Desert farming is the practice of developing agriculture in deserts. As agriculture depends upon irrigation and water supply, farming in arid regions where water is scarce is a challenge. However, desert farming has been practiced by humans for thousands of years. In the Negev, there is evidence to suggest agriculture as far back as 5000 BC. Today, the Imperial Valley in southern California, Australia, Saudi Arabia, Israel and Palestine are examples of modern desert agriculture. Water efficiency has been important to the growth of desert agriculture. Water reuse, desalination, and drip irrigation are all modern ways that regions and countries have expanded their agriculture despite being in an arid climate. History Humans have been practicing and refining agriculture for millennia. Many of the earliest civilizations such as ancient Assyria, Ancient Egypt, and the Indus Valley Civilization were founded in irrigated regions surrounded by desert. As these civilizations grew, the ability to rear crops in the desert became of increasing importance. There are also instances of civilizations that subsisted primarily in the desert with little irrigation or rainfall, such as various western American Indian tribes. Native Americans The Native Americans practicing this agriculture included the ancient and no longer present Anasazi, the long-present Hopi, the Tewa people, Zuni people, and many other regional tribes, including the relatively recently arriving (about 1000 to 1400 CE) Navajo. These various tribes were characterized generally by the Spanish occupiers of the region as Sinagua Indians, sinagua meaning "without water", although this term is not applied to the modern Native Americans of the region. Owing to the great dependence upon weather, an element considered to be beyond human control, substantial religious beliefs, rites, and prayer evolved around the growing of crops, and in particular the growing of the four principal corn types of the region, characterized by their colors: red, yellow, blue, and white. The presence of corn as a spiritual symbol can often be seen in the hands of the "Yeh" spirit figures represented in Navajo carpets, in the rituals associated with the "Corn Maiden" and other kachinas of the Hopi, and in various fetish objects of tribes of the region. American Indians in the Sonoran Desert and elsewhere relied both on irrigation and "Ak-Chin" farming—a type of farming that depended on "washes" (the seasonal flood plains by winter snows and summer rains). The Ak-Chin people employed this natural form of irrigation by planting downslope