Streamflow trends in the United States

Secular trends in streamflow are evaluated for 395 climate-sensitive stream gaging stations in the conterminous United States using the non-parametric Mann-Kendall test. Trends are calculated for selected quantiles of discharg e, from the 0(th) to the 100(th) percentile, to evaluate differences betwee n low-, medium-, and high-flow regimes during the twentieth century. Two ge neral patterns emerge; trends are most prevalent in the annual minimum (Q(0 )) to median (Q(50)) flow categories and least prevalent in the annual maxi mum (Q(100)) category; and, at all but the highest quantiles, streamflow ha s increased across broad sections of the United States. Decreases appear on ly in parts of the Pacific Northwest and the Southeast. Systematic patterns are less apparent in the Q(100) flow. Hydrologically, these results indica te that the conterminous U.S. is getting wetter, but less extreme.