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<br />APPROACH <br /> <br />The approach that has been utilized is similar to the one used in the <br /> <br />N <br />~ <br /> <br />previous evaluation, e.g., a statistical one, using the correlation between <br /> <br />N <br /> <br />a control area and a target area to determine the apparent effectiveness of seeding. <br /> <br />The previous evaluation, which was done for the three winter seasons from 1973-74 <br /> <br />through 1975-76, included the months of November through March. <br /> <br />Dr. Geoffrey Hill of the Utah Water Research Laboratory in his review of <br /> <br />the NAWC evaluation was critical of the results citing the fact that the NAWC <br /> <br />evaluation included the unseeded months of November and December 1973 and December <br /> <br />1974 while it eliminated the month of February 1976 because of. the exceptional <br /> <br />precipitation in the control areas. Hill felt that if February 1976 was to be <br /> <br />considered as an extra-ordinary month, and therefore it should be left out of the <br /> <br />sample, then similar data should be removed from the unseeded data set to eliminate <br /> <br />any positive bias towards seeding. <br /> <br />Hill concluded that the evaluation of the snowpack data was .invalid for the <br /> <br />same reason, i.e., the data set included three months of unseeded precipitation. <br /> <br />In addition, Hill indicated that warm March 1974 temperatures in Nevada and Arizona <br /> <br />affected the snowmelt making it appear that there was more precipitation in Utah <br /> <br />than would be expected based upon Nevada and Arizona data. <br /> <br />To eliminate these concerns, the current approach has been to restrict the <br /> <br />evaluation to only those months which were consistently seeded during all the <br /> <br />seeded years. Therefore, only the months of January, February, and March were <br /> <br />selected for the seeded years of 1974-1978. Likewise, only precipitation amounts <br /> <br />have been used in the evaluation and no attempt has been made to evaluate snowpack <br /> <br />or strea.mflow. <br /> <br />- 2 - <br />