PUBLICATIONS--Policy on documenting the use of ground-water simulation in project reports

To: "File WRD Archive Reston, VA "
Subject: OGW TechMM 96.04
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Date: Thu, 25 Apr 1996 08:50:30 -0400

In Reply Refer To:                                      April 24, 1996
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Subject:  PUBLICATIONS--Policy on documenting the use of 
          ground-water simulation in project reports

It has been more than two decades since Ground Water Branch 
Technical Memorandum No. 75.11 was released on the subject of 
documenting the use of ground-water simulation in project 
reports.  Because of the time lapse, changes in modeling 
techniques, and the frequency of problems found when reports are 
reviewed, a revisit to policy on this subject is appropriate.

There is no rigid checklist or recipe for reporting on the use 
of simulation in a ground-water study.  The appropriate level 
of documentation will vary depending on the project objectives 
and the complexity of the simulations.  The general structure of 
a well-constructed report describing simulation is much the same 
as that for any investigative study.  It should present (1) the 
objectives of the study, (2) a description of the work that 
was done, (3) logical arguments to convince the reader that 
the methods and analyses used in the study are valid, and 
(4) results and conclusions.

Specific topics that should be addressed in reports that 
describe studies in which simulation is used include the 

1. Describe the purpose of the study and the role that 
simulation plays in addressing that purpose.  

The objective of the simulation must be clearly stated.  The 
model should be represented as a tool to help solve specific 
problems or answer specific questions rather than as an end 

2. Describe the hydrologic system under investigation.  

The extent, nature of boundaries, transmitting properties, 
storage properties, sources of water, discharge mechanisms and 
other relevant components of the ground-water system should be 
described as known or conceptualized.  Usually this can be 
accomplished in part by referencing previous works, but major 
relevant system characteristics should be summarized in the 
report that describes the simulation.

3. Describe the mathematical methods used and their 
appropriateness to the problem being solved.  

In most cases, a reference to a readily available publication 
will be sufficient to document mathematical details; however, it 
will usually be desirable to briefly summarize the methods that 
are used.  For a well-documented computer program, this will 
often require only a paragraph or two.  If a documented computer 
program is modified such that computed values are affected, the 
modifications should be documented and evidence that the 
modifications are correct should be supplied.

4. Describe the hydrogeologic character of the boundary 
conditions used in the simulation of the system.  

In many cases, the model boundaries are placed where the aquifer 
terminates against relatively impermeable rocks or is 
intersected by a perennial stream whose 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 modeled area 
may need to extend beyond the project area to accurately 
simulate the natural boundaries of the aquifer system.  If the 
modeled area is arbitrarily truncated at some distance from the 
area of interest, it should be shown that the selection of the 
arbitrary boundary condition does not materially affect the 
ability of the model to simulate the system for the purposes of 
the study.  Internal boundaries such as streams, lakes, and 
pinchouts of important hydrogeologic zones should be identified 
and their representation in the model should be described in the 
report.  A clear, convincing argument of the appropriateness of 
the boundary conditions used in the model to represent the 
actual system should be made for the entire bounding surface of 
the modeled volume or cross section, as well as for any internal 

5. If the method of simulation involves discretizing the system 
(finite-difference and finite-element methods for example), 
describe and justify the discretized network used.  

The spacing and distribution of the blocks, elements, or 
subregions should reflect, in part, the spatial variability of 
the hydraulic parameters and the location of boundaries (for 
example streams, lakes, bed pinchouts), human-made features (for 
example wells and dams), and stresses.  In most cases, a map 
showing the discretized network superimposed on the study area 
is required.  Vertical discretization should be described and/or 
shown on illustrations.  The manner in which time is discretized 
for transient models also should be described.  If a steady-
state model is used to simulate an average or approximate steady-
state condition, discuss the errors that could be introduced in 
the study results as a consequence of using a steady-state model.

6. Describe the aquifer system properties that are modeled.

Explain whatever inferences are made from field data and 
previous studies as to the spatial variation of hydraulic 
properties of aquifers and confining beds and how discretized 
values are computed throughout the simulated area.  During model 
calibration (see item 9), modeled values are often changed; the 
final aquifer system properties that are modeled should be 
described in the report.  This can be through maps or 
descriptions in the text.  Lists of model arrays do not 
generally provide much understanding of the model and 
accordingly should not be included in the report unless it is 
expected that readers will want to repeat the simulations.  If 
lists of arrays are included, they should usually be provided on 
electronic media.  Note that Office of Ground Water Technical 
Memorandum No. 93.01 describes the separate requirement for 
archiving the complete model data sets used in ground-water 

7. Describe all the stresses modeled such as pumpage, 
evapotranspiration from ground water, recharge from 
infiltration, river stage changes, leakage from other aquifers, 
and source concentrations in transport models.  

The relations between observed and modeled stresses should be 
described.  For example, it usually is desirable to provide a 
representative sample of actual pumping histories and the 
corresponding modeled pumping histories, although such 
information would not necessarily be provided for every pumped 
well.  The manner in which stresses are averaged within the 
discretized time and space scheme should also be described.
If a steady-state model is used to simulate an average or 
approximate steady-state condition, describe how the average 
stresses representing this system are calculated.

8. For transient models, describe the initial conditions that 
are used in the simulations.  

Ideally, a transient simulation will start from a steady-state 
condition, and the steady-state initial conditions will be 
generated by a steady-state simulation using the same model.  In 
this case, the steady-state simulation must use the same 
hydraulic and stress parameters that are used in the transient 
simulation, except that the transient stresses are removed.  In 
situations where it is not possible to start a transient model 
from a simulated steady-state condition, it is necessary to 
describe how the initial conditions were derived.  It is also 
important to estimate the error in the derived values and the 
possible impact on the model results.  

9. If a model is calibrated, present the calibration criteria, 
procedure, and results.  

Describe the source of the observed data to which model results 
are compared.  Explain the appropriateness of using these data 
for model comparisons and the rationale for any adjustments made 
to actual observations when making the comparisons.  For 
example, when steady-state models are used to simulate an 
approximate steady-state condition, it is important to explain 
to what extent the observations that have been made at specific 
points in time correspond to the approximate steady-state 
condition being simulated.  Give a representative sample of the 
actual comparisons used for calibration, and show the locations 
of the observation points on maps.  When the number of 
observations is extensive, locations of representative points 
can be shown.  It is important to report and use as many types 
of data as possible for calibration.  For example, in a flow 
model, both head and flow observations are desirable for use in 

10. Discuss the limitations of the model's representation of the 
actual system and the impact those limitations have on the 
results and conclusions presented in the report.  

Evaluating the sensitivity of the computed model responses to 
changes in parameter values that reflect plausible parameter 
uncertainty helps to assess the model reliability.  If the model 
is to be used to make specific projections, it is useful to 
estimate the impacts of the uncertainty of parameter values on 
the projections.  In calibrated models, a concern is non-
uniqueness, which is the extent to which other combinations of 
parameter values or configurations may result in an equally good 
fit to the observed data.  Discuss the extent to which 
nonuniqueness may affect the use of the model in the study.

In summary, a report describing a study in which simulation is 
used should address the above topics; however, there is 
considerable flexibility in the form of such a report.  The 
report should describe the purpose of the simulation and 
convince the reader that the use of simulation is credible.  The 
report should further describe the system being simulated, the 
methods of simulation, and the data that are used.

                        William M. Alley
                        Chief, Office of Ground Water

Distribution: A, B, S, FO, PO

This memorandum supersedes Ground Water Branch Technical 
Memorandum No. 75.11