The productivity of water used in agriculture increased by at least 100% between 1961 and 2001, thanks mainly to increases in crop yields. Irrigated rice yields doubled and rain-fed wheat yields rose by 160% in that period, with little variation in water consumption per kilo of output. Globally, FAO estimates that water needs for food per capita halved between 1961 and 2001, a significant saving and an equally significant gain for other water users.
By one calculation, a 1% increase in water productivity in food production makes available - in theory, at least - an extra 24 litres a day per head of population, while a 10% increase would equal current domestic water consumption. Investing in agriculture and in agricultural water management, therefore, is an attractive strategy for freeing water for other purposes.
Improving water productivity - whether under rain-fed or irrigated conditions - requires, first, an increase in crop yields or values, i.e. the marketable yield of the crop for each unit of water transpired. Also necessary is a reduction of all outflows or "losses" (e.g. drainage, seepage and percolation) except crop transpiration, and more effective use of rainfall, stored water, and water of marginal quality. Loss reduction and water control are considered parts of basin-wide integrated water resource management (IWRM), which gives an essential role to institutions and policies in ensuring that upstream interventions are not made at the expense of downstream water users. These three principles apply at all scales, from plant to field and agro-ecological levels, but options and practices associated with them require different approaches and technologies at different spatial scales.
Harvest index. At the plant level, higher productivity will depend mainly on germplasm improvements, such as stronger seedling vigour, increased rooting depth, increases in the harvest index (the marketable part of the plant as part of its total biomass) and enhanced photosynthetic efficiency. Green Revolution wheat, rice and maize varieties that are insensitive to day-length and of short to medium duration (90-120 days) have proved successful in escaping late-season drought that adversely affects flowering and grain development. As a result, modern rice varieties are three times more productive, in terms of water use, than traditional varieties. Traditional breeding has already made progress in extending these achievements to other crops, and genetic engineering is expected to overcome long-standing obstacles to development of higher-yielding, drought-tolerant crop varieties.
FAO questions common wisdom that water use in rice cultivation is inherently inefficient, pointing out that percolation from the standing water layer on the field surface is often recycled, and that the water productivity of rice generally compares well with that of dry cereals. Nevertheless, water-saving irrigation techniques - such as saturated soil culture and alternate wetting and drying - can reduce the unproductive water outflows drastically and increase water productivity. These techniques generally lead to some yield decline in high-yielding lowland rice varieties, although substantial increases for some local varieties - with average yields over 8 tonnes per ha. - have been obtained using a technique called "system rice intensification", from Madagascar, in which the soil is only kept wet during the reproductive stages and when the plant is producing grains.
In rain-fed agriculture, bridging crop water deficits during dry spells through supplementary irrigation stabilizes production and increases both production and water productivity dramatically. While investments in water harvesting -. e.g. construction of ditches that take the runoff to a storage reservoir - are relatively small, its effectiveness depends on many factors, including topography, soil characteristics and fertilizer availability, and - not least - the involvement of the beneficiaries in design and operation.
Value calculations. At system and river-basin level, options for improving water productivity include better land-use planning, use of medium-term weather forecasts, improved irrigation scheduling, and use of various sources of water. But increases in water productivity may not result in greater economic or social benefits - water in the rural areas of developing countries has many uses, a fact that complicates value calculations. At this larger scale, the effect of agriculture on other water users, human health and the environment becomes at least as important as production issues.
Why not sea water?
Desalinated seawater costs about 50 cents per cubic metre, which is more than twice the price of freshwater used in irrigation (as calculated by a recent study of 23 irrigation systems in 11 countries in Asia, Africa and Latin America). "This source of water is too expensive for virtually all agricultural production," FAO says. "However, its cost has come down to about one-tenth of what it was 20 years ago. Further improvements in the technology of seawater desalination are likely and its cost is also likely to continue falling."
Finally, FAO stresses the need to identify the types of policies and incentives that will work best in promoting adoption of new agronomic and cultural practices and, with them, higher water productivity: "Experience with conservation agriculture indicates that the short-term interests of the farmers often differ from society's long-term interests, and that the financial benefits that accrue from changes in cultural practices often take a long time to materialize. The inconsistent and sometimes contradictory results from studies on the adoption of new practices suggest that the decision-making process of farmers is highly variable and often unacceptably long considering the urgent character of water-scarcity problems. Experience from participatory research and extension could help reduce this delay."