Agriculture in the prehistoric Southwest
After the Hohokam disappeared, Spanish explorers of the 16th century noted the use of canal irrigation in only two areas of the Southwest: eastern Sonora, primarily practiced by the Opata and Lower Pima, and among the Pueblos of northern New Mexico. Irrigation by the Opata and Pueblos had different objectives. In Sonora, with a long growing season, two crops of maize a year were grown in river valleys. The spring crop during the dry season was dependent upon irrigation; the summer and fall crop during the rainy season utilized irrigation to supplement rainfall. The necessity of irrigation probably required a high level of social organization. In New Mexico, with only one crop of maize per year possible, irrigation was supplemental and the lesser dependence on irrigation implies less social organization than in Sonora.
Chetro Ketl
The Ancestral Puebloans relied on regular rainfall to sustain their agricultural society. This proved challenging in Chaco Canyon, even with consistent precipitation. By 1130, the rains had diminished and the maize crops that the Chacoans depended on had begun to fail, as the region increasingly suffered from the effects of a devastating, fifty-year drought. After a continuous occupation of more than six hundred years, the Chacoans began to migrate from the canyon. Fagan states that by 1140 "Chaco was finished". A study of 12th-century burials in the Gallup, New Mexico, area indicates as many as half of the people who lived during the drought died before the age of eighteen, with sixty percent of all deaths occurring before age thirty-five. The study estimated that for every woman, four children were needed to sustain the agriculture workforce.