Sedimentation rate on spate irrigated fields in Sheeb area, eastern Eritrea

Spate irrigation in the lowlands of Sheeb area in eastern Eritrea enables the farmers to harvest a crop without application of fertilizers. But at the same time, the sediment deposition increases the surface level of spate irrigated fields, which often requires raising the field bunds. This study was conducted (1) to determine the sedimentation rate on spate irrigated fields; (2) to assess the impact of sedimentation on the physical and chemical characteristics of the soils and (3) to recommend possible management interventions. The thickness of deposited sediment layers on the fields was measured using sediment pins. Twenty-seven sediment pins were installed at three plots in Sheeb area on a fine, loamy, calcaric Fluvisol. The plots were located at the upstream, midstream and downstream parts along a main diversion canal. The measured sedimentation rates ranged from 8•3 to 31•6 mm y−1in the upstream plot, from 6•0 to 18•0 mm y−1in the midstream plot and from 5•2 to 8•6 mm y−1in the downstream plot. These rates corresponded with an average sedimentation of 143 tons ha−1in the three plots. The sedimentation has improved the physical and chemical characteristics of spate irrigated soils compared with non-irrigated soils. However, nutrient inputs (such as addition of manure and incorporation of crop residues) may be necessary in the near future to replenish the soil nutrient stocks.