WATERSHED EROSION PREDICTION PROJECT (WEPP) SOIL EROSION RATES
for FIA/FHM PHASE 3 PLOTS in IDAHO
M.C. Amacher1, K.P. O’Neill2, and B.L. Conkling3
Rocky Mountain Research Station, 2USDA-FS North Central Research Station, and 3North Carolina State University
WEPP-predicted soil erosion rates on FIA P3 plots
State: Idaho
Disturbance: none (mature forest)
te, tons/acr e
Soil er osion ra
Disturbed WEPP inputs
None
Precipitation erosivity factor (R)
Climate station data
40
Soil texture
Soil erodibility factor (K)
Soil texture
30
% bare soil, % litter cover, %
plant cover, litter thickness, plant
canopy height
Cover factor (C)
Total soil cover = 100 - % bare
soil2
20
Slope factor (LS)
Cultural practice factor (P)
Forest condition class and
ancillary data
Number of plots
None
Slope length
Not applicable
Forest or disturbance type3
Not applicable
1 A = RKLSCP
where A is the predicted soil loss, R is the precipitation erosivity factor, K is the soil erodibility factor, L is slope length, S is
slope gradient, C is the cover and management f actor, and P is the erosion control practice factor.
2 Only % bare soil is needed to run WEPP, the other cover values, litter thickness, and plant canopy height data are needed
for RUSLE, but not for WEPP.
3 Choices are 20-year old forest (mature forest), 5-year old forest (young forest), low-severity fire, high-severity fire, or skid
trails.
Paradise Dam
Soil texture 2
Loam
Element3
2
Slope length
Gradient5
(%)
Length 6
(ft)
Area7
(acres)
20-year old forest
35
112
20-year old forest
35
Cover8
(%)
99
1.33
112
99
Select from climate station database. Use climate station with similar elevation nearest to plot location.
Choices are sandy loam, silt loam, clay loam, and loam. Since slightly different soil textures are collected in the field, use the soil texture
conversion table to match the soil textures collected in the field to those used by WEPP.
WEPP div ides a hillslope into upper and lower areas or elements, but it can be applied to portions of a hillslope (e.g., plot area). Apply
the WEPP model to a rectangular area surrounding the four subplots (fig. 1). Divide the area into two halves with equal slope lengths
(e.g., 112 + 112 = 224 ft). Also, you may want to consider the orientation of the plot on the hillslope when selecting slope lengths since
the area surrounding all four subplots is a rectangle and not a square.
4 Choose a forest or disturbance type. Choices f or forest type are 20-year old forest (mature forest) or 5-year forest (young forest).
Choices for disturbance type are low-severity fires, high-severity fires, or skid trails.
5 Use the mean of the slopes for all f our subplots in the WEPP input table. Normally, the 1st slope of the upper element is the top of the
hillslope (0%) and the 2nd is the slope at the midpoint of the upper element. The 1st slope of the lower element is the midpoint of the lower
element and the 2nd slope is the bottom of the hillslope. Since WEPP is being applied to an area the size of the whole plot, which may
be less than the whole hillslope area, use the same value for all four inputs.
6 Divide the whole plot area into upper and lower elements of equal slope length (e.g., 112 + 112 = 224 ft).
7 Corresponds to a rectangular area surrounding all f our subplots (f ig. 1).
8 Soil cover = 100 - % bare soil. Use mean for all four subplots.
1
2
3
Table 3. Soil textures used by WEPP that correspond to soil textures
collected from P3 plots.
Soil textures in FIA field guide
Soil textures used by WEPP
Coarse sand
Sandy loam
Sand
Sandy loam
Loam
Loam
Clay
Clay loam
Pr
ec
pe ipita
rio t io
d, n
ye r et
ar u rn
s
16
Mature forest
Low-severity fires
High-severity fires
Skid-trails
14
12
10
8
6
Slope length: 224 ft
Area: 1.33 acre
4
2
0
MF
LSF
HSF
ST
Disturbance
10
35
Lower
4
0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100
Range of soil cover, slope, and slope length values
Treatment4
%
10
0
0
Fig. 2. Histograms showing the number of plots found in each range
of soil cover (top), slope (center), and slope length (bottom) values
for FIA/FHM P3 plots in Idaho
Fig. 6. Box plots of WEPP-predicted soil erosion rates for all
FIA/FHM P3 plots in Idaho as a function of disturbance type
assuming average (2-year return period) precipitation levels. The
25 th and 75 th percentiles are shown as a box centered about the
median, the 10 th and 90 th percentiles are shown as error bars, and
the 5 th and 95 th percentiles and outliers are shown as points. MF =
mature forest, LSF = low-severity fires, HSF = high-severity fires, ST
= skid trails
•P3 plot slope values tend to be normally distributed, while soil cover and slope lengths are skewed (Fig.
