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<br />II.
<br />
<br />10.0
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<br />9.0
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<br />8.0
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<br />7.0
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<br />6.0
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<br />5.0
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<br />
<br />1.5
<br />1900
<br />Figure 12.
<br />
<br />10 20 30 40 50 1960
<br />Seasonal precipitation for Durango;
<br />9 year weighted running means
<br />
<br />(a) Winter (November to March): this season is
<br />characterized by large fluctuations, although the
<br />magnitude varies from station to station. A major
<br />low occurred around the turn of the century followed
<br />by a rapid increase to approximately 1908. In the
<br />case of Durango this peak was over 200 percent of
<br />the 1900-1901 low. Precipitation then decreased to
<br />a minor low around 1916 followed by an increase to
<br />approximately 1919-1922. This minor peak was
<br />followed by a drastic fall in precipitation over the
<br />next decade to a period low at approximately 1929-
<br />1932. An equally dramatic rise followed, resulting
<br />in a peak generally around 1936-1938. In the case of
<br />Telluride, 'precipitation rose from a mean of 157 mm
<br />(6.17 inches) in 1930 to a mean of 351 mm (13.83
<br />inches) in 1938. Minor lows then occur at 1945-1946
<br />and 1961-1962 with intermediate peaks around 1948-
<br />1951 and/or 1956-1957.
<br />
<br />In some cases such as Ames, Ignacio, and Silverton,
<br />a definite downward trend is obvious throughout the
<br />record, whereas in other cases, e.g. Rico and Fort
<br />Lewis, the record only trends downward after the
<br />1936.-1938 high point. A number of the records show
<br />no distinct trend.
<br />
<br />In short, the winter season record is one of large
<br />fluctuations with peaks in the record at intervals
<br />of approximately 12-15 years. Lows do not exhibit
<br />such a regular pattern of occurrence. No trend is
<br />common to all stations but a number show a definite
<br />downward trend.
<br />
<br />(b) Spring (April, May): Fluctuations in precipi-
<br />tation are fairly small for this season with major
<br />troughs at the turn of the century, around 1910-1911,
<br />
<br />27.5
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<br />22.5 8
<br /> 7
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<br />12.5
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<br />; SUMMER
<br />I
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<br />20
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<br />10
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<br />15
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<br />4
<br />
<br />3
<br />
<br />2
<br />1915 25
<br />Figure 13.
<br />
<br />35 45 55 1965
<br />Seasonal precipitation for Silverton;
<br />nine year weighted running means.
<br />Solid line = winter; small dashed line
<br />summer; large dashed line = fall,
<br />dotted line = spring.
<br />
<br />1936-1937, and 1961-1963. Major peaks occur around
<br />1905-1907 and 1941-1942. These are also character-
<br />istic of the other seasons with the exception of the
<br />1910-1911 trough which is unique to spring season.
<br />Silverton, Ignacio, and Durango show a slightly ,
<br />falling trend post-1926 but this is not recognizable
<br />in the other records.
<br />
<br />(c) Summer (June, july, August): This is the
<br />season of highest precipitation and lowest snowfall.
<br />A number of stations, in particular, Rico, Silverton,
<br />Ignacio, Fort Lewis, and Trout Lake show a very
<br />marked downtrend in the summer record. Major
<br />troughs occur at 1900, 1922-1924, 1940-1942, and
<br />1950-1952; major peaks 'are around 1927-1929, and
<br />1947-1948. Some stations show minor peaks at
<br />approximately 1936-1938 and 1954-1955. Thus peaks
<br />in the record appear at approximately 8-10 year
<br />intervals.
<br />
<br />I
<br />l
<br />I
<br />I
<br />j
<br />
<br />(d) Fall (September, October): This season exhibits
<br />fluctuations of high frequency. Major troughs occur
<br />at the turn of the century 1921-1922, 1933-1934,
<br />and 1951-1955. Major peaks occur at approximately
<br />1927, 1941, and 1960-1961. Cascade records sho~ that
<br />fall precipitation totals for the 1938-1946 per10d
<br />were 5 times those for the 1930-1938 period. Such
<br />large fluctuations are presumably due to early
<br />snowfall during the fall.
<br />
<br />52
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