UCODE
A COMPUTER CODE FOR UNIVERSAL INVERSE MODELING |
Order
No: FOS 95 |
| Version 3.062: May 2004 |
UCODE is a universal inversion
code, which was jointly sponsored by the U.S. Department of Defense, International
Ground Water Modeling Center, and the U.S. Geological Survey. UCODE performs
inverse modeling, posed as a parameter-estimation problem, using nonlinear regression.
UCODE is not limited to inverting ground-water problems.
UCODE performs inverse modeling,
and was specifically developed to:
- manipulate application model
input files and read values from application model output files
- compare user-provided observations
with equivalent simulated values derived from the values read from the application
model output files using a weighted least- squares objective function
- use a modified Gauss- Newton
method to adjust the value of user selected input parameters in an iterative
procedure to minimize the value of the weighted least-squares objective function
- report the estimated parameter
values
- calculate and print statistics
to be used to
- diagnose inadequate data or identify
parameters that probably cannot be estimated
- evaluate estimated parameter
values
- evaluate how accurately the model
represents the field processes
- quantify the uncertainty of model
predictions.
Any application model or set of models
can be used; the only requirement is that they have numerical (ASCII or text only)
input and output files and that the numbers in these files have sufficient significant
digits. Application models can include pre- processors and post-processors, as
well as models related to the processes of interest (physical, chemical, and so
on), making UCODE extremely powerful. An estimated parameter can be a quantity
that appears in the input files of the application model(s), or that can be used
in conjunction with user-defined functions to calculate a quantity that appears
in the input files. An observation can be any quantity for which a simulated equivalent
value can be produced for comparison with the observation. UCODE can produce simulated
equivalent values using data that appear in the application model output files
and a set of additive and multiplicative functions defined by the user. Prior,
or direct, information on estimated parameters also can be included in the regression.
The nonlinear regression problem is solved by minimizing a weighted least- squares
bjective function with respect to the parameter values using a modified Gauss-Newton
method. Sensitivities needed for the method are calculated approximately by forward
or central differences, and problems and solutions related to this approximation
are discussed. UCODE is intended for use on any computer operating system: it
consists of algorithms programmed in perl, a freeware language designed for text
manipulation, and Fortran90, which efficiently performs numerical calculations.
Authors: Eileen
Poeter and Mary Hill
UCODE is a public domain program. Click
for free download,
or order ($50) to receive hard copy by mail.