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 Soil Loss. Haith (1985) gives an event-based version of the USLE for soil loss on day t:
Xt = 1.29
A K (LS) C P
(1.1.16)
Where:
Xt = soil loss generated (mg)
A = area of erosion source (ha)
K = soil erodibility
LS = topographic factor
C = cropping factor
P = practice factor
K, LS, C, and P are dimensionless
1.29 is a units conversion constant
Rainfall Erosivity. E is a measure of the energy of rainfall and runoff (including snowmelt)
that causes erosion. Rainfall erosivity can be computed from rainfall intensities based on short term
records (15 min or hourly), however, this data intensive approach is seldom practical.
Alternatively, erosivities may also be obtained from daily rainfall data by using a regression
equation developed by Richardson et al. (1983):
E = 6.46a (R1.18)
(1.1.17)
Where:
E = daily rainfall erosivity
R = daily rainfall (cm)
a = coefficient that varies with location and season (dimensionless)
Richardson et al. (1983) have determined cool season (October-March) and warm season
(April-September) values for coefficient a for 11 locations in the eastern United States. Although
regression equations such as equation 1.1.17 are generally convenient predictive models, their
usefulness depends on the confidence intervals for the model development data set. Neither
Richardson et al. (1983) nor Mills et al. (1985) specifically present confidence intervals. However,
Richardson's erosion index versus rainfall distribution indicate that the confidence in this case may be
fairly low (see Figure 1.1.4.). However, it is probably the best available indicator of the rainfall erosion
relationship.
Soil Erodibility. Soil erodibility K) indicates the susceptibility of soil to erosion and is a
function of soil type. K values are available from local soil and water conservation districts, state
offices of the Soil Conservation Service (SCS), and from Stewart et al. (1975).
1.1-15
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