More and more evapotranspiration models, evapotranspiration crop coefficients and associated measurements of evapotranspiration (ET) are being reported in the literature and used to develop, calibrate and test important ET process models. ET data are derived from a range of measurement systems including lysimeters, eddy covariance, Bowen ratio, water balance (gravimetric, neutron meter, other soil water sensing), sap flow, scintillometry and even satellite-based remote sensing and direct modeling. All of these measurement techniques require substantial experimental care and are prone to substantial biases in reported results. Reporting of data containing measurement biases causes substantial confusion and impedance to the advancement of ET models and in the establishment of irrigation water requirements, and translates into substantial economic losses caused by misinformed water management. Basic principles of ET measuring systems are reviewed and causes of common error and biases endemic to systems are discussed. Recommendations are given for reducing error in ET retrievals. Upper limits on ET measurements and derived crop coefficients are proposed to serve as guidelines. The descriptions of errors common to measurement systems are intended to help practitioners collect better data as well as to assist reviewers of manuscripts and users of data and derived products in assessing quality, integrity, validity and representativeness of reported information. This paper is the first part of a two-part series, where the second part describes recommendations for documentation to be associated with published ET data.

This symposium is devoted to understanding mechanisms that could achieve the higher WUE that will allow Irrigated agriculture is a vital component of total agriculture and the world's food production to keep pace with its growsupplies many of the fruits, vegetables, and cereal foods consumed ing population, if that is even possible. Sinclair et al. by humans; the grains fed to animals that are used as human food;(1984) described WUE on various scales from the leaf 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 to the field. In its simplest terms, it is characterized as about 49% of the world's irrigation occurred in India, China, and the crop yield per unit of water use. At a more biological USA. The objectives of this paper are to (i) review irrigation worldlevel, it is the carbohydrate formed through photosynwide in its ability to meet our growing needs for food production, (ii) thesis from CO 2 , sunlight, and water per unit of transpireview irrigation trends in the USA, (iii) discuss various concepts that ration. Brown (1999) has proposed that the upcoming define water use efficiency (WUE) in irrigated agriculture from both benchmark for expressing yield may be the amount of engineering and agronomic viewpoints, and (iv) discuss the impacts water required to produce a unit of crop yield, which of enhanced WUE on water conservation. Scarcely one-third of our is simply the long-used transpiration ratio, or the inverse rainfall, surface water, or ground water is used to produce plants that of WUE. Often the term WUE becomes confounded are useful to mankind. Without appropriate management, irrigated when used in irrigated agriculture. Bos (1980Bos ( , 1985 agriculture can be detrimental to the environment and endanger sustainability. Irrigated agriculture is facing growing competition for low-recommended that WUE for irrigation be based on cost, high-quality water. In irrigated agriculture, WUE is broader in the yield produced above the rainfed or dryland yield scope than most agronomic applications and must be considered on divided by the net evapotranspiration (ET) difference a watershed, basin, irrigation district, or catchment scale. The main for the irrigated crop, which he called the yield/ET ratio. pathways for enhancing WUE in irrigated agriculture are to increaseHe also proposed the irrigated difference from the drythe output per unit of water (engineering and agronomic management land yield divided by the gross applied water, which he aspects), reduce losses of water to unusable sinks, reduce water degracalled the yield/water-supply ratio and is referred to as dation (environmental aspects), and reallocate water to higher priority irrigation WUE (I WUE ) in this paper. These definitions uses (societal aspects).are attractive but difficult to apply because many management factors such as fertility, variety, pest management, sowing date, soil water content at planting, plant-T.A. Howell, USDA-ARS, Conserv. and Productio...

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electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. M aize is one of the most important crops known to humankind, accounting for nearly 30% of the total global grain production. Th e crop is cultivated on more than 142 million ha of land worldwide, producing over 637 million Mg of grain (FAO, 2003). Recently, demand for maize is increasing as it is used to produce ethanol as biofuel, besides being a staple food in many countries and a feed for livestock in the form of forage, silage, or grain. Th e strong demand is putting tremendous pressure on production, hence, competition for available water. At the same time, it increases the price of maize, which in turn has raised food prices in general. Improving the WUE for maize production is therefore of paramount importance to obtain "more crop per drop" with declining worldwide irrigation resources and the uncertainty in precipitation from global climate change.Simulation models that quantify the eff ects of water on yield at the farm level can be valuable tools in water and irrigation management. In the case of maize, many such models have been tested: for example, the CERES-Maize model (Jones and Kiniry, 1986), the Muchow-Sinclair-Bennett (MSB) model (Muchow et al., 1990), the EPICphase model (Cavero et al., 2000), CropSyst (Stöckle et al., 2003), and the Hybrid-Maize model (Yang et al., 2004). Most of these models, however, are quite sophisticated, demanding advanced skills for their calibration and operation, and require large number of parameters; some are so cultivar-specifi c they are not easily measured or accessible to end-users.Th e newly developed AquaCrop model Steduto et al., 2009) is a user-friendly and practitioner-oriented type of model, as it maintains an optimal balance between accuracy, robustness, and simplicity, and requires a relatively small number of parameters. AquaCrop has been parameterized and tested on maize using six seasons' data collected at the University of California Davis, . Th at study showed that AquaCrop was able to properly simulate the CC, biomass development, and grain yield of four maize cultivars over six diff erent crop seasons diff ering in plant density, planting date, and atmospheric evaporative demand, with irrigation treatments that withheld the water up to tasseling, from tasseling onward, intermittently, or completely, under conditions of little rainfall but with the soil at or near fi eld capacity at planting time.AquaCrop simulates the crop green foliage CC (not leaf area index, LAI) from crop emergence through the development and senescence of the canopy. Th e CC and the reference evapotranspiration (ET o ) are then used with the crop coeffi cient for ABSTRACT Accurate crop development models are important tools in evaluating the eff ects of water defi cits on crop yield or productivity. Th e FAO AquaCrop model predicts crop productivity, water requirement, and water use effi ciency (WUE) under water-limiti...