Canal irrigation

A canal is a watercourse made by people that takes water from a source—such as a river, underground water channel (wadi or riverbed), well, or spring—and conveys it to agricultural land or another location where it is needed. The canal operates on the principle of gravitational flow: the ground is excavated so that it slopes gradually downward, allowing water to flow without requiring additional force.

Canals are used in irrigation farming and are also part of flood management, municipal water supply, and in some cases water transportation. A complete irrigation system typically consists of a main canal, secondary branches, and small channels that deliver water to individual fields.

Structure

A typical canal has an intake where water enters, a channel for water flow, and outlet sections. The intake is controlled by a gate or valve to regulate the amount of water entering. The sides of the canal are sometimes lined with clay or stone to prevent water from seeping into the ground or the banks from collapsing. The slope must be moderate: if too slight, sediment and waste accumulate; if too steep, water flows too quickly and erodes the soil.

Types

Canals differ between open channels exposed to the sky and closed pipes or conduits underground. Open canals are relatively easy to clean but lose some water to evaporation and seepage. Closed channels reduce losses but are more expensive to construct. Canals are also classified as perennial, operating year-round, or seasonal, opened only when the river is full.

Management and problems

Canals require continuous maintenance because sediment from the river settles in the channel and obstructs flow. Wind and plants growing along the banks also slow the water. Excessive irrigation without adequate drainage can raise groundwater levels and bring salt to the soil surface, damaging crops. For this reason, many systems integrate an irrigation canal with a drainage canal.

Administration and allocation

Where multiple farmers share a single canal, there is typically a schedule determining how they divide the water—most often by allocating specific times when each user may draw. Management of this type is usually handled by a local committee, government authority, or farmers' association. Disputes can arise over water sharing, especially during drought and when upstream users take more than their share.