In this Volume the determination of the irrigation schedule is explained. The irrigation schedule indicates how much irrigation water has to be given to the crop, and how often or when this water is given.
How much and how often water has to be given depends on the irrigation water need of the crop. How to determine the irrigation water need has been discussed in Volume 3. The irrigation water need is defined as the crop water need minus the effective rainfall. It is usually expressed in mm/day or mm/month. When, for example, the irrigation water need of a certain crop, grown in a hot, dry climate is 8 mm/day (see Figure 1), this means that each day the crop needs a water layer of 8 mm over the whole area on which the crop is grown. This water has to be supplied by means of irrigation.
Figure 1. Irrigation water need of 8 mm/day
An irrigation water need of 8 mm/day, however, does not mean that this 8 mm has to be supplied by irrigation every day. In theory, water could be given daily. But, as this would be very time and labour consuming, it is preferable to have a longer irrigation interval (see Figure 2). It is, for example, possible to supply 24 mm every 3 days or 40 mm every 5 days. The irrigation water will then be stored in the root zone and gradually be used by the plants: every day 8 mm. The irrigation interval has to be chosen in such a way that the crop will not suffer from water shortage. This is shown in the example on page 2.
Figure 2. When to irrigate?
How often to irrigate?
Often enough to prevent the plants suffering from drought.
How much to irrigate?
As much as the plants have used since the previous irrigation.
If it is assumed that the soil is wet (e.g. at field capacity) on day 1 (see Figure 3), the crop will have no difficulty in taking up the water for the first couple of days. When, however, more and more days pass - and no irrigation is given - the crop will have more and more difficulty in taking up the water.
Figure 3. In the absence of rainfall and if no irrigation water is applied, the plants eventually die
In Figure 3 it can be seen that, on this soil, the plants start to suffer after approximately one week. Irrigation water should be given before this happens, in order to allow for optimal production. In general this means that irrigation should at the latest take place when approximately half of the available water content of the root zone (see Volume 1, section 2.3) has been used by the plants. When, for example (Figure 4), irrigation water is given on day 5, on day 9, on day 13, etc., the plants will not suffer from water shortage.
Figure 4. If irrigation water is applied regularly, the plants do not suffer from water shortage
In principle, the amount of irrigation water given in one irrigation application (irrigation depth) is the amount of water used by the plants since the previous irrigation.
The amount of irrigation water which can be given during one irrigation application is however limited. The maximum amount which can be given has to be determined and may be influenced by:
- the soil type
- the root depth
- the irrigation method.
The soil type influences the maximum amount of water which can be stored in the soil per metre depth (see also Volume 1: Section 2.4: Available Water Content). Sand can store only a little water or, in other words, sand has a low available water content. On sandy soils it will thus be necessary to irrigate frequently with a small amount of water. Clay has a high available water content. Thus on clayey soils larger amounts can be given, less frequently.
The root depth of a crop also influences the maximum amount of water which can be stored in the root zone (see Figure 5). If the root system of a crop is shallow, little water can be stored in the root zone and frequent - but small - irrigation applications are needed. With deep rooting crops more water can be taken up and more water can be applied, less frequently. Young plants have shallow roots compared to fully grown plants. Thus, just after planting or sowing, the crop needs smaller and more frequent water applications than when it is fully developed.
Figure 5. Plants with deep roots take up water over a greater depth than shallow rooting plants
How much water can be infiltrated into the soil with the locally used irrigation method has to be checked in the field. For instance, when using basin irrigation, more water can be infiltrated during one irrigation application than when using furrow irrigation. In particular, with small-scale irrigation (small water flows and small fields) it is often the irrigation method which is the most limiting factor when determining the maximum irrigation application.
All these issues, which are important for irrigation scheduling, are explained in more detail in the following chapters.
Chapter 2 deals with the influence of water shortages on yields. This is an important issue, especially for water saving practices. In Chapter 3 various methods to determine the irrigation schedule are discussed. As rice is grown under different conditions than other crops, the determination of the irrigation schedule for rice is dealt with separately in Chapter 4.