Another problem associated with excessive irrigation on poorly drained soils is waterlogging. This occurs (as is common for salinization) in poorly drained soils where water can't penetrate deeply. For example, there may be an impermeable clay layer below the soil. It also occurs on areas that are poorly drained topographically. What happens is that the irrigation water (and/or seepage from canals) eventually raises the water table in the ground -- the upper level of the groundwater -- from beneath. Growers don't generally realize that waterlogging is happening until it is too late -- tests for water in soil are apparently very expensive.
The raised water table results in the soils becoming waterlogged. When soils are water logged, air spaces in the soil are filled with water, and plant roots essentially suffocate -- lack oxygen. Waterlogging also damages soil structure.
Worldwide, as much as 10% of all irrigated land may suffer from water logging. This is an area about the size of Idaho. As a result, productivity has fallen in this area of cropland.
Both waterlogging and salinization could be reduced if the efficiency of irrigation systems could be improved, and more appropriate crops (less water hungry) could be grown in arid and semi-arid regions. In addition, increasing the cost of water to more closely reflect its true value would encourage its conservation, rather than using incentives that essentially encourage wasting water, as some water rights laws in the US do (e.g., the "use it or lose it" approaches discussed above). We can consider excessive irrigation to be another example of humanities' attempt to increase "K" in ways that are unsustainable. For example, one could argue that the Central Valley of CA shouldn't be lush and green; it is one of the most agriculturally productive regions of the world only because of irrigation, which may not be sustainable there.
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