Laserfiche WebLink
<br />lhjlti21 <br /> <br />e <br /> <br />e <br /> <br />hydrologic variability is essential in conserving native riverine biota and aquatic habitat <br />quality. Termed the "Indicators of Hydrologic Alteration" (IRA) approac~ the method is <br />fIrst applied to assess the degree of hydrologic alteration that has occurred temporally or <br />spatially within a river system. Second, range of variability analysis (RV A) is used to <br />establish prelimil~ ary target flow ranges, which, if met by water managers, will begin to <br />restore the hydrologic regime of the system. and provide a solid basis for the development <br />of testable hypotheses regarding species-habitat-flow relations (Richter et ai., 1996; <br />Richter et al., 1997). <br /> <br />The IRA method first!tes a series of biologically-relevant hydrologic attributes that <br />characterize~tra.-annua ariation in water flow conditions and then uses an analysis of the <br />inter-annual vari.ation m ese attributes as the basis for comparing hydrologic regimes <br />~or€; 6 __ after a river has been altered by significant human activities (Richter et al., <br />.t> ....>J 1996). The method is statistically based, software driven, and for river applications, the <br />~ _ basic data unit is mean daily discharge. Stream flow data are typically obtained from U. S. <br />~ Geological Survey (USGS) (ot other appropriate sources) gage records. Application of the <br />method includes four general steps: <br /> <br />1. The data. series is defined for pre. and post-impact periods in the river system of <br />interest, with the period.of-record for each series recommended to be at least 20 <br />years. <br /> <br />2. Values j~)r each of33 ecologically-relevant hydrologic attributes are calculated for <br />each year in each data series (see Appendix B for attribute descriptions). <br /> <br />3 . Inter-annual statistics are computed by calculating measures of central tendency and <br />dispersion for the 33 attributes in each data series, based on the values obtained in <br />Step 2. <br /> <br />4. Values of the Indicators of Hydrologic Alteration are calculated by comparing the <br />64 inter-annual statistics between the pre-and post- impact data series, and each <br />result is presented as a percentage deviation of one time period relative to the other. <br />The method can be used to compare the state of one river system1~teMf over time <br />(e.g. temporal comparisons, pre- versus, ffi)St-impact, as just described); or it can be <br />used to compare the state of one system"" another (e.g. spatial comparisons, a <br />highly altered system to a less altered., reference system). <br /> <br />The 33 attributes evaluated by the IRA are based upon five fuT\.O~mental characteristics of <br />hydrologic regimes: <br /> <br />1. Flow .l'dagnitude: The magnitude of the flow at any given time is a measure of the <br /> <br />3 <br /> <br />E09-~ 8l0/900.d 99v-l <br /> <br />-WOJ~ Wdll:IO EO-BO-l~O <br />