RESEARCH· ·1970·

GEOLOGICAL SURVEY RESEARCH 1970
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EVALUATION OF A METHOD FOR ESTIMATING SEDIMENT YIELD
By LYNN M. SHOWN, Denver, Colo.
•,.
A.1J8t?·a.ct.-A method for estimating annual sediment yield,
developed by the Pacific Southwest Inter-Agency Committee,
wus tested iu 28 small watersheds ranging in size from 0.02 to
37 squn re miles. These watersheds for which hydrologic records
exist nre in nricl nnd seminrid arens of western Colorado,
north-centrnl New Mexico, nnd east-centrnl 'Vyoming, and
represent a vnriety of geologic, climntic, ecologic, and topogt·nphic
conditions. Estimates of sediment yield correlated
closely with sediment yields mensured in reservoirs, but the
estimutes were usunlly lower than the measured yields. The
method wns designed to mnl(e general sediment classifications
for nrens gt·eater than 10 square miles, but it also appears to
work nearly ns well for watersheds that are as small as 0.1
square mile.
Sediment-yield records for drainage areas of less
than 500 square miles in wildland areas of the '¥estern
United States are scarce. :Managers of land in these
areas need estimates of sediment yield to aid in decisions
which concurrently affect water, soil, timber,
forage, and recreation. A sedimentation task force of
the Pacific Southwest Inter-Agency Committee (PSI­
AC) prepared n, guideline method for field evaluation
of sediment yields, in conjunction with a discussion of
factors affecting sediment yields in the Pacific Southwest
area.
The purpose of this investigation was to determine
the usefulness of the PSIAC method for extrapolating
sediment-yield information to areas lacking records.
METHOD
The 1nethod developed by the PSIAC (1968) consists
of rating the watershed on the basis of nine factors
shown in table 1. A numerical rating is arrived at by
summing the values assigned to the factors. A suggested
range of values for each factor is included in
the table. If a factor would be likely to result in a
small sediment yield, n, value near the lower end of
the range is assigned. A value near the upper end of
the range is assigned when the factor would cause the
sediment yield to be large. For exmnple, surface geol-
TABLE 1..-Rating ranges for the factors evaluated in the Pacific
Southwest Inter-Agency Committee method for estimating sediment
yields using terrain characteristics
Factor
Rating
range
Surface geology____ 0-10
Soils _____________ 0-10
Climate__________ 0-10
Itunoff___________ 0-10
Topography_______ 0-20
Ground cover_____ -1Q-10
Land use _________ -10-10
Upland erosion____ 0-25
Channel erosion 0-25
and sediment
transport.
Main characteristics considered
Rock type.
Hardness.
Texture.
Aggregation.
Shrink-swell.
Rockiness.
Weathering.
Fracturing.
Salinity.
Caliche.
Organic matter.
Storm frequency, intensity, and
duration.
Snow.
Freeze-thaw.
V clume per unit area.
Peak flow per unit area.
Steepness of upland slopes.
Relief.
Fan and flood plain development.
Vegetation.
Litter.
Itocks.
Understory development under
trees.
Percentage cultivated.
Grazing intensity.
Logging.
Ito ads.
Rills and gullies.
Landslides.
Wind deposits in channels.
Bank and bed erosion.
Flow depths.
Active headcuts.
Channel vegetation.
ogy would be assigned a value of 10 in an area underlain
by soft fissile shale, and a value of 1 in an area
underlain by several thick basalt flows.
Each factor except topography is paired with another
factor that has a similar influence on sediment
yield. The pairings are : surface geology and soils,
climate and runoff, ground cover and land use, and upland
and channel erosion. Each factor is rated separately,
but the one it is paired with is usually considered
concurrently, and the degree of interdependence of the
two is reflected in the similarity or lack of similarity
in their respective ratings.
U.S. GEOL. SURVEY PROF. PAPER 700-B, PAGES B245-B249
B245