Irrigation is the act of artificially applying water to soil to allow plant growth. This can also include applying water to a lawn or garden. The term usually refers to providing large amounts of water in arid or dry regions to grow crops. The world's rapid population growth has produced more of a need for irrigation. With it, a higher crop yield is possible on older lands.
Although irrigation is most commonly used in areas with up to 20 inches (500 mm) of rainfall annually, sometimes it also is practiced in areas where the annual rainfall is 20 to 40 inches (500 to 1,000 mm). Irrigation helps to protect against droughts where rainfall is plentiful but uncertain. And in many circumstances, irrigation is applied to grow crops on a set schedule of cultivation and harvest. This type of irrigation also is applied to crops as flood irrigation -- water pumps or is brought to the fields along the ground to flood the crops. It is not an efficient watering method [source: United States Geological Survey].
In some places, snowfall and rainfall are the main suppliers of irrigation water, but in other locales, groundwater is essential. Surface water used for irrigation is stored naturally in lakes and ponds and is conveyed by rivers and streams. Groundwater collects in basins of coarse gravel and in aquifers, which are water-bearing rocks. It reaches the surface through springs. The natural sources discharge mainly in the spring and dry up in summer. Because of that natural cycle, artificial surface reservoirs are increasingly being used to store irrigation water. The largest are dams. Water also can be obtained from wells, particularly in Arizona, California and western Texas, where water from storm-filled streams is directed and collected in the ground to fill underground basins.
Of course, anyone with a home garden is aware of drip irrigation, the method used to water ornamental or edible plants. It is one of the more efficient types of irrigation because it can be set up to water directly at the plant, with different water levels set depending on the plant's needs. Water from drip irrigation does not evaporate as much as water from spray irrigation [source: United States Geological Survey]. Then again, you probably have a few plants to water, not acres of them. Oklahoma farmers can water 120 acres of cotton or corn crops with center pivot irrigation, those clever giant sprinklers that can cover an entire field with a circular or angled motion. If programmed correctly and set in the right soil, they're more efficient than flood irrigation [source: Brown].
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