Average Crop Coefficients

The Kc Curve

A simple procedure is used to construct the Kc curve for the complete growing period. This procedure was first presented by Doorenbos and Pruitt [55]. The Kc curve, such as that shown in Fig. 5.5, is constructed using the following three steps:

1. Divide the growing period into four general growth stages that describe crop phenology or development, and determine the lengths of these stages, in days. The four stages are initial period, crop development period, midseason period, late-season period.

2. Identify the three Kc values that correspond to Kcini, Kcmid and Kcend.

3. Connect straight-line segments through each of the four growth-stage periods. Horizontal lines are drawn through Kcini in the initial period and through Kcmid in the midseason period. Diagonal lines are drawn from Kcini to Kcmid within the course of the development period and from Kcmid to Kcend within the course of the late-season period.

Lengths of Crop Growth Stages

The four crop growth stages are characterized in terms of benchmark crop growth stages or cultivation practices:

1. Initial period—planting date to approximately 10% ground cover. The length of the initial period is highly dependent on the crop, the crop variety, the planting date, and the climate. For perennial crops, the planting date is replaced by the green-up date, that is, the time when initiation of new leaves occurs.

2. Crop development—10% ground cover to effective full cover. Effective full cover for many crops occurs at the initiation of flowering or when LAI reaches 3. For row crops, effective cover can be defined as the time when leaves of plants in adjacent rows begin to intermingle or when plants reach nearly full size if no intermingling occurs. For tall crops, it occurs when the average fraction of the ground surface covered by vegetation is about 0.7 to 0.8. For densely sown vegetation, such as winter and spring cereals and grasses, effective full cover corresponds approximately to the stage of heading.

3. Midseason—effective cover to start of maturity. The start of maturity is often indicated by the beginning of the aging, yellowing, or senescence of leaves, leaf drop, or the browning of fruit.

4. Late season—start of maturity to harvest or full senescence. For some perennial vegetation in frost-free climates, crops may grow year round so that the date of termination may be taken as the same as the date of "planting."

The rates at which vegetation cover develops and the time at which it attains effective full cover are affected by weather conditions, and especially by mean daily air temperature [85]. Therefore, the length in time between planting and effective full cover will vary with climate, latitude, elevation, planting date, and cultivar (crop variety). Generally, once the effective full cover for a plant canopy has been reached, the rate of further phenological development (flowering, seed development, ripening, and senescence) is more dependent on plant genotype and is less dependent on weather [71]. In some situations, the time of emergence of vegetation and the time of effective full cover can be predicted using cumulative degree-based regression equations or by more sophisticated plant growth models [85-89].

The lengths of the initial and development periods may be relatively short for deciduous trees and shrubs that can develop new leaves in the spring at relatively fast rates. The Kcini selected for trees and shrubs should reflect the ground condition prior to leaf emergence or initiation because Kcini is affected by the amount of grass or weed cover, soil wetness, tree density, and mulch density.

The length of the late-season period may be relatively short (<10 days) for vegetation killed by frost or for agricultural crops that are harvested fresh. The value for Kcend after full senescence or following harvest should reflect the condition of the soil surface (average water content and any mulch cover) and the condition of the vegetation following plant death or harvest.

Indicative lengths of growth stages are given in FAO Papers [55, 90, 91]. Local observations of the specific stage of plant development should be used, whenever possible, to incorporate effects of plant variety, climate, and cultural practices. Local information can be obtained by interviewing farmers, ranchers, agricultural extension agents, and local researchers, by conducting local surveys, or by remote sensing. When determining stage dates from local observations, the preceding guidelines and visual descriptions may be helpful.

Crop Coefficients

Values for Kcini, Kcmid, and Kcend are listed in Table 5.1 for various agricultural crops. Most of the Kc values in Table 5.1 were taken from Doorenbos and Pruitt [55]

and Doorenbos and Kassam [90] after some modification. Additional information was obtained from Wright [71], Pruitt [92], Snyder etal. [93, 94], Jensen etal. [2], and other sources.

The Kc values in Table 5.1 are organized by crop group type. Usually there is close similarity in Kc within the same crop group because the plant height, leaf area, ground coverage, and water management are usually similar. For several group types, only one value for Kcini is listed and is considered to be representative of the whole group. The Kcini value in Table 5.1 is only approximate because Kcini can vary widely with soil wetting conditions, irrigation, and rainfall.

The Kc values in Table 5.1 represent potential water use by healthy, disease-free, and densely planted stands of vegetation and with adequate levels of soil water. When stand density, height, or leaf area are less than that attained under perfect or normal (pristine) conditions, the value for Kc should be reduced. The reduction in Kcmid for poor stands can be as much as 0.3-0.5 and should be made according to the amount of effective (green) leaf area relative to that for healthy vegetation having normal planting densities.

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