(First edition: 1998, this reprint: 2004). This publication presents an updated procedure for calculating reference and crop evapotranspiration from meteorological data and crop coefficients. The procedure, first presented in FAO Irrigation and Drainage Paper No. 24, Crop water requirements, in 1977, allows estimation of the amount of water used by a crop, taking into account the effect of the climate and the crop characteristics. The publication incorporates advances in research and more accurate procedures for determining crop water use as recommended by a panel of high-level experts organised by FAO in May 1990. The first part of the guidelines includes procedures for determining reference crop evapotranspiration according to the FAO Penman-Monteith method. These are followed by updated procedures for estimating the evapotranspiration of different crops for different growth stages and ecological conditions.

Crop evapotranspiration : guidelines for computing crop water requirements

Prepared by the Task Committee on Standardization of Reference Evapotranspiration of the Environmental and Water Resources Institute of ASCE.

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The ASCE standardized reference evapotranspiration equation

A brief history of development of the 1985 Hargreaves equation and its comparison to evapotranspiration (ET) predicted by the Food and Agricultural Organization of the United Nations (FAO) Penman-Monteith method are described to provide background and information helpful in selecting an appropriate reference ET equation under various data situations. Early efforts in irrigation water requirement computations in California and other arid and semiarid regions required the development of simplified ET equations for use with limited weather data. Several initial efforts were directed towards improving the usefulness of pan evaporation for estimating irrigation water requirements. Similarity with climates of other countries allowed developments in California to be extended overseas. Criticism of empirical methods by H. L. Penman and others encouraged the search for a robust and practical method that was based on readily available climatic data for computing potential evapotranspiration or reference crop evapot...

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History and Evaluation of Hargreaves Evapotranspiration Equation

Irrigated agriculture is a vital component of total agriculture and supplies many of the fruits, vegetables, and cereal foods consumed by humans; the grains fed to animals that are used as human food; and the feed to sustain animals for work in many parts of the world. Irrigation worldwide was practiced on about 263 Mha in 1996, and about 49% of the world's irrigation occurred in India, China, and the USA. The objectives of this paper are to (i) review irrigation worldwide in its ability to meet our growing needs for food production, (ii) review irrigation trends in the USA, (iii) discuss various concepts that define water use efficiency (WUE) in irrigated agriculture from both engineering and agronomic viewpoints, and (iv) discuss the impacts of enhanced WUE on water conservation. Scarcely one-third of our rainfall, surface water, or ground water is used to produce plants that are useful to mankind. Without appropriate management, irrigated agriculture can be detrimental to the environment and endanger sustainability. Irrigated agriculture is facing growing competition for low-cost, high-quality water. In irrigated agriculture, WUE is broader in scope than most agronomic applications and must be considered on a watershed, basin, irrigation district, or catchment scale. The main pathways for enhancing WUE in irrigated agriculture are to increase the output per unit of water (engineering and agronomic management aspects), reduce losses of water to unusable sinks, reduce water degradation (environmental aspects), and reallocate water to higher priority uses (societal aspects).

Enhancing Water Use Efficiency in Irrigated Agriculture

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1834&context=usdaarsfacpub

Evapotranspiration information reporting: I. Factors governing measurement accuracy

Many forms of the Penman combination equation have been proffered for estimating daily evapotranspiration (ET) by the agricultural reference crops grass and alfalfa (Medicago sativa L.). This study was conducted to evaluate popular forms of the Penman equation, and to develop and evaluate general relationships for estimating daily average values of canopy and aerodynamic resistance parameters required by the Penman-Monteith equation. For simplicity and ease of use, resistance relationships were expressed as linear and logarithmic functions of mean plant height. The Penman-Monteith and other forms of the Penman equation were compared at 11 international lysimeter sites, with the Penman-Monteith method and a Penman equation with variable wind function developed at Kimberly, ID providing the best estimates of reference ET across the sites. Ratios of computed alfalfa to grass reference ET during peak months at various locations averaged 1.32, and ranged from 1.12 to 1.43. Values of computed ratios were related to local wind and humidity conditions. The development of relationships for canopy and aerodynamic resistances as functions of reference crop height allowed use of the Penman-Monteith equation in an operational mode, and improved transferability of this resistance form of the Penman equation to a wide variety of climates.

Operational estimates of reference evapotranspiration

Evapotranspiration information reporting: II. Recommended documentation

The ASCE Evapotranspiration in Irrigation and Hydrology Committee in cooperation with the Water Management Committee of the Irrigation Association has defined and established a standardized reference evapotranspiration (ET) equation. The purpose of the equation and standardized calculation of parameters is to bring a commonality to the methodology of reference ET and to the basis used to determine crop coefficients for both agricultural and landscape use. Issues and requirements in the selection and definition of the standardized procedure are that the procedure be understandable, defensible, accepted by science/engineering communities, relatively simple, and use existing and historical data and technology. The primary challenge was to select one or more equations that would satisfy the selection criteria. There have been traditionally two types of reference crops (grass and alfalfa) and six major reference equation types (Kimberly Penman, CIMIS Penman, FAO Penman, ASCE Penman-Monteith, FAO-56 Penman-Monteith and NRCS Penman-Monteith) have evolved over the past thirty years, each having large groups of followers and related crop coefficients. In addition, the reference crop is a living crop and therefore changes in its height, leaf area, stage of growth, age, variety, stomatal feedback, and irrigation frequency impact its ET rate. Other challenges include a myriad of procedures used to predict net radiation, vapor pressure deficit and other equation parameters. Hourly and daily computation are both in common usage, therefore, it is important that hourly calculations, when summed for 24 hours agree relatively closely with calculations made on daily time steps.

Marvin E. Jensen

Papers

Evapotranspiration information reporting: I. Factors governing measurement accuracy

Operational estimates of reference evapotranspiration

Evapotranspiration information reporting: II. Recommended documentation

Issues, requirements and challenges in selecting and specifying a standardized ET equation.

Design and operation of trickle (drip) systems.