INVFD is a two-dimensional, steady-state ground-water flow model, which can be used to estimate parameters by nonlinear regression. The model simulates (leaky) confined flow in a heterogeneous aquifer. External boundary conditions include specified specific discharge normal to the boundary and specified hydraulic head. The model supports node-specific sources or sinks. Parameters are estimated by minimizing a weighted least-squares objective function by the modified Gauss-Newton method. The inverse model can estimate one or more of the following zoned parameters: transmissivity in X- and Y-direction (i.e., anisotropy), vertical leakance, and distributed recharge. The model requires independent information for the head distribution. The model uses an irregular rectangular grid of maximal 30 * 30 nodes.

Model output includes statistics for analyzing the parameter estimates and the model. These statistics can be used to quantify the reliability of the resulting model, to suggest changes in model construction, and to compare results of models constructed in different ways.

The documentation consists of two USGS manuals:

• R.L. Cooley and R.L. Naff. 1990. Regression Modeling of Ground-Water Flow. Techniques of Water-Resources Investigations of the United States Geological Survey, Book 3, Chapter B4;
• R.L. Cooley. 1993 Regression Modeling of Ground-Water Flow. Supplement 1--Modifications to the Computer Code for Nonlinear Regression Solution of Steady-State Ground-Water Problems. Techniques of Water-Resources Investigations of the United States Geological Survey, Book 3, Chapter B4, Supplement 1.

The package also includes the BEALE, RESAN and THEIS programs discussed in the documentation.

INVFD is a public domain program distributed on a DOS-formatted disk, containing source code, executable image, and data sets.

SYSTEM REQUIREMENTS

Intel 80i86 based computer with math coprocessor, 640Kb RAM (low or conventional memory only), hard drive with about 1 Mb free disk space for programs and example data sets, and DOS 3.0 or higher; program does not include graphics.

Author: R.L. Cooley and R.L. Naff (U.S. Geological Survey).