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492 ~. offer no encouragement to the notion that cloud seeding robs Peter to pay Paul. Studies of rainfall downwind from actual summer- convective cloud seeding operations have been incon- clusive, with the evidence tending to favor some in- crease out to distances of 400 km or so. However, the types of operations involved have been so disparate that no general conclusions are possible. Studies of pre- cipitation downwind of winter-orographic cloud-seeding operations confirm the presence of increases at distances of '-'250 km. The evidence, therefore, does not support the notion that stimulation of precipitation in one area deprives another area but suggests that seeding may strengthen existing precipitation systems. A less direct effect of winter-orographic snowpack augmentation on rainfall may come about if the addi- tional water is used to extend irrigation into areas where it is not now practiced. It has been shown (Changnon, 1974) that evapotranspiration from large irrigated tracts is associated with an increase in showery precipitation over neighboring areas. Such an effect, if it were felt, would most likely affect arid to semiarid range and scrubland and high-altitude forests adjacent to the newly opened irrigation districts. Th~ western Great Plains area is subject to frequent periods of hot, dry air that sometimes causes severe crop damage. Successful augmentation of local summer- convective rainfall will cause local increases in humidity close to the ground that will last until the added water is evapotranspired or percolates to deeper soil horizons, or until a later rainfall event erases the effects of the earlier one. Whether or not the resulting decrease in the frequency of periods of hot, dry air will result 111 less crop damage cannot be determined at this time. 4) AMOUNT OF SUNSHINE Precipitation management is not likely to have a dis- cernible effect on the amount of sunshine. Patterns of sunshine and cloudiness are governed by large-scale weather patterns not subject to the effects of cloud seed- ing. Even successful promotion of cloud mergers, affect- ing only a small subset of cloud formations, would not palpably affect the duration of sunshine. 5) GLOBAL-SCALE EFFECT ON HYDROLOGIC CYCLE Grounds for postulating a global-scale effect of wide- spread precipitation management on the hydrologic cycle are extremely speculative. Even internationally widespread precipitation management would probably involve quantities of water substantially less than those already involved in the practice of irrigation, and much less also than the average annual variability of natural precipitation on a hemispheric scale. Even where seed- ing agents have been dispersed for hail suppression in quantities much larger than those used for precipitation management, there has been no evidence of effect on the weather that could be attributed to persistence longer than a day or two. Though the possibility of a global-scale environmental effect of widespread precipi- tation management cannot be entirely ruled out at the J Vol. 58, No.6, June 1977 present time, the likelihood of any significant effect is considered extremely remote. The verification of such an effect, in the face of other natural and man-caused variables, would be very difficult. b. ImjJact on human activities 1) AGRICULTURE Of all applications of preCIpItation management, the most frequent and extensive are in connection with agriculture and, in particular, with alleviation of drought in subhumid and semiarid regions. Under the assumptions stated earlier in this section, agriculture in these regions will continue to be the focus of applica- tion. How might this change the general character of agricultural practice? In particular, might it produce a dependency on the continued success of precipitation management that, if the latter were to fail, could cause a catastrophe? Agriculture in the United States has been continually in a state of adjustment to emerging ideas, usually spurred by stark necessity. The Great Plains have seen, in succession, a catastrophic degree of overgrazing of "public" lands; expansion, collapse, and readjustment of dry farming; the extension of irrigation followed by the sale of irrigation water rights for municipal and industrial use; and lately the substitution of stubble- mulch tillage for deep plowing and intensive farming of selected soils combined with abandonment of others (Auclair, 1976). One can postulate, with a certain degree of rationality, how agriculture might tend to adjust to any real change in rainfall climate achieved by pre- cipitation management. Whether this adjustment will be as postulated, or will be actually discernible and subject to separate management in the face of much larger ad- justments to political, economic, technological, and social changes, is open to question. Perhaps most ques- tionable is the assumption that significant adjustment might take place in anticipation of an effective role for precipitation management. Demonstrated long-range success of precipitation man- agement would bring about an adjustment of land values between the precipitation-managed regions and other lands producing the same or substitutable crops. In the past, such adjustments have taken place in the case of irrigated land. Adjustments due to precipitation management would probably be of lesser magnitude than those due to irrigation but subject to similar po- litical and economic controls. 2) HYDROELECTRIC POWER This is probably second in prominence on the list of applications for precipitation management. The motiva- tion has usually been the increased economic return from existing investments in installed generating ca- pacity through more intensive utilization, especially when the resulting extra power can be sold at peak rates during the hours of highest demand. Once precipitation management becomes an accepted technology, it is likely that the opportunity for increased return on the large capital investment involved in hydroelectric de-