Geology Reference
In-Depth Information
the flatter portion at the bottom of an S-shaped
slope: thus the complexity of Step 3 in Plate 6,
which in the previous views was shown simply
as single uniform slope length and steepness.
The slope schematic in the upper-right corner of
the view displays this complexity. The upper
(management) layer of this schematic shows a
management break about 55 ft (16.8 m) down the
slope. This is caused by the selection in Step 5 of
a pre-defined strip-barrier system, which in this
case puts a single 20-ft strip of poor stand cool-
season grass at the bottom of the slope. In addi-
tion, Step 5 sets that the runoff from the bottom
of the slope feeds to a sediment basin, which is
pre-defined as having an 80% settling efficiency
for a silt loam soil that has not experienced pre-
vious deposition. This last clause is important
because any deposition occurring before the
runoff hits the sediment basin will cause the
coarse material to settle out, thereby reducing
the actual efficiency of the basin. In the specific
case shown here, there will be deposition at the
bottom of the slope caused by both the decreased
slope steepness and the grass strip, so the basin
will not provide 80% efficiency. If they had so
desired, the users could have added additional
complexity to the view to show where the depo-
sition actually occurred, but this was not deemed
worthwhile.
loss (erosion). The results from this research, in
combination with additional crops and cultural
practices data, ultimately provided a repository
of data widely used by engineers and scientists to
evaluate conservation practices. These data were
the foundation of the empirical erosion predic-
tion technologies and ultimately the Universal
Soil Loss Equation (USLE).
The USLE was developed at Purdue University
under the direction of Walter Wischmeier, with
able assistance from Dwight Smith, and was pub-
lished in 1965 and 1978 in two handbooks (AH282
and AH537). The handbooks became widely
accepted for conservation farming (and especially
soil erosion by water) in the US. In the early 1980s
a program to develop technology to replace the
USLE was initiated. The computer-based RUSLE
(Revised Universal Soil Loss Equation) model was
published in 1997. RUSLE incorporated signifi-
cant advances over the USLE and permitted appli-
cation of soil erosion estimation for a greater
variety of crops and management practices
beyond those in the original USLE database.
RUSLE was subsequently revised to include
advanced scientific and interface technology and
subsequently delivered as RUSLE2, along with
expanded databases and more control over the
parameters that specific users could see and
change. The USDA-NRCS has accepted responsi-
bility for the underlying databases within the US,
which include descriptions of climates, vegeta-
tions and soils, along with extensive files describ-
ing common management practices. RUSLE2 is
widely recognized as a major advance in erosion
prediction and conservation technology, and pro-
vides a very flexible tool allowing resource con-
servationists, managers and developers to
compare a broad range of management alterna-
tives in deciding on an optimum resource use.
8.4 Summary
Soil erosion has long been recognized as a serious
problem. Considerable efforts have been expended
to address this problem, beginning in Missouri in
1923 and supported by the US Congress in a 1929
appropriation that initiated intensive soil ero-
sion research. Early efforts to preserve soil and
prevent erosion through the work of pioneers
like H.H. Bennett led to an early period of plot
scale conservation research at sites representing
the ten major farming regions in the US. The 6 ft
(1.8 m) wide by 72.6 ft (22.1 m) long (0.01 acre,
40 m
2
) research plots were constructed to repre-
sent various crops and rotations. Primary meas-
urements included precipitation, runoff and soil
References
Austin, M.E. (1981) Land resource regions and major
land resource areas of the United States.
USDA
Agricultural Handbook No
. 296. 156 pp.
Bengtson, R.L. & Sabbage, G. (1988)
USLE P-factors for
subsurface drainage in a hot, humid climate
. ASAE
