Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Understanding how plants and soil release water into the air, a process called evapotranspiration (ET), is important for managing water resources. Over time, people have used different terms to describe ET, which can be confusing. This guide aims to define and explain ET in simple terms. It also recommends clear, standardized definitions to help everyone understand and talk about ET better. For farmers, a better understanding of ET means managing water for crops more effectively. This leads to higher yields and water savings. For water planners and scientists, it ensures everyone is on the same page when discussing water use and management. In some Western regions of the U.S., clear ET definitions help make fair and legal decisions about water use. This guide also discusses challenges and opportunities in using technology, like remote sensing, to monitor and manage ET over large areas. In summary, this guide is a useful tool for farmers, water managers, scientists, and policymakers. It helps them use and manage water more effectively and sustainably.

Technical Abstract: Evapotranspiration (ET), the combined process of evaporation from soil and plant surfaces and transpiration from plant tissue, plays a pivotal role in the global water and energy balances. Accurately quantifying ET at different spatial scales is important for diverse applications, including irrigation and natural resource management. While efforts to standardize ET methodology have progressed over the last few decades, some confusion and disagreement in terminology persist among communities of researchers and practitioners involved in the measurement, estimation, and simulation of ET. This technical note addresses the historical evolution and standardization of ET terminology, aiming to reduce and mitigate disparities in definitions and usage by advocating for standardized definitions and emphasizing the adoption of reference ET (ETref) terminology to avoid ambiguity. This document provides comprehensive definitions of key terms, including crop (i.e., vegetation cover) coefficients, consumptive use, actual crop evapotranspiration (ETa), and ETref variants for short (ETo) and tall (ETr) reference crops. Practical discussion on several relevant topics is given: 1) single and dual crop coefficient approaches, 2) applications to nonagricultural vegetation, 3) recommended subscripts for terms, 4) practical guidelines and considerations for ETref calculation, 5) encouragement to replace “potential ET” terminology with better terms, 6) clarification on ETmax and Kc max, 7) ET products derived from remote sensing, 8) a brief description of the role of ET in water rights, and 9) a figure illustrating use of the terms defined herein. The conclusion emphasizes the importance of consistent terminology for effective communication among researchers and end-users, which will facilitate the adoption of standardized ET methods and technologies. This technical note was created by the American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI), Evapotranspiration in Irrigation and Hydrology Committee with input and endorsement from other relevant organizations. This note serves as a comprehensive reference guide for ET practitioners and researchers.