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Irrigation is the controlled provision of water in order to successfully cultivate a crop under the following conditions:

• In arid areas
• During drought periods in semi-arid and semi-humid areas
• In areas with erratic rainfall distribution
• To lengthen the effective growing period into the dry season
  

Because climate and soil conditions are specific to each area, it is not possible to give an in-depth account of irrigation systems and scheduling, but only to describe the more important terms, guidelines and practical implications.

Important terms

• Infiltration rate

The rate at which water infiltrates the soil is an inherent property of the soil. It is highest in dry sandy soil and lowest in wet clay soil.

Practical implications
An irrigation system should supply water to the soil at the rate at which it can absorb it.

Excess water leads to reduced air supply (oxygen) in the soil, runoff, erosion and between-row weed contamination.

• Field water capacity

After water has moved downwards through the soil profile (through gravity and soil water gradients, i.e. redistribution), this point represents the highest limit of the soil’s capacity to store water.

Practical implications:
The field water capacity of clay soil is higher than that of sandy soil, and also higher in layered than in uniform soils of the same structure because layered soil limits drainage.

• Visual moisture stress symptoms

This is often used as a measure that indicates the crop is suffering water stress; irrigation should should be done before this point.

• Evapotranspiration

Evapotranspiration : This determines the pattern and rate of evaporation from an open piece of ground, together with the rate of transpiration from the leaf surfaces of the plants.

Practical Implications
While the lucerne seed is germinating and the seedling is becoming established, evaporation from the soil is higher than from the plants, and is dependent on the climate, on the movement of soil water and on soil temperature.

In established stands where the leaves shade the soil, the transpiration rate of the crop is more important than evaporation in the water use of the crop.

Because plants need CO   for photosynthesis (the amount depends on production potential) they have to transpire, with the result that they will necessarily lose water vapour through their stomata.