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<br />o <br />N <br />en <br />'_,,_,1 <br />t.I;\ <br /> <br />SELECTION OF THE CONTROL AREA <br /> <br />In the previous NAWC evaluation, four control areas were selected outside <br /> <br />of the state in order to minimize the possible contamination of the control area <br /> <br />due to seeding. In retrospect, it appears that the control areas were too far. <br /> <br />removed from the targets. Even though they correlated well with the targets, unwanted. <br /> <br />biases were introduced in the relatively small sample size because of exceptional <br /> <br />storms which primarily affected either a control area or one of the target areas. <br /> <br />In order to eliminate this kind of bias it was necessary to look for a control <br /> <br />area closer to the target but outside the main sphere of seeding effects. Seeding <br /> <br />has been conducted over almost all of Utah during the past five years and it is <br /> <br />virtually impossible to find an area that has not been affected to some extent. <br /> <br />After close inspection of several regions, u.ltimately the western desert region <br /> <br />of Utah was selected as the one least affected and yet close enough to the main <br /> <br />target regions to be a representative control. Ten stations with long-term <br /> <br />precipitation records were selected to make up the control area. These are ShO'Vll <br /> <br />in Attachment 1. One station, Lehman Caves National Monument, is actually in <br /> <br />Nevada (near the border) while the rest are in Utah. <br /> <br />The ten stations that comprise the control were not randomly selected with <br /> <br />those stations just north of the Dixie portion purposely excluded since they might <br /> <br />be significantly affected by the seeding that has been done in southwestern Utah. <br /> <br />On the other hand, the control stations were not purposely selected to produce the <br /> <br />most favorable apparent seeding results. Other potential control areas or groups <br /> <br />of stations were not considered. <br /> <br />It is realized that this control area may have been affected by seeding since <br /> <br />it was within the project area from 1974 through 1977, but at the same time it is <br /> <br />felt that seeding effects have been minimal for the following reasons: The area is <br /> <br />relatively flat with only scattered small mountain ranges and isolated high peaks <br /> <br />and therefore is not conducive to orographic seeding. There is a paucity of ground <br /> <br />- 3 - <br />