Irrigation and agriculture

Because agriculture is basic to human existence, irrigation has been practiced since prehistoric times. Essentially, irrigation is the application of water to soil to overcome soil moisture deficiency so that crops can have an adequate water supply for optimal food production. Irrigation is essential to sustained large-scale food production.

Background

Irrigation systems were important to many ancient civilizations. They were the basis of life in the ancient civilizations of Egypt, India, China, and Mesopotamia (modern day Iraq). Some irrigation works in the Nile Valley that date back to around 3000 BCE still play an important role in Egyptian agriculture. In the United States, the first irrigation systems were developed by Native Americans, and traces of ancient water distribution systems, made up of canals, were still visible at the beginning of the twenty-first century.

Scope and Land Requirements

In 1977, the Food and Agriculture Organization (FAO) of the United Nations estimated that the total global land area under irrigation was 223 million hectares. By 2021, about 353 million hectares were irrigated worldwide. In the United States, more than 20.07 million hectares are irrigated for crop production, according to the US Department of Agriculture in 2023. Some form of irrigation is practiced in every country in the world. Although irrigation results in increased food production, it is extremely water intensive. For example, to grow 1 metric ton of grain (adequate for 50 percent of an average person’s supply for five years and six months) requires as much as 1,700 cubic kilometers of water per person per year. In the United States, 40 percent of total freshwater withdrawals is for irrigation. The value of irrigation is that it greatly increases agricultural productivity. For example, in 1979, the FAO reported that although irrigated agriculture represented only about 13 percent of global arable land (agricultural land that, when properly prepared for agriculture, will produce enough crops to be economically efficient), the value of crop production from irrigated land was 34 percent of the global total production.

For irrigation to be economically viable, the land in consideration must be able to produce enough crops to justify the investment in irrigation works. The land must be arable and irrigable; that is, sufficient water for irrigation must exist. Soil suitable for irrigation farming has the following attributes. The soil must have a reasonably high water-holding capacity and be readily penetrable by water; the rate of infiltration (percolation) should be low enough to avoid excessive loss of water through deep percolation beyond the root zone of the crops. The soil must also be deep enough to allow root development and permit drainage of the soil, and it must be free of harmful (toxic) salts and chemicals—especially those that tend to bond to soil and reach dangerously high concentrations. Finally, it must have an adequate supply of plant nutrients.

Land slopes should permit irrigation without excessive runoff accompanied by high rates. The land should be located in an area where irrigation is feasible without excessive pumping or conveyance costs. Generally, the land should permit the planting of more than one type of crop so that the investment in irrigation works can be utilized year-round and ideally should allow the flexibility of planting more economically viable crop types should economic conditions dictate such changes.

Types of Irrigation Systems

Generally, irrigation systems can be classified as nonpressurized systems (also known as gravity or surface systems) and pressurized systems. Historically, nonpressurized systems, in which water was flooded onto the soil surface via open channels, were the first to be constructed. In fact, nonpressurized systems preceded pressurized ones by thousands of years. Nonpressurized systems include canals, open channels, and pipes that are not flowing full. Pressurized systems include all types of sprinkler systems and low-pressure nozzle systems.

There are five basic methods of implementing irrigation systems: flooding, furrow irrigation, subirrigation, trickle irrigation, and sprinkling. Several subcategories exist within these five basic categories. Flooding systems include wild flooding, controlled flooding, check flooding, and basin flooding applications. In all cases, the irrigated area is flooded with water. The degree to which flooding is controlled or administered differentiates the types of flooding. For example, in wild flooding, there is not much control or preparation of the irrigated land. In contrast, check flooding is accomplished by admitting water into relatively level plots surrounded by levees. In check flooding, the check (area surrounded by levees) is filled with water at a fairly rapid rate and the water is allowed to infiltrate into the soil.

Furrow irrigation is used for row crops—hence the name (a furrow is a narrow ditch between rows of plants). In this method, evaporation losses are minimized and only about 20 to 50 percent of the area is wetted during irrigation, in contrast to flooding irrigation. In sprinkler application, water is sprinkled on the irrigated land. The sprinkling is possible because the water is delivered under pressure. Sprinkler systems provide a means for irrigation in areas where the topography does not permit irrigation by surface methods.

Subirrigation methods are useful in areas where there is permeable soil in the root zone and a high water table. In this method, irrigation water is applied below the ground surface to keep the high enough so that water from the capillary fringe is available to crops. Subirrigation has the advantages of minimizing evaporation loss and requiring minimal field preparation. In trickle (or drip) irrigation, a plastic pipe with perforations is laid along the ground at the base of a row of crops. The water issuing from the perforations is designed to trickle. Excellent control is achieved, and evaporation and deep percolation are minimized.

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