Supplemental irrigation for dry-spell mitigation of rainfed agriculture in the Sahel

Unreliable and erratic rainfall combined with inherent low soil fertility in crust prone soils affect crop growth in the Sahel. Past efforts of agricultural water management have primarily focused on maximising rainfall infiltration through soil and water conservation. The challenge of how to cope with dry-spells, short periods of water stress during crop growth, remains largely unsolved. Small-scale water harvesting (WH) for supplemental irrigation may be a tool for small-holder farmers to stabilise crop water supply and thereby increase yields and create incentives for increased investments in fertitisation. Results are presented on the effects on Sorghum (Sorghum Bicolor (L.)) yields under supplemental irrigation and soil nutrient application in an on-farm experiment in northern Burkina Faso. Focus is on dry-spell bridging and water and nutrient interactions on biomass productivity. The water harvesting system consisted of a low-cost manually dug farm pond with a storage volume of 150 m 3, collecting surface runoff as rill and small gully flow from a 1.8 ha catchment area. The on-farm experiment involved five repetitions of two levels of nutrient application (non-fertilised and fertilised) and two levels of supplemental irrigation (non-irrigated and irrigated) including: farmers' traditional practices (TC), supplemental irrigation (1), fertiliser application (F) and supplemental irrigation combined with fertilisation (IF). This paper presents field results from three rainy seasons (1998-2000), receiving a cumulative seasonal rainfall ranging from 418 to 667 mm. Supplemental irrigation ranging from 60 to 90 mm per season was applied based on actual occurrence of dry-spell induced crop water stress. Supplemental irrigation had a significant effect on grain yield over the three crop seasons (**P < 0.001). Similarly, fertiliser application (46.5 kg N ha(-1)) resulted in a significantly higher grain yield (***P < 0.0001). Analysis of variance for interactions between supplemental irrigation and fertiliser application showed significant differences between treatments when analysing the 3 years together (***P < 0.0001). The control grain yield's (TC) average over the 3 years was 455 kg ha(-1). Supplemental irrigation alone resulted in an average grain yield of 712 kg ha(-1), while fertiliser application alone gave an average grain yield of 975 kg ha(-1). Supplemental irrigation combined with fertiliser application resulted in an average grain yield of 1403 kg ha(-1), which is higher than the farmer's normal practice by a factor of three. Total above ground biomass yields followed the same pattern as the grain yields for respective years. All three seasons provided data systematically supporting supplemental irrigation as a dry-spell mitigating and yield gap reducing technology. (C) 2003 Elsevier Science B.V. All rights reserved.