WSP06144

• File Number: 8210.470
• Description: Organizations - Pacific Southwest Inter-Agency Committee - PSIAC
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/1968
• Author: PSIAC
• Title: Pacific Southwest Inter-Agency Committee - Report of the Water Management Subcommittee
• Water Supply Pro - Doc Type: Report/Study

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"concentrated flow erosion". Rill erosion in the last 20 yearEl, has
become predominantlY associated with cultivated cropland erosion in
the phrase "sheet and rill erosion" and the USLE. concentrated
flow erosion is defined as storm related natural flow paths where
sediment is eroded and transported during, storm events. These
paths are not gullies but can eventually become gullies in some
cases. The flow paths can ,laterally develop as well as developing'
more tributary paths. These flow paths enter into major channels
or gullies or may distribute into lower sloping depositional zones.
The result can be ,significant sediment loss over a wide area of
land. This, is the type of sediment yield and erosion process ,that
the PSIAC sediment' yield procedure was developed to characterize.

The reference to landslide erosion should be eliminated. This type
of erosion can be characterized by a direct volume measurement
estimate method and should not be included in the PSIAC procedure.

In the 10 point category the (b) wind erosion portion should be
eliminated. If wind erosion is a significant problem, then a.wind
erosion equation (USDA, SCS, 1984) should be used to characterize
the problem. The wind erosion values can be used in conjunction
with the PSIAC sediment yield and other data to fully characterize
the erosion and management problems of a geomorphic unit.

The percentage value of the surface area of a geomorphic ,unit used
to rate this factor should be developed in relationship to the
percentage value of effective ground cover (f). The sum of upland
erosion area (h) and ground cover (f) cannot exceed 100% and in
reality should not equal 100%. The potential area open to
concentrated overland flow erosion is not the total remaining area
after effective ground cover, is estimated. The, percentage of
surface area assigned to th~sfactor should be developed by direct
observation of the density of concentrated flow paths", the
opportunity for sheet flow sediment yield between developed flow
paths and the opportunity for development of new concentrated flow
paths. This percentage figure should be assigned to the nearest
five percentage points. An Upland Erosion rating chart is inCluded
on the Revised PSIAC Rating Sheet.

Experienced judgement and field observation are the main tools for
developing the FWP and FWOP conditions for this factor. Should an
area that ,is in good management condition be locally analogous to
the one under study, it can be used as a guide for rating the'
future conditions.

Modification 14: A comparison of the rating of factors
(a,b,c,d,e,f,g) that affect sediment yield to the rating of active
sediment yield factors (h, i) as stated in the PSIAC procedures
(1968, page 2) is a recommended step in the rating process that
should occur in the field. It is proposed that a value of 10
points be used as a maximum difference between the two factor
values. Any differential value greater than 10 points should

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