More crop per drop

Irrigated agriculture has long been synonymous with high productivity, the 20 per cent of farmland that is irrigated producing 40 per cent of current food supply. But, surprisingly perhaps, the greatest potential for meeting further burgeoning food demand lies in rain-fed agriculture. Low-cost technologies that allow judicious supplemental irrigation to bridge dry spells include treadle pumps, trickle and seep-hose systems; these offer yield increases of 100 per cent from rain-fed agriculture whereas, according to Johan Rockström, Director of the Stockholm Environmental Institute (SEI), it will be difficult to achieve more than a 10-15 per cent increase from large-scale flood or sprinkler systems, even if extra water is available. And that appears unlikely as competing demands for water arise from industry, commerce, tourist hotels and fast-growing urbanisation. By 2025 the population will have grown by 2 billion and 95 per cent of the growth will be in urban areas.

Irrigation has been described as "one of the most subsidised activities in the world", and studies have both cast doubts on the economic returns on investment in large-scale irrigation systems and pointed to the environmental costs of waterlogging and salinisation of soils. Salinisation now affects 30 per cent of irrigated land and is reducing the existing irrigated area by two per cent per year. However, according to the Food and Agriculture Organization of the UN (FAO), if farmers are offered incentives they will readily adopt low-cost water-saving irrigation technologies that apply small quantities of water directly to the root zone of crops, eliminating waste and avoiding soil degradation. Consequently, the answer to the question "Do we need to find extra river and ground water for extra food production?" is definitely "No", according to Rockström. He was speaking on The Future of Rain-fed Agriculture at a session organised by the Comprehensive Assessment of Water Management in Agriculture (CAWMA) during World Water Week in Stockholm. Indeed, he said, for every increase in yield level less and less 'extra' water is needed. This is because the water used by plants for transpiration is far less than the water lost by evaporation; so, as plant demand increases, it uses proportionately less water. Conversely, in years of crop failure - zero yield - there is very high evaporation and transpiration but no product. Hence the need to use small-scale irrigation to bridge the dry spells that can desiccate crops. Here, rainwater harvesting can make productive use of even irregular rainfall.

One of the controversies to be addressed by the CA is whether and what type of investments should be made in irrigated versus rainfed agriculture. Rainfed proponents point out that irrigation is heavily subsidised, and that substantial yield gains can be had in rainfed systems. On the other hand, others contend that there is ample scope for water productivity gains in irrigated systems, and that there is scope for further developing these.

Boosting rain-fed yields

The world's food production could be doubled if the productivity of rain-fed agriculture could be unlocked, says Suhas Wani of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT). Also speaking on The Future of Rain-fed Agriculture, he pointed out that 95 per cent of population growth will be in the developing countries, where there is low water use efficiency, contributing to a sixfold increase in water demand by 2050. ICRISAT has had a long-term trial in India since 1977, comparing traditional technologies with a farmer-affordable improved system. The traditional system has been shown to have a carrying capacity of 4 people/ha/year, in contrast to the improved system which has the potential to support 18 people/ha/year. Admittedly the improved system requires changes to tillage techniques and judicious use of nitrogen, phosphorus and micronutrients such as boron, sulphur and zinc, but one 500-hectare community that has used the improved system has doubled yields since 1998.

Paul Polak of International Development Enterprises (IDE), the third speaker on The Future of Rain-fed Agriculture, provided practical examples of low-technology options affordable by small farmers. Treadle pumps, drip irrigation systems and simple, efficient water storage facilities can be financially accessible to small farmers, he said, and some are able to generate sufficient extra income to pay for their investment in a single season. Rainwater collection from roofs is perhaps the simplest option, and it can be most efficiently applied to vegetable gardens through a trickle irrigation system. "Water is the key entry point for increasing smallholder income", he said.

Drawing on long experience in Bangladesh, Polak said that when IDE donors had invested US$15 million in treadle pumps, 1.5 million farmers had invested US$37.5 million of their own money. The net yearly cash income to those small farmers had been US$150 million. In India, a 200 square metre drip-irrigated plot now costs a poor rural family US$8 and generates more than US$50/year in net income from vegetables, fruits and other cash crops. Turning to water storage, Polak presented the option of low-cost and evaporation-proof water storage in plastic 'sausages' laid in and supported by a shallow trench. To improve water use still further, said Polak, farmers should be encouraged to use plastic or organic mulch to reduce water loss from the soil.

Providing experience from Zimbabwe, Charity Mhiki of Midlands State University, who is working with the Mvurumanzi Trust, reported on a simple drip irrigation system used by poor farmers, including AIDS orphans. Based on two 650 litre header tanks filled from shallow wells using rope-and-washer pumps, ten lateral drip lines, each with 33 emitters, provide water to 100 square metres of vegetable gardens. Comparison of identical gardens and crops has shown that not only does drip irrigation make much better use of water than the traditional use of watering cans, yields are significantly higher - 50 per cent or more.

As Paul Polak concluded in his presentation, "Access to affordable water is the first step out of poverty."