Multi-Objective Decision Analysis for Siting Green Infrastructure to Reduce Flooding and Minimize Drought Impacts under Climate Change

Abstract:

Rapid development of urban infrastructure has intensified the risks of nutrient pollution and severe flood events around the world. Excessive nutrient runoff can have severe impacts on the environment, economy and human wellbeing, as evidenced by coastal dead zones. Green infrastructure (GI) has been widely proposed to mitigate these risks by taking up nutrients and reducing runoff. However, tradeoffs in reducing high flows and nutrient loads at the expense of exacerbating low flows need to be considered. Furthermore, the construction and maintenance of GI is expensive. Therefore, the location and size of GI should be optimally selected to handle these conflicting objectives effectively over its lifetime, during which the climate is likely to change. In this study, we apply a decision-making framework to analyze the effectiveness of using increased tree canopy as GI for reducing floods, while also mitigating impacts on low flows under climate change uncertainties in Baisman Run, a suburban-forested catchment near Baltimore, Maryland.

We evaluate how reforesting different locations in Baisman Run would affect the intensity of extreme flood events and low flows. Three GI portfolios are selected by increasing tree canopy in up-slope, mid-slope or down-slope areas. A calibrated RHESSys ecohydrological model is used to simulate streamflow with and without reforestation. Daily precipitation and temperature from the 20-year gauge record and GCMs are input to the RHESSys model. The magnitude of high and low flow extremes at the stream gauge location are computed in each simulation. For each GI portfolio, a cost-benefit analysis is performed comparing the costs of reforestation to their benefits in reducing flooding, while minimizing impacts on low flows relative to the current conditions in the catchment. Results indicate that reforesting mid-slopes is most effective to reduce flooding extremes now, but that reforesting up-slopes might be more effective in wetter futures as both down and mid-slope soils saturate. However, reforesting mid-slopes could reduce low flows in dry seasons, both now and in the future. Understanding these tradeoffs can help decision-makers plan long-term GI investments in a changing climate.