ASCE Manual 70 – Second Edition: Evaporation, Evapotranspiration and Irrigation Requirements

Abstract. The 2nd edition of Manual 70 prepared by the ASCE Task Committee on Evapotranspiration (ET) in Irrigation and Hydrology and edited by Marvin E. Jensen and Richard G. Allen has been released in 2015 by the American Society of Civil Engineers (ASCE) after years of work by the editors and six member task committee. The second edition is a significantly expanded version of the first edition published in 1990. It includes expanded focus on subject areas of evaporation from water surfaces, reference crop methodology and standardization, crop coefficient methodology, direct ET methods, regional estimates, quality of ET measurements, selecting appropriate technology, and estimating irrigation requirements and stream flow depletion. Iterative techniques for solving the surface energy balance for the ET component that are now possible with modern computers are introduced as effective replacements for direct applications of the Penman-Monteith equation. The iterative methods resolve inaccuracies associated with simplifying assumptions associated with the development of the Penman-Monteith equation. The appendices have been expanded with more detail on crop coefficients and an appendix on weather data integrity has been added. The list of references has been expanded to cover a wide range of modern citations from the evaporation and ET literature. The second edition incorporates material from Chapter 4 “Evaporation and Evapotranspiration” of the 1996 ASCE Hydrology Handbook and from the 2005 ASCE report on “Standardization of Reference Evapotranspiration Calculation.” The second edition summarizes inter-comparison of reference ET methods and measurements that was an important basis of the first edition. This ASCE practices manual is intended to be used by consulting engineers working on water rights and hydrologic issues and by instructors in agricultural and civil engineering, environmental sciences, and agronomy. It is also intended to serve as a primary reference for agricultural, environmental and engineering students and professionals in water-related studies. This brief paper highlights some aspects of the revised manual.