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Many research papers on crop water requirements of vegetables have been produced since the publication of the FAO56 guidelines in 1998. A review of this literature has shown that determination of crop evapotranspiration (ET c ) using the K c -ET o approach, i.e., the product of the specific crop coefficient (K c ) by the reference evapotranspiration (ET o ), is the most widely-used method for irrigation water management. Consequently, a review was made to provide updated information on the K c values for these crops. The reviewed research provided various approaches to determine K c in its single and dual versions. With this purpose, actual crop ET (ET c act ) was determined with lysimeters, or by performing the soil water balance using measured soil water content and computational models, or by using Bowen ratio energy balance and eddy covariance measurements, or by using remote sensing applications. When determining the basal K c (K cb ), the partitioning of ET c act was evaluated using different approaches, though mainly using the FAO56 dual K c method. Since the accuracy of experimentally-determined K c and K cb values depends upon the procedure used to compute ET o , as well as accuracy in determining and partitioning of ET c act , the adequacy of the measurement requirements for each approach was carefully reviewed. The article discusses in detail the conceptual methodology relative to crop coefficients and the requirements for transferability, namely distinguishing between actual and standard K c and the need to appropriately use the FAO segmented K c curve. Hence, the research papers selected to update and consolidate mid-season and end-season standard K c and K cb were those that computed ET o with the FAO56 PM-ET o equation; and that also used accurate approaches to determine and partition ET c act for pristine, non-stressed cropping conditions. Under these experimental conditions, the reported K c and K cb values relative to the mid-and end-season could be considered as transferable standard K c and/or K cb values after adjustment to the standard climate adopted in FAO56, where average RH min = 45% and average u 2 = 2 m s −1 over the mid-season and late season growth stages. For each vegetable crop, these standard values were then compared with the FAO56 tabulated K c and K cb values to define the updated values tabulated in the current article. In addition, reported ancillary data, such as maximum root zone depth, maximum crop height, and soil water depletion fraction for no water stress, were also collected from selected papers and tabulated in comparison with those given for the crops in FAO56. The presentation of updated crop coefficient results is performed by grouping the vegetables differently than in FAO56, where distinction is made according to their edible parts:(1) roots, tubers, bulbs and stem vegetables; (2) leaves and flowers vegetables; (3) fruit and pod vegetables; and (4) herbs, spices and special crops, with most of them being newly introduced herein. The updated K c and K cb of veget...

This study reviews the abundant research on FAO56 crop coefficients, published following introduction of the FAO56 paper in 1998. The primary goal was to evaluate, update, and consolidate the mid-season and end-season single (K c ) and basal (K cb ) crop coefficients, tabulated for many field crops in FAO56. The review found that the prevalent approach for estimating crop evapotranspiration (ET c ) is the FAO56 K c -ET o approach, i.e., the product of the K c and reference evapotranspiration (ET o ). The FAO56 K c -ET o approach requires use of the FAO56 PM-ET o grass reference equation with appropriate crop-specific K c and/or K cb . Reviewed research provided various approaches to determine K c and K cb and used a variety of actual crop ET (ET c act ) measurements. Significant attention was placed on accessing the accuracy of the field measurements and models used in these studies. Accuracy requirements, upper limits for K c values, and related causal errors are discussed. Conceptual approaches relative to K c transferability requirements are provided with focus on standard crop conditions and use of the FAO56 segmented K c curve. Papers selected to update K c ∕K cb used the FAO56 PM-ET o , provided accurate measurements to determine and partition ET c act , and satisfied transferability requirements. Selected observed K c and K cb values were converted to standard, sub-humid climate as adopted in FAO56. Observed values, with respect to tabulated FAO56 K c and K cb , were used in consolidating updated values for crops within general categories of grain legumes, fiber crops, oil crops, sugar crops, small grain cereals, maize and sorghum, and rice. Ancillary data, e.g., maximum root depth and crop height, were also collected from selected literature and tabulated. Results showed good agreement between updated and original tabulated FAO56 K c and K cb , confirming the reliability of the FAO56 values. This indicates change in the K c (ET c /ET o ratio) of crops has not occurred due to climate change during the past ≈sixty years. New K c ∕K cb data for crops, not included in FAO56, are also now presented for several oil crops and pseudocereals. The approach adopted for rice differs from FAO56 because consideration was given to the numerous rice water management practices currently used and, thus, K c ∕K cb values for the initial season of rice were also presented. The review also observed that many research papers did not satisfy the adopted requirements in terms of ET o method and/or the accuracy of ET c act determinations and, therefore, could not be used. Thus, emphasis is placed on adopting improved accuracy and quality control in future research aimed at determining K c data comparable to presented values. The transferability of standard K c and K cb has been assured for the values tabulated herein. Improved future applications of the FAO56 K c -ET o method should consider remote sensing observations when available, particularly in defining crop growth stages at given locations.

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