Evaluating the effect of persistence on long-term trends and analyzing step changes in streamflows of the continental United States

Streamflow is a very good indicator of long-term hydroclimatic changes. From a water management perspective, the identification of gradual (trend) and abrupt (shift) changes in streamflow are important for planning purposes. This study investigated the detection of comprehensive change, gradual and abrupt, in 240 unimpaired streamflow stations, categorized according to the hydrologic regions in the continental United States. The changes in streamflow volume were analyzed for water–year, autumn, winter, spring, and summer from 1951 to 2010, a 60-year period. The non-parametric Mann–Kendall test, with variations accounting for short term and long-term persistence, was used to evaluate the trends; the non-parametric change-point Pettitt test was used to evaluate the shifts. The field significance was evaluated using the Walker test. The trend results indicated increasing streamflow patterns in the majority of the eastern U.S. regions – the Upper Mississippi, Missouri, Great Lakes and Texas Gulf were field significant – and dominant decreasing streamflow trends in the Pacific Northwest region. The use of different Mann–Kendall test helped in evaluating the spatial distribution of short-term and long-term persistence and their effect on trends. The Pettitt test analysis indicated that statistically significant shifts occurred during the early 1970s and late 1980s. Similar to the trend results, the Midwest as well as the central and southern U.S. had significantly increasing shifts; the Pacific Northwest, Tennessee (winter season only), and South-Atlantic Gulf (spring season only) had decreasing shifts in streamflow. The findings may assist water managers in better planning and management of water resources under climate variability and change.