Geology Reference
In-Depth Information
Subsurface drainage subfactor
There are some
data that suggest subsurface drainage can be effec-
tive in reducing erosion, presumably by reducing
soil moisture and thereby decreasing runoff dur-
ing a storm event (Formanek
et al
., 1987; Bengtson
& Sabbage, 1988). These data show substantial
scatter, but indicate an average erosion reduction
of about 40% for a subfactor
P
these lessons were partially due to the specifics
of the situation, they are also broad enough to be
instructive to other individuals and groups
within or outside the US who are implementing
a program like RUSLE. One of the key elements
in the development of the RUSLE1 computer
program was the close contact between the pro-
gram developers and a variety of user representa-
tives. Although the development began with
defined user requirements, these underwent sub-
stantial changes as the program was presented to
users through a variety of feedback and training
sessions, involving a mixture of skilled and nov-
ice users. Only through that iterative feedback
process did the program begin to meet the true
user needs, as these needs often only became
apparent when users were exposed to the pro-
gram. Based on the RUSLE experience, it simply
does not work to introduce a new model under
the presumed process of setting initial user
requirements and declaring success once those
are met.
Although the RUSLE1 computer program
itself was first deemed ready for full review and
delivery in 1991, the process of developing the
database information necessary to allow full
implementation took an additional 4-5 years.
This included a strong collaborative research
effort sponsored by USDA-NRCS and carried out
by researchers at North Carolina A&T University,
Alcorn State University, and Alabama A&M
University to collect the data required for the
vegetation descriptions, including especially
time-varying data on vegetative canopy cover,
rootmass and biomass.
Substantial effort also went into determining
exactly how the program would be implemented
in the USDA-NRCS field offices, with special
attention paid to consistency of results across
political boundaries, and consistency of use pat-
terns. One of the important concepts developed
during this period was the development of
C
-Factor Zones, which recognized that climatic
differences rather than political boundaries con-
trolled the possible management scenarios, lead-
ing to shared management descriptions across
state lines. National, state and regional NRCS
=
0.6.
Sediment delivery estimate
As the first step in
the evolution from the USLE, RUSLE1 is still pri-
marily geared towards planning based on soil con-
servation. In spite of this, using the techniques
described above for strip-cropping, it does provide
a crude sediment delivery estimate. This is done
by using a value of
B
M
in Equation (8.22), pro-
viding the ratio of sediment delivered over sedi-
ment eroded, or essentially a sediment delivery
ratio. Multiplying this
P
factor for sediment deliv-
ery by the other factors then provides the hillslope
sediment delivery for evaluating off-site impacts.
=
8.2.2
RUSLE1 program implementation
The RUSLE1 computer code was written in the
C programming language. Chapter 7 in AH703
included a fairly detailed description of the
RUSLE1 program layout and operation. This was
deemed necessary because as dissemination of
and training in RUSLE1 proceeded, it quickly
became apparent that a program with this level of
complexity could not be assumed to be intuitive
to a first-time user. Part of this complexity was
inherent in the level of input information required
from the user, while an additional portion was
due to the program structure. This structure was
based on the USLE 'paper' implementation, and
hid nothing from the user.
8.2.3
RUSLE1 implementation history
and experience
Perhaps the two primary lessons learned from
the USDA-NRCS implementation of RUSLE1
were: (1) the importance of an iterative feedback
process in developing the program; and (2) the
sheer scale of the effort necessary to implement
such a model on the national scale. Although
