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<br />many different types of stress. <br /> <br />vi) Indices should be sufficiently sensitive and timely, such that they quickly <br /> <br />register ecosystem responses to stresses and perturbations. <br /> <br />vii) Indices must allow comparison of measured conditions to a reference condition <br /> <br />or scale of reference conditions based on field data or extrapolationS from these <br /> <br />data. <br /> <br />Many of theccharacteristics-:ofhydrologic::data,collection and analysis have facilitated~ <br />, . '_0"""""',. <br /> <br />the development of the IHA method while meeting most of these desired attributes. In~t <br /> <br />to biological monitoring, hydrologic data collection tends to be highly standardized in.lI!OS!';4 <br /> <br />countries; data have been collected for relatively long time periods (which often predate <br />;~/~~.'- , <br />. '::'-'-- ,'-'-{-".'.,- <br />substantial hydrologic alterations); measurements are commonly obtained from continuo~. ,c5::" <br /> <br />recording gauges and summarized as daily means; access to these data is usually centra1izeAf.:: <br /> <br />-., ',",>/'; <br /> <br />~oi->~,',:~-, <br /> <br />and readily available; and; monitoring networks are comparatively dense (e.g., more 1:hllm. ; <br /> <br />20,000 streamgauging stations have been established in the U.S. by the U.S. GeologicaL' .."t <br /> <br />Survey). The availabilitycof long periods;ofrecord and the relative density of monitoriIl&4-c f <br /> <br />networks also greatly facilitates definition ofa pre. impact or less.a1tered.hydrologic reference, <br /> <br />state for a site of interest. <br /> <br />Although the IHA method results in the computation of a suite of biologically.relev.aDt.;,' <br />. _ , ...u_. .,~ <br /> <br /> <br /><:t"-- <br />hydrologic indicators, these indicators are not combined into a single index.. The intentg,~J? <br /> <br />'-','-' <'....,- <br /> <br />lHA method in its current form is to identify the direction and magnitude of change in.sp.ec!!i.s-: <br /> <br />components of a hydrologic regime in response to a perturbation or management action,:!1~!:1e: <br /> <br />provide a single measure of ecosystem integrity. <br /> <br />7 <br />