AN ASSESSMENT OF LONG-TERM CHANGES IN THE OBSERVED MAGNITUDE, DURATION, AND INTENSITY OF PRECIPITATION IN THE ROOT RIVER BASIN OF SOUTHEASTERN MINNESOTA

As climate change progresses many forecasts for the Upper Midwest predict increases in annual precipitation but with a shift in seasonal patterns that will leave the summer months drier with less frequent, higher magnitude storm events. Changes in precipitation patterns have the potential to alter the sediment budget in watersheds. The purpose of this study is to determine the changes in magnitude, duration, and intensity of precipitation in the Root River Basin located in southeastern Minnesota. This analysis was carried out using hourly precipitation data from 1948 to 2013 from 20 sites in Minnesota and adjacent parts of Wisconsin and Iowa.

The hourly precipitation data provide a high-resolution archive that is ideal for analyzing changing patterns in rainfall at multiple scales, including decadal, yearly, monthly, and per storm. For the purpose of this project we have defined a unique storm event as one having a time gap of greater than 3 hours between previous and subsequent precipitation. Based on our analyses southeastern Minnesota is experiencing decreasing storm durations (1948-1979, 4.44 hr; 1980-2013, 2.81 hr), increasing average rainfall intensities (1948-1979, 2.29 mm; 1980-2013, 3.3 mm), increasing maximum rainfall intensities (1948-1979, 3.81 mm; 1980-2013, 4.57 mm), little to no change in the amount of rainfall per storm (1948-1979, 7.62 mm; 1980-2013, 7.87 mm), and a slight increase in average annual precipitation since the 1970s (1970s = 826 mm; 2000s = 886 mm).

Our data demonstrate that significant changes in precipitations patterns have occurred over the past 60 years. The observed changes are consistent with the predictions derived from various climate models and, as such, may lend support to forecasts of additional shifts in precipitation patterns in the coming decades. Understanding and quantifying these changes, particularly the trend of shorter more intense storms, has large implications on the sediment budget of the Root River basin. These changes may be compounded by concomitant shifts in land use from pasture to row crop within the watershed. Future studies should focus on elucidating how higher intensity storms combined with an increasing percentage of row crop land use will affect sediment erosion from upland and floodplain agricultural areas and near channel sediment storage.