UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY RESTON, VIRGINIA 22092 February 5, 1975 GROUND WATER BRANCH TECHNICAL MEMORANDUM NO. 75.11 Subject: QUALITY CONTROL--Reporting and documentation of aquifer modeling studies Ground Water Branch memo No. 75.08 described the status of digital modeling research in WRD, summarized types of problems being considered, and indicated the kinds of numerical approaches being developed. This memo provides guidelines for reporting and documentation of modeling studies, either digital or analog. Reporting of aquifer modeling studies In most ground-water studies modeling analysis enhances understanding of the ground-water flow system. For any modeling analysis the project report should identify for the reader the modeling assumptions and the resultant constraints on use of the model. Therefore, we recommend that reports concerning model analyses include the following information. 1. Define the location of the modeled area if it does not coincide with the project area. 2. Describe the hydrologic character of the model boundaries and how these are represented in the model. In many cases the model boundaries are placed where the aquifer terminates against nearly impermeable rocks or is intersected by a perennial stream show head variation in time and space is known. In other cases, the aquifer may be so extensive relative to the area of interest that the model simulation can be truncated at some arbitrary distance; but if so, the author should show that selection of the arbitrary boundary location has not materially affected the model calibration or model predictions. Internal boundaries such as streams or lakes should be identified and their counterpart representation should be defined in the report. 3. Justify the use of two-dimensional models. In some cases pumping wells tap one or more zones of a multi- layered sequence of extensive sand and clay layers. Although three-dimensional analog models have been developed, a strictly three-dimensional digital model has not been developed. In lieu thereof multi-aquifer systems are handled by a quasi three-dimensional digital model that uses a two-dimensional area model for each aquifer with interconnection of aquifers through one- dimensional leakage terms. Because a strictly three- dimensional digital model has not been developed is not, of itself, adequate justification to use a two- dimensional model. The burden of justification rests with the modeler. 4. Describe the aquifer system properties that are modeled. Inferences are made from field data as to the areal variation of hydraulic properties of aquifers and confining beds. The spatial variation in heads throughout the aquifer system at some initial time and the subsequent hydrologic stresses (see item 5) are simulated and the model-derived responses are compared to field observations. If the degree of comparison is unacceptable, the modeler often will change the modeled values for one or more of the physical parameters and/or characterization of the physical stresses to obtain a better fit. The outcome of this trial-and-error process essentially is an improved conceptualization of the aquifer system and the imposed stresses. The final synthesis should be described in the report. If the system differs appreciably from the conceptual model developed from the pre-model analysis, the modeler is obliged to provide evidence that modifications made to the model to achieve an "acceptable" fit between computed and observed stresses are hydrologically reasonable because "near best" fits between computed and observed responses can be obtained, for example, using physically absurd aquifer parameter values. 5. Describe the stresses and initial conditions modeled. Stresses, in the form of pumpage, evapotranspiration from ground water, recharge from infiltration, river-level changes, leakage from other aquifers, etc. are imposed on the aquifer system. The relations between observed and modeled stresses should be described. For example, it might be desirable to provide a representative sample of actual pumping histories and the corresponding modeled pumping histories, although such data would not be necessary for every pumped well. 6. Describe the discretization network. The numerical techniques most used to solve the equation describing ground-water flow requires that the region of interest be divided into subregions. If awkward to describe in the text, an illustration showing the spatial discretization should be given. If a digital model is used, the manner in which the computations are marched through time also should be described. 7. Describe the mathematical techniques used. In most cases it will be desirable to briefly describe the mathematical equations used to approximate the flow problem and their method of solution. If an analog model was used, the analogy between the various components of the hydrologic system and the electrical analog system should be described. If a digital model was used, and the computer program used is already published, avoid the temptation of including a listing of the program unless your version is substantially different from the published version. You should however specify any significant modifications of the published program. 8. Present the criteria used for model calibration and the limitations of the model's representativeness of the real system. Give a representative sample of the actual comparisons used for calibration. If changes in head at selected times were used for comparison, show these. Describe the data used to develop the observed heads and changes, and at least indicate locations of representative data points on maps showing observed conditions. This provides a basis for the reader to evaluate the control for observed level conditions. In summary, a report discussing calibration of a model should include a description of the aquifer system, the stresses imposed on the system, and the responses of the system to the imposed stresses. Real aquifer systems are too complex to be modeled exactly. The hydrologist develops a conceptual model that simplifies the system to make it manageable and yet retains those features essential to the objectives of the model study. The assumptions made in developing the conceptual model of the aquifer system should be identified and justified and their implications on model predictions specified. The basis for model calibration and validation should be given. Documentation of aquifer modeling analysis Carefully document the trial-and-error calibration of the model-- even though that material is not included in the published report. It is good modeling practice to keep a "log" for all model runs. We recommend that the information documented include the assumptions made for the run, some rationale on the selection of these conditions, and interpretations of the run results. Remember that changes in the modeled parameters correspond to changes in conceptualization of the physical system. Each model run should be logged when the results are evaluated, not months later. This documentation would (a) aid the modeler in keeping track of the essentially adaptive learning process of trial-and- error calibration, (b) be helpful during the project and report review to discussions on the technical aspects of model calibration, and (c) provide a useful permanent record for future analysts of the geohydrology of the area. (s) Charles A. Appel for Gerald Meyer Chief, Ground Water Branch WRD Distribution: A, B, S, FO, POQUALITY CONTROL--Reporting and documentation of aquifer modeling studies