Environmental Engineering Reference
In-Depth Information
Wet
deposition
Particulate
deposition
Timber
harvest
Management
Biomass
Canopy
leaching
Soil surface
Roots
Uptake
Primary
minerals
Decomposition
Weathering
Organic
material
Depletion
Siol solution
Ion exchange
Secondary clay
minerals
Ion exchange
Recrystalization
Redissolution
Amorphous
precipitates
Precipitation
Leaching
Figure 17.8
A box-arrow diagram for the process system in soil, typical for many biogeochemical models. Biogeochemical models
range from describing a small part of this diagram to having it as a small subcompartment.
There are many biogeochemical codes available for
biogeochemical calculations in ecosystems. Some of these
are easy to obtain and use whereas others exist only
on paper and in scientific articles. If the purpose of
your modelling is to predict, assess or design, then using
an existing model is often an efficient way to get the
work done. If the issue is to explain your research,
investigate your own understanding and develop it, it
is recommended that you build your own model rather
than using an existing one. If you use an existing model, do
not start to use it before you have thoroughly understood
the principles and assumptions it applies and preferably
when you agree how it is made. We can define three
groups of models:
developed for issues in agriculture and groundwater
geochemistry, were based on equilibrium processes. This
basis had historical reasons: before the advent of modern
computers, systems of differential equations were
unsolvable for any practical use. The traditional models
still rely a lot on large amounts of data for calibration.
If this requirement is fulfilled, they will produce useful
backcasting and can be used for making extrapolative
predictions. These models quickly run into problems
without large amounts of data to calibrate on, which is
caused by a specific shortcoming that must be pointed
out. The traditional geochemical models use equilibrium
formulations for processes that are valid for soluble
salts, carbonate dominated systems and other reversible
reaction systems, but such models are not formally
valid for kinetically controlled irreversible systems. The
so-called 'thermodynamic equilibrium databases' for soil
and geochemical silicate reactions referred to are not
'thermodynamic' at all - they are steady-state balance
coefficients that have been misinterpreted as equilibria.
models based on process-oriented kinetics;
•
models based on equilibrium principles;
•
empirical models.
•
Here we will be mainly concerned with the first two
types. The traditional soil-chemistry models, originally
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