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<br />0". '"'1":' <br />6 .j <br /> <br /> Greater than 3.0 times { 8 - <br /> UJ the interquartile range <br /> a: 0 FAR OUT Far~out values occur fewer <br /> ::> <br /> "' f- than once in 300,000 times <br /> <( <br /> UJ in a normal distribution <br /> '" * <br /> "- I- Between 1.5 end 3.0 times { <br /> 0 <br /> f'! the interquartile range * OUTSIDE Outside values occur fewer <br /> Z I- { than once in 100 times in a _ <br /> ::> <br /> UJ Between the 75th normal distribtution <br /> ~ percentile and 1,5 <br /> 1i' I- times the interquartile - <br /> 0- range 75th PERCENTilE <br /> 0 { <br /> a: <br />. 0- f- - MEDIAN <br />0- InterquartUe range <br /> <( <br /> ;;; '- <br /> <6 { 25th PERCENTilE - <br /> 0 Between the 25th <br />. t;i <br /> ~ percentile and 1.5 times - <br /> 1i' the interquartile range <br /> UJ <br /> f- <br /> Sz1 - { * -' <br /> a: <br /> 'i Between 1.5 and 3.0 times * <br /> 0 _ the interquartiJe range * , <br /> iO <br /> i!j { <br /> - Less than 3.0 times the 0 <br /> interquartHe range <br /> - - <br /> Figure 3. Explanation of information for a boxplot. <br /> <br />STREAMFLOW CHARACTERISTICS <br /> <br />Knowledge of streamflow characteristics is <br />essential when attempting to evaluate traveltime and <br />gains and losses because streamflow (discharge) data <br />describe the volume of water moving per unit-time in <br />a river system. In the lower Purgatoire River, water- <br />stage data generally were collected at IS-minute <br />intervals, from which surface-water records (daily <br />mean discharge) were computed by standard methods <br />and procedures described by Rantz and others <br />(1982). Most of the available .streamflow records were <br />rated good, with an uncertainty error ofless than <br />:t1O percent, except during winter when ice effects <br />frequently were recorded at the streamflow-gaging <br />stations in the study area. <br />A flow-duration curve, which is a cumulative <br />frequency curve that indicates the percent of time a <br />specified streamflow was equaled or exceeded, is <br />particularly suitable for evaluating the historical <br />record of streamflow for two different periods of time. <br /> <br />Flow-duration analysis of daily mean streamflow was <br />done for two stations: Purgatoire River at Trinidad <br />(site QIA) and Purgatoire River below Trinidad <br />Reservoir (site Ql). These two mainstem stations <br />were selected to represent conditions of streamflow <br />entering the study area during two relatively long-term <br />periods of time before and after Trinidad Dam became <br />operational in 1979. The flow-duration curve for <br />site Q 1 A was computed for 11 continuous years <br />(1957-67) of streamflow data, whereas the flow- <br />duration curve for site Ql was computed for <br />9 continuous years (1984--92) of streamflow data; <br />these two periods were used for most of the study <br />analyses of streamflow gains and losses. <br />Comparison of the flow-duration curves from <br />site QIA and site QI (fig. 4) indicates that (I) daily <br />mean streamflow entering the study area was different <br />between these two periods; (2) large streamflows <br />(greater than 280 ft3/s) were more common during <br />1957-67 at site QIA; (3) base flow (less than 40 ft3/s) <br /> <br />STREAMFLOW CHARACTERISTICS 9 <br />