Agricultural water scarcity in the predominantly rainfed agricultural system of sub-Saharan Africa (SSA) is more related to the variability of rainfall and excessive non-productive losses, than the total annual precipitation in the growing season. Less than 15% of the terrestrial precipitation takes the form of productive ‘green’ transpiration. Hence, rainwater harvesting and management (RWHM) technologies hold a significant potential for improving rainwater-use efficiency and sustaining rainfed agriculture in the region. This paper outlines the various RWHM techniques being practiced in SSA, and reviews recent research results on the performance of selected practices. So far, micro-catchment and in situ rainwater harvesting techniques are more common than rainwater irrigation techniques from macro-catchment systems. 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%. Up to sixfold crop yields have been obtained through combinations of rainwater harvesting and fertiliser use, as compared to traditional practices. Supplemental irrigation of rainfed agriculture through rainwater harvesting not only reduces the risk of total crop failure due to dry spells, but also substantially improves water and crop productivity. Depending on the type of crop and the seasonal rainfall pattern, the application of RWHM techniques makes net profits more possible, compared to the meagre profit or net loss of existing systems. Implementation of rainwater harvesting may allow cereal-based smallholder farmers to shift to diversified crops, hence improving household food security, dietary status, and economic return. The much needed green revolution and adaptations to climate change in SSA should blend rainwater harvesting ideals with agronomic principles. More efforts are needed to improve the indigenous practices, and to disseminate best practices on a wider scale.

Rainwater harvesting and management in rainfed agricultural systems in sub-Saharan Africa - A review

The role of water harvesting to achieve sustainable agricultural intensification and resilience against water related shocks in sub-Saharan Africa

Grain production shortfalls in northern Ethiopia are commonly associated with occurrence of intra-seasonal dry spells or droughts and rapid land degradation which adversely impact crop yields. Suitable practices that use available rainwater more efficiently to mitigate impact of dry spells on crops and that protect soil are needed to stabilize and improve grain yields in the predominately rainfed agriculture. During three cropping seasons on-farm experiments tested conservation tillage techniques implemented with oxen-drawn plows on clay loam soil. Tested tillage techniques are subsoiling, open and tied ridges, no till, and conventional tillage with the local maresha plow (the control). Effectiveness in improving root zone soil moisture, limiting soil erosion, and improving sorghum (Sorghum bicolor (L.) Moench.) and chickpea (Cicer arietinum L.) grain yield were determined. Results demonstrate that performance of the tillage techniques varied with seasonal rainfall distribution and intensity and land slope gradient. Tied and open ridge increased seasonal root zone soil moisture 15‐24%. Subsoiling slightly (3%) increased and no till slightly decreased soil moisture but were not statistically different from conventional tillage. Tied ridge and no till significantly reduced seasonal soil loss by up to 11 Mg ha � 1 during seasons with moderate intensity storms, but during a season with high intensity storms tied ridge on over 9% slope gradient increased soil loss (up to 35 Mg ha � 1 ). The increased soil disturbance of subsoiling led to higher soil loss rates (up to 32 Mg ha � 1 ) than conventional tillage during all seasons. Grain yield decreased and

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Performance of in situ rainwater conservation tillage techniques on dry spell mitigation and erosion control in the drought-prone North Wello zone of the Ethiopian highlands

Effects of soil and water conservation techniques on crop yield, runoff and soil loss in Sub-Saharan Africa: A review

Although rainwater harvesting system (RHS) is an effective alternative to water supply, its efficiency is often heavily influenced by temporal distribution of rainfall and water demand. Since natural precipitation is a random process and has probabilistic characteristics, it will be more appropriate to describe these probabilistic features of rainfall and its relationship with design storage capacity as well as supply deficit of RHS. This paper aims at developing a methodology for establishing the probabilistic relationship between storage capacities and deficit rates of RHS. A simulation model was built to simulate the input rainfall and water release in RHS. Historical rainfall records were then used as input for simulation and the results were used in probabilistic analysis for establishing the relationships between storage capacities and water supply deficits. The city of Taipei was used as study area for demonstration of this methodology and probabilistic distribution curves for storage capacity and deficit rate relationships were presented. As a result, a set of curves describing the continuous relationships between storage capacities and deficit rates under different exceedance probabilities were generated as references to RHS storage design. At a chose exceedance probability of failure, the engineer can decide from the curve on the storage size under a preset deficit rate.

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A probabilistic approach to rainwater harvesting systems design and evaluation

The study was carried out at three sites in Communal Lands of Zimbabwe namely, Mudzi (Natural Region V), Gutu (Natural Region IV) and Chivi (Natural Region V). These areas have marginal rainfall in terms of quantities (below 500 mm per year) and distribution with a normal rainfall year coming once in every five years. Four tillage treatments were experimented on in the three sites. These were tied ridges, infiltration pits, fanya juus and the conventional ploughing on the flat. The tillage treatments were paired at farm level for easy management considering that farmers were participating in the research trials. The conventional tillage technique was used as control.

The objective of the study was to evaluate and recommend rainwater harvesting techniques that ensure effective capture and utilization of rainfall for sustainable crop production. This was achieved through monitoring residual moisture, after every rainfall shower, in the different tillage treatments. Monitoring of residual moisture was done, up to a depth of 30 cm, using a θ-probe. Yields of maize from the different tillage treatments were determined. Farmers also carried out their own evaluation of the treatments at mid-season and at the end of the season. In addition the researchers tested dissemination techniques that ensured maximum adoption of rainwater harvesting techniques.

Tied ridges were ranked best by both the stakeholders’ evaluation and the theta (θ)-probe in retaining moisture compared to all the other treatments in the season under consideration. The farmers who practiced tied ridges realized yields of about 3 t/ha compared to conventional tillage treatments whose yields were about 1.5 t/ha. Yields were statistically significantly different for the different treatments.

The recommendation was for the farmers to adopt the tied ridges in areas that receive marginal rainfall and experience mid-season droughts. Lead farmers should educate others on the advantages of the tillage treatments.

Rainwater harvesting for sustainable agriculture in communal lands of Zimbabwe

Instittute for People, Innovation & Change in Organizations (PICOTeam) International Water Management Institute 3 Institute for Land, Water and Society Charles Sturt University, Australia. University of Zimbabwe Environmental Management Agency, Zimbabwe 6 University of Eduardo Mondlane Sustainable wetlands management Agreed-upon and functional institutional arrangements which facilitate and regulate sustainable wetland utilization and conservation

Kudakwashe E. Motsi

Papers

Rainwater harvesting for sustainable agriculture in communal lands of Zimbabwe

Guideline for sustainable wetland management and utilization: key cornerstones