Evapotranspiration computing (ETo)
Definition / description
Evapotranspiration (ET) consists of two processes which occur simultaneously: (1) evaporation, whereby liquid water is converted to water vapour and removed from an evaporating surface; and (2) transpiration, whereby water contained within plant tissues is transferred to the atmosphere, predominantly through stomata, through which water vapour and gases are exchanged with the atmosphere. In practice, evapotranspiration is generally related to a defined reference surface, represented by a hypothetical grass reference crop with specific characteristics, and expressed as reference evapotranspiration (ETo), also referred to as reference crop evapotranspiration. ET is commonly computed from weather data. A large number of empirical or semi-empirical equations have been developed for assessing crop or reference evapotranspiration from meteorological data. Reference evapotranspiration (ETo) then provides the basis for computing crop evapotranspiration (ETcrop) using a specific crop coefficient (Kc), expressed as: ETcrop = ETo × Kc
Source: [References]
Publications / information
- Hess, T.M. and T.-M. Stein, 2005:
DailyET - A simple ‘calculator’ for estimating daily reference crop evapotranspiration (ETo).-
IRRISOFT Software Descriptions and Reviews at Sakia.org.
VL Irrigation & Hydrology - vl_ref_id_x_0035
Reference
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Allen, R.G., 2002:
Comparison of Cumulative Evapotranspiration at Kimberly, Idaho using Mean Crop Coefficients from Table 6.9 of ASCE Manual 70 for Years of Measurement by Crop and the 1982 Kimberly Penman and ASCE Standardized Penman-Monteith Reference Methods. -
University of Idaho, Kimberly, Idaho 83301. -
Allen, R.G, Pereira, L.S., Raes, D. and M. Smith, 1998:
Crop evapotranspiration - Guidelines for computing crop water requirements. -
FAO Irrigation and drainage paper 56, FAO, Rome. -
Allen, R.G, and Wright,J.L, 2004:
Evaporation Component of ET Estimated from Daily Lysimeter Measurement at Kimberly, Idaho. -
Winrock Water Files. -
Burnett, B., Allen, R.G, Robison, C.W, Tasumi, M., and I. Lorite, 2008:
Estimating the Soil Surface Evaporation and Transpiration Components from Satellite Images in the Absence of a Thermal Band. -
World Environmental and Water Resources Congress 2008: Ahupua’A. -
Dorenbos, J. and W.O. Pruitt, 1977 and 1992:
Crop Water Requirements - Guidelines for Predicting Crop Water Requirements. - FAO Irrigation and Drainage Paper 24, FAO, Rome. -
ICID-CIID, 2000:
Multilingual Technical Dictionary on Irrigation and Drainage. -
CD Version September 2000, International Commission on Irrigation and Drainage, New Dehli. -
Land and Water Development Division, FAO:
CROPWAT - A computer program for irrigation planning and management. -
(free to download), for MS Windows and DOS, FAO, Rome. -
Pereira, L.S., Allen, R.G., Paredes, P., López-Urrea, R., Raes, D., Smith, M., Kilic, A. and M. Salman, 2025:
Crop evapotranspiration – Guidelines for computing crop water requirements. -
Second edition, revised 2025. FAO Irrigation and Drainage Paper, No. 56 Rev.1. Rome, FAO. -
USDA, Soil Conservation Service, 1993:
Irrigation Water Requirements. -
National Engineering Handbook (NEH), Part 623, Chapter 2, National Technical Information Service. -
WCA infoNET:
Topic tree and sources: Crop water requirements. -
WCA infoNET - A Knowledge and Information Dissemination System on Water Conservation and Use in Agriculture, IPTRID/FAO. (discontinued/offline)