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Fig. 16.6 Erosion exceedance
value and effectiveness of
mitigation treatment comparison
for the first five years following a
wildfire predicted by the ERMiT
interface for a 10% exceedance
probability for Example 16.3.
sediment delivery prediction (Robichaud et al .,
2007). Hence the ERMiT-predicted erosion rates
are lower than those observed on silt fence plots
(2.5 Mg ha −1 vs. 17 Mg ha −1 ) and more typical of
the values observed on the larger areas (Table 16.1,
small watershed, <1 Mg ha −1 ). For some of the
selected events, ERMiT predicts more runoff than
precipitation (20-year event, Fig. 16.4). This is
due to melting snow contributing to runoff for
these events. Erosion events in this area are fre-
quently associated with large runoff events from
rain falling on a snow pack (Tonina et al ., 2008),
and such events are included in all the WEPP
predictions.
use GeoWEPP to predict average annual erosion
values, or can use it for risk analysis. GeoWEPP
can be run in two modes, 'Watershed' or 'Flowpath'
(Cochrane & Flanagan, 1999). The watershed
mode is useful in determining sediment delivery
to points of interest downstream from a major
watershed disturbance. In Watershed mode,
GeoWEPP predicts sediment delivery, surface
runoff, and lateral flow from hillslope polygons,
and routes the delivered sediment through the
stream network (Dun et al ., 2009). In Flowpath
mode, GeoWEPP determines distinct flow paths
throughout the watershed, and determines the
distribution of erosion along each flow path, esti-
mating the erosion rate for each pixel in the anal-
ysis. The flowpath mode is useful for determining
the location of the greatest risks of erosion within
a watershed, so that erosion mitigation treat-
ments can be targeted to those areas. With a 30-m
DEM, there are usually two or three flow paths
generated per hectare. For the example GeoWEPP
flowpath run (Fig. 16.7), there were 286 flow paths
identified on a 140-ha watershed.
Elliot et al . (2006) presented methods for apply-
ing risk-based erosion modelling to post-wildfire
16.5 GIS Interface
The GIS interface for WEPP technology is
GeoWEPP (Renschler, 2003). GeoWEPP builds
the stream network from a digital elevation
model (DEM). The user selects the outlet for each
sub-watershed of interest, generally limiting
watershed areas to under 500 ha for a 30-m DEM.
As with the WEPP technology, the user can either
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