The tale of flooding over the central United States

The central United States is a region of the country plagued by frequent catastrophic flooding (e.g., flood events of 1993, 2008, 2011, 2013, and 2014). In the twentieth and twenty-first centuries, flooding has taken a devastating societal and economic toll on the central United States, contributing to dozens of fatalities and causing billions of dollars in damage. Moreover, previous studies have shown that flood damage has been increasing over the past century across this region, and seems to foreshadow a future increase in flood activity. Despite these large repercussions, the use of historical records to ascertain the changes over time in flooding has thus far proved inconclusive. It is therefore of paramount importance to examine whether the characters (i.e., magnitude and frequency) of recent flooding are different from the long-term averages over the central United States. The results of this thesis are based on long-term discharge records at 774 U.S. Geological Survey sites and show limited evidence suggesting increasing or decreasing trends in the magnitude of flood peaks over the study region. In contrast, there is much stronger evidence of increasing frequency of flood events. While the detection of changes in flood characteristics is essential, it is also of critical importance to start exploring what caused these changes. Therefore, in addition to the aforementioned investigation on the stream flow records, precipitation records were used to inspect whether possible changes in flood characteristics can be linked to the changes in heavy precipitation characteristics. The results indicate that there is a stronger signal of change in the frequency rather than in the magnitude of heavy precipitation events, similar to what found for the discharge records. Given that heavy precipitation is responsible for the observed changes in flooding, further analyses were performed to examine the climatic driving forces that are responsible for the observed changes in the frequency of precipitation, and consequently flooding at the seasonal scale; particular emphasis was paid to the role played by the Atlantic and Pacific Oceans. The results of this dissertation indicate that changes in the climate system play a significant role in explaining the variations in the frequency of heavy precipitation and flooding over the central United States at both the seasonal and sub-seasonal scales. The Pacific North American (PNA) teleconnection pattern was found to play a particularly prominent role. Therefore, these results suggest that recent observed changes in the frequency of flood events over the central United States can be largely attributed to changes in the frequency of heavy precipitation events, which were in turn driven by changes in the climate system.