Is Climate Change Evident in U. S. Streamflow?

The study presented here focuses on the changes/trends in U.S. streamflow from 639 unimpaired stations for the period 1951 – 2002. This is particularly relevant since the issue of climate change is of interest to many and studies have also indicated an abrupt change in climate around the year 1976/77. Trends in U.S. streamflow were evaluated using three statistical tests: Spearman's Rho, Mann-Kendall, and Linear Regression. Step changes in the streamflow data were evaluated using the Rank Sum and Student t test. The multiple tests provide robust results in which areas of the U.S. have experienced significant changes in streamflow data. The number of stations experiencing an increasing or decreasing trend/step change in any given water year were evaluated at a 95% confidence level. Results indicated that the Upper and Middle Mississippi River basin has an increasing trend in streamflow quantity. For the Pacific Northwest and some stations in Florida, the streamflow quantity is decreasing.