The USGS Water Science School
Irrigation has been around for as long as humans have been cultivating plants. Man's first invention after he learned how to grow plants from seeds was probably a bucket. Ancient people must have been strong from having to haul buckets full of water to pour on their first plants. Pouring water on fields is still a common irrigation method today -- but other, more efficient and mechanized methods are also used.
Early man would have used this "low-tech" method of irrigating crops -- collect water in a bucket and pour it onto the fields. Today, this is still one of the most popular methods of crop irrigation. The system is called flood irrigation -- water is pumped or brought to the fields and is allowed to flow along the ground among the crops. This method is simple and cheap, and is widely used by societies in less developed parts of the world as well as in the U.S.
A large part, about 39 percent, of all the fresh water used in the United States goes to irrigate crops. After use, much of this water cannot be reused because so much of it evaporates and transpires in the fields. If you consider that the majority of irrigation occurs in the western U.S., where water is relatively scarce, you can see how important it is for farmers to find the most efficient methods of using their irrigation water.
Here are some things that farmers are doing to be more efficient:
For irrigating fruits and vegetables this method is much more efficient than flood irrigation. Water is sent through plastic pipes (with holes in them) that are either laid along the rows of crops or even buried along their rootlines. Evaporation is cut way down, and up to one-fourth of the water used is saved, as compared to flood irrigation.
Spray irrigation is a more modern way of irrigating, but it also requires machinery. This system is similar to the way you might water your lawn at home - stand there with a hose and spray the water out in all directions. Large scale spray irrigation systems are in use on large farms today. These systems have a long tube fixed at one end to the water source, such as a well. Water flows through the tube and is shot out by a system of spray-guns.
A common type of spray-irrigation system are the center-pivot systems. They work in the same way you might water your yard. If you placed a faucet in the center of your yard, you could take a hose, punch holes all along it, and attach a spray gun at the end. Turn the water on, pull it tight, and start spraying (water is also spraying from the holes in the hose at the same time). While you are spraying you are also walking around in a circle (with the faucet at the center of the circle). Using this method you can get a very large circle of lawn watered with just a short hose.
The center-pivot systems have a number of metal frames (on rolling wheels) that hold the water tube out into the fields. And there can be a very big water gun at the end of the tube. Electric motors move each frame in a big circle around the field (the tube is fixed at the water source at the center of the circle), squirting water.
If you've been in an airplane you can easily locate center-pivot irrigation systems on the ground. You can't miss them -- just look for green circles of irrigated land below.
By use of traditional spray irrigation, water basically is just shot through the air onto fields. In the dry and windy air of the western U.S., a lot of the water sprayed evaporates or blows away before it hits the ground. Another method, where water is gently sprayed from a hanging pipe uses water more efficiently.