Temporal and spatial amplification of extreme rainfall and extreme floods in a warmer climate

Abstract This work explores the relationship between catchment size, rainfall duration and future streamflow increases on 133 North American catchments with sizes ranging from 66.5 to 9886 km 2 . It uses the outputs from a high spatial (0.11°) and temporal (1-hour) resolution Single Model Initial condition Large Ensemble (SMILE) and a hydrological model to compute extreme rainfall and streamflow for durations ranging from 1 to 72 hours and for return periods of between 2 and 300 years. Increases in extreme precipitation are observed across all durations and return periods. The projected increases are strongly related to duration, frequency and catchment size, with the shortest durations, longest return periods and smaller catchments witnessing the largest relative rainfall increases. These increases can be quite significant, with the 100-year rainfall becoming up to 20 times more frequent over the smaller catchments. A similar duration-frequency-size pattern of increases is also observed for future extreme streamflow, but with even larger relative increases. These results imply that future increases in extreme rainfall will disproportionately impact smaller catchments, and particularly so for impervious urban catchments which are typically small, and whose stormwater drainage infrastructures are designed for long-return period flows, both being conditions for which the amplification of future flow will be maximized.