Environmental Engineering Reference
In-Depth Information
Table 22.4
Sensitivities of changes in soil loss to changes in average annual precipitation. Sensitivity values are calculated as the
ratio of the percent change in soil loss to the percent change in precipitation. Values represent averages for all simulation runs
associated with the soil, crop, slope, or location listed in the first column. Values greater than zero indicate that soil loss increases
with increased annual precipitation. A value of greater than one indicates a greater percentage change in soil loss than the
percentage change in precipitation.
Scenarios
Normalized sensitivity of soil loss to changes in average annual precipitation
Change in
Change in
Combined changes in
number of wet days
amount of rain per day
Both
Silt loam soil
0.90
2.45
1.72
Sandy loam soil
0.89
2.60
1.82
Clay soil
0.79
2.10
1.46
Grazing pasture
1.02
2.66
1.96
Fallow
0.95
2.22
1.71
Corn and soybean
0.70
2.46
1.48
Wheat winter
0.77
2.18
1.50
S-shape (0%-3%-1%) 40m
0.92
2.47
1.71
S-shape (0%-7%-1%) 40m
0.84
2.40
1.67
S-shape (0%-15%-1%) 40m
0.82
2.27
1.61
West Lafayette, IN
0.74
2.35
1.56
Temple, TX
0.88
2.10
1.50
Corvallis, OR
0.92
2.69
1.93
Overall average
0.85
2.38
1.66
was the only crop treatment for which the sensitivities for
runoff were less than for soil loss.
The difference between a sensitivity of 0.95 for soil loss
and 1.06 for runoff for the fallow scenario of change only
in the number of days of rainfall (Tables 22.4 and 22.5) can
be explained in terms of surface sealing and consolidation
processes. Surface sealing and consolidation occur as a
function of rainfall amount in nature and in the WEPP
model (Flanagan and Nearing, 1995), so that any increase
in rainfall will increase soil resistance to erosion via
consolidation. This process also acts as a feedback effect,
similar to the effect of rainfall-enhanced biomass growth,
which partially offsets the impact of the increased rainfall
on erosion and explains the lesser sensitivity of 0.95 for
soil loss as compared to 1.06 for runoff.
The soil-loss-sensitivity value for fallow conditions for
the scenario of change in amount of rainfall per day
was greater (2.22) than that for runoff (1.99), whereas
for the other crops the trend was reversed (Tables 22.4
and 22.5). Although the effects of surface sealing and
consolidation, as discussed above, are present in this
case, that effect is apparently superseded by yet another
process when rainfall amounts and intensities per day
are increased. These processes were related to rill and
interrill soil-detachment processes. Interrill erosion rates
are represented in the WEPP model as proportional
to the rainfall intensity and the runoff rate (Flanagan
and Nearing, 1995), which are relationships based on
experimental data (Zhang
et al
., 1996). Both of these
variables increase with increased rainfall intensity, so the
effect of increased rainfall intensity on interrill erosion is
greater than unity. Rill erosion also occurs as a threshold
process. Rill detachment occurs proportional to the excess
shear stress of water flow above the threshold critical shear
stress of the soil, rather than to the shear stress of the flow
itself. The overall effect is that the sensitivity of the rill
erosion rate to runoff rate will be somewhat more than
unity, other factors remaining constant. The effect is not
present in the precipitation scenario of changes in the
number of rainfall days because in that case, the average
runoff rate is essentially not changing, but rather only the
frequency of runoff events changes.
These are only a portion of the interactions discussed by
Pruski and Nearing (2002) that were evident in the results
of this study, but they provide a flavour of the types
of information that the process-based model provides,
which the empirical model cannot address. Hopefully the
above discussions of these two model application will
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