Wednesday, 15 May 2002: 1:45 PM
Mann-Whitney U analysis of annual streamflow over the central United States: 1939-1998
Climate projections for the central United States have suggested a more drought-prone future, with projected increases in precipitation (P) offset by even greater increases in evapotranspiration (E). Such projections naturally lead to questions about historical trends in P,E, and soil moisture over these agriculturally important regions, but unbiased records of the balances between those variables are not available for extended periods. Given that constraint, the approach here is to qualitatively infer those balances via the timing and duration of low frequency hydrological drought regimes over the central U.S. between 1939 and 1998. To avoid the problems associated with trend analysis, an alternative statistical screening approach is used here. This process ranks mean annual streamflow rates, calculates Mann-Whitney U statistics from samples of those rankings over running time windows, and then tests those U statistics for significance. Analysis of the records of 42 Hydro-Climatic Data Network streamflow stations over the Great Plains and Midwest shows a fairly consistent pattern of highly ranked annual streamflow during time windows beginning after 1973 and ending in either 1997 or 1998. Comparing daily streamflow characteristics during those late-century high flow time periods and during all other years shows a general tendency to shorter duration periods of below normal streamflow, but also a clear tendency among some stations toward extended periods of above normal streamflow. These results suggest that some parts of the central U.S. may have shifted toward a more drought-resistant climate regime during the closing decades of the 20th century.