The Eritrean spate irrigation system

Eritrea’s coastal zone has been identified as an area of substantial development potential. About 14,000 ha of this 5 million ha area (i.e. <0.3%) has already been developed under a form of spate irrigation known locally as jeriff. This is a water diversion and spreading technique in which wadis (ephemeral streams), springing from Eritrea’s Central Highlands are diverted to irrigate land in the coastal plains. The system as it is applied in Sheeb, an area north-east of Asmara, characterised by agro-pastoral spate irrigation, is described. Under spate irrigation, crop growth is entirely dependent on the residual soil moisture stored in the soil profile. If the basin fields are flooded adequately, the resulting residual soil moisture is sufficient for two or sometimes three crop harvests. The spate irrigation system builds up land by depositing rich sediment on the fields, but therefore, the elevation of the irrigated lands rises every year. Moreover, the system requires huge numbers of trees annually for constructing diversion structures which are subsequently often washed away by heavy floods. In general, the overall irrigation efficiencies of spate schemes are only about 20% because of the difficulty of controlling floods and because water is lost by percolation, seepage and evaporation. Suggestions are made to improve the system and make it more sustainable: permanent flood diversion and distribution structures should be built to effectively divert the floods and to reduce water loss through percolation and seepage, and the basin fields should be properly levelled to distribute the floodwater uniformly over the entire field.