Geology Reference
In-Depth Information
based on the parameters that control the subfac-
tors for surface cover, biomass and roots in the
soil, surface roughness, vegetative canopy cover,
and soil moisture. Once those relationships are
developed using field data, if the RUSLE1 pro-
gram can model the effect of any field mana-
gement operation on those parameters (soil,
vegetation, biomass), it should be able to model
the resulting erosion.
The subfactor approach and the equations con-
trolling it are described in great detail in AH703.
What follows is a brief introduction to the subfac-
tors included in RUSLE1.
account for residue additions or losses by
decomposition.
Canopy cover subfactor (CC)
The canopy-
cover subfactor indicates the effectiveness of the
vegetative canopy in reducing the energy of rain-
fall striking the soil surface. Although most of
the rainfall intercepted by canopy eventually
reaches the soil surface, it usually does so with
much less energy than rainfall directly striking
the ground. The intercepted drops fracture into
smaller drops, or drip from leaf edges, or travel
down crop stems to the ground. The canopy-
cover effect is given as:
Prior land-use subfactor (PLU)
The
PLU
sub-
factor is calculated in RUSLE as the product of
soil consolidation and soil biomass effects:
CC
=
1
−
F
c
·
exp
(
−
0.1
· H
)
(8.11)
where
CC
is the canopy-cover subfactor ranging
from 0 to 1,
F
c
is the fraction of land surface cov-
ered by canopy, and
H
(ft) is the distance that
raindrops fall after striking the canopy.
PLU
C
f
· C
b
· exp-[(
c
ur
·
B
ur
)
=
(8.10)
+
(
c
us
·
B
us
/
C
f
cuf
)]
where
PLU
is the prior land-use subfactor (rang-
ing from 0 to 1),
C
f
is a surface-soil-consolidation
factor,
C
b
represents the relative effectiveness of
subsurface residue in consolidation,
B
ur
is the
mass density of live and dead roots in the upper
100 mm (lb acre
−1
in
−1
),
B
us
is the mass density
of incorporated surface residue in the upper
100 mm of soil (lb acre
−1
in
−1
),
C
uf
represents the
soil consolidation impact on the effectiveness of
incorporated residue, and
c
ur
and
c
us
are calibra-
tion coefficients indicating subsurface residue
impacts.
The
B
u
variables calculate the impact on ero-
sion rates of live and dead roots and incorporated
residue. The effectiveness of such materials can
take two forms. Firstly, roots and residue can
control erosion directly by physically binding soil
particles together and acting as mechanical barri-
ers to soil and water movement. Secondly, roots
and residue exude binding agents and serve as a
food source for micro-organisms that produce
other organic binding agents. These serve to
increase soil aggregation and thereby reduce sus-
ceptibility to erosion. The RUSLE software keeps
track of the biomass in each layer, continuously
adjusting the rootmass and subsurface residue to
Surface cover subfactor (SC)
Surface cover
affects erosion by reducing the transport capacity
of runoff, by causing deposition in ponded areas,
and by decreasing the surface area susceptible to
raindrop impact. This is perhaps the single most
important factor in lowering SLR values. Surface
cover includes crop residue, rocks, cryptogams,
and other non-erodible and non-mobile material
in direct contact with the soil surface. The effect
of surface cover on soil erosion is given as:
SC
=
exp[
−
b
·
S
p
(0.24 /
R
u
)
0.08
]
(8.12)
where
SC
is the surface cover subfactor,
b
is a
coefficient,
S
p
is the percentage of land area cov-
ered by surface cover, and
R
u
is the surface rough-
ness, as will be defined later.
Land area percentage covered by residue can
be estimated from residue weight by the relation-
ship of Gregory (1982):
S
p
=
[1
−
exp (
−α
·
B
s
)] · 100
(8.13)
where
S
p
is percentage residue cover,
is the ratio
of the area covered by a piece of residue to its
α
