Irrigation is the addition of water to the soil to produce near optimum soil moisture conditions for crop growth in regions of water scarcity (Rydzewski and Ward 1989). Irrigated agriculture appears to have commenced on a small scale soon after the domestication of cereals in the Middle East. This domestication probably began about 12,000 years ago, yet by 10,000 years ago archaeological excavations at Jericho have revealed that groundwater from a nearby spring was being used for irrigation (Kenyon 1969-70). However, irrigated agriculture is perhaps most closely associated with the development of urban civilisations in the great river valleys of the Old World, including the Tigris-Euphrates, Indus and Nile (Adams 1965; Wittfogel 1957). The key factor is that irrigation of these huge floodplains using large-scale diversion structures and canals generated the wealth necessary for the construction of complex urban systems and the founding of empires.
Irrigation, by providing more assured crop production and by increasing yields by up to four times those of their rain-fed equivalents, meant that much larger populations could be supported from relatively small areas of cultivated land. In turn, this creation of wealth from agriculture led to the division of labour within society and the growth of specialist occupations, such as soldiers, enquirers and accountants, which are taken for granted today. Irrigation development can, therefore, be regarded as a vital element in the evolution of human societies.
There is a continuum of irrigation practices in terms of the volume of water added, from crops such as wheat, which require relatively little water, even in arid environments, to crops like rice, which can demand very large quantities of water even in humid environments (Burns 1993). The earliest methods of irrigation were probably furrow and flood types. Both employ low technology and require only small amounts of human or animal labour to carry them out. With furrows, a series of v-shaped channels are fashioned across the irrigated area using a form of plough-like implement. The furrows slope in a downslope direction to ensure that the water moves along them under the influence of gravity. With flood irrigation, small banks are constructed to delimit roughly rectangular flat areas of various sizes into which water can be led to flood the ground to a depth of several centimetres. Water is then usually led from one small basin to another in a downslope direction. Basin or flood irrigation is best known from the Nile valley (Hamdan 1961).
Since the end of the Second World War, new methods of irrigation have been developed. They are usually described as pressure systems, as the water is delivered to the plant/crop through a series of pipelines. All these systems require a much higher level of technology than is the case with the traditional furrow and flood systems, and the infrastructure costs associated with them are high (Plusquellec et al. 1994). The commonest of these methods is the sprinkler system, which