In what is today the southwestern United States, which successful farming strategy did early Native American groups use?

The early Native Americans used the successful farming strategy of utilizing a complex irrigation system.

Explanation:

Though the early native Americans were not much civilized, they bore phenomenal knowledge about agriculture and its techniques. It was through their conventional yet partially modernized techniques and intelligent use of available resources, the crops they took rarely failed. The irrigation techniques that made carrying water to the field convenient, do seem complex for their time but the Native Americans seem to have been expert at its use.