Assessment of Erosive Wind Frequency for Portage County, Wisconsin
Behm, David H.
University of Wisconsin-Stevens Point, College of Natural Resources
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Erosive winds are those equal or greater than 18 mph (8 msec^ -1 ) measured at 33 feet (10 m). March accounted for 15.4% of the annual erosive wind energy (EWE) for Portage County, 10.6% more than observed in Eau Claire, WI. April accounted for 18.1% of the annual EWE in Portage County, nearly the same as that for Eau Claire (18.4%). The average maximum erosive wind duration in April was 8.8 hours and the average maximum average wind velocity (1-hour duration) was 28.5 mph (13 msec^-1). Climatic erosivity, C, was calculated to be only 4%, compared to the 6% value presently used. Soil erosion estimates calculated by the wind erosion equation with C=4 will generally be one-third less than those calculated with C=6. Analysis of monthly and weekly erosive wind frequencies (March 15 through December 15) identified critical erosion periods and those most likely to result in abrasive flux damage to susceptible crops. In late May and June the soil surface is susceptible to erosion because vegetative cover is not yet fully established. This erosion can cause significant crop damage to susceptible crop seedlings. Prevailing wind erosion direction (PWED) was either west or southwest throughout the year. During the erosive period (March 15 through December 15) in Portage County, all preponderance values were less than 2.0, which indicates highly variable erosive wind direction. Reliance upon only PWED and preponderance values to orient wind barriers is unwarranted; other factors, such as frequency and direction data of erosive winds during the critical erosive period, need to be considered. Circular and curvilinear wind barriers are analyzed as alternatives to traditional linear designs.