To

As shown in Fig. 5.3, shortly after planting of annuals or shortly after the initiation of new leaves for perennials, the value for Kc is small, often less than 0.4. The Kc begins to increase from the initial Kc value, Kcini, at the beginning of rapid plant development and reaches a maximum value, Kcmid, at the time of maximum or near-maximum plant development. The period during which Kc = Kcmid is referred to as the midseason period. During the late-season period, as leaves begin to age and senescence due to natural or cultural practices occurs, the Kc begins to decrease until it reaches a lower value at the end of the growing period equal to Kcend.

Figure 5.3. Generalized crop coefficient curve during a growing season.

Time of Season, days

Figure 5.3. Generalized crop coefficient curve during a growing season.

0 20 40 60 80 100

Days after Planting or Greenup

Figure 5.4. Crop coefficient definitions showing the basal Kcb, soil evaporation Ke, and time-averaged Kcb + Ke values.

0 20 40 60 80 100

Days after Planting or Greenup

Figure 5.4. Crop coefficient definitions showing the basal Kcb, soil evaporation Ke, and time-averaged Kcb + Ke values.

The form for the equation used in the basal Kc approach is

where Ks is the stress reduction coefficient (0 to 1), Kcb is the basal crop coefficient (0 to ~1.4), and Ke is the soil water evaporation coefficient (0 to ~1.4). Kcb is defined as the ratio of ETc to ET0 when the soil surface layer is dry, but where the average soil water content of the root zone is adequate to sustain full plant transpiration. As shown in Fig. 5.4, the Kcb represents the baseline potential Kc in the absence of the additional effects of soil wetting by irrigation or precipitation. Ks reduces the value of Kcb when the average soil water content of the root zone is not adequate to sustain full plant transpiration. Ke describes the evaporation component from wet soil in addition to the ET represented in Kcb.

Because Eq. (5.53) requires the calculation of a daily soil water balance for the surface soil layer, a simplification is required for routine application, which may involve long time steps or wetting effects averaged over many fields or over many years. The time-averaged Kc is

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