Computer Programs Mary C. Hill UCODE, A Computer Code for Universal Inverse Modeling U.S. Geological Survey Water-Resources Investigations Report 98-4080 By Eileen P. Poeter1 and Mary C. Hill2 1. International Ground Water Modeling Center of the Colorado School of Mines 2. U.S. Geological Survey UCODE performs inverse modeling, posed as a parameter-estimation problem, using nonlinear regression. 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. Observations to be matched in the regression can be any quantity for which a simulated equivalent value can be produced, and simulated equivalent values are calculated using values that appear in the application model output files and a set of additive and multiplicative functions. 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 objective 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. Statistics are calculated and printed for use to diagnosing inadequate data and identifying parameters that probably cannot be estimated; evaluating estimated parameter values; evaluating the model representation of the actual processes; and quantifying the likely uncertainty of model simulated values. 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. The program and documentation can be downloaded from Link to the IGWMC Full PDF Document (wr984080.pdf) and Errata (wr98-4080errata.pdf). Free Download of Adobe Acrobat Reader. mchill@usgs.gov Last Modified: March 17, 1999