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Irrigated agriculture makes a major contribution to food security, producing nearly 40 percent of food and agricultural commodities on 17 percent of agricultural land. Irrigated areas have almost doubled in recent decades and contributed much to the growth in agricultural productivity over the last 50 years. Irrigated agriculture uses more than 70 percent of the water withdrawn from the earth's rivers; in developing countries the proportion exceeds 80 percent.

The scope for further irrigation development to meet food requirements in the coming years is, however, severely constrained by decreasing water resources and growing competition for clean water. While on a global scale water resources are still ample, serious water shortages are developing in the arid and semi-arid regions as existing water resources reach full exploitation. The situation is exacerbated by the declining quality of water and soil resources. The dependency on water has become a critical constraint on further progress and threatens to slow down development, endangering food supplies and aggravating rural poverty.

The great challenge for the coming decades will therefore be the task of increasing food production with less water, particularly in countries with limited water and land resources. Water productivity for food production was a major issue at the Second World Water Forum convened in March 2000 by the World Water Council in The Hague, the Netherlands, where a vision of progress towards water security was presented and an action framework for achieving this was developed. One of its main targets was defined as the need to increase water productivity for food production from rainfed and irrigated agriculture by 30 percent by 2015.

Water stress affects crop growth and productivity in many ways. Most of the responses have a negative effect on production but crops have different and often complex mechanisms to react to shortages of water. Several crops and genotypes have developed different degrees of drought tolerance, drought resistance or compensatory growth to deal with periods of stress. The highest crop productivity is achieved for high-yielding varieties with optimal water supply and high soil fertility levels, but under conditions of limited water supply crops will adapt to water stress and can produce well with less water.

In the context of improving water productivity, there is a growing interest in deficit irrigation, an irrigation practice whereby water supply is reduced below maximum levels and mild stress is allowed with minimal effects on yield. Under conditions of scarce water supply and drought, deficit irrigation can lead to greater economic gains than maximizing yields per unit of water for a given crop; farmers are more inclined to use water more efficiently, and more water-efficient cash crop selection helps optimize returns. However, this approach requires precise knowledge of crop response to water as drought tolerance varies considerably by species, cultivar and stage of growth.

Recognizing the potential of deficit irrigation practices in conserving scarce water resources, increasing farm profitability and enhancing environmental protection, the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture coordinated a research project between 1990 and 1995 entitled "The use of nuclear and related techniques in assessment of irrigation schedules of field crops to increase effective use of water in Irrigation projects". The results of this project were published in 1996 in IAEA-TECDOC-888 Nuclear techniques to assess irrigation schedules for field crops and externally in 1999 by Kluwer Academic Publishers Crop yield response to deficit irrigation (C. Kirda, P. Moutonnet, C. Hera and D.R. Nielsen, eds.).

The past five years have seen substantial progress in the practical application of deficit irrigation for both annual and perennial crops. Recognizing the need for wide dissemination of this new information, the Joint FAO/IAEA Division together with the FAO Land and Water Development Division invited specialists in this sector of research and development to contribute to a new publication to provide a state-of-the-art evaluation for a wide range of crops. Ms L.K. Heng and Mr P. Moutonnet (IAEA, Vienna) and Mr M. Smith (FAO, Rome) implemented this task.

The aim of this publication is to provide further information on the way crops react to stress, leading to practical guidelines to assist extensionists, farmers and decision-makers in minimizing water use for optimal crop production.

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