Page:Collier's New Encyclopedia v. 05.djvu/250

LEFT IRRIGATION 198 IRRIGATION sugar consumed in the United States. It has increased the population and agri- cultural wealth of New Zealand and been greatly extended in Mexico and Central America. Methods — The water used on the greater part of the irrigated area is dis- tributed by gravity. The supply is tapped at a point higher than the land to be watered; thence it is carried in canals or ditches, which follow the con- tour of the country, to the place of use. Many of these canals are important en- gineering achievements. The Cavour Canal, in Italy, cost over $17,000,000, and has doubled the population of the valley of the Po in the past 40 years. The Ganges Canal is the greatest system in India. The upper Ganges has 890 miles of main channel, 3,700 miles of laterals, 282 miles of outlet channels, 31 locks, 202 bridges, and 17 dams. It was com- pleted in 1884, irrigates 1,205,000 acres, and cost $14,644,000. The lower Ganges system embraces 531 miles of main canal and 1,854 of lateral canals. It irrigates 620,000 acres, and cost $7,- 000,000. The W. Jumna Canal, begun in 1875, has cost over $8,000,000, and has a total length of 1,200 miles. In the United States there are several canal systems, and at Lagrange, California, is the highest overfall dam in the world. The lifting of water from wells or streams by hand or by oxen was one of the great means of ancient irrigation. In recent years this has been supple- mented by an important use of steam- pumping plants. There are 1,500 of these in the San Joaquin valley of Cali- , f ornia alone. A large part of the water used in the irrigation of cane in the Hawaiian Islands is raised by pumps. Some of those are of extraordinary size and capacitv, being capable of lifting 20,- 000,000 gallons 500 feet vertically in 25 hours, 'ihe method of distributing wa- ter from canals must vary according to the amount of water available, the na- ture of the soil, the topography of its surface, and the character of the crop, and there can be no single best method under all circumstances. Flooding. — Under the flooding system small parallel ditches are constructed about 100 feet apart, the plans varying with the slope of the ground. These small ditches are usually fed from a lateral from the main canal. Where the surface is broken, they will be irregular in outline, and will generally approxi- mate a right angle to the steepest slope of the soil. They are usually simply fur- rows made with a heavy mould-board plow, and where the crop is grain they are filled back by the plow before har- vesting. In the distribution of water they are cut at convenient points to al- low the water to run out and spread over the surface. The water thus released if left alone would follow the lines of quickest descent, and in doing so spread out as it proceeds, dividing into numer- ous branches or rills as it increases its distance from the opening of the ditch. For the purpose of facilitating its spreading and to insure its application to every portion of the surface, the irriga- tor follows its course and guides it by moving a few shovelfuls of earth here and there, thus separating the various small rills and starting the branches in different directions. This is a cheap but somewhat disagreeable method, because the irrigator has to work in the water and use all his strength and speed in or- der to direct the water so that it will reach all points, which it seldom does in equal volume. It has for its chief claim cheapness and speed, but sacrifices econ- omy. Under this system an experienced irrigator can cover from 10 to 20 acres a day, the area depending on the charac- ter of the land, the uniformity of the surface and the volume of water at his disposal. For such crops as grain, al- falfa, clover, and the various grasses, and for everything which is sown broad- cast and is distributed uniformly over the surface, the flooding system is em- ployed. Checks. — A modification, and in some respects a great improvement, of irriga- tion by flooding consists in the use of what are called compartments or checks. It is really flooding by the use of levees, which restrict the movement of the water to definite areas, thus giving as- surance that it shall remain on this area till it sinks out of sight or is drawn off. The compartments or squares vary in size according to the slope of the ground and with the locality. The Mexican irri- gators in the S. W. make compartments less than 100 feet square, while in Cali- fornia and Arizona they often contain several acres. Furrow Method. — Under this method parallel furrows are plowed which lead from the lateral used across the rows of crop to be irrigated. A small opening is made in the ditch to let the water be- tween each of these rows. There are numerous devices for regulating the turning of water into the furrows from the field lateral, the simplest being the placing of a dam of canvas or earth below the opening through which water is to be run; but in many places the lat- eral is constructed permanently, often consisting of a plank or cement flume with openings in its side, having ad-