Increasing the productivity of rainfed agriculture, which still supplies some 60 percent of the world's food, would make a significant impact on global food production. However, the potential to improve yields depends strongly on rainfall patterns. In dry areas, rainwater harvesting can both reduce risk and increase yields. As the diagram below shows, there are various forms of rainwater harvesting: using microstructures in the field to direct water at specific plants or plant rows (in situ water conservation); capturing and directing external water from the catchment area to the field in which crops are grown (flood irrigation); and collecting external water from the catchment area and storing it in reservoirs, ponds and other structures for use during dry periods (storage for supplementary irrigation).
Ways of dealing with arid conditions
Work in Burkina Faso, Kenya, the Niger, the Sudan and the United Republic of Tanzania has shown that rainwater harvesting can increase yields two to three times as compared with conventional dryland farming. Furthermore, rainwater harvesting often has double or triple benefits: not only does it provide more water for the crop but it also adds to the recharging of groundwater and helps reduce soil erosion.
Effects of subsoiling in trials in Tanzania
Maize yields were increased from less than 1 to more than 4.8 tonnes per hectare by subsoiling with good rains and fertilizer.
Risk management is crucial in rainfed agriculture. The higher the risk of crop reductions from droughts and dry spells, the lower the likelihood that farmers will invest in inputs such as fertilizers, improved varieties and pest management. In situ soil and water conservation contributes relatively little to reducing risks in rainfed agriculture. To make substantial risk reductions, flood irrigation, with the option of supplementary irrigation, has to be introduced. Unfortunately, technologies that reduce risk are usually more expensive and require more know-how to construct.
In dry areas, poor land management can greatly reduce crop yields, even to below 1 tonne per hectare. One reason is that land degradation often affects the soil surface, leading to crust formation and other phenomena that prevent infiltration by rainwater. Most rainfall then simply runs off the land surface, collects in silt-laden torrents and produces severe gully erosion. Crops benefit little.
A major cause is turning the soil, by hand, with animal traction or with a tractor, too often. This leaves the soil exposed and prone to both wind and water erosion. While ploughing techniques developed in temperate regions, with their gentle rains and light winds, are harmless enough, they are often poorly suited to tropical climates and soils.
Alternative forms of tillage - such as turning the soil only along plant lines, deep ploughing to break up soil crusts, building raised ridges that follow the contour, growing crops in pits, and building eyebrow terraces round trees and shrubs - can improve crop yields and reduce erosion. They lead to a much more efficient use of limited rainfall. Trials in the United Republic of Tanzania have shown, for example, that breaking up the plough-pan increased maize yields from 1.8 to 4.8 tonnes per hectare in a year with good rains and if manure was applied as fertilizer. In Damergou in the Niger, 310 hectares were equipped with microcatchments and contour furrows in less than one month using special ploughs. Costs were only US$90/hectare. Average yields were 2 tonnes/ hectare of sorghum with an annual rainfall of only 360 mm.
More needs to be done to cope with the effects of the dry spells that occur every year in arid and semi-arid areas. Although these periods of drought often last less than three weeks, if they occur during sensitive growth stages - such as during flowering or grain filling - there is a high risk of serious yield reductions.
The best way of tackling the problem is to divert rainfall from the surrounding catchment area to the soil in which the crops are being grown. Providing the right infiltration conditions have been established, water can be stored in the soil around the crop roots for considerable periods - certainly for long enough to be of considerable use during a three-week drought. Methods of diverting water within the catchment area towards the crops themselves include diverting streams, using spate flow from wadis, directing runoff with low walls (a system used to great effect by the ancient inhabitants of the Negev desert) and even diverting flow towards crops from roads and paths.
Keita Valley, Niger
Halting land degradation by dune fixation in the Keita Valley, the Niger
In five years the people of the Keita Valley in the Niger, with the help of an Italian-funded FAO integrated development project, transformed nearly
5 000 square kilometres of barren and non-productive landscape into a flourishing garden for crops, livestock and trees. Public participation was the key to the project's success which has benefited from more than 4 million man- and woman-hours of work on planting trees, digging wells, constructing weirs and damming streams, building terraces and bunds, deep ploughing land and erecting dune fences.
The project provided training and helped villagers construct new schools, roads, community centres, clinics and mills. New skills have been introduced into the community, including growing cash crops, producing handicrafts and farm processing. Thousands of people have participated in work teams and hundreds have been trained to manage them.
Finally, there are ways of storing the runoff from rainy periods for use during the dry spells: these include the tanks, ponds, cisterns and earth dams used for supplementary irrigation in China, India, sub-Saharan Africa and many other areas. Although they are more costly and require considerable know-how on the part of the farmers who have to build them, they have the advantage of greatly reducing the risk of small or non-existent harvests as a result of drought.
Small-scale farming can be productive in marginal rainfed areas if supplementary irrigation is available to overcome short-term droughts which are critical to the crop and reduce yield considerably. If there are cost-effective ways to store water before critical crop stages and apply it when the rain fails in these critical stages, crop production can be considerably increased.