|dc.description||Crops can be grown without fertilizers and even without soil, but they cannot survive without water. Without adequate moisture plants cannot mobilize soil nutrients and Jack of water at crucial stages of development will leave plant growth stunted. The demand for higher crop yields is greatest in many countries where water is increasingly scarce, either due to low annual rainfall or to concentrated periods of precipitation with long intervening dry spells. To meet future food needs farmers will have to make better use of all available moisture by employing water harvesting, conservation and efficient irrigation.
The World Meteorological Organization estimates that water use has tripled in the last two decades and has been increasing at twice the rate of population growth. Agriculture is the largest user of a nation's water (see Spore 57.' Water, the limiting resource); urgent action is needed to improve water management so that resources are conserved and used to optimum benefit.
Traditions of water harvesting and irrigation are to be found in many parts of Africa that are centuries, and even millennia, old and evidence survives in land-forms, structures and practices: yet sub-Saharan Africa has the lowest proportion of irrigated land in the world. FAO estimates that the proportion of irrigated to total arable land in Asia is 37 per cent, in North Africa 24 per cent and in Latin America 15 per cent, but in sub-Saharan Africa it is only 4 per cent. Moreover, 75 per cent of all irrigated land in Africa is accounted for by just six countries: Egypt, Madagascar, Morocco, Nigeria, South Africa and Sudan. However, these figures refer to irrigation and do not include the many and diverse techniques of water harvesting and exploitation of residual moisture. These include planting in river beds and seasonal lakes (dambos) in Zambia, and 'draw-down' cropping where planting on river banks follows falling water levels. These traditional opportunistic techniques of using water agriculturally may offer the easiest and most economic route to achieving more trop yield from reduced water availability.
The need for irrigation is obvious but the cost is often far too high for smallholder farmers. Most irrigation equipment in Africa is imported, with only limited quantities manufactured locally. FAO found that locally manufactured and imported water pumps, irrigation pipes and sprinklers in Malawi, Tanzania, Zambia and Zimbabwe are three to ten times more expensive than in Asia. It is not surprising that most of the irrigated land is concentrated under government or parastatal bodies such as the Office du Niger in Mali or the Gezira Authority in Sudan, or is in the hands of large farming companies growing crops for export.
Options for smallholders
Farmers in ACP countries are acutely aware of their dependence on water, and smallholder farmers have three options:
invest in irrigation;
use water harvesting conservation techniques;
plant more drought-tolerant crops.
In the absence of low-cost irrigation equipment a combination of the other two options can still result in significant increases in productivity at minimal cost. One of the first options for farmers anticipating insufficient rainfall is to grow more drought-tolerant crops: where maize has been the main trop it may be necessary to plant sorghum or, in the driest areas, millet. Cassava is also remarkably drought-tolerant. Local food tastes may have to change to reflect what can be grown locally or a premium may have to be paid-for increasingly scarce imported foods.
Water harvesting was developed by the ancient Nabateans over 3,000 years ago in what are today Israel and Jordan, and it is probably not a coincidence that very similar techniques have been developed and have survived in the Red Sea Hills of North-west Sudan and in the Central Darfur region. Depending on the topography, wadis are dammed with earth embankments and the water dispersed over a wide area of crop land; alternatively, water flowing down slopes is caught behind a sequence of low (100 mm high) crescent shaped bunds, forcing the moisture to percolate into the soil before the surplus flows on to fill successive trus, as they are known. Yields of sorghum planted in the trus are 57 bags/acre, compared with only 2-3 bags of millet grown without trus in normal years.
In Niger farmers have improved on their traditional planting pits - tassa - by increasing them in size from 20-30cm in diameter and from 10-25cm in depth. The excavated soil is placed on the down-slope to increase water catchment and storage, and manure and a little fertilizer are added to each tassa before the rains. In Burkina Faso and Mali planting pits are known as zai, which means `water pocket', and these are dug at 0.8 to 1.0 metre intervals along the contour.
In Tanzania, farmers in Maswa District in recent times have developed the Nabatean concept of channelling rain falling on a substantial area of sloping land to water a smaller area of trop land, in this case for rite. They have further refined their technique by deliberately grazing bare the catchment slopes during the dry season so that with the first rains the soils form a crust, which reduces infiltration and maximizes run-off. By channelling and using the water in this way, these farmers have put to good use the very soil condition so often instanced as a failure of soil and water conservation, where rain falling on overgrazed, crusted soil flash-floods with minimum benefit to land or crops.
Finally, the Fanya ju terraces developed in the Machakos District of Kenya are another example of local enterprise demonstrating the efficacy of simple technology in husbanding rainfall to maximize trop growth (see Spore 51 p5: Farming the hillsides).
Constraints on water supplies for agriculture are inevitable, and may well limit conventional irrigation of crops. Already the most advanced growers have increasingly turned to the more water-efficient trickle and drip systems pioneered by US and Israeli farmers but even these systems depend on storage or a reliable water source. Water harvesting and conservation must be a priority if agriculture is to sustain, and hopefully increase yields in the coming years.
(See also Spore 67 p12: Sustaining the soil - indigenous soil and water conservation in Africa).||en_US