Soil Erosion and Conservation - Environmental Modelling: Finding Simplicity in Complexity - page 350

22

Soil Erosion and Conservation

Mark A. Nearing
USDA-ARS Southwest Watershed Research Center, Tucson AZ, USA

of crop-productivity loss. Conservationists refer to this
process as soil loss , referring to the net loss of soil over
only the portion of the field that experiences net loss
over the long-term. Areas of soil loss end where net
deposition begins. Off-site concerns, on the other hand,
are associated with the sediment that leaves the field,
which we term here sediment yield . In this case, we are not
necessarily concerned with the soil loss, or for that matter
the amount of sediment deposited prior to leaving the
field, although estimation of both of these may be used to
estimate sediment yields. Ideally, a model will compute
soil loss, deposition and sediment yield, and thus have
the capability to address both on-site and off-site issues.
Data variability and model uncertainty are two related
and important issues associated with the application of
erosion models. Data from soil-erosion plots contain
a large amount of unexplained variability, which is an
important consideration for using erosiondata to evaluate
soil-erosion models, as well as for interpreting erosion
data. This variability is due both to natural causes and
measurement errors. When comparing measured rates of
erosion to predicted values, a portion of the difference
between the two will be due to model error, but a portion
will also be due to unexplained variance of the measured
sample value from the representative, mean value for a
particular treatment.
Knowledge of variability in soil-erosion data, however,
is somewhat limited, although recent studies have enlight-
ened us to some degree. Only one experimental erosion
study to date has been conducted with a sufficient number
of replicated erosion plots to allow an in-depth analysis
of variability. Wendt et al . (1986) measured soil erosion
rates on 40 cultivated, fallow, experimental plots located

22.1 The problem

Accelerated soil erosion induced by human activities is
the principal cause of soil degradation across the world.
The main culprit behind the problem is agriculture, and
at stake is the long-term viability of the agricultural pro-
duction capacity of the planet. Barring major unknown
scientific advances in the future, and if soil erosion and
population growth remain unchecked from their current
rates, humanitywill eventually lose the ability to feed itself.
Another significant problem associated with soil erosion
is off-site sediment pollution. Costs associated with the
delivery of sediment to streams and other water bodies
worldwide are huge (e.g. Pimentel, 1995). This chapter
will focus on models of soil erosion as they are used for
purposes of soil conservation. In particular, we focus here
exclusively on soil erosion by water (see also Chapter 15).
Models of other agricultural erosion processes, such as
wind erosion and tillage erosion, are certainly important,
but they will not be addressed here.
Models can be used in conservation work for three
primary purposes: (a) to help a land owner or manager
choose suitable conservation practices from among
alternatives, (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.
In selecting or designing an erosion model, a decision
must be made as to whether the model is to be used for
onsite or offsite concerns, or both. On-site concerns are
generally associated with degradation or thinning of the
soil profile in the field, which may become a problem