The Climatology of Flooding in the US

The flood risk in the US is changing. The annual dollar losses and the number of recorded flood events, by multiple sources, imply an increase in the flood risk nationally. Previous research has suggested the changes in the number of flood events is related to many factors including urbanization and changes to precipitation patterns. Previous work has also captured regional climatologies of flood risk, but often these are site specific and calculated at or near a stream gage. Other studies have provided varying definitions on how to classify or quantify a flood event. If flood risk is changing, it is problematic to use present day measures and definitions of a flood for a given location when comparing historical classifications of flood events as in previous studies. This research uses a novel approach whereby flood determinations are based upon flood impact measures from the National Weather Service and compared yearly to quantify the yearly flood occurrences for given locations. Over 3,500 streamflow gages nationwide from 1940 to 2014 are used in this study. At each stream gage, thirty-year flood event climatologies were calculated geospatially across the US. A spatial regression model is computed with precipitation, temperature, land use, average annual discharge, and the drainage area above a gage location as independent variables. The results of the study suggest that some areas of the US are experiencing more flood events per year on average than other areas and by different magnitudes. For example, the Southeast US is experiencing fewer overall flood events, but the upper Midwest is experiencing more. There are clear regional differences in flood events per year and clear changes in the number of floods presently than in previous decades. The regression analysis shows that different sets of independent variables contribute to the changes in flood risk by regions.