Irrigation (in agriculture) is the replacement or supplementation of rainfall with imported or stored water in order to grow crops. (Agriculture that relies only on direct rainfall is referred to as dryland farming[?].)
The water source for irrigation may be a nearby or distant body of liquid or frozen water such as a river, spring, lake, aquifer, or snowpack[?]. Depending on the distance of the source and the seasonality of rainfall, the water may be channelled directly to the agricultural fields or stored in reservoirs or cisterns[?] for later use. In addition, the "harvesting" of local rain that falls on the roofs of buildings or on nearby unfarmed hills and its use to supplement the rain that falls directly on farmed fields also involves irrigation.
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.
Ditch irrigation -- The plants are grown in somewhat raised beds or rows, and the water is distributed throughout the field via unlined ditches 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 simple tractor, or precisely fashioned using laser-guided instruments. 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 permeabilitiy, much water may be lost before it can benefit the plants. 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.
Terracing -- 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).
Overhead irrigation -- Water is piped to one or more central locations within the field and distributed by overhead high-pressure sprayers or lower-pressure sprinklers. The sprayers can be hidden below ground level, if esthetics is a concern, and pop up in response to increased water pressure. Individual sprayers can be designed to rotate in a full or partial circle. At the high-tech end, computerized, automatically moving wheeled setups may irrigate large areas unattended. At the low end, a person may water each plant individually with a watering can. Much water can be lost because of high winds or evaporation, and irrigating the entire field uniformly can be difficult or tedious. 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. However, overhead irrigation is the best solution for watering lawns and golf courses. (See also center pivot irrigation.)
Drip irrigation -- Water is delivered at or near the root zone of plants, drop by drop. This is the most water-efficient method of irrigation, since very little water evaporates or drains off before being used by the plants. Drip irrigation methods range from very high-tech and computerized to low-tech and relatively labor-intensive. Only rather low water pressure is usually needed, 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. The method is not cost-effective (or esthetically pleasing) for lawns and golf courses.
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