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<br />1)1')/: <br />"'" .1...;;.1.. <br /> <br />. <br /> <br />. <br /> <br />3. Change in the variability of flow over the record <br /> <br />Chatlges in the variance of flows over the record were assessed by computing a moving window <br />estimate of the flow variance. This is done by (a) computing the difference between the local mean flow <br />estimated for each month of record using /owess (previous section), and the flow recorded for this month, <br />(b) by squaring each such difference, and (c) computing the local variance by estimating the local mean of <br />the squared differences using /owess with a moving window of 20 years. The results of this calculation for <br />the Archuleta gage are shown in Figure 3. The local variance varies in a manner similar to that of the <br />local mean. It is about 7% higher for the last 20 years than for the full record. Periods oflow flows <br />coincide with periods oflow variability, However, the high flow period in the 1940's exhibited a <br />proportionally higher variability than the 1974-93 period. The trends for the Bluff gage are vel)' similar. <br /> <br />Trend line of flow variability, using a 20 year span <br /> <br />o <br />~ <br />~ <br />"" <br /> <br /> <br /> <br />; <br /> <br />OJ <br />V <br />C <br />OJ <br />C <br />OJ <br />> <br />.., <br />OJ) <br />"" <br />v <br />-< <br /> <br />o <br />~ <br />, <br />o <br />:.: <br /> <br />riaace.ofJuURecord .;. J -15 E 10. <br /> <br />o <br />~ <br />~ <br /> <br />1930 <br /> <br />1940 <br /> <br />1950 <br /> <br />1960 <br /> <br />1970 <br /> <br />1980 <br /> <br />1990 <br /> <br />Year <br /> <br />'. . <br />Figure 3: Local estimates of flow variance for the San Juan River at Archuleta. These estimates are <br />computed as the moving window variance about a moving window mean, using a moving window of240 <br />months and /owess as a tool for estimating the statistic of interest. A factor of 1.8 variation is seen in the <br />20 year moving window local variance over the 65 years of record, suggesting that the variability of 20 <br />year periods can be quite large at this site. Flows are in Acre-feet/month. <br /> <br />May 25, 1997 <br /> <br />4 <br />