Irrigation (in agriculture) is the replacement or supplementation of rainfall with water from another source in order to grow crops. In contrast, agriculture that relies only on direct rainfall is sometimes referred to as dryland farming.
Various types of irrigation techniques differ in how the water obtained from the source is distributed within the field. In general, the goal is to supply the entire field uniformly with water, so that each plant has the amount of water it needs, neither too much nor too little.
The plants are grown in somewhat raised beds or listed rows, and the water is distributed throughout the field via unlined ditches, or furrows, between the rows or beds. Depending on economic and physical factors such as the size of the field, the types of technology available, and the cost of manpower, the ditches can be dug with hand tools, turned with a plow pulled by an animal or tractor, or precisely fashioned using laser-guided instruments. Water can be transported to the furrows via rigid gated plastic or aluminumpipe, layflat plastic with holes punched at each furrow, concrete or plastic lined ditches, or unlined ditches. Where ditches are used, siphon tubes are generally used to move water from the main ditch to the furrow. When pipes are used, water flow can be controlled by turning it on or off at the local source or by using automatic or manually controlled gates to shunt it from one set of ditches to another. Unless the field is small or very level, parts of it may suffer from water-logging while other parts may be too dry. Depending on heat, wind, and soil permeability, much water may be lost before it can benefit the plants. Automatic valves, also known as surge valves, can increase the efficiency of furrow irrigation because they alternately wet the furrows and allow the soil infiltration rate to slow prior to using the furrow for actual irrigation.
Once common in the U.S., many ditch irrigation systems have been replaced because of high labor costs and increasing demands on water resources. Furrow irrigation also has a tendency to raise the water table in some areas and cause soil salination, requiring drainage. These types of systems are still common in other parts of the world.
Large steps are cut into hillsides and supported by stone or concrete walls. The level parts are used as garden plots or small fields. As water flows down the hillside it is channelled to each plot (probably most often by ditch irrigation). Terracing is usually very labor-intensive, since the fields are small and access to them may be steep and narrow (so it's hard to mechanize the work). In addition, the walls need constant maintenance, especially in rainy climates. However, terracing does allow steep mountainsides to be used to grow plants (although it may be more cost effective to use them only for animal pasturage).
In overhead or sprinkler irrigation, water is piped to one or more central locations within the field and distributed by overhead high-pressure sprinklers or guns or by lower-pressure sprays. A system utilizing sprinklers, sprays, or guns mounted overhead on permanently installed risers is often referred to as a solid-set irrigation system. Some sprinklers can also be hidden below ground level, if esthetics is a concern, and pop up in response to increased water pressure. This type of system is commonly used in lawns, golf courses, cemeteries, parks, and other turf areas.
Sprinklers that spray in a fixed pattern are generally called sprays or spray heads. Sprays are not usually designed to operate at pressures above 30 psi (200 kPa), due to misting problems that may develop, while higher pressure sprinklers that rotate are usually called rotors. Rotors are usually driven by a ball drive, gear drive, or impact mechanism. Rotors can be designed to rotate in a full or partial circle. Guns are similar to rotors, except that they generally operate at very high pressures of 40 to 130 psi (275 to 900 kPa) and flows of 50 to 1200 gal/min (3 to 76 L/s), usually with nozzle diameters in the range of 0.5 to 1.9 inches (10 to 50 mm). Guns are used not only for irrigation, but also for industrial applications such as dust suppression and logging.
Sprinklers may also be mounted on movable platforms connected to the water source by a hose. At the high-tech end, computerized, automatically moving wheeled systems may irrigate large areas unattended. At the low end, such as in a small greenhouse or landscape, a person may be watering each plant individually with a hose end sprinkler or even a watering can.
One drawback of overhead irrigation is that much water can be lost because of high winds or evaporation, and irrigating the entire field uniformly can be difficult or tedious if the system is not properly designed. Water remaining on plants' leaves may promote fungal and other diseases. If fertilizers are included in the irrigation water, plant leaves can be burned, especially on hot, sunny days.
Overhead irrigation is generally the best solution for watering lawns and golf courses, although drip irrigation is gaining in popularity in some lawn applications. (See also center pivot irrigation.)
Manually assembled systems of piping that are broken down to permit tillage and harvesting are sometimes called "hand set" or "hand move pipe". These are also commonly used on athletic fields where permanently installed sprinklers or outlets are not desired or where lower initial costs are a factor.
Water is delivered at or near the root zone of plants, drop by drop. This type of system can be the most water-efficient method of irrigation, if managed properly, since evaporation and runoff are minimized. In modern agriculture, drip irrigation is often combined with plastic mulch, further reducing evaporation, and being also the means of delivery of fertilizer. The process is known as fertigation.
Deep percolation, where water moves below the root zone, can occur if a drip system is operated for too long of a duration. Drip irrigation methods range from very high-tech and computerized to low-tech and relatively labor-intensive. Lower water pressures are usually needed than for most other types of systems, with the exception of low energy center pivot systems and surface irrigation systems, and the distribution can be adjusted for uniformity throughout a field or for precise water delivery to individual plants in a landscape containing a mix of plant species. Although it is difficult to regulate pressure on steep slopes, the field does not have to be level. High-tech solutions involve precisely calibrated emitters located along lines of tubing that extend from a computerized set of valves. Both pressure regulation and filtration to remove particles are important. The tubes are usually black (or buried under soil or mulch) to prevent the growth of algae. But drip irrigation can also be as low-tech as a porous clay vessel sunk into the soil and occasionally filled from a hose or bucket. Subsurface drip irrigation has been used successfully on lawns, but it is more expensive than a more traditional sprinkler system. Surface drip systems are not cost-effective (or esthetically pleasing) for lawns and golf courses.
By the middle of the 20th century, the advent of diesel and electric motors led for the first time to systems that could pump groundwater out of major aquifers faster than it was recharged. This can lead to permanent loss of aquifer capacity, decreased water quality, ground subsidence, and other problems. The future of food production in such areas as the
North China Plain, the Punjab, and the Great Plains of the US is threatened.