From AMS Glossary
The artificial application of water to land to promote the growth of crops.
First practiced in arid lands to supplement deficient rainfall, it is now used extensively in more humid areas to ensure proper timing of water supply for maximum crop yield. The earliest form of irrigation consisted of diverting flood flows of streams onto the cropland. More common practice today is the storage of floodwaters in reservoirs from which the water may be withdrawn for use as needed. Extensive irrigation projects have been developed that use groundwater pumped from wells.