Evaporation is one of the main reasons why crops need to be continuously irrigated. Can you list
some other reasons?
Answers
Crops need water for transpiration and evaporation (see Volume 1, Section 4.2).
The plant roots suck or extract water from the soil to live and grow. The main part of this water does not remain in the plant, but escapes to the atmosphere as vapour through the plant's leaves and stem. This process is called transpiration. Transpiration happens mainly during the day time.
Water from an open water surface escapes as vapour to the atmosphere during the day. The same happens to water on the soil surface and to water on the leaves and stem of a plant. This process is called evaporation (Fig. 4).
The water need of a crop thus consists of transpiration plus evaporation. Therefore, the crop water need is also called "evapotranspiration".
The water need of a crop is usually expressed in mm/day, mm/month or mm/season.
Suppose the water need of a certain crop in a very hot, dry climate is 10 mm/day. This means that each day the crop needs a water layer of 10 mm over the whole area on which the crop is grown (Fig. 5). It does not mean that this 10 mm has to indeed be supplied by rain or irrigation every day.
It is, of course, still possible to supply, for example, 50 mm of irrigation water every 5 days. The irrigation water will then be stored in the root zone and gradually be used by the plants: every day 10 mm.
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