Streamflow trends in the United States

Secular trends in streamflow are evaluated for 395 climate-sensitive streamgaging stations in the conterminous United States using the non-parametric Mann-Kendall test. Trends are calculated for selected quantiles of discharge, from the 0(th) to the 100(th) percentile, to evaluate differences between low-, medium-, and high-flow regimes during the twentieth century. Two general patterns emerge; trends are most prevalent in the annual minimum (Q(0)) to median (Q(50)) flow categories and least prevalent in the annual maximum (Q(100)) category; and, at all but the highest quantiles, streamflow has increased across broad sections of the United States. Decreases appear only in parts of the Pacific Northwest and the Southeast. Systematic patterns are less apparent in the Q(100) flow. Hydrologically, these results indicate that the conterminous U.S. is getting wetter, but less extreme.