Evapotranspiration Terminology and Definitions

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 balance. Accurately quantifying ET at various 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 disagreements 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 (ET ref) terminology to promote consistency and accuracy and to avoid ambiguity. This document provides comprehensive definitions of key terms, including crop (i.e., vegetation cover) coefficients, consumptive use (CU), actual crop evapotran-spiration (ET a), and ET ref variants for short (grass, ET o) and tall (alfalfa, ET r) 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 ET ref calculation, (5) encouragement to replace "potential ET" terminology with better terms, (6) clarification on maximum ET (ET max) and maximum crop coefficient (K c max) terms, (7) ET products derived from remote sensing, (8) a brief description of the role of ET in water rights, and (9) a figure illustrating the 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 in the United States and internationally. This note serves as a comprehensive reference guide for ET practitioners and researchers.. This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/. Practical Applications: Understanding evapotranspiration (ET) is crucial for the effective management of water as a natural resource for agriculture and other managed landscapes. The overall motivation of this technical note is to provide practical insights and recommendations on the terminology used in ET applications, fostering clearer communication and establishing a foundation for discussion among stakeholders in the field. This technical note addresses historical discrepancies in the use of terminology by clarifying the terms and recommending standardized definitions, thereby providing a reliable foundation for communications in water resource planning and irrigation management. The use of standardized reference ET terminology is emphasized to avoid ambiguity and to enhance clarity of communication among stake-holders. For farmers and landscapers, this translates to improved water management and enhanced irrigation scheduling, optimizing yields or providing acceptable landscape appearance while conserving water resources. Water planners, hydrologists, and modelers will benefit from consistent ET communication within their professional communities. Water rights systems, especially in regions governed by the Prior Appropriation Doctrine or in the case of interstate compacts, will benefit from accurate and clearly defined consumptive use estimations, ensuring fair and legal water allocations. Consistent and clear definitions will help policymakers and regulatory agencies clearly understand the complex nature of ET, its quantification methods, and the diverse applications of ET information. Additionally, this technical note discusses the challenges in remote sensing of ET, offering opportunities for technological advancements in monitoring and managing water consumption at larger scales. Ultimately, this technical note offers substantive information to foster a common language for practitioners, policymakers, and ET researchers regarding sustainable water use and management across diverse managed landscapes and natural ecosystems and for use in hydrologic system design.