Assessing the impact of spatial allocation of bioretention cells on shallow groundwater – An integrated surface-subsurface catchment-scale analysis with SWMM-MODFLOW

Well-designed and implemented green infrastructure (GI) can help to recover the natural hydrologic regime of urban areas. A large-scale GI planning requires a good understanding of the impact of GI spatial allocation on surface-subsurface hydrologic dynamics. This study, firstly, developed a coupled surface-subsurface hydrological model (SWMM-MODFLOW) that can simulate fine-temporal-scale two-way interactions between GI and groundwater at catchment scale. The model was calibrated and validated using the monitoring data at one urban catchment within Kitsap County, WA, US. Based on the validated model, a series of hypothetical simulations was then performed to evaluate how spatial allocation of bioretention cells (BCs), one type of GI, influences and correlates with surface runoff and groundwater table dynamics. The spatial allocation was represented by implementation ratio (i.e., area), aggregation level (i.e., density) and location of BCs. The hydrologic dynamics were quantified by peak and volume reductions of surface runoff, as well as groundwater table rise and standard deviation of groundwater levels. A small number of BCs can raise groundwater table locally and regionally. However, it may not affect the spatial uniformity of groundwater levels (represented as the standard deviation of groundwater levels) if being properly allocated. Although the impact of aggregation level of BCs was relatively low compared to the implementation ratio and relative location of BCs, more-distributed BCs resulted in lower peak groundwater table rises but higher temporally-averaged groundwater table rises. Allocating BCs upstream resulted in higher groundwater table rises regionally, which is recommended for areas of deeper groundwater tables. While, allocating BCs downstream is more recommended for areas of shallower groundwater tables. BCs of greater surface runoff control efficiencies lead to higher groundwater table rises, which highlights the importance of considering the tradeoff between surface runoff control and groundwater protection in GI planning.