Kc Kcb Ke554

where Kcb + Ke represents the sum of the basal Kcb and time-averaged effects of evaporation from the surface soil layer, Ke.

Typical shapes for the Kcb, Ke, and Kcb + Ke curves are shown in Fig. 5.4. The Kcb curve in the figure represents the minimum Kc for conditions of adequate soil moisture and dry soil surface. The Ke spikes in the figure represent increased evaporation when precipitation or irrigation has wetted the soil surface and has temporarily increased total ETc. The spikes generally reach a maximum value of 1.0 to 1.2, depending on the climate, the depth of soil wetting, and the portion of soil surface wetted. When summed, the values for Kcb and for Ke represent the total crop coefficient, Kc, shown as the dashed line in

Figure 5.5. Crop coefficient curve and stage definitions.

Time of Season, days

Figure 5.5. Crop coefficient curve and stage definitions.

Fig. 5.4. The Kc or Kcb + Ke curve lies above the Kcb curve, with potentially large differences during the initial and development stages, depending on the frequency of soil wetting. During the midseason period, when the crop is likely to be near full cover, the effects of soil wetness are less pronounced.

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