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<br />While fully cognizant of the replication issue, we have based the IHA method on the <br /> <br />simple design of comparing the central tendency and dispersion of hydrologic attributes at a <br /> <br />single site before and after a putative perturbation -- the method allows the magnitude of <br /> <br />impacts to be estimated, but does not enable strong inferences regarding the cause. We take <br /> <br />this simpler approach for two reasons. First, in many locations where the method might be <br /> <br />applied, the availability of an appropriate control site may be limited. Second, causal )W<I <br /> <br />inference, while desirablef.may not always;be;a.necessary prerequisite to prescribing--,,, ,ill <br /> <br />management.or res.toration actions to mitigate for observed effects. However; the IHA-metbOd <br /> <br />is robust ~d can be easily adapted to more sophisticated experimental designs, and we ' ""i.l <br /> <br />encourage such lIP,P!ication when control sites are available. <br /> <br />~1~~~ <br /> <br />'. To ensUIe consistency in the application of the lHA, users of the method should:c''''''ly, <br />- ....;...,~- ,~. . ,- - - <br /> <br /><C..:," <br /> <br />identify the presumed cause of the impact(s) being evaluated, e.g., the .impact of an UpstreaRri <br /> <br />reservoir orirri~tion diversions on streamflow, or the effects of ground water pumping Oll,,~, <br /> <br />wetland pond levels. In this manner, the time period prior to the presumed perturbation can be <br /> <br />defined as the .pre.impact~"pe[iod,.an(l.the.time since initiation of the presumed perturbation:- <br /> <br />can be defined as the .post-impact" period. Once pre- and post-impact time periods have:beCn <br /> <br />defined, the hydrologic regimes from the two periods can.be characterized and compared. <br /> <br />In the IHA method, the pre-impact and post-impact hydrologic regimes are <br /> <br />Y~"l~" <br /> <br />characterized separately by computing, for each year, in the pre- or post-impact record, the<,'J,1 <br /> <br />, <br /> <br />value of each of the 32 hYdrologic attributes listed in Table 1. Then" the inter-annual variation <br /> <br />of ' each attribute within the p're- or post-impact time period is determined by computing the.":" <br />, - . .. - -.' . . <br /> <br />central tendency fUld a measure of dispersion of annual values. The assessment of hydrologic" <br /> <br />13 <br /> <br />J. .- <br />