2).
•WEPP-predicted soil erosion rates for P3 plots in mature forest (median cover = 99%) are negligibly small
(Fig. 3) even on steep slopes (median slope = 35 %), although rates increase with slope steepness and
precipitation return period (Fig. 4). Median soil erosion rate for an average precipitation year = 0 tons/acre. Ma ximum
soil erosion rate for a 100-year return period precipitation year = 0.7 tons/acre.
•Disturbances greatly increase potential soil erosion rates (Fig. 5, 6 & 7), especially high-severity fires on
steep slopes during wet years (Fig. 5). High-severity fires produce the greatest soil erosion potential followed by skid
trails followed by low-severity fires. The maximum predicted soil erosion rates for a 100-year return period precipitation year on
Idaho P3 plots disturbed by high-severity fires, skid trails, and low-severity fires are 85, 41, and 13 tons/acre, respectively for
plots on the steepest slopes.
•Soil erosion potentials are not sensitive to soil texture for undisturbed areas (Fig. 8). Silt loam soils have the highest soil
erosion potential under high-severity fire conditions (Fig. 8).
Summary and Conclusions:
•The WEPP model can be used to calculate soil erosion potentials on FIA/FHM P3 plots. WEPP offers
several advantages including use of climate station databases and calculation of soil erosion rates for a
range of precipitation return periods and disturbance scenarios. Recommendations are presented for
using the WEPP model with P3 plot data.
•Preserving soil cover is the key to soil conservation. Since it is not possible to predict when high return
period episodic precipitation events will occur, managing forest resource use to minimize disturbance
impacts to soil cover is the best way to forestall catastrophic soil erosion events. Prompt post-disturbance
implementation of effective rehabilitation practices will minimize the time bare soils are exposed to erosive
events.
FHM Posters home page
Matu re forest
Low-severity fires
High-severity fires
Skid trails
| FHM 2002 Posters
0
70 60
50 40
30
20
10
0
0
Fig. 5. Three-dimensional scatter plot of WEPP-predicted soil
erosion rates vs. slope and precipitation return period for all
FIA/FHM plots in Idaho assuming high-severity fire
disturbances.
WEPP-predicted soil erosion rates on FIA P3 plots
for different soil textures, disturbance histories,
and episodic climate event return periods
State: Idaho
Disturbances: none - mature forest (MF),
high-severity fires (HSF)
100 yr
120
High-severity fires
50 yr
20 yr
10 yr
100
Precipitation
return period
80
100 yr MF Erosion
50 yr MF Erosio n
20 yr MF Erosio n
10 yr MF Erosio n
5 yr MF Erosion
100 yr HSF Erosion
50 yr HSF Erosion
20 yr HSF Erosion
10 yr HSF Erosion
5 yr HSF Erosio n
60
40
20
Mature forest
5 yr
0
Clay loam Silt loam Sandy loam
MF
LSF
HSF
ST
Disturbance
Slope length: 224 ft
Area: 1.33 acre
Results:
100
80
60
40
20
Slope length: 224 ft
Area: 1.33 acre
Fig. 4. Three-dimensional scatter plot of WEPP-predicted soil
erosion rates vs. slope and precipitation return period for all
FIA/FHM plots in Idaho assuming mature forest conditions (no
disturbances).
100
80
60
40
20
0
60
50
40
30
20
10
0
40
30
20
10
0
30
25
20
15
10
5
0
25
20
15
10
5
0
40
Sl ope 20
,%
Slope length: 224 ft
Area: 1.33 acre
Fig. 3. Three-dimensional scatter plot of WEPP-predicted soil
erosion rates vs. soil cover and precipitation return period for all
FIA/FHM P3 plots in Idaho assuming mature forest conditions
(no disturbances).
20
35
Upper
6
0
0
S lop e len gth: 224 ft
A rea : 1.3 3 acre
30
Table 2. Example of a data input table for the Disturbed WEPP model.
