In Reply Refer To:                                                March 22, 2005 
Mail Stop 411


Subject: PUBLICATIONS--Policy on documenting the results of new simulations using previously published ground-water models

The Office of Ground Water has received numerous inquiries about the appropriate level of documentation required for additional numerical simulations made using a previously published ground-water model (a flow or transport model). The existing policy on documenting the development of a ground-water model in project reports is stated in Office of Ground Water Technical Memorandum No. 96.04 ( However, if a ground-water flow or transport model has already been developed and documented in a published report to aid in solving a previous problem, that model may be useful in addressing new questions raised by our cooperators in managing their resources. The documentation required for additional numerical simulations made using a previously published ground-water model follows the intent of the original policy but can be condensed. In this memorandum, “original model” refers to a model that was previously developed and documented in a published report, and “scenario model” refers to new model simulations using a previously developed, documented, and published ground-water model. As used here, scenario model results are based on new stresses, such as the simulation of additional wells, pumping rates, or chemical sources that differ from the original model. A scenario model does not involve major changes to the structure of the original model (such as changing the grid, boundary conditions, transmitting properties, or transport properties used in the model), which would require substantial documentation and recalibration of the original model.

Scenario model simulations must be documented in a published report to ensure that the simulations are appropriate for the model, the stresses applied in the simulations are fully described, and the results are available to the public on an equal basis. Models always involve simplifications and are developed to answer specific questions. The appropriateness of the model to address new questions must be evaluated and described in the report. In addition, as stated in Water Resources Division Memorandum No. 94.19, “Longstanding USGS policy requires that information be released to all interested parties (the public) on an equal basis, and that no interpretive information be released to the public without prior approval of the Director.” This requirement to publish new findings is reiterated in the U.S. Geological Survey Manual ( and in Office of Water Information Technical Memorandum No. 2002.11 on “Reemphasizing the importance of public release of investigation results”

In documenting scenario model results, only the aspects of the original model that are changing or are particularly relevant to the discussion of the adequacy of the model to simulate the new stresses need be described in the report. A complete re-documentation of the original model construction is not necessary. The original model can simply be referenced in the scenario model report. All the details of the construction and calibration of the original model need not be reproduced in the new report documenting the scenario model results. The main components of a scenario model report are:

  1. A description of the new problem to be evaluated.
  2. A justification of the appropriateness of using the original model (with specific attention to the appropriateness of the boundary conditions) to adequately simulate the new problem.
  3. A description of the new stresses being simulated and any other changes made to the original model. This description should be complete enough that a reader could independently reproduce the stresses and other changes for the simulation.
  4. For transient models, a description of the initial conditions and how they were determined.
  5. A description of the simulation results.
  6. A discussion of model limitations and the usefulness of the simulation results in answering the problem posed.

In addition, the scenario model must be archived in accordance with Office of Ground Water Technical Memorandum No. 00.02 on the archiving of models (

In many cases, reports documenting scenario models can be relatively short and concise, and can be released online-only, if desired. These reports, whether online-only or printed, must address the critical elements listed above, and they must go through the complete USGS report review and approval process.

William M. Alley /s/
Chief, Office of Ground Water

Distribution: A, B, S, FO, PO,
Regional and District Ground Water Specialists, NR, CR, SR, and WR