|Agricultural Water Use|
The images of seemingly endless crop fields of the American Midwest and the lush San Joaquin Valley of central California are powerful symbols of the agricultural might of the United States. In the past century, the United States has become the greatest producer of food in the world.
Water has always been a vital part of agriculture. Just like humans, crops need water to survive and grow. The process where dry land or crops are supplied with water is called irrigation. A century ago, the relatively small fields of a local farmer in many areas of the United States could receive enough moisture from rainwater, along with water that could be diverted from local streams, rivers, and lakes.
The growth of huge corporate farms that are thousands of acres in size has taken the need for water to another scale. For these operations, water needs to be trucked in, pumped up from underground, and obtained from surface water (freshwater located on the surface) sources in large quantities.
In modern times, in countries such as the United States and Canada, agriculture is not the largest user of water but is the largest consumer of water. Other activities such as the oil industry use more water than does agriculture. But, in these other industries, much of the water is put back into the ground or surface water after being used. Agriculture consumes water; the water does not go back to the surface or to the groundwater.
Uses of water in agriculture
There are four main areas of water use in agriculture: growing of crops, supplying drinking water to livestock, cleaning farm buildings and animals, and supplying drinking water for those who work on the farm. The amount of each category varies according to the type of farm.
For example, farms in the eastern part of North America usually receive enough rainfall and water from melting snow to meet most of the water needs. But drier areas, such as the U.S. and Canadian prairies, regions of Mexico, and some mountainous regions of the West do not receive sufficient natural moisture. On these farms, water must be supplied through irrigation.
Nearly 60% of the world’s freshwater that is used by humans is used for irrigation. Of this water that is applied to crop fields, only about half returns to surface water or groundwater sources. The rest is lost by natural processes such as evaporation (when liquid water changes to water vapor) and transpiration (when water from plant leaves is transformed into water vapor), and accidental occurrences such as leakage from pipes or spillage.
There are various methods to irrigate crops. The oldest, "low-tech" way is to flood the field. This flood type of irrigation has been used for centuries and remains popular for crops like rice. Field flooding is very wasteful, since only about half of the water used actually gets to the plant.
The efficiency of flood irrigation can be improved by making the land contoured, such as eliminating small hills and putting steps (terraces) on larger hills to prevent water from flowing over certain portions of the field and gathering in another part of the field. The flooding of a field can also be controlled by releasing water from dams (barriers) alongside the field, adding water to the field only when needed. Water that flows off of a field can be captured in a pond and re-used.
A newer and much more efficient technique of water use is called drip irrigation. In drip irrigation, water runs through pipes that have tiny holes in them. When buried underground, water can ooze out of the pipe into the soil near the roots of the plants. The loss of water is reduced and less water is required to grow the crops.
A popular means of irrigation is spraying. Water flows through a tube and is shot out through a system of spray nozzles positioned along the length of the tube. The tube can be fixed in one position or can be moved manually or automatically from place to place. A visual example of a spray irrigation system is a green circle seen from an airplane passing over farmland.
The green circles are crops that are being irrigated by a circular sprayer. Spray irrigation is sometimes wasteful, as water that is sprayed can evaporate or be blown away before hitting the crop. Some farmers now use an irrigation method where water is gently sprayed from pipes that are suspended over the crop. This method allows about 90% of the water to reach the crop.
With the knowledge that surface and ground waters are resources that can be overused, agricultural scientists and modern farmers are paying attention to methods of conserving and re-using water while maintaining the growth of their crops.