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Introduction
Irrigation water which is not evapotranspirated by the crops can raise the groundwater table. Especially under arid and semiarid conditions and poorly drained soil, water can rise from a high groundwater table to the soil surface by capillary rise and accumulate there (see figure 1).
Water logging, another effect of groundwater table rise, prevents aeration in the soil pores. As some air in the root zone is needed by crops, decreased aeration reduces yields. In addition, water logging creates unfavourable conditions for the use of agricultural machinery.
Effect on crop yield
High salt content in the soil water prevents water uptake by crops due to the high osmotic pressure, leading to lower yields or even crop death. The high sodium content of the salt also causes a deterioration of the soil structure which results in weak root growth of the crops and less water supply to the roots.
Worldwide, about 10% of all irrigated land suffers from water logging and poor drainage. As a result, productivity has fallen about 20% in this area. Growers often do not realize that water logging is happening until it is too late. Tests for water in soil are apparently very expensive, but other preventive methods are more effective.
Implication to cope with water logging
Leaching
Leaching is applying additional water to the crop water requirement which helps to wash out salts below the root zone. The excess water should be extracted through subsurface drainage.
To prevent negative results of water logging or to make waterlogged soils utilizable, drainage systems are required. Drainage releases excess water from irrigated or non-irrigated lands to lower the water table, and collect and dispose of excess water. Salt can run-off through the drainage with the excess water. Large drainage systems with saline excess water can have a significant impact on the salt content downstream.
Change of crops
A change to less water hungry crops in arid and semi-arid regions can reduce water logging and other water related soil problems.
References and further reading
FAO: waterlogging & salinity. http://www.fao.org/nr/water/topics_qual_waterlogging.html [accessed 22 April 2013]
GIZ (1993): Irrigation and the environment. A review of environment issues. Part I & II: Environmental considerations in planning and operations.
