Stream flow in Minnesota: Indicator of climate change

Stream flow records (up to the year 2002) from 36 USGS gauging stations in five major river basins of Minnesota were studied. Seven annual stream flow statistics were extracted and analyzed: mean annual flow, 7-day low flow in winter, 7-day low flow in summer, peak flow due to snow melt runoff, peak flow due to rainfall as well as high and extreme flow days (number of days with flow rates greater than the mean plus one or two standard deviations, respectively). The Mann-Kendal non-parametric test was used to detect significant trends over time windows from 90 to 10 years in combination with the Trend Free Pre-Whitening (TFPW) method for correcting time series data for serial correlation. Streamflows in the state of Minnesota have varied over the period of record. Trends differed significantly from one river basin to another, and became more accentuated for shorter time windows. Periodicity was detected in the trends for the Red River of the North, the Mississippi River, and the Minnesota River basins for six of the statistics studied. Periods were on the order of 13-15 and 25 years, and the amplitudes were particularly strong after 1980. Peak flow due to snowmelt, typically the highest flow in each year, appears to be the only streamflow statistic that has not changed at a significant rate. Peak flows due to rainfall events in the summer are increasing, as well as the number of days with higher flows (high flow days). Increases in low flow (base flow) in summer and in winter have been significant. Wetter summers and more frequent snow melt events due to warmer winters are the likely cause. Stream flows in Minnesota reflect observed changes in precipitation with increases in mean annual precipitation, a larger number of intense rainfall events, more days with precipitation and earlier and more frequent snowmelt events. For water resources management the results suggest that the threat of snowmelt flooding has not increased, but floods due to rainfall events are more likely. Higher summer and winter base flows may benefit water quality.