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Irrigation is essential for agricultural production in arid and semi-arid regions where rainfall is inadequate to sustain crop growth. Indeed, in more humid areas irrigation has now become the primary tool to increase and stabilize agricultural production in view of uncertainties of rainfall and frequent droughts, and to feed an ever increasing world population.

Worldwide, large investments in irrigation engineering works have been made over the past decades and irrigated areas have almost doubled. Water resources have, however, become increasingly scarce and the competition for good quality water is diverting more water from agriculture to human and urban consumption.

Water shortages have become more frequent and farmers often face deficiencies in water deliveries, resulting in reduced yields and incomes. Furthermore, poor management of available water for irrigation, both at system and farm level, has led to a range of problems and further aggravated water availability, and has reduced the benefits of irrigation investments.

Excessive use of water has led to a range of environmental problems such as waterlogging, leaching of agro-chemicals and consequent groundwater pollution. Salinization of soil and groundwater resources due to inappropriate water use is threatening large areas of productive irrigated land.

Irrigation scheduling is the primary tool to improve water use efficiency and raise yields, and in turn will lead to higher incomes and greater availability of water resources, and provoke a positive effect on the quality of soils and groundwater. Research results over the past decades have importantly improved our knowledge on plant-water relations and provided a large number of tools which can potentially enhance irrigation management and substantially improve irrigated crop production and water use efficiency. Despite considerable efforts to promote the introduction of modem irrigation scheduling tools, their application in practice has so far fallen well below expectations.

The Internation Commission on Irrigation and Drainage (ICID) and the Food and Agriculture Organization of the United Nations (FAO), in a joint effort, organized a workshop on "Irrigation Scheduling: From Theory to Practice" on the occasion of the 46th Meeting of the ICID Executive Council in September 1995. The Workshop aimed specifically at bringing to light the various factors which determine the applicability of irrigation scheduling methods and included the respective constraints in relation to farmer information, on-farm irrigation systems, water supply systems and the socio-cultural and institutional environment. The selected papers of the workshop presented here are aimed at making the relevant information available to the technical and scientific community.

All papers have been peer-reviewed and edited to assure the scientific soundness of the contributions. The editors wish to thank the authors for their outstanding efforts in making available the wealth of information and their specific experiences, and in reviewing the papers of their colleague authors.

The editors:

M. Smith, FAO, Rome, Italy
L.S. Pereira, Lisbon University, Portugal
J. Berengena, University of Cordoba, Spain
B. Itier, Département de Bioclimatologie, France
J. Goussard, Grenoble, France
R. Ragab, Institute of Hydrology, Wallingford, UK
L. Tollefson, Canadian Water Resources Association, Canada
P. Van Hofwegen, Institute of Hydraulic Engineering, The Netherlands

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