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<br />alteration is then based on the magnitude of the difference in central tendency and dispersion <br /> <br />for each of the attributes between the two time periods. This results in calculation of 64 <br /> <br />deviations between the pre- and post-impact periods, which together represent the "Indicators <br /> <br />of Hydrologic Alteration. " <br /> <br />In impact assessment, we are generally more interesied in the size of an effect aDd ' <br /> <br />. - '~""C <br />whether itmatters biologically than we are in whether or not an effect can be detected.' Tliat' <br /> <br /> <br />.~':--;'':lQt",..t <br />is, we want to estimate .the'sizeofthe:effecus~welhas the precision of the estimate, rather: ", , <br /> <br />than test the,null hypothesis that the central tendency or dispersion has not changed be~1l <br /> <br />. ,,~'t,"-;''1 <br />the pre- andpost~impact time periods. Accordingly, the results of the IHA method are most' <br /> <br />usefully presented in terms of the magnitudes of the differences in-central tendency (or -!-",,~ <br /> <br /> <br />dispersion) between the pre- and post-impact periods (see Figs; 8.:12 for-illustration)' along: <br />with the confidence limits. for this difference, rather than as the .e value for-the null hyp6ih~si'i <br />that the central tendencies are the same. Hypothesis testing may be'valUable for 'specific'~ ' <br /> <br />J _'- "i....~'f~ <br />in which biologically relevant thresholds to hydrologic change can be identified. In these' ," <br /> <br />. h~''''~'' <br />cases, an equivalence. test:(McBride;etal."ol993). can: be used to test the null'hypothesis tha'ttlie <br /> <br />observed difference is greater than some user-identified biologically significant value. <br /> <br />,yJ. ii1Jb <br /> <br />-"::-"'~' <br /> <br />Development: of Pre- and Post-Impact Data Sets <br /> <br />':~-:'-. - <br /> <br /> <br />When adequate hydrologic records are available for both the pre-impact and post-impacf1iihe.~., <br /> <br /> <br />periods, application of the IHA method will be relatively straightforward using the statistical"" , <br /> <br /> <br />procedures--described above, When pre- or post-impact records are non-existent or iriadeijiiiit2' <br /> <br /> <br />in length, however; various data reconstruction or estimation procedUres willneed to lle --~ <br /> <br />- .'.,- <br /> <br />14 <br />