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<br />e <br /> <br />001023 <br /> <br /> <br />~1f"l' <br /> <br />5. IlIA Growp #5; Rate and requency of Flow Change: The three attributes in this <br />group measure the mean. ate of both positive and negative flow changes from one <br />day to th~~ next, and th number of flow reversals. <br /> <br />analysis described above by using the range of histori~- <br />development) vnnability' the 33 different hydrologic attributes as the basis for setting <br />water managexm:nt targe (Richter et at., 1997). The RV A target range for each attribute is <br />normally based upon sel cted percentile levels or a simple multiple of the parameter <br />standard deviations for e l.....~~ -",MIIIal, Ii!IIJ' pre-development,.streamflow regime. The <br />m.anagement o~iective . s not to have the river attain the targeted range every year, but to <br />attain the targetl:d range at the same frequency as occurred. in the ~U.ll.. . ~1-@ pre- <br />development flow regime. For example, attainment of an RV A target range defined by the <br />75th and 25th per<~entile values of a particular attribute would be expected in only 50% of <br />years, while a range derIDed by (+) or (-) one standard deviation would be expected to be <br />attained in about 68% of years, assuming normally distributed data. The degree to which <br />the R V A target range is not attained is a measure of "hydrologic alteration", calculated as: <br /> <br /> <br />((ObseIVed - Expected)/Expected) * 100 <br /> <br />when expressed as a percentage, where "observed" is the count of years in which the <br />observed value of the hydrologic parameter fell within the targeted range and "expected" is <br />the count of ye~lI'S for which the value is expected to fall within the targeted range. <br />Hydrologic alteration is equal to zero when the observed frequency of post-development <br />annual values filling within the RV A target range equals the expected frequency. A positive <br />deviation indicates the target range is being attained more often than ex.pected, while a <br /> <br />~ve;:7~:e;en~s~e~~~ <br /> <br />Application of IHA to the San Juan River <br /> <br />oJJ <br /> <br />To address study objectives. the IRA approach was applied to the stream flow record for the <br />San Juan Rive:l~ near Bluff, UT gage. This U. S. Geological Survey (USGS) gage has a long <br />period-of-record (1929 - 2002) and was used extensively in developing the SJRIP flow <br />recommendations (Holden (ed.), 1999). The period-of-record was divided into three time <br />periods, 1929 to 1961 (pre-Navajo Dam), 1965 to 1991 (post-Navajo Dam), and 1992 to <br />2002 (SJRIP). ~ the guidance ofRicbter et aih998) and Holden (ed.)(1999), the <br />assumption@!i made that water development activitf prior to the construction of Navajo <br />Dam had a reiatively minor affect on the river's hydrograph and that an 11 year record was <br />adequate to de:scribe the hydrologic regime for the SJRIP period.. The 1962 to 1964 water <br />years were de leted from the record as N avaj 0 Reservoir was filling during this period. <br />Following thl~ strong recommendation of The Nature Conservancy and Smythe Scientific <br /> <br />5 <br /> <br />EOS-d BlO/BOO'd 99v-l <br /> <br />-WDJd WdBl:lO EO-BO-t~o <br />