To tie or not to tie ridges for water conservation in Rift Valley drylands of Ethiopia

The Rift Valley drylands of Ethiopia are characterized by sandy loam soils that have poor fertility and unreliable rainfall conditions. The aim of this study was to examine the potential benefit of rainwater harvesting by tied-ridges and improved soil fertility on maize productivity through field experimentation and simulation with the FAOʹs AquaCrop model. The effect of tied-ridges with and without manure on maize yield at smallholder farms was studied during the years 2009 (very dry, 96% probability of exceedance) and 2010 (normal year, 46% probability of exceedance). During a normal rainfall, the mean yield of maize grain was 4.0 Mg ha−1 when tied-ridges were applied in combination with manure (at 4.5 Mg ha−1 rate); 3.4 Mg ha−1 when tied-ridges were applied without manure; 3.5 Mg ha−1 when traditional tillage was applied in combination with manure; and 2.7 Mg ha−1 when traditional tillage was applied without manure. Long-term simulations with the FAOʹs AquaCrop showed that the root zone soil water may exceed field capacity for consecutive days during above average rainfall seasons in the shallow sandy loams. The question thus is when to tie or not to tie ridges. Therefore, upon proper calibration of the FAOʹs AquaCrop model, the effect of tied-ridges and improved soil fertility was simulated in response to different amounts of seasonal rainfall, number of rainfall days and sowing times. Simulations revealed that, during below average rainfall seasons (280–330 mm), tied-ridges are more effective at improving crop yields than enhancing the fertility level of the soil. But during above average rainfall seasons, the rainwater that is held in tied-ridges can be more effectively utilised when the current fertility level of the soil is improved. The simulated rainwater use efficiency of maize was 6.1–6.5 kg ha−1 mm−1 for traditional tillage without any fertiliser, 6.8–7.3 kg ha−1 mm−1 for tied-ridges without any fertiliser and 11.0–12.9 kg ha−1 mm−1 for tied-ridges with optimum fertiliser (96% soil fertility level). It is, therefore, concluded that combined use of tied-ridges and farmyard manure can enhance maize yield under wide range of rainfall conditions (annual rainfall ranging from 280 mm to 680 mm). The Maresha-modified ridger used in this study can be popularized among Ethiopian farmers due to its simplicity and effectiveness.