With rainfed crops, salinization is not a problem because the salts are naturally flushed away. But when irrigation water is applied to crops and returns to the atmosphere through plant transpiration and evaporation, dissolved salts concentrate in the soil, where they inhibit plant growth.
The practice of applying about 10 million L irrigation water per ha each year results in approximately 5 t salts per ha being added to the soil (Bouwer 2002). The salt deposits can be flushed away with added fresh water, but at a significant cost (Bouwer 2002).
Worldwide, approximately half of all existing irrigated soils are adversely affected by salinization (Hinrichsen et al. 1998). The amount of world agricultural land destroyed by salinized soil each year is estimated to be 10 million ha (Pimentel et al. 2004a). In addition, drainage water from irrigated cropland contains large quantities of salt.
Waterlogging is another problem associated with irrigation. Over time, seepage from irrigation canals and irrigated fields causes water to accumulate in the upper soil levels (Pimentel et al. 2004b). Because of water losses during pumping and transport, approximately 60% of the water intended for crop irrigation never reaches the crop (Wallace 2000).
In the absence of adequate drainage, water tables rise in the upper soil levels, including the plant root zone, and crop growth is impaired. Such irrigated fields are sometimes referred to as ―wet deserts‖ because they are rendered unproductive (Pimentel et al. 2004a).
For example, in India, waterlogging adversely affects 8.5 million ha of cropland and results in the loss of as much as 2 million tons grain every year (Pimentel et al. 2004a). To prevent both salinization and waterlogging, sufficient water and adequate soil drainage must be available to ensure that salts and excess water are drained from the soil.
Water Run off and Soil Erosion
Because more than 99% of the world‘s food comes from the land, an adequate global food supply depends on the continued availability of productive soils (FAO 1998).
Erosion adversely affects crop productivity by reducing the availability of water; by diminishing soil nutrients, soil biota, and soil organic matter; and by decreasing soil depth (Pimentel et al. 2004a).
The reduction in the amount of water available to growing plants is considered the most harmful effect of erosion, because eroded soil absorbs 87% less water through infiltration than uneroded soil (Pimentel et al. 2004a).
Soybeans and oats intercept approximately 10% of the rainfall in areas where they are planted, whereas tree canopies intercept 15% to 35% (Pimentel et al. 2004a). Thus, the removal of trees increases water runoff and reduces water availability.
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Given a total rainfall of 800 mm per year, a water runoff rate of about 30% causes significant water shortages for growing crops such as corn, ultimately lowering crop yields (Pimentel et al. 2004a).
In summary, the reduction in the amount of water available to growing plants is considered the most harmful effect of erosion. Irrigation requires a significant expenditure of fossil energy both for pumping and for delivering water to crops.
The processes of carbon dioxide fixation and temperature control require plants to transpire enormous amounts of water.
Rainfall patterns, temperature, vegetative cover, high levels of soil organic matter, active soil biota, and water runoff all affect the percolation of rainfall into the soil, where it is used by plants. With rainfed crops, salinization is not a problem because the salts are naturally flushed away.