RESEARCH· ·1970·

B248
HYDROLOGIC TECHNIQUES
type in the upper area. Another 20 percent of the watershed
has a silty-clay soil derived from Cody Shale.
Roots penetrate this soil about 1 foot. This area is
covered with Nuttall saltbrush and bottlebrush squirreltail
(Sitanion hystrix). The remaining 10 percent of
the watershed is comprised of high, grass-covered
plateaus and of bottomlands covered with greasewood
(Sarcobatus vermiculat1ts). Average annual precipitation
is estimated to be about 10 inches (N.J. King, oral
commun., 1969). · Runoff occurs from snowmelt and
rains in the spring, and from occasional torrential summer
storms (Peterson, 1961). Additional hydrologic
information, including a 16 ... year record of sediment
yield through July 1968, was obtained from King
(1959) and from N.J. King and G. C. Lusby (written
commun., 1969).
RE,SULTS AND DISCUSSION
Figure 2 shows the relationship of estimated sediment
yields obtained with the PSIAC method to
measured sediment yields for three sets of watersheds.
In all three sets, with the exception of the low-sediment-yielding
watersheds in Wyoming, the estimates
tend to be lower than reservoir values, as most of the
points lie above the 1 :1 lines in figure 2. Also, the mean
sediment-yield estimate for all sites is 1.4 acre-feet
per square mile, and the mean for the reservoir records
is 1.73.
The tendency for the estimated sediment yields to
be lower ·than reservoir records is attributed to the
author's bias in rating some of the factors lower than
they should have been, but no factor was consciously
rated consistently low. After the watersheds had been
rated, it was apparent that the channel erosion and
sediment transport factor had not been rated high
enough for some of the steeper watersheds having raw
gullies. The ratings were accordingly increased for
some of the Badger Wash and New Mexico watersheds,
resulting in smaller differences between the estimated
and measured sediment yields. The correlation of measured
runoff with values for the runoff factor in the
method was rather poor; the coefficient of correlation
(1·) was 0.34. No objective means of adjusting values
for the runoff factor was found.
The correlation of watershed area with the difference
between reservoir measurements and the estimates was
very poor ( r = 0.13). Apparently, on very small watersheds,
the PSIAC method sufficiently accounts for expected
larger sediment yields per unit area caused by
greater average slope, intense storms covering the
whole watershed, and small transit losses. This indicates
that the method may have application in the de-
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ESTIMATED SEDIMENT YIELD, IN
ACRE-FEET PER SQUARE MILE
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FIGURE 2.-Graphs showing reLationship of estimated
sediment yields to measured sediment
yi,elds, determined by the Pacific Southwest
Inter-Agency Committee method, for reservoirs
in three sets of watersheds: A, Badger Wash,
Colo.; B, New Mexico; C, Wyoming. All correlation
coefficients ( 1·) are significant at the 1-
percent level. The solid lines are lines of
equality where the estimated sediment yields
equal the measured sediment yields. The
dashed lines are regression lines fitted to the
points.
sign of structures for small watersheds and in proposals
for land treatment, in addition to its use for 1naking
broad sediment classifications of larger areas.
In applying this method, good records from gaged
areas, where available, should be used to check the estimates.
If the estimates and records do not reasonably
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