The Seasonal Nature of Extreme Hydrological Events in the Northeastern United States

Abstract

Recent analyses of extreme hydrological events across the United States, including those summarized in the recent U.S. Third National Climate Assessment (May 2014), show that extremely large (extreme) precipitation and streamflow events are increasing over much of the country, with particularly steep trends over the northeastern United States. The authors demonstrate that the increase in extreme hydrological events over the northeastern United States is primarily a warm season phenomenon and is caused more by an increase in frequency than magnitude. The frequency of extreme warm season events peaked during the 2000s; a secondary peak occurred during the 1970s; and the calmest decade was the 1960s. Cold season trends during the last 30-50 yr are weaker. Since extreme precipitation events in this region tend to be larger during the warm season than during the cold season, trend analyses based on annual precipitation values are influenced more by warm season than by cold season trends. In contrast, the magnitude of extreme streamflow events at stations used for climatological analyses tends to be larger during the cold season: therefore, extreme event analyses based on annual streamflow values are overwhelmingly influenced by cold season, and therefore weaker, trends. These results help to explain an apparent discrepancy in the literature, whereby increasing trends in extreme precipitation events appear to be significant and ubiquitous across the region, while trends in streamflow appear less dramatic and less spatially coherent.

River flood seasonality in the Northeast United States: Characterization and trends

“…However, there is an increased frequency of floods in the warm season (Jun–Oct)—historically flood‐poor months at nearly all gages (Figures and ). This trend generally matches the seasonality of regional upward trends in precipitation totals, extremes, and persistence (Frei et al, ; Guilbert, Betts, Rizzo, Beckage, & Bomblies, ; Huang, Winter, & Osterberg, ; Huang, Winter, Osterberg, Horton, & Beckage, ), indicating flooding is becoming more likely during a time of year when evapotranspiration and antecedent soil moisture conditions have historically not favoured flooding (Ivancic & Shaw, ). The increase in Jun–Oct flood counts also supports the findings of Frei et al () who found warm season high streamflow in the Northeast has increased in frequency.…”

“…This trend generally matches the seasonality of regional upward trends in precipitation totals, extremes, and persistence (Frei et al, ; Guilbert, Betts, Rizzo, Beckage, & Bomblies, ; Huang, Winter, & Osterberg, ; Huang, Winter, Osterberg, Horton, & Beckage, ), indicating flooding is becoming more likely during a time of year when evapotranspiration and antecedent soil moisture conditions have historically not favoured flooding (Ivancic & Shaw, ). The increase in Jun–Oct flood counts also supports the findings of Frei et al () who found warm season high streamflow in the Northeast has increased in frequency. Interestingly, Mallakpour and Villarini () showed an increased frequency of summer floods in the eastern part of the central United States—an area proximal to the Northeast United States that has a similar climate and where the summer season has historically been a minor contributor to flood occurrence.…”

“…To investigate whether previously documented regional trends in annual flood counts (Armstrong et al, , ) were driven by changes in frequency during one time of year, I decomposed annual POT flood series into cold (Nov–May) and warm (Jun–Oct) season counts for 65 gages with no missing data from 1941 to 2013. This time period was chosen for comparability with the analyses of Frei et al () while also including a majority of the 90 study gages (i.e., less than half of the gages have data going back to 1935). The count series were then pooled across sites, and regional trends for the three series (water year, Nov–May, and Jun–Oct) were assessed by the Regional Kendall test via the R package “rkt” (Helsel & Frans, ; Marchetto, ).…”

“…Their analysis also partially explains the well‐documented phenomenon that upward trends in Northeast U.S. precipitation extremes are stronger than increasing flood trends when both are evaluated on an annual basis (Wehner, Arnold, Knutson, Kunkel, & LeGrande, ). Frei et al () showed that cold season (Nov–May) streamflow extremes, for which upward trends in magnitude and frequency have been weaker, are more numerous in this part of the country than warm season events and thus dominate the annual signal (Frei et al, ).…”

“…and (2) decomposition of annual POT time series into cold (Nov–May) and warm (Jun–Oct) season subseries to examine the relative importance of each season to documented increases in POT per water year across the region (Armstrong, Collins, & Snyder, , ). The second approach follows the analyses of Frei et al (), but with a more restrictive definition of what constitutes a streamflow extreme as described below.…”