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Patent Office, followed by the Signal Office in 1875. Regular Weather Bureau records began in 1896. In the San Juan area, useful data are nonexistent prior to 1850 and the two decades 1850-70 are virtually devoid of records. Fort Massachusetts, Fort Garland, and Fort Lewis are the earliest sources in the immediate area (Figure 1). Hermosa is the first recording station (1875-82) within the Project area. The longest 19th century pre- cipitation sequence in southern Colorado is that of Fort Massachusetts and Fort Garland (October 1852- October 1883 with gaps principally in the mid- 1860's). However, an examination of the monthly summary charts indicates that precipitation events are frequently noted, but amounts are not always given. Thus, many months are minimum estimates only (Figure 2). The use of data from one meteorological station alone is thus subject to some errors and clarification of the record may be achieved only by comparisons with other stations over a larger area. Attempts were made to extend the temporal range of data for the Project area by spatial correlations with stations in adjacent states where longer records may be available, for example, Santa Fe. However, preliminary analysis of existing data indicated that correlations may.change over time (Figure 3). Thus a correlation coefficient between two stations de- rived from the last forty years of record may not be applicable to the previous forty year period. Balanced against this is the problem of data homogeneity for the early period. For example, the calculation of mean monthly temperatures has altered over time due to changes in recording times (Table 1), and this clearly affects data homogeneity. This problem is discussed further in the section of this report called "Analysis of precipitation data, San Juan area." Literature Review Much of the literature pertaining to the climatic history of the western U.S. involves dendro- chronological evidence (Schulman 1938, 1954, 1956; Keen 1937; Fritts 1965; Fritts et al. 1971). Schulman (1938) notes five major drought periods in the Southwest since the beginning of the eighth century -- at approximately A.D. 715, 1100, 1290, 1585; the last occurred during the early part of the twentieth century. However, he goes on to point out that there is evidence for especially stormy periods to follow on the heels of such droughts...in the unsmoothed annual record, very sharp transitions from a prevailing wet to a prevailing dry type, or the reverse, occur. WYOMING F t. Laramie- Ft. Fred Steele - NEBRASKA Salt ~t. Bridger Lake City - -Douglas Camp Ft. Sanders _ Fl, D.A. R,ussell Sidney Chevenne- I .Barracks North Platte - Ft.- Mc Pherson . Ft. Hartsuff COLORADO - Ft. Duchesne Georgetown - UTAH Montrose - -Denver Forest Home. 400 N Monument - -Ft. Wallace KANSAS - Dodge City 370 OKLAHOMA N 1 1 j 1 Pikes Peak . - Colorado Springs Los Animas - - Ft. Lyon ~t. Massachusetts Hermosa - Ft.-Lewis Ft. Garland ARIZONA Buriwin Camp NEW MEXICO Ft. Defiance_ Santo Fe- Hol brook - - Albuquerque -Ft. Verde 1090W Ft. Union - Ft. Elliott - _Gallinas Spring TEXAS 1030W ~ Figure 1. Stations recording precipitation for at least four successive years before 1890. " 44