Low Frequency Streamflow Regimes over the Central United States: 1939–1998

Intra- to multi-decadal (6–30 year) streamflow regimes over the central United States during the late 20th century are identified and investigated here through a two-stage analysis. The first stage ranks mean annual flow rates during 1939–1998, calculates Mann–Whitney Ustatistics from samples of those rankings over running time windows, and then tests those U statistics for significance. This analysis of the records of 42 Hydro-Climatic Data Network stations over the Great Plains and Midwest reveals consistent patterns of highly ranked annual streamflow during time windows at the end of the century, with most beginning during the late 1960s and early 1970s and ending in either 1997 or 1998. Many of these stations are located in a critical agricultural region known as the Corn Belt. The second stage of analysis compares both the duration of abnormal flow periods and the frequency of hydrological surplus and drought conditions during the high flow years indicated by the first stage, relative to the remaining years of 1939–1998. Among gage stations in the climatologically drier western Corn Belt during the 1980s and 1990s there is a clear tendency toward extended periods of above normal flow, which results in more than a doubling in the average annual frequency of hydrological surplus days relative to previous years. These stations also show more than a 50% reduction in the average annual frequency of hydrological drought days relative to previous years. Similar but less pronounced shifts in hydrological regime are evident in the central and eastern Corn Belt, and in the Mississippi River at Vicksburg during 1973–1998. These results indicate that many areas of the central United States have shifted toward a climate regime of relative hydrological surplus during the closing decades of the 20th century.