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<br />llIA Group #5: Rate and Frequency of Change in Water Conditions <br /> <br />The four attributes included in this group measure the number and mean rate of both positive <br /> <br />and negative change in water conditions (Fig. 7). The rates and frequency of change in water <br /> <br />conditions can be described in terms of the abruptness and number of intra-annual cycles of <br />.,;! <br />environmental variation, and provide a measure of the rate and frequency of intra-annual ' <br /> <br />environmental change. <br /> <br />,~",tiJ <br /> <br />. ".:!:fm:c' <br /> <br /> <br />Methods for' Assessing Hydrologic Alteration <br /> <br />. .nz!lllU <br /> <br />: '. ~ t~.r:w <br /> <br />In assessing the impact of a perturbation on the hydrologic regime, we want to. . "',, " <br />.~~~.. <br />determine whether the state of the perturbed system differs significantly from what it would',.';~:;; <br />.;t;; .-., + ...~.~1 ~~,!-pr~:I:~~,~',}~,-_.- <br />have been in the absence of the perturbation. In particular, we want to test whether the central:.. . " <br /> <br />tendency or degree of inter-annual variation of an attribute of interest has been altered by Ihe:,: <br />".1\-, <br />~ ;.I.i~.1 <br />perturbation (Stewart-Oaten et al. 1986). The assessment of impacts to natural systems often' <br />.,'.mT <br />pose difficult statistical problems, however, because the perturbation of interest cannot be <br /> <br />replicated:~~ randomly assigned to'experimental units:(Carpenter 1989; Carpenter et al. 1989; <br />...' . ',' 'UJtsd <br />Hurlbert i984; Stewart-Oaten et al. 1986). The lack of replication does not hinder estimation <br /> <br />"+..1i"lcl:. <br /> <br />,'-.}i~' <br /> <br />of the magnitude of an effect, but severely Iimi~ any inference mat can be made regarding,thc:: <br /> <br />;"";-';":'__"~' , _ . )lO:j.~' <br />cause of the effect. This issue haS received considerable attention recently, and more ..;~;<: <br /> <br />.. <br />!)i'~' <br />sophisticated ~~perimental designs that incorporate replication overtime and sa~plingat ;~': ''v <br />" ,," , ,..,;,:n~ <br />"control" and an "impact" site have been suggested to address it (Stewart-Oaten et a1.1986, <br />,:..;,~::_, <br /> <br />1992). <br /> <br />.,- <br /> <br />"L,,!~jtto <br /> <br />- j,'-..-,' <br /> <br />12 <br />