ASTM D5611-94(2008)
(Guide)Standard Guide for Conducting a Sensitivity Analysis for a Groundwater Flow Model Application
Standard Guide for Conducting a Sensitivity Analysis for a Groundwater Flow Model Application
SIGNIFICANCE AND USE
After a model has been calibrated and used to draw conclusions about a physical hydrogeologic system (for example, estimating the capture zone of a proposed extraction well), a sensitivity analysis can be performed to identify which model inputs have the most impact on the degree of calibration and on the conclusions of the modeling analysis.
If variations in some model inputs result in insignificant changes in the degree of calibration but cause significantly different conclusions, then the mere fact of having used a calibrated model does not mean that the conclusions of the modeling study are valid.
This guide is not meant to be an inflexible description of techniques of performing a sensitivity analysis; other techniques may be applied as appropriate and, after due consideration, some of the techniques herein may be omitted, altered, or enhanced.
SCOPE
1.1 This guide covers techniques that should be used to conduct a sensitivity analysis for a groundwater flow model. The sensitivity analysis results in quantitative relationships between model results and the input hydraulic properties or boundary conditions of the aquifers.
1.2 After a groundwater flow model has been calibrated, a sensitivity analysis may be performed. Examination of the sensitivity of calibration residuals and model conclusions to model inputs is a method for assessing the adequacy of the model with respect to its intended function.
1.3 After a model has been calibrated, a modeler may vary the value of some aspect of the conditions applying solely to the prediction simulations in order to satisfy some design criteria. For example, the number and locations of proposed pumping wells may be varied in order to minimize the required discharge. Insofar as these aspects are controllable, variation of these parameters is part of an optimization procedure, and, for the purposes of this guide, would not be considered to be a sensitivity analysis. On the other hand, estimates of future conditions that are not controllable, such as the recharge during a postulated drought of unknown duration and severity, would be considered as candidates for a sensitivity analysis.
1.4 This guide presents the simplest acceptable techniques for conducting a sensitivity analysis. Other techniques have been developed by researchers and could be used in lieu of the techniques in this guide.
1.5 This guide is written for performing sensitivity analyses for groundwater flow models. However, these techniques could be applied to other types of groundwater related models, such as analytical models, multi-phase flow models, non-continuum (karst or fracture flow) models, or mass transport models.
1.6 This guide is one of a series on groundwater modeling codes (software) and their applications, such as Guide D5447 and Guide D5490. Other standards have been prepared on environmental modeling, such as Practice E978.
1.7 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
1.9 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a projec...
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Designation: D5611 − 94(Reapproved 2008)
Standard Guide for
Conducting a Sensitivity Analysis for a Groundwater 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope and Guide D5490. Other standards have been prepared on
environmental modeling, such as Practice E978.
1.1 This guide covers techniques that should be used to
conduct a sensitivity analysis for a groundwater flow model.
1.7 The values stated in inch-pound units are to be regarded
The sensitivity analysis results in quantitative relationships
as standard. The values given in parentheses are mathematical
between model results and the input hydraulic properties or
conversions to SI units that are provided for information only
boundary conditions of the aquifers.
and are not considered standard.
1.2 After a groundwater flow model has been calibrated, a
1.8 This standard does not purport to address all of the
sensitivity analysis may be performed. Examination of the
safety concerns, if any, associated with its use. It is the
sensitivity of calibration residuals and model conclusions to
responsibility of the user of this standard to establish appro-
model inputs is a method for assessing the adequacy of the
priate safety and health practices and determine the applica-
model with respect to its intended function.
bility of regulatory limitations prior to use.
1.3 After a model has been calibrated, a modeler may vary
1.9 This guide offers an organized collection of information
the value of some aspect of the conditions applying solely to or a series of options and does not recommend a specific
the prediction simulations in order to satisfy some design
course of action. This document cannot replace education or
criteria. For example, the number and locations of proposed experience and should be used in conjunction with professional
pumpingwellsmaybevariedinordertominimizetherequired judgment. Not all aspects of this guide may be applicable in all
discharge.Insofarastheseaspectsarecontrollable,variationof circumstances. This ASTM standard is not intended to repre-
these parameters is part of an optimization procedure, and, for sent or replace the standard of care by which the adequacy of
the purposes of this guide, would not be considered to be a a given professional service must be judged, nor should this
sensitivity analysis. On the other hand, estimates of future document be applied without consideration of a project’s many
conditionsthatarenotcontrollable,suchastherechargeduring
unique aspects. The word “Standard” in the title of this
a postulated drought of unknown duration and severity, would document means only that the document has been approved
be considered as candidates for a sensitivity analysis.
through the ASTM consensus process.
1.4 This guide presents the simplest acceptable techniques
2. Referenced Documents
for conducting a sensitivity analysis. Other techniques have
been developed by researchers and could be used in lieu of the
2.1 ASTM Standards:
techniques in this guide.
D653Terminology Relating to Soil, Rock, and Contained
Fluids
1.5 This guide is written for performing sensitivity analyses
D5447GuideforApplicationofaGroundwaterFlowModel
forgroundwaterflowmodels.However,thesetechniquescould
to a Site-Specific Problem
be applied to other types of groundwater related models, such
D5490Guide for Comparing Groundwater Flow Model
as analytical models, multi-phase flow models, non-continuum
Simulations to Site-Specific Information
(karst or fracture flow) models, or mass transport models.
