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Part II of this manual describes how the water needs of the various crops, grown on an irrigation scheme, can be calculated.

The crop water need, or in other words the amount of water needed by a certain crop to grow optimally, mainly depends on:

· the climate: in a sunny and hot climate crops need more water per day than in a clouded and cool climate

· the crop type: crops like rice or sugarcane need more water than crops like beans or wheat

· the growth stage of the crop: fully grown crops need more water than crops that have just been planted.

The amount of water needed can be supplied to the crops by rainfall, by irrigation, or by a combination of both. Usually the irrigation water supplements or adds to the rainwater. Only in desert or arid areas - or in the dry season - will all the water needed by the crops have to be supplied by irrigation.

The irrigation water need is the difference between the crop water need and that part of the rainfall which can be used by the plants (effective rainfall). The irrigation water need calculation provides the basis for the determination of the irrigation schedule (usually by agronomists) and the design of the irrigation scheme, e.g. canal dimensions (usually by engineers). An overview of all the subjects that are dealt with in this manual is given.

Fig. 1 Irrigating maize

Overview of the determination of the reference crop evapotranspiration (ETo), the crop water need (ET crop) and the irrigation water need

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