Geology Reference
In-Depth Information
factor. For example, during cool and wet peri-
ods, higher antecedent soil water is likely to
increase runoff and soil erosion, thus
K
should
be higher. Similarly, increased temperature is
expected to increase evapotranspiration, lead-
ing to lower antecedent soil moisture, lower
runoff, and reduced
K
values. Relationships in
RUSLE2 capture the main effects of seasonal
variation in
K
at each location based on the
ratio of temperature and precipitation values at
each location to the average annual values at
the reference location (Columbia, MO). For
identical soil descriptions, these adjustments
will increase the annual effective
K
at locations
that are cooler and wetter than Columbia, MO,
while average
K
values will be lower than the
nomograph value at locations that are hotter
and drier than Columbia, MO.
In RUSLE2, inclusion of the CREAMS (Foster
et al
., 1980) sediment transport and deposition
relationships requires knowledge of the sedi-
ment size distribution at the point of detach-
ment, so the diameter, specific gravity, and
primary particle composition of each of five size
classes is calculated as a function of soil clay
using equations similar to those in CREAMS
(Foster
et al
., 1985). The effect of drainage on
runoff and sediment transport is discussed below
with regard to the
P
factor.
where the ratio
K
r
/
K
i
is the inherent rill to inter-
rill soil erodibility ratio computed as a function
of soil texture (as discussed in the text following
Equation (8.3) ); the term
c
pr
/
c
pi
reflects the effect
of prior land use on the rill to inter-rill erosion
ratio; the ratio exp(-
b
r
f
g
)/exp(-0.025
f
g
) reflects
how ground cover affects rill erosion more than it
affects inter-rill erosion,
b
r
and 0.025 are coeffi-
cients (%
−1
) that express the relative effectiveness
of ground cover for reducing rill erosion and inter-
rill erosion, and
f
g
is ground cover expressed as a
percentage. The last term is the same as Equation
(8.3). Equation (8.24) shows how RUSLE2 takes
the information stored in the topographic, man-
agement, and soil objects and uses it to calculate
needed coefficients, thus reducing the need for
users to specify unfamiliar parameters. The fact
that the rill to inter-rill erosion ratio, as calcu-
lated from Equation (8.24), is independent of slope
length (when it really is not) illustrates the price
that RUSLE2 pays for the ability to retain the
simple and familiar USLE equation structure.
Complex slopes can be represented in RUSLE2
to provide a better approximation of topography.
A broad range of process-based routines allows
for calculation of deposition caused by either
management or topographic changes. This means
that, for RUSLE2, the hillslope is defined as from
where runoff begins until it enters a concentrated
flow channel, which is the same definition as for
WEPP.
(iii) Changes to the topographic description
Whereas the rill to inter-rill erosion ratio in
RUSLE1 was selected by the user, in RUSLE2 this
ratio is calculated internally based on soil tex-
ture, prior land use (soil biomass and soil consoli-
dation) effects, ground cover and slope steepness.
This ratio determines the slope length exponent,
m
, in Equation (8.2), which controls the sensitiv-
ity of sheet and rill erosion to slope length. Instead
of using Equation (8.3), the ratio of rill to inter-rill
erosion in RUSLE2 is computed from (USDA-
ARS, 2008a):
(iv) Changes to the management description
One significant change from RUSLE1 to RUSLE2
was the grouping of field operations and vege-
tations into a separate management object or
description. Management objects comprise
descriptions of field operations (their dates of
occurrence, and their effects on surface cover and
surface roughness) with vegetation descriptions
whose growth is begun by the operation (if any)
and the yield expected for that vegetation, and
the amount and type of external residue added to
the surface if a mulching operation. Management
descriptions result in daily tracking of an exten-
sive suite of variables that affect sheet and rill
erosion, including canopy cover, standing residue,
æ
ö æ
ö
æö
c
exp(
-
b f
)
K
s
/ 0.0896
æ
ö
pr
r g
b
=
r
ç
÷ ç
÷
ç÷
ç
÷
ç
÷ ç
÷
0.8
Kc
exp( 0.025 )
-
f
3
s
+
0.56
è
ø
è
è
ø è
ø
i
pi
g
(8.24)
