|
<br />160
<br />150
<br />
<br />140
<br />130
<br />
<br />120
<br />If)
<br />~IIO
<br />o
<br />100
<br />
<br />90
<br />
<br />80
<br />70
<br />
<br />
<br />1928-29 32-33 36-37 40-41 44-45 48-49 52-53 56-57
<br />
<br />Figure 32.
<br />
<br />Frequency of types Em' A, B, and Bn_c
<br />(October-May) .
<br />
<br />CONCLUSIONS FOR THE SAN JUAN ECOLOGY PROJECT
<br />
<br />It is clear that the Target Area has experienced
<br />temperature fluctuations over the last 100-120 years
<br />which are different from those generally noted for
<br />large parts of the Northern Hemisphere (viz. cooling
<br />to 1930 then a reversal). At high elevations, this
<br />trend resulted in near threshold values for growth
<br />about 40 years ago.
<br />
<br />Seasonal precipitation fluctuations show rapid
<br />transitions from relatively dry to relatively wet
<br />modes and some stations indicate an overall trend
<br />toward increasing aridity superimposed on these
<br />fluctuations. In terms of cloud seeding, it is
<br />important to consider these frequency changes from
<br />"wet" to "dry" conditions. Random seeding in a
<br />"wet" period may result in an above-normal snowpack
<br />very early in the season. Even if cloud seeding
<br />was then suspended, natural events could lead to
<br />extremely high snowpack by the end of the winter
<br />season. In anyone year, therefore, seeding could be
<br />associated with overall seasonal snowpack increases far
<br />in excess of the 15 percent average figure generally
<br />considered by the Ecology Project. Further, such
<br />conditions would persist throughout one of these
<br />"wet-mode" periods. In a "dry-mode" period, on the
<br />other hand, random seeding CQuld occur throughout
<br />the season without the normal values of snowpack
<br />being reached; this was true in 1971, 1972 and 1974.
<br />
<br />At presen~ seeding operations are randomly selected
<br />throughout the winter period when certain pre-
<br />selected meteorological conditions occur. Although
<br />a study of the frequency of these seeding criteria
<br />on a monthly basis through historical time is out-
<br />side the scope of this study, the broad-scale
<br />synoptic situations on seedable days were examined.
<br />In historical times the frequencies of these types
<br />varied considerably from month to month (see Table
<br />12) with highest frequencies at the beginning and
<br />end of the winter period. If seeding were to be
<br />implemented by randomly chosen decisions on seedable
<br />days, then it might be expected that seeding would be
<br />most frequent in October, and March to April. This
<br />potential asymmetry of seeding events may be of
<br />-ecological significance, since a definite seasonal
<br />distribution of precipitation augmentation seems
<br />likely.
<br />
<br />References
<br />
<br />Bradley, R. S. 1976. Secular Changes of Precipi-
<br />tation in the Rocky Mountains and Adiacent Western
<br />States. Westview Press, Boulder (in press).
<br />
<br />and Barry, R. G. 1973. Climatic
<br />fluctuations in southwestern Colorado since the
<br />mid-nineteenth century. Monthly Weather Review,
<br />101, 3, 264-270.
<br />
<br />Bruns, R. J. (ed) 1897. Gold and Silver of the
<br />San Juans. County Commissioners of San Juan County.
<br />
<br />Clark, E. T. 1959. Objectivity in the classifi-
<br />cation of North American weather types. Unpub-
<br />lished Ms. Sci. Thesis, U. S. Naval Post-graduate
<br />School, Monterey, California.
<br />
<br />Cornelius, o. F. 1933. Pioneer History and
<br />Reminiscences of the San Juan Basin.
<br />
<br />I
<br />
<br />'j
<br />
<br />Elliott, R. D. 1949. Weather types of North
<br />America. Weatherwise ~ (1-6).
<br />
<br />1951. Extended-range forecasting
<br />by weather types in Compendium of Meteorology,
<br />American Meteorological Society, Boston, p. ~36.
<br />
<br />Fritts, H. C. 1965. Tree ring evidence for
<br />climatic changes in western North America.
<br />Monthly Weather Review, 93: 421-443.
<br />
<br />1972. Tree rings and climate.
<br />Scientific American 226 (5):92-101.
<br />
<br />, Blasing, T. J., Hayden, B. P. &
<br />Kutzbach, J. E. 1971. Multivariate techniques
<br />for specifying tree-growth and climate relation-
<br />ships and for reconstructing anomalies in Palaeo-
<br />climate. Monthly Weather Review, 10: 845-864.
<br />
<br />Keen F. P. 1937. Climatic cycles in eastern Oregon
<br />as'indicated by tree rings. Monthly Weather
<br />Review, 65: 175-188.
<br />
<br />Kohler, M. A. 1949. On the use of double mass
<br />analysis for testing the consistency of meteorolo-
<br />gical records and for making required adjustments.
<br />Bull. of the Am. Meteorol. Soc., 30: 188-189.
<br />
<br />Leopold, L. B. 1951. Rainfall frequency: an
<br />aspect of cli~tic variation. American
<br />Geo~hysical Union Transactions, 32: 347-357.
<br />
<br />Schulman, E. 1938. Nineteen centuries of rainfall
<br />history in the Southwest. Bull. of the Am.
<br />Meteorol. Soc., 19: 51-55.
<br />
<br />1954. Dendroclimatic changes in semi-
<br />arid regions. Tree Ring Bulletin, 20: 26-30.
<br />
<br />1956. Dendroclimatic Changes in
<br />Semi-arid America. Tucson: University of
<br />Arizona Press, 142 pp.
<br />
<br />Sellers, W. D. 1968. Climatology of monthly pre-
<br />cipitation patterns in the western United States,
<br />1931-1966. Monthly Weather Review, 96: 585-595.
<br />
<br />Thomas, H. E. 1959. Reservoirs to match our
<br />climatic fluctuations. Bull. of the Am. Meteorol.
<br />Soc., 40: 240-249.
<br />
<br />66
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