Geology Reference
In-Depth Information
Fig. 16.2
Return period analysis
output screen from WEPP
Windows interface for
Example 16.1.
volume, as WEPP considers daily conditions of
the vegetation and soil water content before mak-
ing runoff and erosion predictions. A detailed
examination of the WEPP output for this sedi-
ment delivery event showed that it occurred on 2
August, year 32, when 43.3 mm of precipitation
resulted in 4.5 mm of runoff. This storm was the
25-yr return period precipitation event, but the
24-h runoff depth was less than that for a 2-yr
event (Fig. 16.2). The peak runoff rate predicted
for the storm was 29.2 mm h
−1
, the value for the
20-y return period peak runoff rate. Since the
WEPP simulations show that there were no major
runoff events in the four months prior to this
storm, the hydrology was driven by the large pre-
cipitation event only.
If model users wish to consider the risk of
exceeding a given level of erosion for an entire year,
as a function of the variability in weather, then the
user can request the detailed annual output from
the WEPP model, and note the annual erosion rates
for 50 or more years. These can either be analysed
using a return period analysis technique, or simply
ranked, with the year with the highest value serv-
ing as an estimate for the erosion with a probability
of occurrence of one in the length of run, and the
second largest value having a probability of occur-
rence of two in the length of run, and so on. This
process has been programmed into the online
Disturbed WEPP Interface (Elliot, 2004).
16.4 Online Interfaces
Two online interfaces have been developed that
incorporate risk-based erosion prediction for forest
conditions (Elliot, 2004), and they can be accessed
at http://forest.moscowfsl.wsu.edu/fswepp. One
interface, Disturbed WEPP, provides both the
average annual runoff and erosion estimates, and
the annual return period values for precipitation,
runoff, upland erosion, and sediment delivery.
Example 16.2
The climate, soil, topography and
vegetation cover conditions described for Example
16.1 were entered into the online input screen for
Disturbed WEPP. A 50-year run was selected and
the recommended cover calibration was carried
out (Elliot, 2004). The model was then run and
the output screen presented (Fig. 16.3).
The return period analysis (top part of Fig. 16.3)
shows that following a wildfire, there is a 1 in 10
chance that annual sediment delivery from this
hillslope will exceed 13.0 Mg ha
−1
. The second out-
put from Disturbed WEPP (bottom of Fig. 16.3) is
that there is an 82% probability that there will be
sediment delivered in the year following a wildfire.
The average predicted erosion rate is 4.3 Mg ha
−1
.
The predicted erosion rate is greater than the
value predicted in Example 16.1 because it is for
a full year and not a single storm, and maybe
because there are different versions of WEPP
