QUALITY CONTROL--Reporting and documentation of aquifer modeling studies
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, PO