ASTM D5611-94(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D5611–94 (Reapproved 2002)
Standard Guide for
Conducting a Sensitivity Analysis for a Ground-Water Flow
Model Application
This standard is issued under the fixed designation D5611; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 1.7 The values stated in inch-pound units are to be regarded
as the standard. The SI units given in parentheses are for
1.1 This guide covers techniques that should be used to
information only.
conduct a sensitivity analysis for a ground-water flow model.
1.8 This standard does not purport to address all of the
The sensitivity analysis results in quantitative relationships
safety concerns, if any, associated with its use. It is the
between model results and the input hydraulic properties or
responsibility of the user of this standard to establish appro-
boundary conditions of the aquifers.
priate safety and health practices and determine the applica-
1.2 After a ground-water flow model has been calibrated, a
bility of regulatory limitations prior to use.
sensitivity analysis may be performed. Examination of the
1.9 This guide offers an organized collection of information
sensitivity of calibration residuals and model conclusions to
or a series of options and does not recommend a specific
model inputs is a method for assessing the adequacy of the
course of action. This document cannot replace education or
model with respect to its intended function.
experience and should be used in conjunction with professional
1.3 After a model has been calibrated, a modeler may vary
judgment. Not all aspects of this guide may be applicable in all
the value of some aspect of the conditions applying solely to
circumstances. This ASTM standard is not intended to repre-
the prediction simulations in order to satisfy some design
sent or replace the standard of care by which the adequacy of
criteria. For example, the number and locations of proposed
a given professional service must be judged, nor should this
pumpingwellsmaybevariedinordertominimizetherequired
document be applied without consideration of a project’s many
discharge.Insofarastheseaspectsarecontrollable,variationof
unique aspects. The word “Standard” in the title of this
these parameters is part of an optimization procedure, and, for
document means only that the document has been approved
the purposes of this guide, would not be considered to be a
through the ASTM consensus process.
sensitivity analysis. On the other hand, estimates of future
conditionsthatarenotcontrollable,suchastherechargeduring
2. Referenced Documents
a postulated drought of unknown duration and severity, would
2.1 ASTM Standards:
be considered as candidates for a sensitivity analysis.
D653 Terminology Relating to Soil, Rock, and Contained
1.4 This guide presents the simplest acceptable techniques
Fluids
for conducting a sensitivity analysis. Other techniques have
D5447 Guide for Application of a Ground-Water Flow
been developed by researchers and could be used in lieu of the
Model to a Site-Specific Problem
techniques in this guide.
D5490 Guide for Comparing Ground-Water Flow Model
1.5 This guide is written for performing sensitivity analyses
Simulations to Site-Specific Information
for ground-water flow models. However, these techniques
E978 Practice for Evaluating Mathematical Models for the
couldbeappliedtoothertypesofground-waterrelatedmodels,
Environmental Fate of Chemicals
such as analytical models, multi-phase flow models, non-
continuum (karst or fracture flow) models, or mass transport
3. Terminology
models.
3.1 Definitions:
1.6 This guide is one of a series on ground-water modeling
3.1.1 boundary condition—a mathematical expression of a
codes (software) and their applications, such as Guide D5447
stateofthephysicalsystemthatconstrainstheequationsofthe
and Guide D5490. Other standards have been prepared on
mathematical model.
environmental modeling, such as Practice E978.
This guide is under the jurisdiction of ASTM Committee D18 on Soil and
Rockand is the direct responsibility of Subcommittee D18.21 on GroundWater and
Vadose Zone Investigations. Annual Book of ASTM Standards, Vol 04.08.
Current edition approved Sept. 15, 1994. Published October 1994. Annual Book of ASTM Standards, Vol 11.05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D5611–94 (2002)
3.1.2 calibration—the process of refining the model repre- ample, estimating the capture zone of a proposed extraction
sentation of the hydrogeologic framework, hydraulic proper- well),asensitivityanalysiscanbeperformedtoidentifywhich
ties, and boundary conditions to achieve a desired degree of modelinputshavethemostimpactonthedegreeofcalibration
correspondence between the model simulations and observa- and on the conclusions of the modeling analysis.
tions of the ground-water flow system.
