Irrigation is the controlled application of water to land to support plant growth, chiefly in agriculture. It supplements inadequate or irregular rainfall and replenishes moisture used by crops. Irrigation also serves parks, gardens, nurseries, and other managed landscapes. Some applications have additional purposes, including frost protection and removal of salts from soil. Systems range from hand watering to extensive networks that divert, store, convey, and distribute water across agricultural regions. (usgs.gov)
Development and water supply
Irrigation has been practiced for thousands of years. Early surface systems diverted streams onto adjacent floodplains using simple barriers and canals. In Mesopotamia, irrigated farming supported settlements on otherwise dry plains. Modern earthmoving machinery and pumps subsequently extended irrigation to uplands and areas distant from natural watercourses. (fao.org)
Water sources include rivers, lakes, ponds, springs, and groundwater, as well as reclaimed wastewater in some systems. Groundwater supplies are drawn from aquifers. Irrigation infrastructure may include diversion structures, storage facilities, wells, canals, pipelines, pumps, and field outlets. The reliability, quantity, and quality of the supply influence both system design and agricultural operations. (usgs.gov)
Application methods
Surface irrigation distributes water across a field by gravity. Basin irrigation floods level areas enclosed by low embankments; furrow irrigation carries water through channels between crop rows; border irrigation directs flow along strips of land. Water enters the soil as it advances. Performance depends on field slope, soil infiltration, flow rate, and the time at which inflow stops. Land leveling can improve distribution, while poorly controlled applications may produce excess drainage or surface runoff. (fao.org)
Sprinkler irrigation distributes water through pressurized pipes and nozzles, producing droplets over the crop or soil. It accommodates many field shapes and uneven terrain with less leveling than surface methods. Wind can disrupt distribution, and equipment requires maintenance and energy for pressurization. Sprinkler designs include fixed and movable installations, including rotating center-pivot systems. (fao.org)
Drip irrigation, also called trickle irrigation, releases small quantities through emitters near individual plants or crop rows. It wets a restricted portion of the root zone rather than the entire field surface. Frequent applications can maintain relatively stable moisture conditions. Filtration and maintenance are important because sediment and other material can obstruct emitters. Its installation costs and management requirements differ from those of surface systems. (fao.org)
Subirrigation supplies moisture by controlling the groundwater level close enough to crop roots for water to move upward through the soil. Unlike drip delivery, it operates through management of the water table and drainage network. Its suitability depends on local soil and groundwater conditions. No application method is universally superior: crop characteristics, topography, water availability, labor, and costs all affect suitability. (fao.org)
Crop requirements and scheduling
Irrigation scheduling determines when water is applied and how much is delivered. A principal component of crop water demand is evapotranspiration, comprising evaporation from soil and transpiration by vegetation. Atmospheric demand, crop development, rooting depth, and soil properties affect the rate at which the root zone loses available moisture. Consequently, identical irrigation intervals are not appropriate for every crop or season. (fao.org)
Soil stores water between applications. Field capacity describes the moisture remaining after substantial downward drainage has ceased; the permanent wilting point marks the lower limit of water that plants can extract. Their difference, combined with rooting depth, determines total available root-zone water. Crops generally begin experiencing moisture stress before this entire quantity is depleted. (fao.org)
A soil-water balance accounts for rainfall, irrigation, upward groundwater contributions, evapotranspiration, runoff, and drainage below the roots. Scheduling uses this balance to estimate depletion and the required replacement depth. The gross volume supplied may exceed the net root-zone requirement because conveyance and application are imperfect, or because additional water is needed to leach accumulated salts. (fao.org)
Efficiency and water accounting
Irrigation efficiency describes relationships between water supplied and water beneficially used or stored, with different measures applying to conveyance and field application. Distribution uniformity is a separate consideration: an adequate average application can still leave some areas under-watered and others excessively wet. Design and operation matter alongside the nominal method used. (fao.org)
Field-level reductions in withdrawals do not automatically represent equivalent savings across a drainage basin. Seepage and drainage can become return flows that replenish groundwater or supply downstream users. Consumptive water use, especially evapotranspiration, removes water from immediate local reuse. A technology that reduces recoverable return flows while increasing crop consumption may therefore reduce withdrawals without reducing basin-wide consumption. (usgs.gov)
Environmental effects and governance
Excess irrigation and inadequate drainage can cause waterlogging. Irrigation water also introduces dissolved salts; evaporation and plant water uptake leave much of this salt behind. Without sufficient leaching and drainage, soil salinization can impair crop production. Excessive groundwater pumping can lower water tables, while nutrient movement into water bodies can contribute to eutrophication. (fao.org)
Shared systems require arrangements for allocation, operation, maintenance, and financing. Water user associations, public irrigation agencies, and mixed-management organizations perform these functions in different settings. Responsibilities may be divided between an agency operating major canals and farmers managing local distribution. Water-use entitlements, delivery schedules, fees, and maintenance obligations determine how a shared supply reaches individual fields. (fao.org)