Road segment effect on soil loss prediction
Figure 5 illustrates the GeoWEPP “Flowpath” mode simulated onsite soil
loss for the study area under the post-fire conditions. The paved road
surface showed minimal soil loss and can be distinguished from other
areas with the yellow or green deposition or zero erosion categories. In
some locations, deposition (yellow pixels) was predicted to occur on the
road surface when a flow path reached a road segment. Detailed
deposition on the surface of road 1 was simulated by running 22
different hillslope profiles that intersected with Road 1 in WEPP
Windows. From these runs, the average annual sediment deposition along
the road segment within the fire was calculated as 40.4 kg
m-2, or a sediment deposition depth averaging 4 cm on
the Road 1 surface.
To evaluate the surface hydrology unit effect on soil loss due to road
influence, the onsite soil loss was classified into 6 categories and
overlaid with sub-catchments. Table 5 shows that sub-catchments that
originated from the ridge resulted in a larger fraction of the high soil
loss category (greater than 4 Mg ha-1yr-1) than the sub-catchments below roads. The average
soil loss rate also showed that sub-catchments below the main ridgeline
had the greatest onsite soil loss. Similarly, the sub-catchments below
roads showed relatively more deposition and much lower average soil loss
rates than those above roads. For sub-catchments below road 2, the
average predicted onsite soil loss was negative, indicating that
deposition was the dominant process.
To explore soil erosion variation with terrain positions in more detail,
the onsite soil loss rate for different distances from ridge and road
segments were measured by buffer analysis. The predicted onsite soil
loss rate was extracted for each 10-m-width belt, and then the average
value was calculated and plotted with distance (Figure 6). It can be
seen that the average onsite soil loss rate below ridge increased
exponentially with the downslope distance. On the other hand, the
average soil loss rates below both road 1 and road 2 showed a
logarithmically declining trend with the downslope distance. The first
10-m buffer belt below roads showed remarkably higher soil loss rate
compared to belts further below. The average onsite soil loss within the
first 10-m belt below road 1 is close to that of the last belt below the
ridge. Similarly, the average onsite soil loss rate within the first
belt below road 2 was similar to the last belt below road 1. When the
flow path goes beyond 50 m downslope from road 2, deposition tends to
occur. This is consistent with the earlier results that measured erosion
by sub-catchment and observations in the field. Note that the average
onsite soil loss rate 70 m below road 2 (Figure 6.c) showed an
abnormally low value due to the high deposition rates predicted within
the channels. This low value was not included when developing the
regression equation shown on the graph.
The GeoWEPP Watershed analysis divides the study area into hillslope
polygons, each with a representative hillslope profile (Flanagan et al.,
2013). From the GeoWEPP watershed outputs, one of the hillslope polygons
that passed through road 1 was selected and run on its own in WEPP
Windows to generate a typical soil loss graph along the hillslope
profile (Figure 7). Figure 7 shows that the erosion rate generally
increased as the slope length increased from 0 to 140 m due to runoff
accumulation. When reaching the road segment, the erosion rate
dramatically decreased and deposition occurred on the road surface
section. As runoff exited the road and continued flowing downslope, soil
loss rate increased sharply within a short distance below road segment,
likely due to the steep fillslope and clearer water. The loss reached a
maximum value for the profile, and then gradually declined as the
increase in runoff due to increased slope length was offset by a
decrease in the hillslope steepness.