Evaluating the impacts of climate variability and change versus methodological approaches on stormwater control measures rainfall thresholds. A case study from North Carolina, USA

Climate change can significantly affect the effectiveness and resilience of Stormwater Control Measures (SCMs) in urban environments, as changes in rainfall intensity and depth can impact SCMs' capacity to mitigate flooding and reduce pollutant loads in water bodies. Thus, increasingly adaptive and robust SCM designs need to be informed by a continued understanding of changing precipitation patterns. ​In this study we evaluate SCMs precipitation thresholds based on two main methods: 90% rainfall capture depth and 90^th percentile rank of daily precipitation over the period 1980-2023 in North Carolina, USA. We sought to address the questions of whether changes in daily precipitation are detected over time and, if so, whether these changes result in different thresholds depending on the method of choice.  Our results indicate over the entire timeseries (1980-2023) both methods result in thresholds consistent with the current North Carolina Department of Environmental Quality SCM standard of 25.4 mm/day in central and western North Carolina and 38.1 mm/day in the eastern coastal plains. However, when the data is sliced in four 11-year periods the evolution of precipitation thresholds show a positive trend in both methods. We also find that the capture depth method is considerably more sensitive to extreme precipitation events than the 90th percentile method.  Our results indicate that current water quality event standards in North Carolina may underestimate pollutant load treatment due to observed precipitation changes in recent years, suggesting a need for decadal adjustments. ​Defined SCM threshold need also to account for differences arising from the choice of calculation method, and practitioners using the capture depth methodology in particular, may want to revisit established thresholds.