Agriculture in Sub-Saharan Africa is mainly rain fed, characterized by rainfall variability and low inputs of nutrients and other agrochemicals. This results in low productivity in comparison to other areas with similar climate.
While often thought of as inherently dry regions, research has shown that the annual rainfall availability in Sub-Saharan Africa is sufficient to support a significant increase of agricultural production.
The problem thus, is not about rainfall being in short supply each year, but about a lack of ways to make use of the water that is available. Annual water supplies are sufficient for increasing production, but seasonal rainfalls mean that direct availability is erratic through the year.
"Water harvesting systems, where water resources are redirected and stored in ponds to be used for supplementary irrigation during droughts and dry spell periods, have been shown to have great potential for increasing productivity in different regions in Sub-Saharan Africa. But there have been few studies looking at the effects of water harvesting on the larger landscape"
Yihun Dile, author
Where and how much?
In a study that was recently published as a part of his Doctoral thesis "Intensifying Agricultural Water Management in the Tropics – A cause of water shortage or a source of resilience?" Dile and colleagues developed and tested a decision support system for determining size and location of water harvesting ponds, using the Soil and Water Assessment Tool (SWAT). This is a model that is used to predict the impact of management practices, such as water harvesting systems.
The system is useful when determining whether or not to build a water harvesting system, and to determine the appropriate size and location of ponds. It takes into account parameters such as the appropriate slope of the land and the suitable soil types, as well as the amount of water needed to meet irrigation requirements.
Dile and his colleagues tested their system on an area in the Lake Tana basin in Ethiopia, using the decision support system to plan for water harvesting, and the SWAT model to predict what the effects would be of implementing what the decision support system suggested.
They found that using their system to determine where to build ponds for water harvesting and how big to make the ponds had positive effects on crop production. Irrigation from water harvesting combined with the addition of nutrients increased the production of teff, a traditional Ethiopian grain, by up to three times.
The bigger picture
The study also shows that on a landscape level, water harvesting can have positive effects both upstream and downstream.
Upstream, water harvesting makes it possible to provide crops with a more constant water supply throughout the growing season. This has been shown to increase yields substantially, improving both food security and income from selling cash crops. The increased availability of food also benefits people living downstream.
For downstream users the risk of flooding during the rainy season is also decreased as water is redirected. The water flow is also made more constant during the year, decreasing the risk for severe droughts during the dry season.
Other possible benefits relate to farmers' behaviour:
"Previous studies have shown that due to unpredictable weather, risk of droughts and other obstacles, farmers do not add nutrients to their crops because of low confidence in generating positive results. Water harvesting reduces this uncertainty to a degree, and may in that way inspire farmers to invest more in their crops," says Dile who defended his doctoral thesis on water management in the tropics on 4 June, 2014.
As agricultural productivity increases in already cultivated areas, there is also a decreased need to convert more land for food production. This serves as a sort of protection for the ecosystem services that are produced in the landscape, and that might be diminished by land use changes.
"The benefits we found to be associated with water harvesting clearly motivate further studies, looking more closely at the economic feasibility of implementing such systems, and the ecological impacts of doing so," Dile concludes.
Dile, Yihun. 2014. Intensifying Agricultural Water Management in the Tropics: A cause of water shortage or a source of resilience? PhD thesis. Stockholm Resilience Centre, Stockholm university.
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