Geology Reference
In-Depth Information
mass (acre lb
−1
), and
B
s
is the dry weight of crop
residue on the surface (lb ac
−1
). If more than one
type of residue is present, the resulting total sur-
face area cover is calculated as:
R
u
=
0.24
+
[
D
r
(
R
i
−
0.24)]
(8.16)
where
R
u
is in inches. Since many field operations
affect only a portion of the surface,
R
u
is also the
roughness of that field portion left undisturbed
by the current operation.
For that surface portion affected by the field
operation, the resulting roughness has been found
to be a function of subsurface biomass present in
the top 4 in. of soil. The relationship is:
S
p
=
{1
−
exp[
−Σ
(
α
i
·
B
si
)]} · 100
(8.14)
where
a
i
is the ratio of area covered to the mass of
that residue for each type encountered. The sum-
mation is for each type of residue, as each residue
type may have a unique
a
i
value.
R
a
=
0.24
+
(
R
t
−
0.24)
Surface roughness subfactor (SR)
Surface rough-
ness has been shown to affect soil erosion directly,
and also to affect it indirectly through the impact
of residue effectiveness as controlled by the
b
value in Equation (8.12). The surface roughness
subfactor is a function of the surface random
roughness, which is defined as the standard devi-
ation of surface elevations across the slope, when
changes due to land slope or non-random tillage
marks (such as dead furrows, traffic marks, and/
or disk marks) are removed from consideration.
A rough surface has many depressions and barri-
ers. During a precipitation event, these trap water
and sediment, causing rough surfaces to erode at
lower rates than do smooth surfaces under simi-
lar conditions. Increasing surface roughness
decreases transport capacity and runoff detach-
ment by reducing flow velocity.
Roughness and cloddiness of soils also affect
the degree and rate of soil sealing from raindrop
impact. Soils that are left rough and cloddy typi-
cally have greater infiltration rates. Soils that are
finely pulverized are usually smooth, seal rapidly,
and have low infiltration rates. RUSLE assumes
that roughness decreases with the time since till-
age by the relationship:
{0.8 [1
−
exp (
−
0.0012
B
u
)]
+
0.2}
(8.17)
where
R
a
is the roughness after biomass adjust-
ment (in.),
R
t
is the original roughness based on
the assumption of ample subsurface biomass
such as is found with high-yielding US-type corn,
and
B
u
is total subsurface biomass density in the
top inch of soil (lb ac
−1
in
−1
), with
B
u
=
B
ur
+
B
us
as
used in Equation (8.10).
The adjusted tillage roughness is then com-
bined with that of the undisturbed portion of the
surface as follows:
R
n
=
R
a
F
d
+
R
u
F
u
(8.18)
where
R
n
is the net roughness following the field
operation (in.) and
F
d
and
F
u
are respectively the
fractions of the surface disturbed and undisturbed,
such that their sum equals one.
Similarly, the roughness decay coefficient
must be adjusted to reflect that only a portion of
the field is disturbed using the relation:
D
e
=
D
r
F
u
+
1.0 F
d
(8.19)
where
D
e
is the equivalent roughness decay coef-
ficient. RUSLE then reorganizes the relation-
ships described above to calculate the
R
t
,
P
t
and
EI
t
values corresponding to the equivalent rough-
ness decay coefficients, under the assumption
of a constant
EI
t
/
P
t
ratio. If a site is clean-tilled
and left without human intervention, two
things will happen: (1) the tillage roughness will
decrease as defined previously; and (2) as time
passes, vegetation will tend towards its climax
D
r
=
exp[ ½(
−
0.14
P
t
)
+
1/2(
−
0.012 ·
EI
t
)] (8.15)
where
D
r
is the dimensionless roughness decay
coefficient,
P
t
is the total inches of rainfall since
the most recent soil-disturbing surface operation,
and
EI
t
is the total
EI
amount since that operation.
If the initial roughness is defined as
R
i
, then
surface roughness just before a new tillage opera-
tion (
R
u
) can be defined as:
