Standard single and basal crop coefficients for vegetable crops, an update of FAO56 crop water requirements approach

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

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2020
Publication Date: 9/24/2020
Citation: Pereira, L.S., Parades, P., Lopez-Urrea, R., Hunsaker, D.J., Mota, M., Mohammadi Shad, Z. 2020. Standard single and basal crop coefficients for vegetable crops, an update of FAO56 crop water requirements approach. Agricultural Water Management. 243. Article 106196. https://doi.org/10.1016/j.agwat.2020.106196.
DOI: https://doi.org/10.1016/j.agwat.2020.106196

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 20 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 vegetable crops and for some other vegetables not tabulated in the original FAO56. The methodology included an extensive review and analysis of hundreds of research papers on vegetable crop coefficients that have been published since 1998. The results provide a straightforward and clear collection of vegetable crop coefficient upgrades to the FAO56 method for use into the extended future. This updated information provides vegetable growers guidance for irrigation decision-making and for increasing irrigation efficiencies.

Technical Abstract: 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 (ETc) using the Kc-ETo approach, i.e., the product of the specific crop coefficient (Kc) by the reference evapotranspiration (ETo), is the most widely-used method for irrigation water management. Consequently, a review was made to provide updated information on the Kc values for these crops. The reviewed research provided various approaches to determine Kc in its single and dual (Kcb) versions. With this purpose, actual crop ET (ETc 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 Kc (Kcb), the partitioning of ETc act was evaluated using different approaches, though mainly using the FAO56 dual Kc method. Since the accuracy of experimentally-determined Kc and Kcb values depends upon the procedure used to compute ETo, as well as accuracy in determining and partitioning of ETc 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 Kc and the need to appropriately use the FAO segmented Kc curve. Hence, the research papers selected to update and consolidate mid-season and end-season standard Kc and Kcb were those that computed ETo with the FAO56 PM-ETo equation; and that also used accurate approaches to determine and partition ETc act for pristine, non-stressed cropping conditions. Under these experimental conditions, the reported Kc and Kcb values relative to the mid- and end-season could be considered as transferable standard Kc and/or Kcb values after adjustment to the standard climate adopted in FAO56, where average RHmin = 45% and average u2 = 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 Kc and Kcb values to define the consolidated values tabulated in the current article. In addition, reported ancillary data, such as maximum root zone depth, maximum crop height, and 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 consolidated crop coefficient results is performed by grouping the vegetables differently than in FAO56, were distinction is made according to their edible parts: 1) roots, tubers, bulbs and stem vegetables; 2) leaves and flowers vegetables; 3) fruits and pods vegetables; and 4) herbs, spices and special crops, with most of them being newly introduced herein. The updated Kc and Kcb of vegetable crops based on this review are generally coincident with those in FAO56, although slightly lower for several crops. Close agreement of selected paper values with FAO56 values provides good evidence of their quality and also confirms the reliability of the original FAO56 tabulated values. It is noteworthy that many papers surveyed from the past 20 years did not satisfy the adopted Kc requirements in terms of ETo computation method nor provide solid evidence of measurement accuracy for ETc act. It is recommended that future Kc research of vegetables should sufficiently address these issues with objectives broadened to provide more transferable data to other regions. Also, new data on vegetable Kc and Kcb values should be carefully scrutinized in the context of these results and those provided in FAO56.