Culture and Ecology of Chaco Canyon and the San Juan Basin
Culture and Ecology of Chaco Canyon and the San Juan Basin
Culture and Ecology of Chaco Canyon and the San Juan Basin
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32 <strong>Chaco</strong> Project Syn<strong>the</strong>sisA few years later, Br<strong>and</strong> (1937c:43-45) acceptedFisher's estimated number <strong>of</strong> frost-free days <strong>and</strong>lengths <strong>of</strong> growing season. He also reported that <strong>the</strong>peak rainy season occurred in July, August, <strong>and</strong>September (46 percent <strong>of</strong> total annual rainfall).Variation in annual precipitation ranged from less than15.24 cm (6 in) in <strong>the</strong> western areas <strong>of</strong> <strong>the</strong> canyon tomore than 38.10 cm (15 in) in higher sou<strong>the</strong>rn <strong>and</strong>eastern areas. Annual precipitation could vary asmuch as 50 percent from <strong>the</strong> mean. Temperature <strong>and</strong>winds shifted diurnally. Comparison <strong>of</strong> <strong>Chaco</strong> datawith Koeppen's system indicated that <strong>Chaco</strong> wasconsidered during normal years to be a cold desertbordering on steppe. With an average increase <strong>of</strong> only2.54 cm (1 in) more rain per year, <strong>the</strong> canyon wouldbe considered a steppe environment.Gordon Vivian <strong>and</strong> Ma<strong>the</strong>ws (1965:8-11) had 32years <strong>of</strong> temperature <strong>and</strong> precipitation figures toexamine. The highest temperature recorded was 106degrees F; <strong>the</strong> lowest was minus 38 degrees F. Therewere also large daily variations during <strong>the</strong> summermonths, but <strong>the</strong> 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]), <strong>and</strong> <strong>the</strong>re seemed to be a pattern <strong>of</strong> twoto four or five years <strong>of</strong> below mean precipitation (22.1cm [8.71 in]) separated by a single year above <strong>the</strong>mean. They inferred that <strong>the</strong>re would be several yearswith only 12.7 to 17.8 cm (5 to 7 in) <strong>of</strong> rainfallfollowed by a year with destructive rainfall. Theyrecognized <strong>the</strong> effects <strong>of</strong> precipitation <strong>and</strong> frost-freeperiods on agricultural production. When <strong>the</strong>y compared<strong>the</strong> available data from <strong>Chaco</strong> with Hack's(1942) observations on <strong>the</strong> Hopi, <strong>the</strong>y recognized thatrecent precipitation in <strong>Chaco</strong> was slightly more thanhalf <strong>of</strong> what was needed. Extensive water collection<strong>and</strong> conservation systems would thus have been anecessity during <strong>the</strong> height <strong>of</strong> <strong>the</strong> Pueblo agriculturaladaptation. Although <strong>the</strong> wea<strong>the</strong>r station at PuebloBonito was in a cold desert, Chacra Mesa (at around2,113 m, or 7,000 ft) enjoyed a steppe climate wherepiiion, juniper, <strong>and</strong> sagebrush provided ground cover.Because <strong>the</strong> wea<strong>the</strong>r station was moved in 1960(Gillespie 1985: 19), it was important to re-evaluateboth <strong>the</strong> data available prior to <strong>the</strong> inception <strong>of</strong> <strong>the</strong><strong>Chaco</strong> Project (e.g., Br<strong>and</strong> et al. 1937; Fisher 1934;Toulouse 1937; Gordon Vivian <strong>and</strong> Ma<strong>the</strong>ws 1965)<strong>and</strong> data ga<strong>the</strong>red by scholars during <strong>the</strong> past fewdecades to exp<strong>and</strong> our underst<strong>and</strong>ing <strong>of</strong> <strong>the</strong> role <strong>of</strong>climate on <strong>the</strong> <strong>Chaco</strong>. Gillespie (1983, 1985) examinedclimatologic data