Comparing Flood Projection Approaches Across Hydro-Climatologically Diverse United States River Basins

Abstract:

The challenge of estimating design flood magnitude under climate change has led to the development of multiple approaches to long-term flood projection: stationarity, informed-parameter (comprised of both trend informed and climate informed), and hydrologic simulation. This is the first study to compare these approaches across a large set of hydro-climatologically diverse basins located throughout the contiguous United States, grouped into distinct clusters based on catchment and flood characteristics. The comparison is achieved using a split-sample test conducted over the historic period; the climate (specifically, precipitation) informed and hydrologic simulation (specifically, Sacramento Soil Moisture Accounting Model – SACSMA) approaches are forced with observations as well as downscaled general circulation model (GCM) simulations. The results provide a quantitative perspective on key long-term flood projection issues. The precipitation informed approach can be informative for projections of regional change, while the hydrologic simulation approach can be informative for direction of change for a single basin. Both approaches struggle to skillfully represent basins in the upper Midwest, the western side of the Appalachian Mountains, and the Northeast. When forced with GCM outputs, precipitation informed is both more accurate and more precise than hydrologic simulation, after accounting for model error. For hydrologic simulation, forcing with GCMs will add positive bias for basins in the Rocky Mountains and Southeast, and will add negative bias with large uncertainty across GCMs for basins in the lower Midwest, the South, and the mid-Atlantic seaboard. The results illustrate the need for continued improvement in long-term flood projection approaches and the necessity of design paradigms incorporating uncertainty.