Prevalence and Magnitude of Trends in Peak Annual Flow and 5-, 10-, and 20-Year Flows in the Northeastern United States

Abstract

This study assessed trends in flows of return periods of 5, 10, and 20 years and in the peak annual flow (PAF). The U.S. Geological Survey (USGS) stream-gauging stations in the northeastern United States (n=93) were analyzed, including 34 gauges in the USGS’s Hydroclimatic Data Network 2009 (HCDN), which are mostly unaffected by land-use change. For each gauge, flood flows were determined for a series of sequential blocks, each 10 years long. Approximately 26% of HCDN gauges and approximately 17% of non-HCDN gauges showed a statistically significant (alpha=0.05) increasing trend for at least one return period on the basis of the Mann-Kendall test; no gauge showed a decreasing trend. The percentage of gauges with increasing trends was approximately the same, regardless of return period. Regarding magnitude of these statistically significant trends assessed via Sen slope, the average rate of increase for gauges was ∼1.8% per year, again regardless of return period, which is larger than the average rate for PAF (0.9%). Rates were approximately equal for HCDN and non-HCDN gauges. The difference in trend magnitude between PAF and flood flows indicates that a study of trends in PAF may not produce results that correspond to flows of higher return periods.