4.2 If variations in some model inputs result in insignificant
3.1.2.1 Discussion—During calibration, a modeler may changes in the degree of calibration but cause significantly
vary the value of a model input to determine the value which
different conclusions, then the mere fact of having used a
produces the best degree of correspondence between the calibrated model does not mean that the conclusions of the
simulation and the physical hydrogeologic system. This pro-
modeling study are valid.
cess is sometimes called sensitivity analysis but for the
4.3 Thisguideisnotmeanttobeaninflexibledescriptionof
purposes of this guide, sensitivity analysis begins only after
techniques of performing a sensitivity analysis; other tech-
calibration is complete.
niques may be applied as appropriate and, after due consider-
3.1.3 calibration targets—measured, observed, calculated,
ation,someofthetechniqueshereinmaybeomitted,altered,or
or estimated hydraulic heads or ground-water flow rates that a
enhanced.
modelmustreproduce,atleastapproximately,tobeconsidered
calibrated.
5. Sensitivity Analysis
3.1.4 ground-water flow model—an application of a math-
5.1 The first step for performing a sensitivity analysis is to
ematical model to represent a ground-water flow system.
identify which model inputs should be varied. Then, for each
3.1.4.1 Discussion—This term refers specifically to model-
input: execute calibration and prediction simulations with the
ing of ground-water hydraulics, and not to contaminant trans-
value of the input varied over a specified range; graph
port or other ground-water processes.
calibration residuals and model predictions as functions of the
3.1.5 hydraulic properties—intensive properties of soil and
valueoftheinput;anddeterminethetypeofsensitivitythatthe
rock that govern the transmission (that is, hydraulic conduc-
model has with respect to the input.
tivity, transmissivity, and leakance) and storage (that is, spe-
5.2 Identification of Inputs to be Varied:
cific storage, storativity, and specific yield) of water.
5.2.1 Identify model inputs that are likely to affect com-
3.1.6 residual—the difference between the computed and
puted hydraulic heads and ground-water flow rates at the times
observed values of a variable at a specific time and location.
and locations where similar measured quantities exist, and
3.1.7 sensitivity—the variation in the value of one or more
therebyaffectcalibrationresiduals.Also,identifymodelinputs
output variables (such as hydraulic heads) or quantities calcu-
that are likely to affect the computed hydraulic heads upon
lated from the output variables (such as ground-water flow
which the model’s conclusions are based in the predictive
rates) due to variability or uncertainty in one or more inputs to
simulations.
a ground-water flow model (such as hydraulic properties or
5.2.2 Usually, changing the value of an input at a single
boundary conditions).
node or element of a model will not significantly affect any
3.1.8 sensitivity analysis—a quantitative evaluation of the
results.Therefore,itisimportanttoassemblemodelinputsinto
impact of variability or uncertainty in model inputs on the
meaningful groups for variation. For example, consider an
degree of calibration of a model and on its results or conclu-
4 unconfined aquifer that discharges into a river. If the river is
sions.
4 represented in a finite-difference model by 14 nodes, then
3.1.8.1 Discussion—Anderson and Woessner use “calibra-
varying the conductance of the river-bottom sediments in only
tion sensitivity analysis” for assessing the effect of uncertainty
one of the nodes will not significantly affect computed flow
on the calibrated model and 88prediction sensitivity analysis”
into the river or computed hydraulic heads. Unless there are
for assessing the effect of uncertainty on the prediction. The
compelling reasons otherwise, the conductance in all river
definition of sensitivity analysis for the purposes of this guide
nodes should be varied as a unit.
combines these concepts, because only by simultaneously
5.2.3 Coordinated changes in model inputs are changes
evaluating the effects on the model’s calibration and predic-
madetomorethanonetypeofinputatatime.Inground-water
tions can any particular level of sensitivity be considered
flow models, some coordinated changes in input values (for
significant or insignificant.
example, hydraulic conductivity and recharge) can have little
3.1.9 simulation—one complete execution of a ground-
effect on calibration but large effects on prediction. If the
watermodelingcomputerprogram,includinginputandoutput.
model was not calibrated to multiple hydrologic conditions,
3.2 For definitions of other terms used in this guide, see
sensitivity analysis of coordinated changes can identify poten-
Terminology D653.
tial non-uniqueness of the calibrated input data sets.
5.3 Execution of Simulations:
4. Significance and Use
5.3.1 Foreachinput(orgroupofinputs)tobevaried,decide
4.1 After a model has been calibrated and used to draw
upon the range over which to vary the values. Some input
conclusions about a physical hydrogeologic system (for ex-
values should be varied
...