Crop Coefficient Concept

The Kc is the crop coefficient for a given crop and is usually determined experimentally. The Kc values represent the integrated effects of changes in leaf area, plant

Table 1 Values for Cn and Cd in eqn [2]

Grass reference (ETo)

Alfalfa reference (ETr)

Time step

Units for ETo and ETr

Units for Rn and G

Daily

900

0.34

1600

Hourly during daytime

37

0.24

66

Hourly during nighttime

37

0.96

1.20

1.00

0.20

Initial

Crop development

Mid-season

Late season

Mid-season

Late season

Time of season (days or weeks after planting) Figure 2 Schematic representation of increase and decrease in crop coefficient based on different plant development stages.

height, crop characteristics, irrigation method, rate of crop development, crop planting date, degree of canopy cover, canopy resistance, soil and climate conditions, and management practices. Each crop will have a set of specific crop coefficient and will predict different water use for different crops for different growth stages. An example of a Kc curve as a function of days or weeks after planting for a plant for initial, development, mid-season, and end-season stages is given in Figure 2.

In general, crop growth stages can be divided into four main growth stages: initial, crop development, mid-season, and late season. The length of each of these stages depends on the climate, latitude, elevation, planting date, crop type, and cultural practices. Local field observations are best for determining the growth stage of the crop and adjust the empirical Kc values accordingly. Early in the growing season, during the crop germination and establishment stage, most of the ET occurs as evaporation from the soil surface. As the crop canopy develops and covers the soil surface, evaporation from the soil surface decreases and transpiration component of the ET increases.

Early in the season when plant is small, the water-use rate and Kc value are also small (Kc initial stage) and the crop ET rate increases as the plant develops (Figure 2). For agronomical plants, the crop ET rate is at the maximum level when plant is fully developed (Kc mid-season). The ET rate decreases again when plant completes development and reaches physiological maturity towards the end of the season (Kc end season).

For perennial crops a similar pattern can occur as the plant starts to develop canopy area, grow new shoots, and develop fruit. The percentage of leaf area, soil water status, and climatic conditions will drive the rate of crop (ET) at a given growth stage. Usually, the maximum canopy cover coincides with the time of year when the solar radiation and temperature are at their peak values

(usually mid-season) and the maximum ET therefore occurs during that period. The Kc values for many different crops have been published in numerous literatures.

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