Method applied to determine the erosion threshold
Knowledge of the influence of erodibility factors on soil erosion
(both environmental and related to agricultural management) can
be used to determine erosion thresholds, defined as the value (of
the erodibility factor) above which the reduction in erosion is
statistically significant. The erosion threshold approach is intended
to facilitate decision making with a view to improving the planning
of agricultural activity from the standpoint of soil conservation and
avoiding possible arbitrary decisions arising from the use of
empirical methods.
To establish the erosion threshold, we consider the hypothesis
that water erosion does not increase at a constant rate with respect
to erodibility factors. The conditions under which a research study
is conducted are of crucial importance in this respect; research
conditions in the
field (agricultural plots) differ greatly from those
in the laboratory and can make it difficult to isolate the influence of
each erodibility factor on the overall erosion. For example, Roose
and Ndayizigiye (1997) showed that erosion did not increase
linearly with the slope gradient, due to the existence of a stonier
soil and less crusting on the steeper slopes, which reduced the
degree of erosion. That is, the erosion trend observed with respect
to a particular factor may be influenced by other factors, the
interference from which is difficult to isolate, given the working
conditions in the
field plots. Nevertheless, these are the actualconditions that must subsequently be addressed when the
research outcomes are used as the basis for management
recommendations. From this standpoint, it seems most appropriate
to conduct the study under the same conditions as those to
which the results will later be transferred.
It is important to recognize that reducing erosion from a
statistical point of view is not synonymous with the effective
control of erosion, because a ‘statistical’ reduction may take place
at high absolute values of erosion. To assess whether the erosion
threshold used corresponds to effective erosion control, we applied
a verification methodology based on the use of visual indicators to
assess the intensity of erosion by means of a qualitative
classification (Table 2). It has been established that an erosion
threshold, determined from a statistical standpoint, reflects the
effective control of erosion when a plot with the specified
characteristics presents an erosion intensity of category 0 or
category 1; in contrast, it is not effective when the category is 2 or
3. In the latter case, the erosion threshold cannot be used for the
purpose for which it was designed. As background to this
verification procedure, we cite the study by Mutekanga et al.
(2010), who examined satellite images to assess the risk of erosion
and validated the results obtained in a
field study using an erosion
intensity classification based on the presence/absence of visual
cues and their size.