Enough water must be available to permit rapid flooding of each border strip (the area between the dikes). A minimum of 225 gallons per minute per acre in the largest border is recommended. Since one man can irrigate a relatively large acreage in a day, larger irrigation streams than those specified may be desirable in order to utilize labor efficiently.

Advantages of the contour border method are: Efficient and uniform distribution of irrigation water is easily obtained, maximum utilization can be made of rainfall, adequate drainage facilities are easily provided, operation of the system is simple and easy, initial installation costs are low, and operational costs are relatively low.

Limitations are: The contour borders are restricted in use by the required combination of soil characteristics and topographic conditions we mentioned, land smoothing is required, relatively large irrigation streams are usually required, and the maintenance of ditches and structures is a problem.

The surface of the area to be irrigated should be smoothed by two or more passes with a landplane. The number of passes depends on the relative smoothness of the existing surface.

Smoothing tends to permit better alinement of the levees and a more uniform application of water and provides for more effective drainage.

GRADED OR PARALLEL BORDER irrigation systems have parallel strips of land separated by low earth ridges on fields that have previously been leveled to a desired grade. The strips are transversely level between borders and normally run down the predominant slope. If the land has been leveled in benches, the strips run perpendicular to the predominant slope.

A large stream of water is introduced at the head of the border strip. A sheet of water, confined by the ridges or borders, is advanced down the slope. It enters the soil as it goes. The desired amount of the water is applied to the strip, usually before the sheet has reached the lower end. The water is then cut off; when the receding sheet of water has reached the lower end, the entire strip has been irrigated.

The graded border method is adapted to most soils whose profile and topography permit the required leveling at a reasonable cost and without lasting reduction in productivity.

It can be used efficiently on all soils except those that have extremely high or extremely low intake rates. Soils with high intake rates require excessively large irrigation streams and very short strips if losses from deep percolation at the upper ends of the strips are to be kept within reason. Soils with low intake rates require relatively small irrigation streams, and it is hard to avoid excessive runoff from the lower ends of the strips and to get adequate coverage over the land.

This method is recommended for the irrigation of close-growing crops that are not damaged by temporary flooding. It is used primarily for irrigating hay crops, pastures, and small grains.

The topography of land to be irrigated by this method must be flat to very gently sloping. The slopes should be sufficiently uniform to permit land leveling within reasonable and economic limits. Slopes should ordinarily not exceed 2 feet per 100 feet.

The method has a number of advantages. The initial investment is relatively low, because the border ridges can be constructed with ordinary farm implements. The greatest initial expense is for land leveling; if that cost can be kept low, the total initial investment will be modest. With a properly installed system, one man can irrigate a relatively large acreage. Labor costs thus are relatively low. If pumping is involved, this cost will be low, since the system does not operate under pressure. Uniform distribution and resulting high water-use efficiencies may be obtained through proper design, layout, and operation. Relatively large irrigation streams may be used efficiently. Operation of the system is simple and easy. Properly graded border strips give excellent surface drainage if adequate outlet facilities are provided.

On the other hand, a more precise land-leveling job is required for graded borders than is needed for other surface methods. Relatively large irrigation streams are needed usually in excess of 900 gallons per minute. Light applications, less than 2 inches deep, cannot ordinarily be made without difficulty and without sacrificing efficiency. Difficulties often are encountered on soils that tend to bake or crust after becoming wet. The border ridges present a maintenance problem and may hinder the use of machines.

A precise land-leveling job is a normal prerequisite to successful irrigation by the graded border method. The land should be graded to a slope of not more than 1 foot per 100 feet (in the direction of irrigation) for hay crops and small grains. Slopes up to 2 feet per 100 feet are permissible for irrigating permanent pastures.

If the crop to be irrigated is to be rotated with row crops irrigated by the furrow method, the permissible slopes for furrows in the direction of irrigation will be applicable.

The border strips should be transversely level if possible. The cross slope should never exceed 0.10 foot in the width of one strip.

The width, or distance between border ridges, is largely governed by the slope of the land and the size of the irrigation stream that is available and can be safely turned into the head of the strip. The selected border strip width whenever possible should be a multiple of the width of the least flexible farm implement to be used.

SUBIRRIGATION is a method of applying water beneath the ground surface rather than on the surface.

Subirrigation is accomplished usually by controlling the water table and maintaining it at some predetermined depth below the surface. Moisture then reaches the plant roots through capillary movement upward.

This method has several major advantages. Subirrigation can be used effectively on soils that are difficult and expensive to irrigate by other methods. Except where water is introduced through tile drains, the installation costs are low. Labor requirements are low. Subirrigation is adaptable to a large variety of crops and does not interfere with tillage practices.

Subirrigation also has limitations. The main one is the unusual combination of natural conditions required. Only water of good quality may be used. Soils may become saline unless careful control is exercised. High levels of fertility may be hard to maintain.

Several conditions must be met. An adequate supply of water relatively free of salts must be available throughout the growing season. The topography must be nearly level and comparatively smooth. Land leveling or smoothing is required in most cases for best results. A layer of soil must exist immediately below the surface soil, which is sufficiently permeable to permit the free and rapid movement of water both laterally and vertically. A barrier against excessive losses through deep percolation must exist in the soil profile. This barrier may be in the form of a relatively impervious layer in the substratum or a permanently high natural water table on which an artificial table can be maintained. An adequate drainage outlet is needed.

Relatively few places exist in the humid area where all these conditions occur together. The use of subirrigation therefore is limited. Among the more well-known areas where sub-irrigation is practiced are the Everglades of southern Florida, the Flatwoods of the Florida Coastal Plain, and smaller, scattered localities with organic soils in Michigan, Indiana, Minnesota, Wisconsin, and Ohio.

The principles of Subirrigation are the same in all areas, although the means of introducing water into the soil profile may differ. The controlled water table is usually 12 to 30 inches below the surface.