Geology Reference
In-Depth Information
personnel developed the required management
descriptions to describe the bulk of schemes used
in these areas, which in turn defined the opera-
tion and vegetation descriptions that had to be
developed. In other words, broad implementation
required both local expertise and substantial
cooperation and oversight.
In spite of all the work that needed to be done
and all the decisions that needed to be made (and
remade!), full NRCS implementation of RUSLE1
began in 1993, using version 1.04 of the program,
which is the version represented and documented
in far more detail in AH703. This was actively
used for conservation planning throughout the
US, and for the Conservation Compliance por-
tion of NRCS responsibilities associated with the
1985 and 1990 US Farm Bills.
that the erosion results could vary by up to 30%
between the two approaches.
8.2.5
RUSLE1 program weaknesses
In addition to the weaknesses inherent in the
RUSLE1 science development, some more gen-
eral weaknesses in the program operation became
apparent during implementation.
The first program weakness was that the
RUSLE1 structure was based on science rather
than on how the user saw things. For example,
one parameter used in the
LS
calculations is the
soil texture, which affects the susceptibility of
the soil to develop rills, thereby impacting the
LS
b
value (Equation (8.2) ). In spite of this, the user
will clearly think of texture as a soil property, and
not as something related to topography. This is
one of many examples in RUSLE1 where there
was a need to approach things more from the
user's viewpoint, and not from the modelling
viewpoint.
Another weakness of the RUSLE1 approach is
that any user could change any database value.
Although NRCS had put substantial effort into
developing specific databases for climates and
vegetations, any user could change the values,
either intentionally or by accident. This resulted
in many implementation headaches, such that
two users using the same inputs would get very
different results because one of the underlying
database files had been modified.
Finally, the DOS-based interface used in
RUSLE1 was already dated at the time of its deliv-
ery, and users repeatedly asked for a Windows
®
-
based or similar graphical user interface, with
which they were becoming increasingly familiar.
8.2.4
Science problems with RUSLE1
As the USLE came into general use, it quickly
became apparent that the impact of management
on erosion could vary greatly among periods
within a year or among years within a rotation.
This was recognized by the later USLE methods,
with AH537 using a time-varying SLR based on
cropping periods. RUSLE1 carried this further,
using a daily time-step for the
C
-factor calcula-
tions, and also for some of the
P
-factor calcula-
tions. However, due to user requirements that
the structure of RUSLE1 reflect that of a 'paper
implementation' of the USLE, this was not car-
ried to its logical extreme. The time-varying val-
ues of each of the individual factors were
aggregated over the year, and the resulting annual
values were multiplied as shown in Equation
(8.1). Unfortunately, this aggregated approach is
not correct, as the sum of products is not equal to
the product of sums, which can be seen in the
simple calculation (2
8.3 RUSLE2
+
3) × (4
+
5)
=
45
≠
(2 × 4)
+
(3 × 5)
23. Clearly, the proper approach was to
take any time-varying values and multiply these
for each day or period, then add the daily products
to get the total erosion. This was recognized as a
problem early in RUSLE1 development, but it
could not be dealt with while retaining the 'paper
implementation' capability. Calculations showed
=
As RUSLE1 developed, it quickly became appar-
ent that there were some scientific weaknesses
with the approach taken that were primarily
caused by its close linkage to the methodology
used in the USLE. In addition, through the train-
ing and implementation process, some lessons
were learned about how the general program
