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These guidelines constitute the efforts of eight years of deliberations and consultations by the authors, who together formed the working group to pursue the recommendations of the FAO expert consultation that was held in May 1990 in Rome. The consultation was organized to review the then current FAO guidelines to determine Crop Water Requirements, published in 1977 as FAO Irrigation and Drainage paper No. 24 (FAO-24) and authored by J. Doorenbos and W. Pruitt. The conceptual framework for the revised methodologies introduced in this publication came forth out of the advice of the group of eminent experts congregated in the 1990 meetings and who have importantly contributed to the development of the further studies conducted in the framework of the publication. Members of the 1990 FAO expert consultation included Dr P. Fleming of Australia, Dr A. Perrier of France, Drs L. Cavazza and. L. Tombesi from Italy, Drs R. Feddes and J. Doorenbos of the Netherlands, Dr L. S. Pereira of Portugal, Drs J. L. Monteith and H. Gunston from the United Kingdom, Drs R. Allen, M. Jensen and W. O. Pruitt of USA, Dr D. Rijks from the World Meteorological Organization and various staff of FAO.

Many other experts and persons from different organizations and institutes have provided, in varying degrees and at different stages, important advice and contributions. Special acknowledgements for this are due in particular to Prof. W. O. Pruitt (retired) of the University of California, Davis and J. Doorenbos of FAO (retired) who set the standard and template for this work in the predecessor FAO-24, and to Prof. J. L. Monteith whose unique work set the scientific basis for the ETo review. Prof. Pruitt, despite his emeritus status, has consistently contributed in making essential data available and in advising on critical concepts. Dr James L. Wright of the USDA, Kimberly, Idaho, further contributed in providing data from the precision lysimeter for several crops. Further important contributions or reviews at critical stages of the publication were received from Drs M. Jensen, G. Hargreaves and C. Stockle of USA, Dr B. Itier of France, and various members of technical working groups of the International Commission on Irrigation and Drainage (ICID) and the American Societies of Civil and Agricultural Engineers.

The authors thank their respective institutions, Utah State University, Instituto Superior de Agronomia of Lisbon, Katholieke Universiteit Leuven and FAO for the generous support of faculty time and staff services during the development of this publication.

The authors wish to express their gratitude to Mr H. Wolter, Director of the Land and Water Development Division for his encouragement in the preparation of the guidelines and to FAO colleagues and others who have reviewed the document and made valuable comments.

Special thanks are due to Ms Chrissi Redfern for her patience and valuable assistance in the preparation and formatting of the text. Mr Julian Plummer further contributed in editing the final document.

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