The most important element in any irrigation system is a dependable water supply. Groundwater from deep wells is also used for irrigation, especially in desert countries such as Israel, Kuwait, and Saudi Arabia. Generally the need for irrigation water is highest in the dry season when river flows are lowest. To ensure continuous supply, water must be stored on a seasonal and sometimes annual basis. By erecting a dam, an artificial lake, or reservoir, is created from which water can be released as required.
Some reservoirs are capable of storing billions of gallons of water.
Systems of dams and reservoirs along a river and its tributaries are developed with the purpose of providing comprehensive and integrated water resources management in an entire basin. A modern large-scale irrigation system consists of the main dam and a network of diversion structures to distribute the water to where it is needed. Such structures include canals, pumping stations, and barrages. The water flows from the reservoir into a main canal that conveys water to the irrigation sites. The canal usually runs downhill, so the water flows by gravity.