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 justable gates by which the amount of water delivered to each furrow can be regulated. In the application of this water the attention of an irrigator is required to see that the proper amount is kept in each furrow, and that it does not break out and flood the surface, leav ing the furrow beyond the break without moisture. The success of this system de pends on having a soil in which vertical percolation is not excessive and lateral movement is adequate. The furrows must be so constructed as to maintain the velocity of the water within the safe limit permitted by the character of the soil; otherwise there will be an injurious erosion of the furrows. Where these conditions are secured it has several ad vantages; it thoroughly soaks the soil and accomplishes the distribution of moisture without saturating the surface and causing it to run together, as it does where water stands on it. In cultivating crops the displacement of the soil is re duced to a minimum; hence, the labor is reduced and the cost is less than by any method which distributes water so evenly and thoroughly. There is less loss from evaporation because a less area of water is exposed to the action of the air and sun.
Subirrigation.—Introducing water be low the surface for the watering of crops has several obvious theoretical ad vantages, such as the preventing of loss by evaporation, distributing water at levels which will encourage plants to strike their roots deep in the soil, and the reducing of frequent surface cultivation, which have caused inventors and culti vators to labor to secure a system which would be effective and cheap enough for general adoption, but thus far without complete success.
Irrigation Laws and Institutions.—In France, Spain, Italy, Egypt, and India all the more important irrigation works are owned and controlled by the govern ment. In Italy, especially, irrigation is highly developed. Most of the water for irrigation purposes is drawn from the Po river and from lakes Maggiore and Como. In undeveloped and sparsely set tled countries like Australia nearly all that has been done is the result of pri vate enterprise. In Canada all streams are state property and all irrigation works are built under licenses issued by the government, and in accordance with the plans prepared or approved by it. The Reclamation Service in the United States, operated by the national govern ment, was established by the Reclamation Act of June 17, 1902. By 1920 some 3,000,000 acres of land located in semi arid and arid regions had been made valuable both from the standpoint of agri culture and of homesteading. The irriga tion projects are in Ariz., Cal., Colo.. Idaho, Kan., Mont., Nebr., Nev., N. M., N. Dak., Okla., Ore., S. Dak., Utah, Wash., Wyo., Texas. The funds for this work come mainly from the sale of public lands and from oil-leasing royalties. According to the Reclamation Extension Act of Aug. 13, 1914, the money expended is returned to the fund by the settlers, through pay ments in fifteen or twenty annual install ments, without interest. Over 400,000 persons are living on the 1,780,000 acres irrigated by the Reclamation Service. The value of the crops in 1920 on the project land was nearly $114,000,000. In 1002 the average value of the desert land was not more than $10 per acre. Since the government has undertaken the irri gation of this territory, the average value has increased to $200 per acre, or a total of over $350,000,000. More than 13,000 miles of canals and ditches have been built on the projects. The Reclamation Service in 1920 served 2,845,000 acres of land, including 1,662,000 acres for which the government furnished the total water supply, and 1,183,000 acres to which the government sent stored water as an aid to the supply of private streams. The Mexican Government has taken an inter est in the question of irrigation, and has appropriated over $20,000 to be used for the improvement of the Yaqui River Val ley, Sonora. The table on page 199 shows the principal United States irrigation projects and their cost up to June 30,,1919.