Geology Reference
In-Depth Information
0
.70
*
1 - very fine granular
2 - fine granular
3 - med or coarse granular
4 - blocky, platy, or massive
10
.60
90
% OM = 0
1
2
*
Soil structure
1 2 3 4
20
.50
80
3
4
30
70
60
.40
40
.30
50
.20
.70
50
60
.60
.10
40
70
0
.50
30
Percent sand
(0.10-2.0 mm)
Permeability
6
5
43
21
80
.40
2
15
10
5
0
90
.30
6 - very slow
5 - slow
4 - slow to mod.
3 - moderate
2 - mod. to rapid
1 - rapid
100
.20
.10
0
Fig. 8.2
The soil erodibility nomograph that gives
K
in US Customary Units (see Wischmeier & Smith, 1978).
1981). Detailed information on the selection of
slope calculations is given in AH703 Chapter 4.
The difficulty in defining slope length is, how-
ever, substantial enough to have served as the
primary impediment for employing GIS-based
systems in using RUSLE. The topographic data
available to populate GIS databases generally
does not have the spatial resolution necessary to
pick out the small concentrated flow channels
commonly found at the bottom of a USLE/RUSLE
hillslope. As a result, slope lengths computed
using these data are almost always far too long. In
fact, most attempts to use GIS with USLE/RUSLE
recognize this and simply cut off the slope lengths
at some arbitrary value. This poor resolution also
causes the GIS system to miss the flat floodplains
often found at the bottom of a hillslope, where
substantial deposition may occur. This may
change as higher-resolution topographic data
(such as those collected using Lidar) become
available, although how best to use these exten-
sive datasets must still be decided.
In using the USLE, the slope length was defined
as beginning at the top of the hillslope where run-
off starts, and extending down to where the sheet
and rill flow reaches either a concentrated flow
channel or a depositional area. This limit of the
depositional area was required because such dep-
osition rarely occurred on the plots used to col-
lect USLE data. Deposition can be caused by
anything that slows the runoff and causes sedi-
ment to deposit, such as an increase in roughness
caused by a management change (e.g. a strip of
dense vegetation), or a decrease in slope grade.
