ABSTRACT
Water is a crucial input in all cultivation of plants; without an adequate supply of this basic resource, any cultivation project would fail. Irrigation, in all its simple and complex forms, is the human technique of manipulating water as an input in cultivation projects. Historians of all kinds have placed great emphasis on the role of irrigation systems in the ancient states, although the topic has been highly controversial. Irrigation is one of the available technologies for human interception in the Hydrological Cycle. In many regions, however, local small-scale and even farm-size systems predominate, as in the oases of the Sahara and Arabia, in southeast India or in Ceylon. The Sonjo of northern Tanzania described by R. F. Gray inhabit an arid zone of meagre and unreliable rainfall. The Sonjo have built only small earth dams reinforced with revetments of small timbers from which small dykes conduct the water into the nearby fields.
