The C-factor is based on the concept of deviation from a standard, in this case an area under clean-tilled continuous-fallow conditions. The Soil Loss Ratio (SLR) is then an estimate of the ratio of soil loss under actual conditions to losses experienced under the reference conditions.
"C" represents the effects of plants, soil cover, soil biomass, and soil disturbing activities on‘ erosion. RUSLE uses a subfactor method to compute soil loss ratios, which are the ratios at any given time in a cover management sequence to soil loss from the unit plot. Soil loss ratios vary with time as canopy, ground cover, roughness, soil biomass and consolidation change. A "C" factor value is an average soil loss ratio weighted according to the distribution of R during the year. The subfactors used to compute a soil loss ratio values are canopy, surface cover, surface roughness, prior land use and antecedent soil moisture.
Surface cover is material in contact with the soil surface that intercepts raindrops and slows surface runoff. The total percent of the surface covered is the characteristic used by RUSLE to compute how surface cover affects erosion. Surface cover includes all cover that is present, including rock fragments, live vegetation, cryptograms, and plant residue. The only minimum size requirement for material to be counted as surface residue is that it either be of sufficient size or attached to the surface such that is not removed by runoff.
RUSLE accounts for surface roughness in the C value calculation. Surface roughness ponds water in depressions and reduces erosivity of raindrop impact and water flow. If the deposition are sufficiently deep, much deposition occurs in them. Over time, roughness disappears as the depressions fill with sediment. and one soil subsides after the tillage operations that formed the depressions.
Roughness is reduced in RUSLE as a function of cumulative rainfall after the last tillage operation. Roughness also indicates the degree of clodiness and the likelihood that the surface will seal, producing increased runoff and soil erodibility. Accounting for roughness, such as with the plow plant tillage system, is one reason some C values are lower with RUSLE.
If a C factor of 0.15 represents the specified cropping management system, it signifies that the erosion will be reduced to 15 percent of the amount that would have occured under continuous fallow conditions. The C factor in Michigan ranges from 0.005 for continuous low residue grass/hay to 0.50 for continuous row crops with low residue returned to the soil. C factors are also avaliable for pastureland, wildlife land, idle land, and woodland uses.
There are four regions representing Michigan's RUSLE C & K factor zones: 100a, 102a, 102b and 102c. Each zone is represented by the following cities for historical weather data as follows: 100a, Marquette; 102a, Gladwin; 101b, Grand Rapids: 102c, Norwalk, Ohio.
From Technical Guide to RUSLE use in Michigan, NRCS-USDA State Office of Michigan.
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