Location 1
Slope
8
0
40
0
50 40
30 20
S
WEPP-predicted soil erosion rates for FIA P3 plots
State: Idaho
% Slope
Slope length
40
WEPP-predicted soil erosion rates for FIA P3 plots
State: Idaho
Average soil erosion rates
0
10
70 60
lo pe ,
20
60
So il co 40 20
ve r, %
Soil cover
10
% Slope
100
80
60
40
20
Soil erosion rate, tons/acre
RUSLE
Soil cover, slope, and slope lengths on FIA P3 plots
State: Idaho
Disturbance: none (mature forest)
80
Soil erosion rate, tons/acre
Field data
0.0
Fig. 1. FIA/FHM P3 plot diagram. The rectangular area shows the
proposed soil erosion plot area for determining slope gradient and
slope length for soil erosion modeling. The mean of the soil cover
values from the four subplots was used as the input soil cover value
for Disturbed WEPP.
0.0
10 0
80
60
0 .2
inputs1
60
0.2
0.4
Table 1. Soil erosion data collected from P3 plots and input data needed for the RUSLE and Disturbed WEPP
models.
80
0.4
0 .6
•Demonstrate how the WEPP model can be used to calculate soil
erosion potentials using 1999 P3 plot data from Idaho as an example.
100
0.6
0 .8
Objective:
0.8
P re
c
pe ipita
rio tio
d, n r
ye etu
a r rn
s
WEPP-predicted soil erosion rates on FIA P3 plots
State: Idaho
Disturbance: none (mature forest)
WEPP-predicted soil erosion rates on FIA P3 plots
State: Idaho
Disturbance: high-severity fires
, tons/acr e
Soil erosion rate
224 ft
tons/acr
Soil erosion rate,
1.33 acres
•Rates of soil erosion are an important indicator of forest soil health.
•The Revised Universal Soil Loss Equation (RUSLE) was initially
selected to assess soil erosion potential on FIA/FHM P3 plots and data
collection methods were developed accordingly.
•The Watershed Erosion Prediction Project (WEPP) model for assessing
soil erosion potential can also be applied to P3 plot soil data. This model
has many advantages including use of climate station data, calculation of
soil erosion potential for ranges of precipitation return periods, and
calculation of soil erosion potential for different forest disturbances.
Soil erosion rate, tons/acre
Introduction:
e
258 ft
Pr
ec
i
p e p ita
r io tio
d, n r
ye e tu
ar r n
s
1 USDA-FS
Fig. 7. Box plots of WEPP-predicted soil erosion rates for all
FIA/FHM P3 plots in Idaho as a function of disturbance type
and precipitation return period.
Loam
Soil texture
Climate station: Fenn Ranger Station, Idaho
Slope: 75 %
Slope length: 224 ft
Area: 1.33 acre
Cover: 99 % in late succession forest, 45 % after high severity fire
Fig. 8. WEPP-predicted soil erosion rates for an FIA/FHM P3
plot near the Fenn climate station as a function of soil texture
and precipitation return period assuming mature forest and
high-severity fire conditions.
Recommendations:
•Apply the WEPP model to the entire plot area (fig. 1) instead of to each subplot area individually. Divide the whole plot area
into equal halves for the slope lengths for the upper and lower elements in the WEPP input table. Additional enhancement: Consider the
orientation of the rectangular area surrounding the whole plot area on the hillslope for selecting slope lengths. Note: A new slope length protocol
is being developed for a future version of the field guide.
•Discontinue collecting the litter cover and plant cover estimates as part of the soil erosion indicator. These data are not
needed for WEPP and can be obtained from field data collected as part of the vegetation indicator for use in RUSLE. Note: This
recommendation was included in the change proposal for version 1.5 of the field guide.
•Delete the 4-ft radius soil erosion miniplots from the soil indicator. The litter thickness and plant canopy height data collected from
these mini-plots are not needed for WEPP and can be deri ved from data collected by the down woody debris and vegetation indicators for use in
RUSLE. Note: This recommendation was included in the change proposal for version 1.5 of the field guide.
•Develop a procedure to automatically match the location of each P3 plot to the nearest climate station with an elevation
similar to the plot.
•Change soil texture designations in FIA field guide to match those used by WEPP.
References:
Renard, K.G., G.R. Foster, G.A. Weesies and others. 1991. RUSLE: Re vised universal soil loss equation. J. Soil Water Conserv. 46:30-33.
Elliot, W.J., D .E. Hall, D.L. Scheele. 2000. Disturbed WEPP: WEPP interface for disturbed forest and range runoff, erosion, and sediment
delivery. http://forest.moscowfsl.wsu.edu/fswepp/docs/distwepp.doc.html.