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INTRODUCTION TO SECOND EDITION

In 1990, the American Society of Civil Engineers (ASCE) published the first edition of Manuals and Reports on Engineering Practice No. 70, which was prepared by the Irrigation Water Requirements Committee of the Irrigation and Drainage Division. In 1995, the Committee was renamed the "Evapotranspiration in Irrigation and Hydrology Technical Committee" to include evapotranspiration on nonirrigated lands. The mission of the committee is to advance the science and practice of measuring and estimating evapotranspiration with emphasis on developing and evaluating procedures and tools for integrated water management needed in planning, design, and management of irrigation projects, hydrologic units, and water resources systems. In 2000, when the stock of printed copies had been sold, the Committee appointed a Task Committee to prepare an updated revised edition of the manual

Since its publication, Manual 70 has served as a valuable reference for specialists working in agrometeorology, hydrology, and irrigation planning and management. The first edition contained equations for estimating evapotranspiration, both theoretical and empirical, and equations to calculate parameters in the ET equations and for various components of the energy balance. Because the general knowledge base of users has improved greatly over the past two decades, the manual has been restructured for greater ease in applying new technology and standardized calculations, and new information has been included. The manual's current focus is on state-of-the-art technology with less emphasis on older empirical equations and comparisons among various methods of estimating evapotranspiration (ET).

This revised edition includes extensive material from the ASCE Hydrology Handbook (Allen et al. 1996), which was a product of the previously noted ET technical committee. Additional material on the standardization of reference ET calculation and clarification of its application developed by a task committee of the ET committee from 1999 through 2004 (ASCE 2005) has been brought into this edition. Because integrated water management often involves estimating evaporation from water bodies such as storage and regulating reservoirs, new material on estimating evaporation (E) from water surfaces has been added.

The revised edition presents various methods for estimating ET and example calculations as a guide to users in applying the new technology. Graduate students preparing to enter one of these fields should find this manual to be useful as a source of basic information. Tables of basic air and water properties, conversion factors, and meteorological tables are retained for reference purposes.