Geology Reference
In-Depth Information
into what users can expect of models in terms of
robustness, accuracy and data requirements, and
by raising the questions that users need to ask
when selecting a model that is appropriate to the
type and scale of their problem. It is important
that users understand both the advantages and
limitations of erosion models.
The Handbook is arranged in two main parts.
The first part introduces the user to some impor-
tant generic issues associated with erosion mod-
els. Chapter 2 sets out the various stages that a
user should go through when selecting and apply-
ing an erosion model, and shows that these are
much the same as erosion scientists adopt when
developing their models. There is much common
ground between model developers and model
users, probably more so than most users are aware
of. The next four chapters take key issues and dis-
cuss them in detail, along with solutions which
model users might adopt. Chapter 3 looks at the
question of calibration. This is a controversial
topic with opinions ranging from those who con-
sider that it is impossible to calibrate the more
complex, physically-based models and those who
believe that calibration is essential. This chapter
is broadly in favour of calibration, showing how it
can improve the quality of predictions both in
terms of erosion rates and the spatial distribution
of erosion. Chapter 4 raises the issue of uncer-
tainty in model predictions. After discussing why
we should worry about uncertainty, various
approaches are described which can be used to
reduce the level of uncertainty. How successful
these are depends on the causes of the uncertainty,
and model users need to be encouraged to appreci-
ate and understand these. Uncertainty is taken
further in Chapter 5, which shows how one
approach is used in practice with reference to the
application of one specific erosion model. Chapter
6 reviews the issues posed by scale. Many prob-
lems faced by users relate to a single scale, be it
field, hillslope, small catchment or large catch-
ment, but others need to be addressed at a range of
scales. This chapter looks at the problems involved
when moving from one scale to another with the
difficulty of modelling interconnectivity between
hillslope and river systems. At present there are
few solutions to the problems that arise when
modelling across a range of scales, but several
ideas for further research are presented whereby
model development and data collection need to
become more fully integrated. Chapter 7 shows
the importance of choosing the right model for a
specific problem and scale, and the implications
of using inappropriate models. A frequent occur-
rence is the misunderstanding by the user of either
the problem being addressed or what specific
models are able to achieve. Although a dynamic
process-based model is often the best choice, there
are many situations in which it will not perform
better than a simpler statistical model.
Part 2 of the Handbook looks at specific appli-
cations and shows how models are used in prac-
tice. Each chapter is really a case study in which
a problem commonly faced by environmental
planners, consultants and managers is presented.
An appropriate model is then chosen and the user
is taken through the various steps involved in
setting-up and applying the model and interpret-
ing its output. Table 1.2 lists the applications
under broad subject headings and for each one
identifies the relevant chapter and the spatial
scale (erosion plot, field, catchment, region) of
the problem being considered. Additional infor-
mation is provided on the temporal scale, which
ranges from individual events to mean annual
conditions and long-term landform evolution.
Taking each chapter in turn, Chapter 8
reviews the issues typically faced by field officers
of the Natural Resources Conservation Service
of the US when predicting erosion from agricul-
tural land and planning soil protection measures.
Chapter 9 takes a specific example of a small
watershed in southwest Missouri and shows
how modelling can assist in designing a strategy
for sustainable management under both present
land use and climatic change. In Chapter 10,
modelling is used to predict rates of soil loss in
Brazil from hillslopes on forest roads in São
Paulo State and from agricultural land under dif-
ferent management systems in Minas Gerais
State. Chapter 11 examines how a physically-
based erosion model can be used to assess
soil erodibility and evaluate different soil
