Estimating the Impacts of Climate Change and Population Growth on Flood Discharges in the United States

This study reflects a portion of the riverine analysis for a Federal Emergency Management Agency initiative to estimate the economic risks associated with climate and land use change to the U.S. National Flood Insurance Program. Specifically, this paper investigates how the 1% annual chance flood discharge, $Q_{1\%}$ (equivalent to a 100-year return period flood), may change based on climate change and population projections through the year 2100. Watershed characteristics and observations of climate indicators at 2,357 U.S. Geological Survey gauging stations were used to develop regression relationships to estimate $Q_{1\%}$. Projections of the climate indicators that measure extremes in temperature and precipitation from a suite of global climate models were then used within a Monte Carlo sampling framework to estimate future changes to $Q_{1\%}$ throughout the United States, while also translating the uncertainty resulting from multiple climate model projections into uncertainty in estimating the future $Q_{1\%}$. Population growth models consistent with climate model emission assumptions were used to estimate increases to impervious area over the next century, along with corresponding contributions to the $Q_{1\%}$ estimates. The study provides a screening-level analysis of possible changes to flow and suggests spatial trends across the United States. The results suggest that $Q_{1\%}$ may increase substantially over many areas of the United States over the next century, especially in the Pacific Northwest, the Northeast, and highly urbanized areas.