Irrigated agriculture has driven much of the increase in global food production over recent decades. While only 20% of the world's farmland is irrigated, it produces 40% of our food supply. The highest yields obtained from irrigation are more than double the highest yields from rainfed agriculture - even low-input irrigation is more productive than high-input rainfed farming.
Why then is irrigated agriculture unlikely to expand as fast it has over the past? One consideration is cost: irrigation has been described as "one of the most subsidized activities in the world", and some studies have cast doubts on the economic returns on investment in large-scale irrigation schemes. The environmental costs of conventional irrigation are also high. High-intensity schemes are often blamed for waterlogging and soil salinization, which now affect 30% of irrigated land. Salinization is reducing the existing area under irrigation by up to 2% a year. To increase irrigation's contribution to food production, FAO says, what is needed is improved efficiency in the use of irrigation water.
Drip, drip, drip... If incentives are in place, farmers will adopt water-saving irrigation technologies. The main technologies likely to be used in developing countries, where labour is normally abundant but capital scarce, are underground and drip irrigation. Both technologies depend on the frequent application of small amounts of water as directly as possible to the roots of crops. A major advantage of water-saving technologies, particularly drip irrigation, is that as well as saving water they can increase yields and reduce the rate of salinization. Furthermore, since neither system brings water into contact with foliage, they can be used with brackish water for crops that are not too sensitive to salinity.
Some underground irrigation systems do not require costly equipment but are labour intensive. Indeed, one of the oldest methods of irrigation is placing porous clay jars in the soil around fruit trees and along crop rows. Porous or perforated pipes buried underground serve the same purpose, and can usually be used to irrigate two rows of crops, one on either side of the pipe. The rate of application cannot be controlled (although the frequency can) since it depends on the size of the perforations and soil characteristics.
Drip irrigation - a pressurized system that forces water through perforated pipes running above ground - has been applied to only a small part of the area for which it is suited. Though the technology is relatively simple, it does require both investment and maintenance - water emitters can easily become clogged. However, results from many countries show that farmers who switched from sprinkler irrigation to drip systems have cut their water use by 30-60%. Because plants are effectively 'spoon-fed' the optimal amount of water (and often fertilizer) when they need it, crop yields often increase at the same time.
In South Asia, inexpensive treadle pumps have been successful in extracting irrigation water from shallow aquifers, allowing farmers to make good use of the household labour and increase production and income. The farmer has full control over the timing and amount of the pumped water which, given the effort involved, is used sparingly. "Treadle pumps are intrinsically pro-poor," FAO says, "as richer farmers would not be able to persuade household members to use them." Positive experience has also been reported with bucket drip-irrigation kits, which are suitable for the irrigation of small plots of vegetables and fruit trees in peri-urban areas. In Kenya, where farmers have bought more than 10 000 kits, the return on an investment of about US$15 per kit was some US$20 per month.
A range of other modern small-scale and supplementary irrigation systems could help increase productivity in rainfed areas. Pumping water with small-scale diesel or electric engines is less labour-intensive than using a treadle pump, and can be more economic than large-scale schemes that rely heavily on centralized control. Furthermore, because individual farmers are in full control of their own systems, they can often maximize production to suit their own lifestyles - something impossible with large, centrally-controlled schemes.
Drainage problems. Drainage of irrigated land serves two purposes: to reduce waterlogging and to control and reduce salinization. Proper drainage also allows crop diversification and intensification, the growth of high-yielding varieties, and effective use of inputs such as fertilizers. Drainage problems are serious on about 100-110 million hectares of irrigated land located in the world's semi-arid and arid zones. At present, about 20-30 million hectares of irrigated land are seriously damaged by the build-up of salts and 0.25-0.5 million hectares are estimated to be lost from production every year as a result of salt build-up. However, even efficient drainage causes problems - drainage effluent is often contaminated with salts, trace elements, sediments and traces of agricultural inputs, which need to be safely disposed of.