Geology Reference
In-Depth Information
(i) Rainfall erosivity factor (
R
)
The most com-
mon way of presenting rainfall erosivity informa-
tion in the US has been through the use of
isoerodent maps, allowing the reader to interpo-
late the corresponding
R
value for a specific loca-
tion. The isoerodent maps in AH703 were
calculated using the same criteria as in AH537,
namely summing the storm kinetic energy times
the maximum 30-minute intensity for storms
larger than 0.5 in (12 mm), unless at least 0.25 in
(6.5 mm) fell in 15 min. These calculations were
computed for all non-snow storms within a period
of
N
years. Normally at least 22 years of storms
were included for the calculations (see AH537 for
details), but longer periods are advisable when
the coefficient of variation of annual precipita-
tion is large. A total of 181 key precipitation loca-
tions with 15-min data were used for the map in
AH537, and a few additional locations to fill in
gaps were added to produce Figure 2.1 in AH703.
AH537 included very little erosivity information
for the western US, with only 11 western station
isoerodent values used to estimate the two-year
6-h precipitation amount. A power relationship
developed by Wischmeier (1974) to fit those
values provided some measure of the expected
erosivity, but the results were not thought to
be very accurate, or to reflect adequately the
known intermontane climate variability. Through
an agreement between Oregon State University,
USDA-ARS, USDA-SCS and the National
Weather Service, data from 713 stations with
15-min measurement intervals were used to
calculate
EI
values, and thereby to construct
new isoerodent maps for the western US in
AH703, although all storms were included in the
western erosivity calculations (excluding snow).
Analysis of these records showed that 225
precipitation-measuring locations had records
longer than 12 years and precipitation resolutions
of 0.01 in. (0.25 mm). Values of the coefficient of
determination (R
2
) in excess of 0.8 were obtained
with the model
EI
15
had record lengths of 20 years or longer. These
data were adjusted to a 15-min measurement
interval using the cited correction.
R
factors were
also adjusted to equivalent break-point data using
the Weiss (1964) relationship
R
1.0667 (
R
15
). The
isoerodent map was prepared by hand contouring
on large-scale maps, reflecting the major topo-
graphic influences in mountain and range topog-
raphy. The newer isoerodent maps (Figures 2-2,
2-3 and 2-4 of AH703) were thus felt to be a sig-
nificant improvement over those in AH537.
In addition, seasonal
EI
distributions were
developed for 84 climate zones in the western US
(Fig. 2-7, AH703). The distributions were devel-
oped for calculating the time-varying
C
factor in
RUSLE1, building on the crop growth stage
approach found in AH537.
City database files were then developed in
RUSLE1 to provide the climatic data needed for
erosion calculations. This included the
R
-factor
value, the
EI
distribution values for 24 bimonthly
periods, and the 10-yr frequency storm maximum
EI
that was needed for calculating the
P
factor
credit for contour farming. Maps of these values
were calculated for precipitation gauge locations
and are presented in Figures 2-9 to 2-12 in
AH703.
Two additional modifications to the classical
USLE
R
-factor approach were included in RUSLE1
to address specific geographical needs. In areas
with very low relief and high rainfall intensities
(such as in the Mississippi River delta), research
has found that runoff ponds to substantial depths
before running off, and that this ponded water
absorbs some of the raindrop impact that could
cause detachment (Mutchler, 1970). Based on
these data, RUSLE1 included a term to adjust
downwards the erosivity experienced by the soil,
based on slope steepness and rainfall erosivity
(taken as a surrogate for intensity). The other
modification to the
R
factor was for frozen and
thawing soils, encountered in the Pacific
Northwest (Northwest Wheat and Range Region
(Austin, 1981) ), and in some of the southern
plains of Canada. In these cases, a soil with much
weakened structure exposed to even a low-erosivity
event will experience high erosion rates, so an
=
b
(
EI
60
). Values of the regres-
sion parameter
b
ranged from 1.08 to 3.16, vary-
ing widely among climate zones.
To supplement this work, 1082 hourly stations
were used to calculate
EI
60
. Of these stations, 790
=