for <strong>Chaco</strong> <strong>Canyon</strong> <strong>and</strong> <strong>the</strong><strong>San</strong> <strong>Juan</strong> <strong>Basin</strong>, which are characterized by somewhatcool, semiarid environments. There is considerablefluctuation in both daily <strong>and</strong> yearly temperatures, <strong>and</strong><strong>the</strong>re is generally low <strong>and</strong> variable precipitation. Therainfall tends to be seasonal, falling in two peakperiods separated by droughts in <strong>the</strong> months <strong>of</strong> June<strong>and</strong> November. The higher summer peak is from lateJuly through September, <strong>and</strong> <strong>the</strong> lower peak is in <strong>the</strong>winter <strong>and</strong> early spring. Humidity tends to be low,<strong>and</strong> <strong>the</strong>re are occasional strong winds. The <strong>San</strong> <strong>Juan</strong><strong>Basin</strong> is in a sensitive transitional location where <strong>the</strong>reare fluctuations between a relatively winter-dominantpattern <strong>of</strong> precipitation to <strong>the</strong> north <strong>and</strong> a summerdominantpattern to <strong>the</strong> south. Depending on latitude,elevation, <strong>and</strong> geographic location in relation toatmospheric circulation features <strong>and</strong> surroundingorographic barriers, precipitation values <strong>and</strong> seasonaldistribution vary.Precipitation comes into <strong>the</strong> area from varioussources due to different circulation features. Winterprecipitation comes from two sources. Moisture from<strong>the</strong> nor<strong>the</strong>rn Pacific brings cyclonic storms thatusually pass north <strong>of</strong> <strong>the</strong> Four Comers area butsometimes dip into <strong>the</strong> Southwest. Tropical Pacificlow-pressure systems also occasionally enter <strong>the</strong>Southwest, but <strong>the</strong>se are rare in <strong>the</strong> <strong>San</strong> <strong>Juan</strong> <strong>Basin</strong>.The summer precipitation peak is caused by <strong>the</strong>summer monsoonal circulation pattern, which brings<strong>the</strong> western edge <strong>of</strong> moisture from <strong>the</strong> Gulf <strong>of</strong> Mexico<strong>and</strong> tropical Pacific adjacent to Central America to <strong>the</strong>highl<strong>and</strong>s <strong>of</strong> <strong>the</strong> Colorado Plateau by mid-July. Itsnor<strong>the</strong>rly boundary seems to be a line across sou<strong>the</strong>asternUtah <strong>and</strong> northwestern Colorado, <strong>and</strong> into <strong>the</strong>Great Plains. Because <strong>the</strong> <strong>San</strong> <strong>Juan</strong> <strong>Basin</strong> is on <strong>the</strong>northwestern edge <strong>of</strong> this monsoonal flow pattern,<strong>the</strong>re is less moisture than in <strong>the</strong> central areas, <strong>and</strong> itarrives later <strong>and</strong> departs earlier in <strong>the</strong> season. Thereis a distinct gradient in <strong>the</strong> percentage <strong>of</strong> rainfalloccurring in <strong>the</strong> summer, it is heavier in <strong>the</strong> sou<strong>the</strong>rnpart <strong>of</strong> <strong>the</strong> <strong>San</strong> <strong>Juan</strong> <strong>Basin</strong> (38 to 40 percent) than in<strong>the</strong> north (about 20 percent in <strong>the</strong> Mesa Verde).Rainfall is also heavier in <strong>the</strong> east than it is in <strong>the</strong>west. Because <strong>Chaco</strong> is on <strong>the</strong> leeward side <strong>of</strong> <strong>the</strong>Chuska Mountains, less precipitation is available in<strong>the</strong> canyon. It drops approximately 15 mm/lOO m(0.18 in/10 ft) in <strong>the</strong> lower parts <strong>of</strong> <strong>the</strong> basin. Irel<strong>and</strong>