ASTM D5611-94(2002)

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.

1.1 This guide covers techniques that should be used to conduct a sensitivity analysis for a ground-water 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 ground-water 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 ground-water flow models. However, these techniques could be applied to other types of ground-water 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 ground-water 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 the standard. The SI units given in parentheses are for information only.

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 judgement. Not all aspects of this guide may be applicalbe 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 unique aspects. The word "Standard" in the title of this document means only that the document has been approved through the ASTM consensus process.