Topographic Factor. The topographic factor (LS) combines source area slope length and
steepness into a single factor for estimating soil erosion by water. Wischmeier and Smith (1978)
provide a table, figure and an English units equation for determination of source area LS. Data for
computing LS are available from topographic maps or county soil surveys.
Cover and Management Factor. The cover and management factor (C) describes the
protection of the soil surface by plant canopy, crop residues, mulches, etc. The maximum C value is
1.0, corresponding to fallow cropland with no protection. Cropland C values change substantially
during the year in response to seasonal activities such as crop planting, tillage, growth, and harvest.
Cover and management factor values are available for each crop stage in different management systems
(Wischmeier and Smith, 1978) and Mills et al. (1985). The average annual values from Stewart et al.
(1975) are adequate for an initial approximation screening study. Wischmeier and Smith (1978) have
also developed C factors for construction
Figure 1.1.4. General Relationship Between Rainfall Amount and Erosion Index for
Individual Storms. (after Richardson et al., 1983).
sites; pasture, range and idle land; undisturbed forests; and mechanically prepared woodland sites.
Supporting Practice Factor. The supporting practice factor (P) is a measure of the effect of
soil conservation practices such as contouring and terracing on cropland erosion. Stewart et al. (1975)