Geology Reference
In-Depth Information
organization applies to both the computer pro-
gramming and the way that data are input by the
user. The RUSLE2 developers made an effort to
group and consolidate information needed by
RUSLE2 into objects or descriptions that reflected
how users think about the USLE factors. In the
example mentioned above, with RUSLE1 the user
had to use soil-related information not only in
determining the
K
factor, but also in determining
the
LS
factor, where the user chose among soil
classes differing in their relative susceptibility to
rill or inter-rill erosion (Table 4-5, Renard
et al
.,
1997). In RUSLE2, all soil-related information is
included in a soil description, and all manage-
ment information is contained in a management
description. RUSLE2 combines these descrip-
tions with the topographic description to define
another description, that of a hillslope profile
object, and extracts the information it needs from
the descriptions to make erosion computations
based on climate information contained in a
location description.
Databases are maintained at the object level.
Objects may contain other objects and sub-
objects. For example, a management object is
composed of the dates of occurrence of opera-
tion objects (like tillage, planting, or other soil-
disturbing operations) and vegetation objects.
Vegetation objects contain descriptions of
growth patterns, and canopy and residue charac-
teristics needed by RUSLE2 to compute the veg-
etation's influence on erosion. RUSLE2 does not
simulate the growth of vegetation, but rather
takes the information contained in the vegeta-
tion description and accounts for its effect on
the
L
,
C
and
P
factors through numerous influ-
ences on variables tracked or calculated inter-
nally by RUSLE2, including soil biomass, surface
residue cover, surface roughness, canopy cover,
Manning's roughness, and the runoff curve
number. In the USLE, all the factors were inde-
pendent of each other; the
K
,
L
,
S
and
P
factors
were annual constants, while the
R
and
C
fac-
tors were broken down into crop growth phases.
In RUSLE1, the
R
,
K
and
C
factors varied among
24 half-month periods but remained largely
independent of each other, although the
LS
and
ground-cover effects varied with the ratio of rill
to inter-rill erosion, which in turn varied with
soil texture, slope steepness and cover-manage-
ment variables. In RUSLE2, all factors except
S
vary on a daily basis, and there are numerous
interactions among the factors (USDA-ARS,
2008a). Annual averages of the RUSLE2 factors
can be calculated, but the products of these aver-
ages will not equal the average annual erosion
predicted by RUSLE2.
A major improvement in RUSLE2 is that the
user can now define any number of steepness,
soil, or management breaks along the slope, and
the program will accordingly break the slope into
segments representing each combination, and
complete the calculations on those. RUSLE2
overcame limitations in describing complex hills-
lopes that existed in USLE and RUSLE1 by con-
ceiving of hillslopes as being composed of three
layers: topography, soil, and management. Each
of these layers can be segmented independently
to represent any complex one-dimensional hills-
lope situation. RUSLE2 then defines slope seg-
ments as each unique combination of topography,
soil, and management layers. Because of the
inclusion of deposition routines that were not
part of the USLE or RUSLE1, RUSLE2 applies
to hillslopes that include concave areas where
sediment deposition occurs. Also, channels at
the slope bottom, terraces with channels within
hillslopes, impoundments, and sediment basins
may all be described. These features allow
RUSLE2 to compute sediment deposition and
fine-particle enrichment of delivered sediment
using process-based equations. Currently RUSLE2
does not simulate erosion in channels.
This ability to consider slope segments has
also enabled RUSLE2 to deal nicely with the
application of terraces or diversions as a manage-
ment alternative. From a USLE/RUSLE perspec-
tive, the terrace channel becomes the concentrated
flow channel defining the bottom of an upper
hillslope profile, while the top of the terrace itself
defines the beginning of a new lower profile.
Within RUSLE2 this is handled automatically,
defining not only the profiles, but allowing the
user to specify the type of concentrated flow
