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

Abstract

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.

Prediction of crop coefficients from fraction of ground cover and height: Practical application to vegetable, field and fruit crops with focus on parameterization

“…Crop DOI of original article: https://doi.org/10.1016/j.agwat.2020.106197. coefficient curves have been developed and reported for a wide range of agricultural crops (Allen et al, 1998(Allen et al, , 2007Jensen and Allen, 2016) and their standard values were recently updated for vegetable and field crops (Pereira et al, 2021a(Pereira et al, , 2021b, as well as for fruit tree and vine crops (Rallo et al, 2021). Standard K c represents the relative fraction of ET o that is governed by the amount, type and condition of a given crop type under standard, pristine conditions, and is regarded as generally transferable among regions, subject to adjustment for local climate, under the assumption that the ET o accounts for nearly all weather-related ET c variation.…”

“…The derivation of K c and K cb values from field data requires that ET c is accurately determined as discussed in the recent K c reviews (Pereira et al, 2021a(Pereira et al, , 2021bRallo et al, 2021). Those field-based ET c measurements are appropriate for research purposes but are generally impractical for grower use due to complexity and expense.…”

“…Values reported in literature of M L and F r were analysed. Optimal M L and F r values were derived by a numerical search that minimized the differences between K cb A&P with standard K cb for vegetable, field, and fruit crops as tabulated by Pereira et al (2021aPereira et al ( , 2021b and Rallo et al (2021). Sources for f c were literature reviews supplemented by a remote sensing survey.…”

Estimating Processing Tomato Water Consumption, Leaf Area Index, and Height Using Sentinel-2 and VENµS Imagery

“…Where water is a limited resource, optimal water management is vital for food security. Crop coefficient (K c )-based estimation of crop water consumption is one of the most commonly used irrigation management methods [3,4]. K c is defined as the ratio between the actual evapotranspiration from a crop field and the environmental evaporative demand [3].…”

Balanço Hídrico Do Solo E Uso Da Água No Trigo (Triticum Aestivum L.): Um Caso De Estudo Em Condições Mediterrânicas

“…As perdas de água por evaporação, processo essencialmente físico, verificamse sobretudo a partir da superfície do solo; as perdas por transpiração, processo essencialmente fisiológico, ocorrem a partir dos estomas das plantas. Dado serem diferentes as características da atividade fisiológica dos vários tipos de plantas, bem como as condições de exploração e do meio (tipo de solo, rega, fertilizações, controlo de doenças), são estabelecidos vários conceitos de evapotranspiração (Allen et al, 1998;Pereira et al, 1999Pereira et al, , 2021): (i) a evapotranspiração de referência (ETo) corresponde à taxa de evapotranspiração de uma cultura de referência hipotética muito semelhante à evapotranspiração de uma extensa superfície de relva verde de altura uniforme, crescendo ativamente, cobrindo totalmente o solo e bem provida de água; a evapotranspiração da cultura (ETc) é a taxa de evapotranspiração de uma cultura livre de doenças, bem (Monteith and Unsworth, 1990), e nos coeficientes Kc, que integram todas as diferenças físicas e fisiológicas das plantas cultivadas, com enfoque na separação dos processos de evaporação (Ke) e transpiração (Kcb) (Allen et al, 1998 (Allen et al, 1998):…”