Soil Erosion and Conservation - Environmental Modelling: Finding Simplicity in Complexity - page 360

Table 22.5 Sensitivities of changes in runoff to changes in average annual precipitation. Sensitivity values are calculated as the
ratio of the percent change in runoff to the percent change in precipitation. Values represent averages for all simulation runs
associated with the soil, crop, slope, or location listed in the first column. Values greater than zero indicate that runoff increases
with increased annual precipitation. A value of greater than one indicates a greater percentage change in runoff than the
percentage change in precipitation.

Scenarios Normalized sensitivity of runoff to changes in average annual precipitation

Change in Change in Combined changes in

number of wet days amount of rain per day both

Silt loam soil 1.32 2.57 2.00

Sandy loam soil 1.31 2.80 2.17

Clay soil 1.15 2.17 1.75

Grazing pasture 1.54 3.09 2.41

Fallow 1.06 1.99 1.60

Corn and soybean 1.32 2.51 1.97

Wheat winter 1.21 2.43 1.91

S-shape (0%-3%-1%) 40m 1.32 2.59 2.03

S-shape (0%-7%-1%) 40m 1.29 2.49 1.98

S-shape (0%-15%-1%) 40m 1.23 2.42 1.91

West Lafayette, IN 1.16 2.61 1.94

Temple, TX 1.19 2.25 1.73

Corvallis, OR 1.50 2.64 2.23

Overall average 1.28 2.50 1.97

provide the reader with a sense of how each type of model
might be used to advantage depending upon the desired
application.

which model to use then becomes a matter of (a) what
type of information we would like to know, and (b)
what information (data) we have for the particular site
of application. We know from our discussions above that
the USLE provides only estimates of average annual soil
loss on the portion of the field that experiences a net loss
of soil. If we have an interest in offsite impacts, then we
probablywant to choose either RUSLE, whichwill provide
us with a rough idea of the sediment leaving the profile, or
WEPP, if we want more comprehensive sediment-yield
informationor if we aremodelling a small watershed. If we
have an interest in obtaining other, auxiliary information
about our choice of management strategy, such as soil
moisture or crop yields, we might also decide to use
WEPP. On the other hand, if data are limited for the
situation to be modelled, then the USLE might be the
best option in any case, and one would be forced to move
to other options for assessing information not supplied
by the USLE. At the current time most applications of
WEPP are possible in the United States because of the
availability of soil, climate and crop information, but in
other areas this might not be the case.
Making broad-scale erosion surveys in order to under-
stand the scope of the erosion problem over a region

22.4 Lessons and implications

At the start of this chapter we listed three primary uses for
soil erosion models: (a) to help a land owner or manager
choose suitable conservation, (b) to make broad-scale
erosion surveys in order to understand the scope of the
problem over a region and to track changes in erosion
over time, and (c) to regulate activities on the land for
purposes of conservation compliance. Let's look at each
of these goals in turn.
Choosing how to manage land, from the practical
perspective, is often a matter of choosing between an
array of potential options. Often, therefore, what we need
to know is not necessarily the exact erosion rate for a
particular option to a high level of accuracy, but rather we
want to know how the various options stack up against
one another. We may certainly be interested to have
a general quantitative idea of the erosion rate, but for
purposes of land management, it is not critical. Choosing