E978Practice for Evaluating Mathematical Models for the
1.6 This guide is one of a series on groundwater modeling
Environmental Fate of Chemicals (Withdrawn 2002)
codes (software) and their applications, such as Guide D5447
1 2
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock For referenced ASTM standards, visit the ASTM website, www.astm.org, or
and is the direct responsibility of Subcommittee D18.21 on Groundwater and contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Vadose Zone Investigations. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Sept. 15, 2008. Published November 2008. Originally the ASTM website.
approved in 1994. Last previous edition approved in 2002 as D5611 –94 (2002). The last approved version of this historical standard is referenced on
DOI: 10.1520/D5611-94R08. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5611 − 94 (2008)
3. Terminology 3.1.9 simulation—one complete execution of a groundwater
modeling computer program, including input and output.
3.1 Definitions:
3.1.1 boundary condition—a mathematical expression of a 3.2 For definitions of other terms used in this guide, see
stateofthephysicalsystemthatconstrainstheequationsofthe
Terminology D653.
mathematical model.
4. Significance and Use
3.1.2 calibration—the process of refining the model repre-
sentation of the hydrogeologic framework, hydraulic
4.1 After a model has been calibrated and used to draw
properties, and boundary conditions to achieve a desired
conclusions about a physical hydrogeologic system (for
degree of correspondence between the model simulations and
example, estimating the capture zone of a proposed extraction
observations of the groundwater flow system.
well),asensitivityanalysiscanbeperformedtoidentifywhich
3.1.2.1 Discussion—Duringcalibration,amodelermayvary
modelinputshavethemostimpactonthedegreeofcalibration
the value of a model input to determine the value which
and on the conclusions of the modeling analysis.
produces the best degree of correspondence between the
4.2 If variations in some model inputs result in insignificant
simulation and the physical hydrogeologic system. This pro-
changes in the degree of calibration but cause significantly
cess is sometimes called sensitivity analysis but for the
different conclusions, then the mere fact of having used a
purposes of this guide, sensitivity analysis begins only after
calibrated model does not mean that the conclusions of the
calibration is complete.
modeling study are valid.
3.1.3 calibration targets—measured, observed, calculated,
4.3 Thisguideisnotmeanttobeaninflexibledescriptionof
or estimated hydraulic heads or groundwater flow rates that a
techniques of performing a sensitivity analysis; other tech-
modelmustreproduce,atleastapproximately,tobeconsidered
niques may be applied as appropriate and, after due
calibrated.
consideration, some of the techniques herein may be omitted,
3.1.4 groundwater flow model—an application of a math-
altered, or enhanced.
ematical model to represent a groundwater flow system.
3.1.4.1 Discussion—This term refers specifically to model-
5. Sensitivity Analysis
ing of groundwater hydraulics, and not to contaminant trans-
5.1 The first step for performing a sensitivity analysis is to
port or other groundwater processes.
identify which model inputs should be varied. Then, for each
3.1.5 hydraulic properties—intensive properties of soil and
input: execute calibration and prediction simulations with the
rock that govern the transmission (that is, hydraulic
value of the input varied over a specified range; graph
conductivity, transmissivity, and leakance) and storage (that is,
calibration residuals and model predictions as functions of the
specific storage, storativity, and specific yield) of water.
valueoftheinput;anddeterminethetypeofsensitivitythatthe
3.1.6 residual—the difference between the computed and
model has with respect to the input.
observed values of a variable at a specific time and location.
5.2 Identification of Inputs to be Varied:
3.1.7 sensitivity—the variation in the value of one or more
5.2.1 Identify model inputs that are likely to affect com-
output variables (such as hydraulic heads) or quantities calcu-
puted hydraulic heads and groundwater flow rates at the times
lated from the output variables (such as groundwater flow
and locations where similar measured quantities exist, and
rates) due to variability or uncertainty in one or more inputs to
therebyaffectcalibrationresiduals.Also,identifymodelinputs
a groundwater flow model (such as hydraulic properties or
that are likely to affect the computed hydraulic heads upon
boundary conditions).
which the model’s conclusions are based in the predictive
simulations.
3.1.8 sensitivity analysis—a quantitative evaluation of the
impact of variability or uncertainty in model inputs on the 5.2.2 Usually, changing the value of an input at a single
node or element of a model will not significantly affect any
degree of calibration of a model and on its results or conclu-
sions. results.Therefore,itisimportanttoassemblemodelinputsinto
meaningful groups for variation. For example, consider an
3.1.8.1 Discussion—Anderson and Woessner use “calibra-
unconfined aquifer that discharges into a river. If the river is
tion sensitivity analysis” for assessing the effect of uncertainty
represented in a finite-difference model by 14 nodes, then
on the calibrated model and ''prediction sensitivity analysis”
varying the conductance of the river-bottom sediments in only
for assessing the effect of uncertainty on the prediction. The
one of the nodes will not significantly affect computed flow
definition of sensitivity analysis for the purposes of this guide
into the river or computed hydraulic heads. Unless there are
combines these concepts, because only by simultaneously
compelling reasons otherwise, the conductance in all river
evaluating the effects on the model’s calibration and predic-
nodes should be varied as a unit.
tions can any particular level of sensitivity be considered
significant or insignificant. 5.2.3 Coordinated changes in model inputs are changes
made to more than one type of input at a time. In groundwater
flow models, some coordinated changes in input values (for
example, hydraulic conductivity and recharge) can have little
Anderson, Mary P., and Woessner, William W., Applied Groundwater
effect on calibration but large effects on prediction. If the
Modeling—Simulation of Flow and Advective Transport,Academic Press, Inc., San
Diego,1992. model was not calibrated to multiple hydrologic conditions,
...
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