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.
Lomas
In many locations the lomas were over-exploited for agriculture and grazing. One example is that, in prehistoric times, north of Ilo, Peru, far from any other source of water, four lomas-fed springs permitted about of irrigated agriculture plus grazing for llamas and alpacas. Hundreds of people of the Chiribaya culture benefited from this unlikely agriculture in a rainless land. Later, during the 17th century, Spanish colonists pastured 200 mules in these lomas. As late as 1951, a few tara trees still lived although the lomas were by then nearly devoid of all vegetation and population.