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Part B - Crop evapotranspiration under standard conditions

Chapter 5 - Introduction to crop evapotranspiration (ETc)
Chapter 6 - ETc - Single crop coefficient (Kc)
Chapter 7 - ETc - Dual crop coefficient (Kc = Kcb + Ke)

This part examines crop evapotranspiration under standard conditions (ETc). This is the evapotranspiration from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions and achieving full production under the given climatic conditions.

The effects of various weather conditions on evapotranspiration are incorporated into ETo (Part A). The effects of characteristics that distinguish the cropped surface from the reference surface are integrated into the crop coefficient. By multiplying ETo by the crop coefficient, ETc is determined.

Typical crop coefficients, calculation procedures for adjusting the crop coefficients and for calculating ETc are presented in this part. Two calculation approaches are outlined: the single and the dual crop coefficient approach. In the single crop coefficient approach, the difference in evapotranspiration between the cropped and reference grass is combined into one single coefficient. In the dual crop coefficient approach, the crop coefficient is split into two factors describing separately the differences in evaporation and transpiration between the crop and reference surface.

As discussed in Chapter 5 and summarized in Table 10, the single crop coefficient approach is used for most applications related to irrigation planning, design, and management. The dual crop coefficient approach is relevant in calculations where detailed estimates of soil water evaporation are required, such as in real time irrigation scheduling applications, water quality modelling, and in research.

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