Irrigation

Irrigation refers to the controlled application of water to agricultural land, landscapes and other cultivated areas to support plant growth. It has formed a core component of agriculture for more than five millennia, with early systems emerging in ancient civilisations such as those of Mesopotamia, Egypt, China and the Indus Valley. Throughout history, irrigation has enabled farming in arid regions, improved crop yields and supported the development of complex societies by ensuring reliable food production even where rainfall is limited or seasonal.

Purpose and Functions

The primary purpose of irrigation is to supplement natural rainfall and provide plants with sufficient moisture to support healthy growth. It plays a crucial role in maintaining crop productivity in dry climates, during droughts or in areas with irregular precipitation. Beyond crop cultivation, irrigation supports landscape maintenance, the revegetation of disturbed soils and the sustainability of lawns, parks and urban green spaces.
Irrigation serves several additional agricultural and environmental functions. It can protect crops from frost by using water to regulate temperature around the plant, suppress weed growth in cereal fields and prevent soil crusting. In some agricultural contexts, irrigation helps cool livestock, reduce dust and facilitate mining operations. Drainage systems, which remove excess surface or subsurface water, are frequently integrated into irrigation planning to prevent waterlogging and maintain soil health.

Major Irrigation Methods

Irrigation techniques vary widely according to water availability, soil characteristics, crop type and local environmental conditions. The principal methods include the following:

• Surface irrigation: One of the oldest and most widely used methods, surface or gravity irrigation relies on water flow across the soil surface. Water is channelled through canals or furrows and allowed to infiltrate the soil naturally. Its effectiveness depends on land levelling and soil permeability.
• Sprinkler irrigation: Water is pumped through a system of pipes to one or more elevated points and distributed by high-pressure sprinklers. This method simulates rainfall and is suited to many crop types, although it typically requires greater energy input than surface irrigation.
• Microirrigation: Operating at low pressure, microirrigation uses a network of pipes to deliver small, controlled amounts of water to individual plants. It includes drip irrigation and micro-sprinklers. This approach minimises water loss and is well suited to orchards, vineyards and areas with limited water supplies.
• Drip irrigation: Water is delivered directly to the plant root zone through perforated tubes or emitters. It increases water-use efficiency and reduces evaporation and runoff, making it valuable in semi-arid and arid regions.
• Subirrigation: Used particularly in high water table regions, subirrigation artificially raises groundwater levels to moisten the soil beneath plant roots. It has been applied widely in field crops and controlled environment agriculture.

Water sources vary and include groundwater extracted from springs or wells, surface water diverted from rivers, lakes or reservoirs and non-conventional supplies such as reclaimed wastewater, desalinated water, drainage water and fog collection. In some regions, especially coastal or humid areas, condensation methods also contribute small but meaningful volumes of irrigation water.

Environmental Effects

The environmental impacts of irrigation arise from alterations to local and regional hydrology. Large-scale water withdrawals can contribute to the depletion of aquifers, especially where pumping exceeds natural recharge. Overirrigation may result in nutrient leaching, chemical runoff and waterlogging. Rising groundwater levels can mobilise salts, leading to soil salinity and reduced agricultural productivity. Effective drainage systems are essential in managing these risks by maintaining an appropriate water table and preventing salt accumulation.
Irrigation can also affect downstream ecosystems by changing river flows, sediment transport and water quality. These shifts may influence aquatic habitats, biodiversity and access to freshwater resources for communities located further downstream.

Global Extent of Irrigation

By the year 2000, approximately 2,788,000 square kilometres of land were equipped for irrigation worldwide, with the majority located in Asia. Smaller but significant areas were found in the Americas, Europe, Africa and Oceania. High-density irrigated regions include the Ganges and Indus Basins in South Asia, the major river basins of China, the Nile Valley in Egypt and Sudan and agricultural zones across the Mississippi–Missouri system and the western United States.
By 2012, irrigated land had expanded to roughly 3,242,917 square kilometres, nearly the size of India. Approximately 20% of global farmland accounts for 40% of food production, highlighting the critical role of irrigation in global food security. Over the twentieth century, irrigated acreage increased dramatically, rising from around 8 million hectares in 1800 to more than 235 million hectares by 1990. This expansion supported major agricultural transformations, including the Green Revolution.
Global data from the early 2020s show continuous growth in irrigated land, reaching more than 350 million hectares. Asia accounts for the majority share, followed by the Americas and Europe. India and China possess the world’s largest irrigated areas, reflecting their reliance on irrigation for staple crop production and economic development. Africa shows the fastest relative growth in irrigation investment, though from a smaller base.

Water Sources and Innovations

Irrigation systems rely on diverse water sources, including:

• Groundwater: Extracted from wells or springs, often supplying regions without major surface water bodies.
• Surface water: Diverted from rivers, lakes or reservoirs through canals and pumping systems.
• Non-conventional sources: Such as treated wastewater, desalinated seawater, drainage water or fog collection.

Innovative irrigation practices continue to expand in response to water scarcity. Examples include the use of condensation from humid night air in vineyards, fog-collecting installations in dry coastal environments and using condensate from air conditioning units in urban agriculture. Experimental saline agriculture has also emerged, with seawater used for flood irrigation of salt-tolerant crops, contributing benefits such as soil remediation and carbon sequestration.

Pressure on Water Resources

The global demand for freshwater has increased substantially due to population growth, higher living standards and dietary changes including increased consumption of water-intensive foods. Industrialisation and urban expansion further intensify competition for limited water supplies. Agriculture remains the largest global consumer of freshwater, making efficient irrigation a central concern for food security and environmental sustainability.
Farmers are under increasing pressure to improve productivity while utilising water resources more efficiently. This challenge is heightened in regions experiencing declining rainfall, reduced river flows or overdrawn aquifers.

Overview of Irrigation Methods in Practice

The overarching goal of any irrigation system is to distribute water as uniformly as possible so that each plant receives the correct amount of moisture. The choice of method depends on economic resources, agricultural objectives, land topography and climate. Modern irrigation tends to integrate technologies such as moisture sensors, automated scheduling and precision application to reduce waste and improve crop yields.