Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 223

Cover and Management Factor (C) The factor C is the ratio of soil loss from
an area with specified cover and management to that from an identical area in tilled
and continuous fallow. It is a measure of the combined effects of all cover and
management and is primarily affected by crop stage period, canopy cover, applied
mulch, residue mulch, tillage, and crop residuals. The value of C usually ranges
from 0.01 to 1.4.

Support Practice Factor (P) The factor P is the ratio of soil loss with a specific
support practice to the soil loss on a unit plot. For croplands, the value of P for con-
touring is about 0.6-0.9, for strip-cropping 0.3-0.9, for contour-farmed and terraced
fields 0.05-0.9.

Modified Forms of USLE Williams ( 1975 ) replace the R factor in USLE by
a runoff factor and interpret the other USLE factors on a watershed-wide basis
(referred to as MUSLE). Thus, MUSLE is a watershed, rather than an upland, sedi-
ment yield model. Onstad and Foster ( 1975 ) modified the R factor in USLE to allow
individual storm estimation of upland soil loss. Other factors of USLE retain their
original meaning and interpretation.

6.5.6 Sample Workout Problems on Sediment Yield Estimation

Example 6.4

A farmer plowed his field in spring and planted grain corn. The soil is sandy loam
with medium organic matter content. The climate of the area is semi-humid. The
field is 400 m long with a 5% slope. Contour farming was performed in the field.
Estimate the soil loss from the field. Assume standard value of the variables, if
needed.

Solution

= × × × ×

We know soil loss, A P (t/ha/yr)

We get rainfall and runoff factor, R , which can be expressed as

R K LS C

R = 0.417 P 2.17

where P is the 2-yr, 6-h rainfall amount.
For the given semi-humid area, assume P =50mm
Then, R [50] 2.17

= 0.417 × = 2, 027

Assumed values of other factors (along with their conceptual judgment) are

summarized below: