Spate irrigation

Spate irrigation (from the English word spate, meaning: a. a flood or inundation. b. a river flooding its banks) uses seasonal floods of rivers, streams, ponds and lakes to fill water storage canals. If irrigation is the manipulation of water for use in growing crops, spate irrigation is perhaps the most ancient method devised. Evidence of basic spate irrigation engineering can be traced back beyond recorded history and include the Ancient Egyptian diversion of the Nile River for storage at times of heavy rainfall for use in the dryer times of the year.

In spate irrigation, water is diverted from normally dry river beds when the river is in spate. The flood water is then diverted to the fields. This may be done by free intakes, by diversion spurs or by bunds, that are built across the river bed. The flood water, typically lasting a few hours or a few days, is channeled through a network of primary, secondary and sometimes tertiary flood channels. Command areas may range from anything between a few hectares to over 25,000 hectares (62,000 acres).

Spate water management is more common to arid and semi-arid regions, typically in underdeveloped countries in the Middle East, North Africa, West Asia, East Africa and parts of Latin America, but is used in countries with sufficient precipitation to manage water elevation in canals.

Spate irrigation systems require huge management efforts to control and optimize the flow of water. Because fast-moving water is capable of moving very large amounts of sediment, the heights of river banks and the composition of their beds can change rapidly. Diverting the flow of a powerful force which is capable of moving rocks, soils and other materials used to divert the path of the water can prove difficult.

The number of public programs to support spate irrigation has been relatively limited. One reason has been the difficulty to justify investments in civil engineering works on systems dominated with low-value farming. A second reason has been that it has been hard to identify successful interventions in spate systems, because spate systems are often hydraulically and socially generally very complex.

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