Environmental Science: Agricultural Uses of Water
Agriculture, the growing of plants as food, is the largest consumer of fresh water on Earth, accounting for nearly 70 percent of all freshwater withdrawal. One of the biggest challenges of farming in some regions of the world is locating enough fresh water to support crops.
In the drive to meet the food needs of growing human populations, farmers have extended their croplands into drier regions that are farther from natural, seasonal sources of water. As a result, farmers have to build irrigation systems to bring water to the crops in these drier regions.
Irrigation systems come in many different forms, depending on the landscape, the regional water availability, and the water needs of the crops. A few of the most common types of irrigation systems are
Furrow irrigation: Furrow irrigation involves digging furrows, or channels, alongside rows of crops. It’s one of the oldest methods of irrigation and was used by ancient civilizations in Egypt and Mesopotamia. By digging shallow ditches along a gentle slope, farmers rely on the pull of gravity to transport the water from a nearby river or stream into their crop fields.
The main problem with furrow irrigation is that it isn’t the most efficient way to water crops. In some regions, as much as 35 percent of the water transported to the crops evaporates or runs off the field without being absorbed into the soil.
Flood irrigation: Flood irrigation uses a natural source of nearby flowing water, such as a river or stream, and periodically diverts the water to flood agricultural fields. This irrigation method allows the water to completely cover and soak into the fields. It’s more efficient than using furrows because it loses only 15 to 20 percent of the water to evaporation or runoff.
Drip irrigation: Drip irrigation applies small amounts of water more directly to the plants that need it. This localized irrigation system uses hoses and pipes to drip water onto (or just below) the soil surface.
Losing only 5 percent of the water to evaporation, drip systems are very efficient. They work best in fields that don’t need to be plowed every season because the drip hoses are woven through the field at or below the soil surface.
Sprinklers: Like drip irrigation systems, sprinklers use pipes and hoses to move water. But unlike drip irrigation systems, sprinklers spray water over the fields from above and, thus, require a form of energy to pump the water through the pipes.
The efficiency of sprinkler systems varies: Some systems lose up to 25 percent of the water, while others lose only about 5 percent. In large agricultural fields, farmers often use sprinklers that are mounted on wheeled systems that move through the fields.
Another common sprinkler is the traveling sprinkler system, which sprays water from a long arm that pivots around a center point. If you’ve ever seen a bird’s-eye view of agricultural fields, you may have noticed fields laid out in circles across the landscape; this circular layout is a result of the traveling sprinkler system, which effectively waters a circle of crops from its center pivot point.
Determining which irrigation system is most sustainable for a particular region depends on many factors, including the availability of water and energy resources, the size and layout of the coverage area, the system costs, and the overall efficiency (which depends, in part, on local soil and weather conditions).
The development of hydroponic agriculture offers a new approach to reducing agricultural water use. A hydroponic system grows crops in a greenhouse without using soil. Instead of soil, the crops are “planted” in nutrient-rich water.
The water not used by the plants is recycled and reused, and the growing conditions are controlled from above (by the greenhouse) and below (by the nutrient solution) to be ideal for maximum crop production. Hydroponic agriculture requires extra costs upfront to set up the greenhouse facility, but in the long term, the method saves water and soil resources and also reduces the need for pesticides.