The main aim of this study is to develop a suitable method for
the calibration and validation of mathematical models of large and complex
aquifer systems. Since the calibration procedure depends on the
nature of the model to be calibrated and since many kinds of models are
used for groundwater, the question of model choice is broached first.
Various aquifer models are critically reviewed and a table to compare
them as to their capabilities and limitations is set up. The need for a
general calibration method for models in which the flow is represented by
partial differential equations is identified from this table.
The calibration problem is formulated in the general mathematical
framework as the inverse problem. Five types of inverse problems that
exist in modeling aquifers by partial differential equations are identified.
These are, to determine (1) parameters, (2) initial conditions,
(3) boundary conditions, (4) inputs, and (5) a mixture of the above.
Various methods to solve these inverse problems are reviewed, including
those from fields other than hydrology. A new direct method to solve the
inverse problem (DIMSIP) is then developed. Basically, this method consists
of transforming the partial differential equations of flow to algebraic
equations by substituting in them the values of the various
derivatives of the dependent variable (which may be hydraulic pressure,
chemical concentration or temperature). The parameters are then obtained
by formulating the problem in a nonlinear optimization framework. The
method of sequential unconstrained minimization is used. Spline functions are used to evaluate the derivatives of the
dependent variable. Splines are functions defined by piecewise polynomial
arcs in such a way that derivatives up to and including the order
one less than the degree of polynomials used are continuous everywhere.
The natural cubic splines used in this study have the additional property
of minimum curvature which is analogous to minimum energy surface. These
and the derivative preserving properties of splines make them an excellent
tool for approximating the dependent variable surfaces in groundwater
flow problems.
Applications of the method to both a test situation as well as to
real -world data are given. It is shown that the method evaluates the
parameters, boundary conditions and inputs; that is, solves inverse problem
type V. General conditions of heterogeneity and anisotropy can be
evaluated. However, the method is not applicable to steady flows and has
the limitation that flow models in which the parameters are functions of
the dependent variable cannot be calibrated. In addition, at least one of
the parameters has to be preassigned a value.
A discussion of uncertainties in calibration procedures is given.
The related problems of model validation and sampling of aquifers are
also discussed.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/617585 |
| Date | 06 1900 |
| Creators | Sagar, Budhi |
| Contributors | Department of Hydrology & Water Resources, The University of Arizona |
| Publisher | Department of Hydrology and Water Resources, University of Arizona (Tucson, AZ) |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Technical Report |
| Source | Provided by the Department of Hydrology and Water Resources. |
| Rights | Copyright © Arizona Board of Regents |
| Relation | Technical Reports on Hydrology and Water Resources, No. 17 |
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