Enhancing productive use of alluvial aquifers

Alluvial aquifers and food security

Agricultural yields in Sub-Sahara Africa have remained stagnant over the last decades. Dominant constraints are soil nutrients (large heterogeneities), water (highly variable), energy (severely limited), markets (volatile and unreliable), and outdated agricultural extension. Insufficient and irregular availability of water for crops stands out as a major challenge in arid and semi-arid zones. This lack of water security explains persistent low fertilizer application and stagnant crop yields. This is compounded by climate change which further increases rainfall variability (floods and droughts).

Water insecurity often leads to food insecurity, a major concern for communities living in Africa’s arid to semi-arid lands. One key to unlocking Sub-Sahara Africa’s agricultural potential is enhancing water security through increased ability of farming systems to buffer water variability by means of increased capacity to store water. This can simultaneously increase the efficient use of limited water resources.

The river beds of seasonal rivers provide opportunities for nature-based water storage. The sandy river beds contain significant amounts of water throughout the dry season and has the potential for intensifying irrigated agriculture. The numerous ephemeral (seasonal) rivers and streams in these areas form a natural buffer when water infiltrates in alluvial river channels and adjacent river banks during the rainy season. Communities use the water during the dry season by scoop holes, hand pumps, dug wells or other simple abstraction means. These aquifers have a distributed storage potential that is currently under-utilised (Love et al., 2010).

Water storage in these rivers is mostly unmanaged. Knowledge of the recharge, infiltration, water loss and storage mechanisms would be an important basis to optimize the use of this resource by using appropriate pumps, constructing check-dams and sub-surface dams, sand storage dams, by managed releases from upstream reservoirs, and other interventions.

There are some interesting examples of larger scale use of riverbed alluvial aquifers. In southern Zimbabwe, some large- and small-scale farmers pump water from the alluvial aquifers in the Mzingwane and Shashi tributaries of the Limpopo river, using different technologies to irrigate cash crops and fruit trees (e.g. Masvopo, 2008). In eastern Kenya people have built so-called sand dams in the Kitui river, which hold the shallow groundwater that is used for domestic purposes (Lasage et al., 2007). In northern Ethiopia, treated streams/rivers with different methods such as check-dams, terracing and biological measures are becoming important sources of water even during drought periods.

Initiatives to use water from alluvial aquifers are often implemented by NGOs as isolated activities, not based on proper research and feasibility analyses, and mostly framed in terms of poverty alleviation. Alluvial aquifers are not yet recognized as an opportunity for rural entrepreneurs to produce agricultural products for local and regional markets, and thus for socio-economic development (Makurira, 2014)