Testing validity of stormwater infrastructure design methods under changing climate conditions: analysis of changes in homogeneous sub-daily rainfall intensity regions in Pennsylvania

Changing climate conditions can be a menace to current infrastructure planning methods which rely on historical information to describe the expected behavior of climate. Especially for stormwater infrastructure planning, hourly and high spatial resolution climate projections are needed because of the high variability of rainfall intensity at the planning scales. In practice, to overcome the lack of long observational records at the sufficient spatial resolution, regions with homogeneous rainfall characteristics are identified. Assuming that rainfall intensity at any point within the identified regions is described by a common probability distribution, rainfall intensity at an ungauged site can be estimated. However, the continued use of this approach to design infrastructure might prove insufficient if the homogeneity assumption does not hold under a future climate. In this study, we made use of the 13-years long, high resolution Contiguous United States Weather Research Forecast (WRF) Model simulations of current and future climate to first analyze the changes in rainfall intensity at each delineated homogeneous region used for design in Pennsylvania. We then investigate whether the spatial extent of these regions would change under future climate conditions. Given the short simulation period, a Bayesian inference approach was employed to spatially analyze extreme sub-daily rainfall intensity. The Generalized Pareto distribution was adopted to estimate the intensity change for different return periods and durations under the WRF Pseudo-Global Warming scheme used to simulate the future climate. The results show that WRF is able to replicate the spatial patterns of homogeneous rainfall intensity and that the distribution of rainfall intensity at each homogeneous region will shift to higher intensities in a future climate. Boundary changes at urban settings imply that stormwater infrastructure designed using an homogeneous regions approach, might experience even higher changes in rainfall intensity, increasing exposure in vulnerable areas.