Rainwater harvesting for dryland agriculture in the Rift Valley of Ethiopia

The Ethiopian drylands occupy about 65% of the total land mass (close to 700,000km²) of the country. The predominantly rainfed agriculture in these drylands is highly constrained due to erratic rainfall, long dry-spells and excessive loss of rainwater through non-productive pathways (surface runoff, evaporation and deep drainage). Apart from this, deteriorated dryland soils have low infiltration and water holding capacity, shallow depths and are sensitive to crusting. Therefore, to keep in pace with the demand for food for the burgeoning population, the Ethiopian drylands should be made more productive through appropriate rainwater harvesting and management techniques. Therefore, the objective of this study was to develop appropriate rainwater harvesting and management techniques in the Central Rift Valley (CRV) drylands of Ethiopia through a participatory planning and development process.

Primarily, an overview of the various rainwater harvesting and management techniques in sub-Saharan Africa was made. Moreover, the biophysical performances and socioeconomic implications of the most common practices were synthesized. The sub-Saharan Africa is actually the birthplace of a range of indigenous rainwater harvesting and management (RWHM) techniques. The micro-catchment and in situ RWHM techniques are more commonly applied than the macro-catchment techniques for supplemental irrigation on farm lands. Depending on rainfall patterns and local soil characteristics, appropriate application of in-situ and micro-catchment techniques could improve the soil water content of the rooting zone by up to 30%. Smart combinations of rainwater harvesting and soil improvements enable to increase crop yields by 200-600% as compared to the traditional farming without them. Following the implementation of rainwater harvesting techniques, the cereal-based smallholder farmers could shift to diversified crops, hence improving household food security, dietary status, and economic return.

The interplay between drought vulnerability and the changing trends in land-use/cover and land management in the CRV drylands of Ethiopia was made using a combination of GIS/remote sensing, meteorological drought analyses and surveying techniques. Given late onset of rainfall seasons (for both livestock farming and crop cultivation) and long dry-spells (for crop cultivation only) as the perceived causes of drought, pastoral system was vulnerable to severe drought once in seven years while mixed crop-livestock farming was vulnerable to severe drought once in twenty eight years. Over the last 5 decades, cultivated lands increased to threefold while the dense acacia coverage declined from 42% in 1965 to 9% in 2010. Although conversion from pastoral life to mixed crop-livestock system was perceived important to cope with drought, long-term tillage using the traditional Maresha cultivation caused deteriorates of the soil water properties. Infiltration rate of the surface soil layer increases significantly immediately after conversion of acacia-based grasslands to cultivation. However, there is a weak decreasing trend in infiltration rate and a significant increase in soil evaporation with increases in cultivation durations. Thus, it was implied that improved soil management and appropriate tillage are needed to maximize rainwater use efficiency and achieve sustainable agricultural production in the CRV of Ethiopia.

Two-year field experimentation was undertaken to calibrate and examine the role of the FAO’s AquaCrop model in simulating the effect of rainwater harvesting techniques in response to different rainfall patterns and soil fertility levels in the CRV of Ethiopia. After proper calibration of the FAO’s AquaCrop model, it was possible to simulate the effect of tied-ridges and soil fertility improvements on maize yield and water use efficiency in response to different rainfall patterns. The model simulation revealed that the effect of tied-ridges alone performed better than soil fertility improvements during below-average rainfall seasons. During above-average rainfall seasons, the combined use of tied-ridges and soil fertility improvement was found very effective to substantially improve maize yield. This is because the excess water held in the tied-ridges can be best utilized due to the enhanced water uptake capacity of maize growing in the fertilized soils. Depending on the seasonal rainfall patterns, the combined use of tied-ridges and optimum level of soil fertilizer doubles the rainwater use efficiency of maize. A field experiment during a normal rainfall in 2010 revealed that the combined use of farmyard manure (4.5 Mg ha^-1) and tied-ridges increased maize yield by 47% while tied-ridges in isolation increased maize yield by 26%. Moreover, long-term simulation revealed that the effect of tied-ridges on yield improvements was higher for sowing in April than for sowing in May.

Finally, this study enabled to develop a participatory planning approach for rainwater harvesting and management. The approach starts with investigation of the priority agro-meteorological determinants for crop production and identification of the existing knowledge and opportunities. The proposed approach also enables to plan an integrated rainwater harvesting and soil improvement techniques. The application of this new approach in the CRV implied that any effort on the introduction of new in situ rainwater harvesting techniques should assess existing tillage, hoeing and associated land management practices. The existing Dirdaro furrow system could be taken as a basis to introduce and develop tied-ridges using the Maresha-modified ridger. Overall, this approach may augment the recent efforts of dissemination of rainwater harvesting and management techniques for improved agricultural development in the vast drylands of Ethiopia.