Environmental Engineering Reference
In-Depth Information
In applying Equation (6.27), it should be noted that
the proportion between rainfall and runoff erosiv-
ity may vary greatly between regions, and that
Equation (6.27) takes into account the influence
of peak runoff rate on soil erosion. Equation
(6.27) has been used to link rainfall runoff models
with erosion prediction.
Making use of the unit energy relationship given by
Equation (6.25), Renard et al. (1991) provided the
details of a method for computing
R
-values for
individual storm events. Froehlich (2009) used the
NRCS nondimensional rainfall distributions and
showed that the erosivity factor,
R
(=
EI
30
) for
storms of 24-hour duration could be estimated by
the relations
TABLE 6.8. Magnitude of Soil Erodibility Factor,
K
(t·h·MJ
−
1
·mm
−
1
)
Organic Matter Content
Soil texture
0.5
2
4
Sand
0.05
0.03
0.02
Fine sand
0.16
0.14
0.10
Very fine sand
0.42
0.36
0.28
Loamy sand
0.12
0.10
0.08
Loamy fine sand
0.24
0.20
0.16
Loamy very fine sand
0.44
0.38
0.30
Sandy loam
0.27
0.24
0.19
Fine sandy loam
0.35
0.30
0.24
Very fine sandy loam
0.47
0.41
0.33
Loam
0.38
0.34
0.29
Silt loam
0.48
0.42
0.33
Silt
0.60
0.52
0.42
Sandy clay loam
0.27
0.25
0.21
0.0232
P
2.229
for Type I storms
24
Clay loam
0.28
0.25
0.21
Silty clay loam
0.37
0.32
0.26
0.0090
P
2.278
for Type IA storms
Sandy clay
0.14
0.13
0.12
24
Silty clay
0.25
0.23
0.19
R EI
=
=
(6.28)
30
Clay
-
0.13-0.29
-
0.0657
P
2.161
for Type II storms
24
Source
: Stewart et al. (1975).
0.0424
P
2.18
7
for Type III storms
24
Ranzi et al. (2012) assumed
K
= 0.022 t·h·MJ
−1
·
mm
−1
based on data for watershed units in Viet-
nam's northern highlands, and their assumed value
was also in agreement with
K
values under similar
conditions in China.
LS.
The slope length factor,
LS
(dimensionless), in
Equation (6.23) is a function of the overland
runoff length and slope, and it adjusts the soil loss
estimates for effects of length and steepness of the
field slope. The
LS
factor can be estimated using
the following relation (Stewart et al., 1975):
where
EI
30
is in MJ·mm·ha
−1
·h
−1
·yr
−1
and the 24-
hour precipitation,
P
24
is in mm. Equation (6.28)
is applicable for estimating
R
-values for single 24-
hour duration storms. For storms with durations
other than 24-hour, approximate analytic expres-
sions developed by Froehlich (2009) can be used
to estimate
R
-values.
An alternative equation developed by Loureiro
and Coutinho (2001) for estimating
R
(MJ·
mm·ha
−1
·h
−1
·yr
−1
) is given by
N
12
m
1
L
∑
∑
=
(
)
(
)
LS
2
(6.30)
R
=
7.05
⋅
rain
−
88 92
.
⋅
days
(6.29)
0 065 0 04579
.
+
.
S
+
0 0065
.
S
10
10
m i
,
N
22 1
.
i
=
1
m
=
1
where
N
is the number of observation years, rain
10
is monthly rainfall [mm], when ≥10 mm, otherwise
it is set to zero, day
10
is the monthly number of
days with rainfall ≥10 mm.
K.
The
soil erodibility factor
,
K
(t·h·MJ
−1
·mm
−1
), in
the RUSLE is a measure of potential erodibility
of soil relative to erosion over a 22-m (72-ft) long
overland flow length on a 9% slope in clean-tilled
continuous fallow soil. This factor is a function of
soil texture, organic matter content, and permea-
bility, and values of
K
can be estimated using Table
6.8. Site-specific values of
K
can be considerably
different that those given in Table 6.8. For example,
where
L
is the length (m) from the point of origin
of the overland flow to the point where the slope
decreases to the extent that deposition begins or
to the point at which runoff enters a defined
channel,
S
is the average slope (%) over the runoff
length, and
m
is an exponent dependent on the
slope steepness, as given in Table 6.9. If the average
slope is used in calculating the
LS
factor, the
average slope will underestimate the
LS
factor
when the actual slope is convex and overestimate
the erosion when the actual slope is concave. To
minimize these errors, large areas should be
broken up into areas of fairly uniform slope, and
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