Standard single and basal crop coefficients for field crops. Updates and advances to the FAO56 crop water requirements method

Authors: PEREIRA, LUIS - Universidade Nova De Lisboa; PARADES, PAULA - Universidade Nova De Lisboa; Hunsaker, Douglas - Doug; LOPEZ-URREA, RAMON - Provincial Technical Institute Of Agronomy (ITAP); MOHAMMADI SHAD, ZEINAB - University Of Arkansas

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2020
Publication Date: 10/3/2020
Citation: Pereira, L.S., Parades, P., Hunsaker, D.J., Lopez-Urrea, R., Mohammadi Shad, Z. 2020. Standard single and basal crop coefficients for field crops. Updates and advances to the FAO56 crop water requirements method. Agricultural Water Management. 243. Article 106466. https://doi.org/10.1016/j.agwat.2020.106466.
DOI: https://doi.org/10.1016/j.agwat.2020.106466

Interpretive Summary: “Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements,” better known as FAO56, was released to the scientific and technical community in 1998. The FAO56 crop coefficient method for estimating crop water use has been well-received over the past 21 years, is being used by irrigators worldwide, and is cited in technical papers over 18,000 times. However, since release, some users have requested additional validation of the crop coefficient values provided in the FAO56 tables. Many others have requested crop coefficient information for newer crops that have been developed in recent years. The objective of this paper is to deliver consolidated updates and advances on crop coefficients for all major field crops and for some new crops, like quinoa and camelina, not tabulated in the original FAO56. The methodology included an extensive review and analysis of hundreds of research papers on field crops that have been published since 1998. The results provide a straightforward and clear collection of crop coefficient and other upgrades to the FAO56 method for use into the extended future. This updated information provides growers guidance for irrigation decision-making and for increasing irrigation efficiencies.

Technical Abstract: The current study reviews the abundant research on FAO56 crop water requirements of field crops, with a focus on the crop coefficients published after issue of the FAO56 guidelines in 1998. The primary goal was to evaluate and update the single (Kc) and basal (Kcb) crop coefficients tabulated in FAO56 for field crops considering the published values generated in research papers over the past 21 years. The review has shown that the most popular approach for the determination of the crop evapotranspiration (ETc) in the irrigation and water management practices is the FAO56 Kc-ETo approach, i.e., the product of the specific crop coefficient by the reference evapotranspiration (ETo). The Kc-ETo approach requires appropriate use of the FAO56 PM-ETo equation and improved and updated knowledge of the Kc and Kcb values. The reviewed research provided various approaches to determine Kc, both in its single and dual versions, and has used a variety of field measurements of crop ET (ETc act), namely, by performing the soil water balance, using the Bowen ratio energy balance, with eddy covariance measurements, or with remote sensing surface energy balance models or vegetation indices. Particular attention was paid to evaluating the accuracy of measurements and models in determining crop ET, accuracy of the derived Kc values, and the accuracy in partitioning crop ET for deriving the basal Kcb. These accuracy requirements are fully discussed along with the upper limits for Kc values, and related causal errors. The conceptual approaches relative to crop coefficients and related requirements for transferability are also discussed, namely distinguishing between actual and standard Kc and focusing on the need to appropriately use the FAO segmented Kc curve. The published papers selected to update and consolidate mid-season and end-season Kc/Kcb used ETo computed with the FAO56 PM-ETo equation, adopted accurate approaches to determine and partition ETc act, and satisfied transferable, standard conditions of Kc/Kcb relative to the mid-season and end-season. The reported observed Kc and Kcb values were converted to the standard sub-humid climate as adopted in FAO56. These values were compared for each crop with the FAO56 tabulated Kc and Kcb to define the respective consolidated values tabulated in the current paper. The reported ancillary data, e.g., maximum root depth and crop height, and depletion fraction for no water stress, were also collected from the selected literature and, then, used to produce the updated tabulated values. The presentation of consolidated results is done by grouping the crops into grain legumes, fiber crops, oil crops, sugar crops, small grain cereals, maize and sorghum, and rice. Their updated Kc and Kcb values are generally close to those tabulated in FAO56, which confirms its excellency. The approach adopted for rice differs from FAO56 because consideration was given to the main water management practices currently used: flooded paddies, flooded with dry seeding, flooded with anticipated cutback, intermittent flooding, aerobic rice with surface and/or sprinkler irrigation, and rainfed paddy rice. Naturally, Kc/Kcb values for the initial season of rice were considered in addition to those for mid- and end-season. The review observed that many research papers did not satisfy the adopted requirements in terms of ETo method and/or the accuracy of crop ET determinations and, therefore, could not be used. Thus, an emphasis is again placed on adopting improved accuracy and quality control in future research aimed at determining appropriate crop coefficients comparable to these tabulated values. However, since the transferability of standard Kc and Kcb has been assured for the values tabulated herein, new research may serve better by focusing on improved management practices that lead to water savings, higher water productivi