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In view of the vital importance of water in man's life, precise knowledge of not only total rainfall but also of the utilisable or effective part of rainfall is essential for the economic and efficient planning of water resources. There is abundant literature on the subject of total rainfall. The topic of effective rainfall has remained somewhat neglected, because of the several disciplines involved. Literature on effective rainfall in general and on its role in agriculture in particular is limited.

This paper on concepts, assessment and the application of effective rainfall in irrigated agriculture has been prepared at the recommendation of the Ad Hoc Consultation Meetings on Crop Water Requirements held under the auspices of the Land and Water Development Division, FAO, Rome. Dr. N.G. Dastane, Project Co-ordinator, Indian Agricultural Research Institute, New Delhi, India, very willingly accepted the task of preparing this paper as FAO Consultant. Helpful suggestions Mere received from Panel Members: Messrs. A. Aboukhaled, Lebanon; G. van den Berg and P.E. Ritjema of the Netherlands; J. Damagnez, Prance; O.N. Ashford, WMO, Geneva; W.O. Pruitt, U.S.A.; and C.E. Houston and J. Doorenbos, FAO, Rome.

This publication presents a number of concepts of effective rainfall currently in use. A distinction is made between effective rainfall and the effectiveness of rainfall being two different entities. The definition of effective rainfall has been examined and a . modified concept of effective rainfall is proposed for use in irrigated agriculture. A brief history as well as the criteria for the assessment and factors influencing effective rainfall are presented. In addition, methods of measuring effective rainfall and its different components, as well as merits and limitations of each of these methods are discussed. Also the application of derived data on effective rainfall, together with several calculations and reference are given for guidance in developing and using the information in planning as wall as in practice of water management. Finally, a number of practices to increase effective rainfall are listed.

I am pleased to have this opportunity of expressing to Dr. Dastane and the members of the FAO Consultative Group on Crop Water Requirements my sincere appreciation for the time and effort they have devoted to the preparation of this valuable publication.

Edouard Saouma Director
Land and Water Development Division

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