WSP01538

• File Number: 8210.470
• Description: Pacific Southwest Interagency Committee
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 4/1/1974
• Author: Unknown
• Title: Report of the Water Management Technical Subcommittee - Erosion and Sediment Yield Methods - April 1974
• Water Supply Pro - Doc Type: Report/Study

K, the soil-erodibility factor, is the erosion rate per unit of
erosion index for a specific soil in cultivated continuous
fallow, on a 9-percent slope 72.6 feet long. The reasons for
selection of these conditions as unit values is explained in
the detailed discussion of this factor.
L, the slope-length factor, is the ratio of soil loss from the
field slope length to that from a 72.6-foot length on the
same soil type and gradient.
S, the slope-gradient factor, is the ratio of soil loss from the
field gradient to that from a 9-percent slope.
C, the cropping-management factor, is the ratio of soil loss
from a field with specified cropping and management to that
from the fallow condition on which the factor K is evaluated.
P, the erosion-control practice factor, is the ratio of soil
loss with contouring, stripcropping, or terracing to that
with straight-row farming, up-and-down slope.
Numerical values of each of the six factors have been extensively
determined from research data in areas east of the Rocky Mountains.
Scarce data has limited the usefulness of the method in the West.
The discussion which follows details the use of the relationship
in a semiarid rangeland with sparse vegetative cover receiving intense
convective Bummer thunderstorms producing the annual runoff.

Rainfall Factor (R)

Iso-erodent maps are available only for portions of the United States
east of the 104th meridian. Therefore, it was necessary to use precipitation
data from the Walnut Gulch Watershed to compute storm EI (total kinetic
energy of the storm times its maximum 30-minute intensity). The sum of
the computed storm EI values for a given time period is a numerical
measure of the erosivity of all the rainfall within that period. The
rainfall erosion index (R) at a particular location is the longtime-average
yearly total of the storm EI values.

The highly variable nature of the air-mass, convective thunderstorms
has been well documented. Accordingly, the rainfall factor associated
with such precipitation would also be expected to be highly variable.

Table 2.--Annual EI values on Walnut Gulch.

Index values normally exceeded once in

2 years

5 years

1 year

10 years

Gage No.

67
140

76
210

22
60

35
23

52
67

Table 2 illustrates the variability of the EI values from two gages
selected from the precipitation network on Walnut Gulch. Interestingly

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