The arid environment which ranges from the northern areas of rain-fed agriculture to the south where irrigation of agriculture is essential if a surplus energy returned on energy invested (EROEI) is to be obtained. This irrigation is aided by a high water table and by melting snows from the high peaks of the northern Zagros Mountains and from the Armenian Highlands, the source of the Tigris and Euphrates Rivers that give the region its name. The usefulness of irrigation depends upon the ability to mobilize sufficient labor for the construction and maintenance of canals, and this, from the earliest period, has assisted the development of urban settlements and centralized systems of political authority.