An irrigation system consists of various structures and devices for supplying, conveying, and applying irrigation water. In the United States and Canada, and in many other countries, irrigation systems usually are owned by individual farmers or by ditch companies—private companies or farmers' cooperatives. The largest irrigation systems may be built by government agencies such as the Bureau of Reclamation of the U.S. Department of the Interior. In some countries, irrigation systems are entirely government-owned.

The main source of irrigation water in some localities is mountain snowfall, supplemented to varying degrees by rainfall. In many places, ground water is an important source.

Surface waters that may be used for irrigation are stored naturally in lakes and ponds. Ground water collects beneath the surface in basins of coarse gravels, and in aquifers (water-bearing rocks). Surface waters are conveyed to farms by rivers and streams, and ground water reaches the surface through springs. These natural sources of supply tend to discharge heavily in the springtime and dry up in midsummer. Therefore, irrigation water is increasingly being stored in artificial surface reservoirs, from which it can be released as needed through canals. Artificial reservoirs are created by various means. The largest are made by dams that flood river valleys.

Water for irrigation can also be obtained from wells. In some places—notably in California, Arizona, and western Texas—water from streams filled by summer storms is diverted and allowed to soak into the ground to refill underground basins.

Canals that carry irrigation water usually are called ditches. Main ditches may be hundreds of miles long and carry millions of gallons of water a day. To prevent loss of water by seepage, many ditches are lined with concrete or bentonite, a type of clay.

A branch of a main ditch, leading to a group of farms, is called a lateral. Each farm takes its allotted portion of water from the lateral at a headgate or take out, where sliding wooden or metal gates are used to regulate the flow. Water is led to various parts of the farm through field ditches. Small dams, or flags, of canvas, metal, or wood, easily placed and removed by hand, are used to direct the flow of water onto the fields.

Various mechanical devices are used to measure the amount of water flowing in main ditches and laterals. Measurements are often given in terms of the acre-inch or the acre-foot, defined as the amount of water that will cover one acre to a depth of one inch or one foot, respectively. One acre-inch equals about 100 cubic meters.

Principal methods of applying water to crops are flooding, furrow irrigation, sub-irrigation, sprinkling, and drip irrigation.

Flooding is done by covering the field with a sheet of water until the ground is soaked. This method is used mainly for pasture, grains, and hay. There are four types of flooding:

Ordinary flooding is practiced where water is abundant and inexpensive. The water is allowed to overflow from a ditch onto the fields.

Border-strip flooding requires the division of land into strips separated by low levees, or borders. Water flows slowly toward the lower end of the strip, soaking the ground as it goes.

Check flooding allows water to stand (be checked) for a time on parcels of level land separated by borders, often along contour lines.

Basin flooding is an adaptation of check flooding to orchards. Water is pooled around each tree or group of two to five trees.

Furrow Irrigation is a method of delivering water to crops that are planted or seeded in rows. The water reaches the plant roots from furrows (shallow depressions) between the rows. The surface of the rows remains dry, saving water costs and permitting cultivation soon after irrigation.

Sub-irrigation is the application of water to the root zone from below, leaving the field surface dry. In natural sub-irrigation, water is allowed to seep from deep-cut ditches into the porous subsoil. In artificial sub-irrigation—an expensive method—a system of pipes is laid below ground.

Sprinkling produces a flow of water similar to that of rainfall. Usually, the water is sprayed under pressure from sprinkler heads placed along a pipe. The pipe may be portable or fixed in place. In a center-pivot sprinkler system, a long straight pipe with sprinkler heads is mounted on wheeled supports and connected at one end to a fixed pivot, through which water is supplied to the pipe. The pipe, powered by hydraulic pressure or electric motors, slowly moves around the pivot and sprinkles a large circular area.

Drip (or Trickle) Irrigation is a method of delivering a very slow but steady flow of water to plants individually. The water flows either from small holes or from nozzlelike “emitters” placed along a narrow, flexible plastic tube lying on the ground. The holes or emitters are spaced so that only the ground immediately around each plant is watered.