R Factor 

R is the rainfall-runoff erosivity factor. It is the average annual summation (EI) values in a normal year's rain. The erosion-index is a measure of the erosion force of specific rainfall. When other factors are constant, storm losses from rainfall are directly proportional to the product of the total kinetic energy of the storm (E) times its maximum 30-minute intensity (I). Storms less than 0.5 inches are not included in the erosivity computations because these storms generally add little to the total R value. R factors represent the average storm EI values over a 22-year record. R is an indication of the two most important characteristics of a storm determining its erosivity: amount of rainfall and peak intensity sustained over and extended period.

The erosivity of rainfall varies greatly by location. The R factor in Michigan ranges from 75 to 135. Approximately 70 percent of the erosivity occurs in the months of May, June, July and August in Michigan. 

R Factor Map for Michigan

From Technical Guide to RUSLE use in Michigan, NRCS-USDA State Office of Michigan.

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