Irrigation

DEFINITION: Watering of land through human-created means

Like many other human modifications to natural ecosystems, the use of water for irrigation achieves some remarkable but temporary advantages that are complicated by long-term environmental problems.

The demands of feeding and clothing the rapidly expanding world population require the production of increasing amounts of food and fiber. One important strategy for achieving the necessary levels of production has been the use of irrigation techniques to supply additional water to arid and semiarid regions where few, if any, crops could otherwise be grown.

The United Nations Food and Agriculture Organization (FAO) calculated that by 2009, approximately 311 million hectares (768.5 million acres) of land worldwide were irrigated. FAO reported that by 2020, this number had increased to 5 billion hectares (12 billion acres). Only about 20 percent of the Earth's land is used as irrigated cropland, but 40 percent of the world's food is grown on this land.

An often-cited example of irrigation success is that of the Imperial Valley in southern California. The valley, more than 12,900 square kilometers (5,000 square miles) in size, was originally considered to be a desert wasteland. The low annual rainfall resulted in a typical desert, with cacti, lizards, and other arid-adapted plants and animals. In 1940, however, engineers completed the construction of the All-American Canal, which carries water 130 kilometers (80 miles) from the Colorado River to the valley. The project converted the Imperial Valley into a fertile, highly productive area where farmers grow fruits and vegetables all year.

Methods

All types of irrigation are expensive, requiring advanced technologies and large investments of capital. In many cases, irrigation systems convey water from sources hundreds of miles distant. In the United States, such vast engineering feats are largely financed by taxpayers. Typically, water from a river is diverted into a main canal and from there into lateral canals that supply each farm. From the lateral canals, various systems are used to supply water to the crop plants in the field.

Flood irrigation supplies water to fields at the surface level. Using the sheet method, land is prepared so that water flows in a shallow sheet from the higher part of the field to the lower part. This method is especially suitable for hay and pasture crops. Row crops are better supplied by furrow irrigation, in which water is diverted into furrows that run between the rows. Both types of flood irrigation cause erosion and loss of nutrients. However, erosion can be reduced in the latter type through the contouring of the furrows.

Sprinkler irrigation systems, though costly to install and operate, are often used in areas where fields are steeply sloped. Sprinklers may be supplied by stationary underground pipes, or a center pivot system may be used, in which water is sprinkled by a raised horizontal pipe that moves slowly around a pivot point. Aside from its expense, another disadvantage of sprinkler irrigation is the loss of water by evaporation. In drip irrigation, in contrast, water is delivered by perforated pipes at or near the soil surface. Because water is delivered directly to the plants, much less water is lost to evaporation than is the case with sprinkler irrigation.

Much of the water utilized in irrigation never reaches the plants. It is estimated that irrigation practices are between 40 and 95 percent efficient in delivering the water to the root systems of crop plants. The remaining water is lost to evaporation, supplies weeds, seeps into the ground, or runs off into nearby waterways.

In the 2020s, several cutting-edge irrigation techniques have emerged to improve water efficiency and crop yields amid growing water scarcity. Smart irrigation systems began to use IoT sensors and artificial intelligence to monitor soil moisture and weather conditions, delivering precise amounts of water only when needed. Subsurface textile irrigation places water directly in the root zone through specialized fabric layers, reducing evaporation and runoff. Other innovations include wireless, low-cost moisture sensing networks, bio-inspired hydrophobic sand mulches that slow surface evaporation, and advanced controllers that integrate real-time data with automated valves. These technologies aimed to maximize water use while supporting sustainable agriculture in a warming climate.

Negative Impacts

As freshwater evaporates from irrigated fields over time, a residue of salt is left behind. The process, called salinization, results in a gradual decline in productivity and can eventually render fields unsuitable for further agricultural use. Correcting saline soils is not a simple process. In principle, large amounts of water can be used to leach salt away from the soil, but in practice, the amount of water required is seldom available, and if it is used, it may waterlog the soil. Also, the leached salt usually pollutes groundwater or streams. One way in which farmers address the problem of salinization is by using genetically selected crops adapted to salinized soils.

As the number of hectares of farmland requiring irrigation increases, so does the water demand. When water is taken from surface streams and rivers, the normal flow is often severely reduced, changing the ecology downstream and reducing its biodiversity. Also, less water becomes available for other farmers downstream, a situation that often leads to disputes over water rights. In other cases, water is pumped from deep wells or aquifers. Drilling wells and pumping water from such sources can be expensive and may lead to additional problems, such as the sinking of land over aquifers. Such land subsidence is a major problem in several parts of the southern and western United States. Subsidence in urban areas can cause huge amounts of damage as water and sewer pipes, highways, and buildings are affected. In coastal areas, depletion of aquifers can cause the intrusion of saltwater into wells, rendering them unusable. In the United States, the federal government spends millions of dollars each year to repair damage to irrigation facilities.

Like many other human modifications to natural ecosystems, the use of water for irrigation achieves some remarkable but temporary advantages that are complicated by long-term environmental problems. Assessments of the total financial costs and environmental impacts of irrigation are continuously weighed against gains in production.

Bibliography

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