MODFLOWP is a parameter estimation package that can be used in conjunction with MODFLOW to improve model construction and calibration. MODFLOWP is an extension of the USGS modular three-dimensional, finite-difference ground-water flow model (MODFLOW), that includes the Parameter-Estimation Package using nonlinear regression. MODFLOWP functions nearly identically to MODFLOW when the Parameter-Estimation Package is not used. Parameters are estimated by minimizing a weighted least-squares objective function by the modified GaussNewton method or by a conjugate-direction method.

Parameters used to calculate the following MODFLOW model inputs can be estimated: transmissivity and storage coefficient of confined layers; hydraulic conductivity and specific yield of unconfined layers; vertical leakance; vertical anisotropy (used to calculate vertical leakance); horizontal anisotropy; hydraulic conductance of the River, Streamflow-Routing, General Head Boundary, and Drain Packages; areal recharge rates; maximum evapotranspiration; pumping rates; and the hydraulic head at constant-head boundaries. Any spatial variation in parameters can be defined by the user.

Data used to estimate parameters can include existing independent estimates of parameter values, observed hydraulic heads or temporal changes in hydraulic heads, and observed gains or losses along head-dependent boundaries (such as streams). 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 IGWMC interactive preprocessor for MODFLOW (PREMOD) facilitates data entry and editing of all MODFLOWP input packages except the Stream-Routing (STR1) and Parameter Estimation packages. The IGWMC post-processor (POSTMOD) reformats the model binary output files of head, drawdown, and cell-by-cell flow terms into an x-y-z format. The reformatted output is compatible with commercial contouring packages.

The MODFLOWP program package is distributed by IGWMC complete with MODFLOWP source code and executables for MODFLOWP, PREMOD and POSTMOD. Distribution includes installation instructions and the MODFLOWP documentation. MODFLOW documentation is optional ($55).

SYSTEM REQUIREMENTS

• Intel 80386/80486 based computer
• 4 Mb RAM
• DOS 3.0 or higher
• at least 5 Mb free disk space
• math coprocessor

Author: M.C. Hill (U.S. Geological Survey)