Pipes may be substituted for canals or flumes, either to convey water across depressions or under streams, as already mentioned, or to prevent losses from both evaporation and seepage. Either riveted steel or wood staves are the materials most commonly used for such pipes, being preferable to cast iron on account of their relative lightness, and consequent ease of transportation in rough country, remote from railways. Where the water is under little or no pressure either vitrified clay or cement pipes are sometimes used, particularly in southern California.
Fig. 7. HEAD-GATE FOR SMALL IRRIGATION CANAL.
Works for Final Distribution consist chiefly of open or closed channels, generally the former, leading from the main or branch canal or other conduit to the land to be irrigated. For the most part small ditches are employed, with permanent or movable gates, or temporary earth dams, to divert the water to or from the minor channels. Except as modified by the topography, the application of water to land is chiefly a detail of agriculture rather than engineering, depending on the crop and the soil and also the ideas of the cultivator. Subsurface irrigation is generally considered impracticable because of the difficulties experienced in securing a thorough spreading of the water, besides which the pipes may clog and the construction prove expensive. Surface application, therefore, is almost universally employed. Broadly speaking, the latter is effected either by flooding the whole surface or sending the water through furrows. Neither involves much engineering skill, but it is best to have the main channels located with the aid of a level, particularly where the areas are large or have an irregular surface. Where pipe systems are employed for final distribution thin wrought iron or steel, or vitrified pipe may be used, and hydrants must be provided for drawing out the water.
Fig. 8. TAPPOONS (WOOD AND METAL) FOR USE IN IRRIGATION.
The distribution of water by means of underground pipes, standpipes, and hose is, however, too expensive for irrigation on a large scale. The open ditch, which takes the water out of the larger canal or reservoir to the land to be irrigated, is made to follow the contour of the land, so that the flow is moderate and uniform and the water can be readily distributed to lateral ditches or flumes at any desired point. The simplest method of turning water from a ditch is to cut a hole in the side and to use earth to make a dam in the ditch. An improvement on this method is the use of portable cloth, wood, or metal dams or ‘tappoons.’ The water is spread over the land by a variety of methods, which belong, as a rule, in three main classes: (1) Flooding, (2) furrow irrigation, and (3) sub-irrigation.
Fig. 9. IRRIGATION BY FLOODING.
Fig. 10. CHECKWORK IRRIGATION.
Wickson describes the following methods
practiced in the Western United States: (1) Free
flooding, or running water on the land without
restraint, except that of the banks of the
lateral conveying it. In this method the ditches
or laterals are carried along the higher parts of
the field and the water is released by spade cuts
at intervals in the banks, or it is made to overflow
the banks by means of dams, as described
above. This is the oldest and simplest method of
irrigation. It is best suited to small grains and
forage plants which are sown broadcast, and is
most effective with nearly level ditches and on
land of uniform grade. On account of the labor
involved and the difficulty of securing uniform
irrigation, this method has been superseded in
many places by one of the check systems. (2)
Flooding in contour checks or irregular areas
of land inclosed by low embankments, the size
and shape of these areas being determined by the
inequalities of the surface. This method is best
adapted to land of very gentle slope. Its first
