In some parts of sub-Saharan Africa there is plenty of water available. The challenge is to increase the amount of available water that is ‘harvested’ for crop growth. Such water harvesting can be done at a field, farm or watershed level. In some places there is a potential for ground water extraction using boreholes. There is also a need to increase the amount of irrigation (using dams or run-of-the-river systems) from the current 6% of arable land, in a sustainable manner. In other regions, water is very scarce; approximately 70% of the world’s poor live in rural areas with little option but to rely on rain-fed agriculture to sustain their livelihoods. An estimated 200 million people in sub-Saharan African (18% of the population) face serious water shortages.
Furthermore, climate change continues to pose a challenge to water management in agriculture. Most agriculture in sub-Saharan Africa is rain-fed and prolonged periods of drought are becoming increasingly frequent. This increases pressures on valuable water resources and agricultural irrigation. In response, water conservation policies, strategies and activities offer solutions to manage and protect fresh water as a sustainable resource and to meet current and future human demand.
Practically, water harvesting involves gathering water from the ‘catchment area’ (the area around a water source where rainfall naturally collects) and channelling it to the required area. This can be accomplished in many ways such as harvesting water from an external catchment, rooftop or semi-circular bunds, but all are suitable under different conditions depending upon the environment, climate and the labour and financial resources available to the farmer. Some techniques can also work at a smaller scale to conserve water within the cropped area by reducing run-off and keeping the water where it falls as much as possible.
Four different methods – contour harvesting, earth basins, planting pits and drip irrigation – highlight a spectrum of approaches currently available to farmers:
Contour harvesting: Ploughing (turning up the earth before seeding) and furrowing (making a rut, groove, or trail in the soil), then planting along the ridges or contours rather than up and down the slope conserves water by reducing surface run-off and encouraging filtration of water into the crop area. Several water-harvesting techniques are based along contours including: contour ploughing; contour ridges; stone lines; grass strips; and terraces.
Earth basins: Square or diamond shaped basins with earth ridges on all sides, earth basins can be constructed on any gradient and whilst most suitable for growing trees, they may also be used for other crops.
Planting pits: Easy to construct small pits in which individual or small groups of plants are sown. The pits catch rainwater run-off and concentrate soil moisture around the roots.
Drip irrigation: Drip irrigation carries water to fields through a system of plastic tubes where the water is slowly dripped onto the soil through small perforations in the tube. A small petrol pump can be used to push the water along the tubes for larger areas, but this will add a fuel cost and will need servicing. Therefore, drip irrigation is more likely to be used on smaller areas of high value crops that require regular watering.Contribution to Sustainable Intensification
The challenge of meeting global food demand requires an increase in the level of agricultural water productivity and some increases in global water use. Finding ways of saving water or using ‘less drop per crop’ must become a mainstay of agricultural production. Suitable water conservation methods minimise negative environmental impacts of drought such as soil erosion and desertification, whilst increasing crop yields throughout the year. Soil nutrients will only be used efficiently if a crop has sufficient water and water conservation can increase the volume of water available, bridging seasonal rainfall variability. Water conservation contributes to Sustainable Intensification by allowing water to be used efficiently, resulting in higher agricultural production throughout the year and improved resilience to drought, improving farmers’ livelihoods and food security.
Combining water conservation with soil testing could help farmers gain a better understanding of the soil types in a specific location and ensure that the appropriate water conservation method is implemented. For example, when water drains out of the soil, essential nutrients such as nitrogen are also washed out. This problem is greater on light sandy soils, but can be reduced by adding compost. Manures or plant residues will eventually increase the amount of water a soil can retain. Farmers can be better equipped to select the correct methods of water conservation and prevent both a loss of soil moisture and nutrients with a combined understanding of their soils.
Combining water conservation with good agricultural practices can help farmers to make effective use of soil water reserves and improve soil health. Deeper rooting crops, such as grasses or cereals will exploit soil water reserves more effectively than shallower rooting crops and therefore can be grown in drier periods. Good agricultural practices such as avoiding ploughing too deeply or when the soil is wet, will also promote the efficient use of soil water reserves. Adding mulch or manure to break up the intensity of rainfall will reduce the tendency of the soil to form a crust, minimising run-off.
Contour harvesting, requires a great deal of hard labour and training to build and continuous labour to maintain the structures. Apart from labour and small hand tools or hoes, no additional cost for construction equipment (such as pipes for drip irrigation) is required. Different contour methods are only applicable for certain types of crops or trees and may reduce yields due to larger spacing of trees within the contours.
Whilst earth basins are simple and cheap to install on almost any slope, the level of maintenance required to keep them free of unwanted vegetation limits their use. Additionally, the system can be badly damaged if there is a storm, which requires additional labour for repairs. In an evaluation of earth basin water harvesting systems comparing their effectiveness for collecting and storing water at different scales, the efficiency of the system varied from more than 85% to as little as 7%, depending on the size of the catchment and the root zone capacity. Therefore, it is important that the location, design and the crop are properly selected, which requires training or extension services to help farmers to build the basins correctly. The low crop densities required for this method mean that farmers get a low yield in return, compared with other water conservation methods.
Planting pits are labour intensive to prepare. One study found that planting pit construction required 76.5 days per hectare for the clay soils and 51.5 days per hectare for sandy soils, compared to 0.6 days per hectare under conventional tillage. However, many smallholder farmers lack draught power – either from animal traction or machinery – so despite the initial labour, digging planting pits is often easier than ploughing. Further, pits can be dug over several months before the rainy season, spreading out the labour requirements over time. Once the pits are dug, they need to be restored for the next planting season. Weeding in planting pits requires 40% more labour compared with conventional tillage (12 human days per hectare due to high weed densities).
Drip irrigation may be laborious to set up and initial costs for equipment may be high for smallholder farmers, yet the systems are comparatively cheap, efficient and simple to use compared to large-scale irrigation. A drip irrigation system costs an estimated US$1,565 compared to US$924 for a watering can-based system (otherwise known as a ‘bucket drip kit’). Whilst expensive, a comparison study between drip irrigation and hand-watering of aubergines in Niger found that drip irrigation reduced labour-hours from 4.7 to 1.1 per day per 500m2 of farmed land. Where labour accounted for 45% of the cost of food production when irrigated by hand, the value per m2 of aubergine crop increased from US$0.1 to US$1.7 with drip irrigation. With a drip irrigation system, water is also saved; for each cubic metre of water applied, 7.3kg of aubergine was obtained using drip irrigation, as opposed to 2.4kg using hand watering. Based on these calculations, an average of 25% less water can be used.
Drip-irrigation systems also face risks. The sun can damage the tubes and shorten their usable life. The system should include a simple wire mesh filter between the storage tank and the drip irrigation pipes that requires regular cleaning as it may get clogged up with algae. Without sufficient leaching of water onto the soil, salts applied with the irrigation water may build up in the root zone. To avoid these pitfalls, support should be made available to farmers after implementation. A study in Zimbabwe by the International Crop Research Institute for Semi-Arid Tropics (ICRISAT) found that 60% of households reported that they needed additional advice after implementation, despite that more than 90% of households received basic training.
Awareness of water conservation methods and their benefits is low. Numerous farmers are not reached by agricultural extension services or those services do not always address methods for rainwater conservation. Further, national policies generally do not provide sufficient incentives—such as land rights—to encourage farmers to invest in improved land and water management. Scaling-up outreach and extension services for locally designed water conservation plans would help to increase the uptake of water conservation measures in remote rural areas.