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15 Conceptual Models and Computer Models
HORST D. SCHULZ
Upon recording processes of nature quantitatively,
the term model is closely related to the term system.
A system is a segment derived from nature with
either real or, at least, imagined boundaries. Within
these boundaries, there are processes which are to
be analyzed. Outside, there is the environment exerting an influence on the course of the procedural
events which are internal to the system by means
of the boundary conditions. A conceptual model
contains principle statements, mostly translatable
quantitatively, with regard to the processes in a
system and the influence of prevalent boundary
conditions. If the systems to be reproduced are
especially complex, any significant realization of
the conceptual model is often only possible by
applying computer models.
Depending on the field of interest and the
nature of the task, the processes in a model will be
either somewhat more physical/hydraulic, geochemical, biogeochemical, or biological. The systems and processes studied might then expand in
size over the great oceans and their extensive
stream patterns down to small segments of deep
ocean floor measuring only few cubic centimeters,
including the communities of microorganisms
living therein. Consequently, there are no predefined dimensions or structures which are valid
for the systems and their models, instead, these
are exclusively determined by the understanding
of the respective processes of interest and their
quantitative translatability. Since geochemical and
biogeochemical reactions occur relatively fast,
and are thus bound to spaces of smaller dimensions, the models dealt with in this chapter will be
preferentially concerned with dimensions encompassing few centimeters and meters of sediment,
to some hundred meters of water column.
In the field of applied geological sciences,
models frequently assume a primarily prognostic
character. For instance, after accomplishing an
adequate calibration, an hydraulic model applied
to groundwater, or a model of solute transport in
groundwater, is primarily supposed to predict the
system’s reaction as precisely as possible under
the given boundary conditions. Such models will
find very practical and often well paid applications. In marine geochemistry such a prognostic
function of models occurs rather seldom; here, the
objective of obtaining a quantitative concept of a
system predominates in most cases, which is
‘function’ of a particular segment of nature. Computer models can then check the plausibility of
model concepts of complex systems, can draw
attention to the areas not sufficiently understood,
and might help discover parameters with which
the system can be described reasonably well and
effectively.
In the following, zero-dimensional geochemical
reaction models will be distinguished from onedimensional models, in which transport processes
are coupled by various means to geochemical and
biogeochemical reactions. The subsections will
then introduce approaches to very different
models, including models that have advanced to
very different stages of development.
15.1 Geochemical Models
15.1.1 Structure of Geochemical Models
Zero-dimensional geochemical models are concerned with the contents of solutions of an
aquatic subsystem (e.g. pore water, ocean water,
precipitation, groundwater), the equilibria between
the various dissolved species as well as their
adjoining gaseous and solid phases. A computer
model by the name of WATEQ (derived from
water-equilibria) was published for the first time
by Truesdell and Jones (1974). Unfortunately, this
was done in the computer language PL1 which has
15 Conceptual Models and Computer Models
HORST D. SCHULZ
Upon recording processes of nature quantitatively,
the term model is closely related to the term system.
A system is a segment derived from nature with
either real or, at least, imagined boundaries. Within
these boundaries, there are processes which are to
be analyzed. Outside, there is the environment exerting an influence on the course of the procedural
events which are internal to the system by means
of the boundary conditions. A conceptual model
contains principle statements, mostly translatable
quantitatively, with regard to the processes in a
system and the influence of prevalent boundary
conditions. If the systems to be reproduced are
especially complex, any significant realization of
the conceptual model is often only possible by
applying computer models.
Depending on the field of interest and the
nature of the task, the processes in a model will be
either somewhat more physical/hydraulic, geochemical, biogeochemical, or biological. The systems and processes studied might then expand in
size over the great oceans and their extensive
stream patterns down to small segments of deep
ocean floor measuring only few cubic centimeters,
including the communities of microorganisms
living therein. Consequently, there are no predefined dimensions or structures which are valid
for the systems and their models, instead, these
are exclusively determined by the understanding
of the respective processes of interest and their
quantitative translatability. Since geochemical and
biogeochemical reactions occur relatively fast,
and are thus bound to spaces of smaller dimensions, the models dealt with in this chapter will be
preferentially concerned with dimensions encompassing few centimeters and meters of sediment,
to some hundred meters of water column.
In the field of applied geological sciences,
models frequently assume a primarily prognostic
character. For instance, after accomplishing an
adequate calibration, an hydraulic model applied
to groundwater, or a model of solute transport in
groundwater, is primarily supposed to predict the
system’s reaction as precisely as possible under
the given boundary conditions. Such models will
find very practical and often well paid applications. In marine geochemistry such a prognostic
function of models occurs rather seldom; here, the
objective of obtaining a quantitative concept of a
system predominates in most cases, which is
‘function’ of a particular segment of nature. Computer models can then check the plausibility of
model concepts of complex systems, can draw
attention to the areas not sufficiently understood,
and might help discover parameters with which
the system can be described reasonably well and
effectively.
In the following, zero-dimensional geochemical
reaction models will be distinguished from onedimensional models, in which transport processes
are coupled by various means to geochemical and
biogeochemical reactions. The subsections will
then introduce approaches to very different
models, including models that have advanced to
very different stages of development.
15.1 Geochemical Models
15.1.1 Structure of Geochemical Models
Zero-dimensional geochemical models are concerned with the contents of solutions of an
aquatic subsystem (e.g. pore water, ocean water,
precipitation, groundwater), the equilibria between
the various dissolved species as well as their
adjoining gaseous and solid phases. A computer
model by the name of WATEQ (derived from
water-equilibria) was published for the first time
by Truesdell and Jones (1974). Unfortunately, this
was done in the computer language PL1 which has