T-COMP—A suite of programs for extracting transmissivity from MODFLOW models

Techniques and Methods 6-A54

Prepared in cooperation with the U.S. Department of Energy, National Nuclear Security Administration, Nevada Site Office, Office of Environmental Management, under Interagency Agreement, DE-NA0001654/DE-AI52-12NA30865


  • Document: Report (1.6 MB pdf)
  • Appendixes:
    • Appendix A Aquifer Tests and Comparisons between Probability Distributions of Transmissivities from Hydraulic-Conductivity Limits and Aquifer-Test Results
    • Appendix B T-COMP Programs, Pre-Processing Tools, and an Example
    • Appendix C Source Codes for T-COMP Programs
    • Appendix D T-COMP_Compare–A Workbook for Comparing Simulated Transmissivities Sampled with T-COMP to Specified Values
    • Appendix E Results from T-COMP Verification
  • Download citation as: RIS | Dublin Core


Simulated transmissivities are constrained poorly by assigning permissible ranges of hydraulic conductivities from aquifer-test results to hydrogeologic units in groundwater-flow models. These wide ranges are derived from interpretations of many aquifer tests that are categorized by hydrogeologic unit. Uncertainty is added where contributing thicknesses differ between field estimates and numerical models. Wide ranges of hydraulic conductivities and discordant thicknesses result in simulated transmissivities that frequently are much greater than aquifer-test results. Multiple orders of magnitude differences frequently occur between simulated and observed transmissivities where observed transmissivities are less than 1,000 feet squared per day.

Transmissivity observations from individual aquifer tests can constrain model calibration as head and flow observations do. This approach is superior to diluting aquifer-test results into generalized ranges of hydraulic conductivities. Observed and simulated transmissivities can be compared directly with T-COMP, a suite of three FORTRAN programs. Transmissivity observations require that simulated hydraulic conductivities and thicknesses in the volume investigated by an aquifer test be extracted and integrated into a simulated transmissivity. Transmissivities of MODFLOW model cells are sampled within the volume affected by an aquifer test as defined by a well-specific, radial-flow model of each aquifer test. Sampled transmissivities of model cells are averaged within a layer and summed across layers. Accuracy of the approach was tested with hypothetical, multiple-aquifer models where specified transmissivities ranged between 250 and 20,000 feet squared per day. More than 90 percent of simulated transmissivities were within a factor of 2 of specified transmissivities.

Suggested Citation

Halford, K.J., 2016, T-COMP—A suite of programs for extracting transmissivity from MODFLOW models: U.S. Geological Survey Techniques and Methods, book 6, chap. A54, 17 p.,

ISSN: 2328-7055 (online)

Study Area

Table of Contents

  • Preface
  • Abstract
  • Introduction
  • Field Estimates—Aquifer-Test Results
  • Generalizing Aquifer-Test Results
  • Constraining Simulated Transmissivity Estimates
  • T-COMP
  • Validation
  • Conclusions
  • References Cited
  • Appendixes A-E

Additional publication details

Publication type:
Publication Subtype:
USGS Numbered Series
T-COMP—A suite of programs for extracting transmissivity from MODFLOW models
Series title:
Techniques and Methods
Series number:
Year Published:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Nevada Water Science Center
vii, 17 p.
Larger Work Type:
Larger Work Subtype:
USGS Numbered Series
Larger Work Title:
Section A: Groundwater in Book 6: Modeling Techniques
Public Comments:
This report is Chapter 54 of Section A: Groundwater in Book 6: Modeling Techniques.
United States
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