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

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 (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...

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

“…The selection of F r and M L values is performed through a numerical search for the values of the parameters that make K cb A&P to match the tabulated K cb TAB values. The K cb A&P are those computed with the A&P approach and the K cb TAB are those tabulated by Pereira et al (2021aPereira et al ( , 2021b and Rallo et al (2021). The procedure consists of: 1.…”

“…The target standard K cb TAB values for vegetable, field, tree and vine crops have been reviewed and updated in three papers of the current Special Issue (Pereira et al, 2021a(Pereira et al, , 2021bRallo et al, 2021). Ancillary data published in these papers, as well as f c and h data published in Allen and Pereira (2009), were used to define the fraction of ground cover and crop height used with the current application of the A&P approach.…”

“…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.…”