Culture and Ecology of Chaco Canyon and the San Juan Basin

32 Chaco Project SynthesisA few years later, Brand (1937c:43-45) acceptedFisher's estimated number of frost-free days andlengths of growing season. He also reported that thepeak rainy season occurred in July, August, andSeptember (46 percent of total annual rainfall).Variation in annual precipitation ranged from less than15.24 cm (6 in) in the western areas of the canyon tomore than 38.10 cm (15 in) in higher southern andeastern areas. Annual precipitation could vary asmuch as 50 percent from the mean. Temperature andwinds shifted diurnally. Comparison of Chaco datawith Koeppen's system indicated that Chaco wasconsidered during normal years to be a cold desertbordering on steppe. With an average increase of only2.54 cm (1 in) more rain per year, the canyon wouldbe considered a steppe environment.Gordon Vivian and Mathews (1965:8-11) had 32years of temperature and precipitation figures toexamine. The highest temperature recorded was 106degrees F; the lowest was minus 38 degrees F. Therewere also large daily variations during the summermonths, but the estimated frost-free period remainedat 150 days. There was a wide fluctuation inprecipitation from year to year (8.5 to 45.8 cm [3.35to 18.02 in]), and there seemed to be a pattern of twoto four or five years of below mean precipitation (22.1cm [8.71 in]) separated by a single year above themean. They inferred that there would be several yearswith only 12.7 to 17.8 cm (5 to 7 in) of rainfallfollowed by a year with destructive rainfall. Theyrecognized the effects of precipitation and frost-freeperiods on agricultural production. When they comparedthe available data from Chaco with Hack's(1942) observations on the Hopi, they recognized thatrecent precipitation in Chaco was slightly more thanhalf of what was needed. Extensive water collectionand conservation systems would thus have been anecessity during the height of the Pueblo agriculturaladaptation. Although the weather station at PuebloBonito was in a cold desert, Chacra Mesa (at around2,113 m, or 7,000 ft) enjoyed a steppe climate wherepiiion, juniper, and sagebrush provided ground cover.Because the weather station was moved in 1960(Gillespie 1985: 19), it was important to re-evaluateboth the data available prior to the inception of theChaco Project (e.g., Brand et al. 1937; Fisher 1934;Toulouse 1937; Gordon Vivian and Mathews 1965)and data gathered by scholars during the past fewdecades to expand our understanding of the role ofclimate on the Chaco. Gillespie (1983, 1985) examinedclimatologic data for Chaco Canyon and theSan Juan Basin, which are characterized by somewhatcool, semiarid environments. There is considerablefluctuation in both daily and yearly temperatures, andthere is generally low and variable precipitation. Therainfall tends to be seasonal, falling in two peakperiods separated by droughts in the months of Juneand November. The higher summer peak is from lateJuly through September, and the lower peak is in thewinter and early spring. Humidity tends to be low,and there are occasional strong winds. The San JuanBasin is in a sensitive transitional location where thereare fluctuations between a relatively winter-dominantpattern of precipitation to the north and a summerdominantpattern to the south. Depending on latitude,elevation, and geographic location in relation toatmospheric circulation features and surroundingorographic barriers, precipitation values and seasonaldistribution vary.Precipitation comes into the area from varioussources due to different circulation features. Winterprecipitation comes from two sources. Moisture fromthe northern Pacific brings cyclonic storms thatusually pass north of the Four Comers area butsometimes dip into the Southwest. Tropical Pacificlow-pressure systems also occasionally enter theSouthwest, but these are rare in the San Juan Basin.The summer precipitation peak is caused by thesummer monsoonal circulation pattern, which bringsthe western edge of moisture from the Gulf of Mexicoand tropical Pacific adjacent to Central America to thehighlands of the Colorado Plateau by mid-July. Itsnortherly boundary seems to be a line across southeasternUtah and northwestern Colorado, and into theGreat Plains. Because the San Juan Basin is on thenorthwestern edge of this monsoonal flow pattern,there is less moisture than in the central areas, and itarrives later and departs earlier in the season. Thereis a distinct gradient in the percentage of rainfalloccurring in the summer, it is heavier in the southernpart of the San Juan Basin (38 to 40 percent) than inthe north (about 20 percent in the Mesa Verde).Rainfall is also heavier in the east than it is in thewest. Because Chaco is on the leeward side of theChuska Mountains, less precipitation is available inthe canyon. It drops approximately 15 mm/lOO m(0.18 in/10 ft) in the lower parts of the basin. Ireland