological functions of urban green spaces

Study region: A typical urban residential compound in China. Equipped with an extensive underground parking. With considerable green space area providing potential benefits of runoff mitigation via imperviousness disconnection. Study focus: This study focuses on the impacts of the underground parking structure on the hydrological processes and the green space functions. Using a fully-distributed and coupled surface-subsurface model, we analyzed the changes in soil saturation, runoff, and evapotranspiration caused by the underground structure. New hydrological insights for the region: Results show that the underground parking structure causes soil water accumulation on its top in long term, leading to higher runoff and evapotranspiration. The magnitude of these effects relies on water supply to the green space. Lower water supply (e.g., when the climate is dryer or soil permeability is lower) would result in less soil water accumulation, thereby weakening the hydrological effects. In contrast, these effects are amplified when imperviousness disconnection is implemented, as it increases water supply to the green space through focused infiltration. Moreover, for the condition of imperviousness disconnection, these effects are not confined to the disconnection point, as the water accumulation zone can develop via subsurface lateral preferential flow along the underground parking slab, potentially influencing runoff generation patterns over a larger area. These findings highlight the importance of considering urban underground structure in stormwater management, especially for regions with wet climate.