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. . . . . . . . . . . . I. . ., . . . .' . . . . . . . . . . . . . . . . . .' . . . . . . 5.0 EVALUATIONS OF SEEDING EFFECTIVENESS The task of determining the effects of cloud seeding has received considerable attention over the years, Evaluating the results of a cloud seeding program for a particular season is rather difficult The primary reason for the difficulty stems from the large natural variability in the amounts of precipitation that occur in a given area, and between one area and another during a given season, Since cloud seeding is normally feasible only when existing clouds are near to (or already are) producing precipitation, it is not usually obvious if, and how much, the precipitation was actually increased by seeding, The ability to detect a seeding effect becomes a function of the magnitude of the seeding increase and the number of seeded events, compared with the natural variability in the precipitation pattern. Larger seeding effects can be detected more easily, and with a smaller number of seeded cases, than are required to detect small increases, Historically, the most significant seeding results have been observed in wintertime seeding programs in mountainous areas, However, the apparent differences due to seeding are relatively small, being of the order of a 5-20 percent seasonal increase, In part, this relatively small percentage increase accounts for the significant number of cases required to establish these results (often five years or more), Despite the difficulties involved, some techniques are available for evaluation of the effects of operational seeding programs, These techniques are not as rigorous or scientifically desirable as is the randomization technique used in research, where roughly half the sample of storm events is randomly left unseeded, Most of NAWC's clients do not wish to cut the potential benefits of a cloud seeding proj ect in half in order to better document the effects of the cloud seeding project The less rigorous techniques do, however, offer an indication of the long- term effects of seeding on operational programs, A commonly employed technique, and the one utilized by NA WC in this assessment, is the "target" and "control" comparison, This technique is one described by Dr. Arnett Dennis in his book entitled "Weather Modification by Cloud Seeding, 1980", This technique is based on the selection of a variable that would be affected by seeding (such as liquid precipitation or snowpack), Records of the variable to be tested are acquired for an historical period of several 44