Geology Reference
In-Depth Information
Soil from
upslope
Transport
capacity
of rain
Soil particle
detachment
by rain
Transport
capacity
of runoff
Soil particle
detachment
by runoff
Soil detached
on slope segment
Total
detached soil
Total
transport capacity
compare
Fig. 2.4
The Meyer-
Wischmeier model:
a conceptual framework
for erosion modelling
(after Meyer & Wischmeier,
1969).
if transport < detachment
if detachment < transport
Soil carried
downslope
Meyer and Wischmeier (1969) carried out their
simulations for relatively simple conditions in
which the soil was bare, the slope planar, and
there was no removal of sediment at the base of
the slope, for example, by a river. There was also
no addition to sediment on the slope over the
long term through the breakdown of the underly-
ing rock into soil by weathering processes.
Although the authors showed their approach was
feasible and that it could reproduce the patterns
of erosion and deposition observed in simple
landscapes, its limitations meant that further
research was necessary before it could be devel-
oped into an erosion model that could satisfy the
objectives of many potential users. In particular,
the effects of crop or vegetation cover and soil
management needed to be accounted for. Further,
during the 1970s and 1980s it became increas-
ingly clear that the USLE was no longer able to
meet the demands of many users as their atten-
tion changed from one of conserving soil in a field
to sustain long-term agricultural productivity, to
concerns over the transfer of sediment from agri-
cultural fields, construction sites and recreational
areas to water bodies. The inability of the USLE
to compute sediment yields from areas larger
than single fields or for specific time periods
instead of an average year meant that different
models were required. In their absence, some work-
ers used the USLE regardless of its limitations
and generally with unsatisfactory results. The
USLE suffered much adverse publicity because it
could not deal with these new issues. This caused
Wischmeier (1978) to publish a warning about its
use and misuse. His paper is still relevant today
because it illustrates the importance of using a
model which is fit-for-purpose (see Chapter 7 for
a fuller discussion of this issue) and that if, for
any reason, an inappropriate model is used and
does not work, it is the fault of the user and not
the model. The most common reasons why an
inappropriate model is chosen are that an appro-
priate model does not exist, or the data to operate
one are not available.
The first set of erosion models that adopted
the Meyer-Wischmeier approach to describe the
processes of erosion and simulate the movement
of sediment over the landscape operated by bolting
