HEA [WORKS of some kind are required for nearly all canals. flumes, or pipe - lines. In America they are generally of timber, but in much of the foreign work they are permanent structures of stone. The essential features are a bulkhead, gates, and wasteway. Where there is a dam at the head of the canal, the head works may be at one end of it, or form a part of it.
pima be substituted for canals i r flumes, either to convey water across depressions or under streams, a; already mentioned. or to prevent losses from both evaporation and seep age. Either riveted steel or wood staves are the materials most •omnionly used for such pipes, being preferable to east iron on account of their relative lightness, and consequent ease of trans portation in rough country, remote from rail so that the flow is moderate and uniform and the Nutter can be readily distributed to lateral ditches or flumes at any desired point. The simplest ways. Where the water is tinder little or no pressure either vitrified clay or cement pipes are sometimes used, particularly in southern Cali fornia.
WoaNs FOR FINAL DISTRIBUTION consist chief ly of open or closed channels, generally the former, leading front the main or branch canal or other conduit to the land to he 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 engi neering. depending on the crop and the soil and also the ideas of the cultivator. Subsurface irrigation is generally considered impracticable beeans of the difficulties experienced in securing a thorough spreading of the water, besides which the pipes may clog and the eonstniction prove expensive. Surface application, therefore, is al most universally employed. Broadly speaking. the latter is effected either by flooding the whole surface or sending the water through furrows. Neither involves much 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 he used, and hydrants must be provided for draw ing nut the water.
The distribution of water by means of under ground 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 irri gated, is made to follow the contour of the land, 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.
Wi•kson describes the following methods prac ticed 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 neater is released by spade cuts at intervals in the banks, or it is made to over flow 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. (In account of the labor involved and the difficulty of securing uniform irrigation, this method has been superseded in many places by one of the cheek systems. (2) Flooding in contour cheeks 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 cost is considerable, but it permits more effective irrigation with less labor than free flooding. In this method the highest check is filled from the ditch or lateral, and the water is either allowed to overflow into the next lower cheek, or is drawn off into it by means of gates provided for the purpose. (3) Flooding in rectangular checks, or level areas of approximately the same size inclosed by low embankments. This method is little used except for orchard, vineyard, garden, and rice irrigation, being largely superseded for other purposes by the contour cheek method.
Unless the land is quite level, its preparation for the method involves the shifting of a large amount of earth, and the levees of irregular heights which are required interfere with the use of power machinery in cultivating. (4) De pressed bed method, in which the ditches are car ried on the tops of the levees and the water is allowed to soak out into the checks inclosed by the levees. This is a garden modification of the rectangular cheek system, and is used in the growing of vegetables and small fruits. It is best suited to porous soils, which require fre quent irrigation. A primitive form of this method is ridge irrigation, in which plants are grown on the sides or at the base of raised ditches. (a) Furrow irrigation, or running water in furrows between the rows of crops, is the simplest, cheapest, and most widely used method of irrigating crops which can be grown to advantage in rows, and is adapted to a wide range of slope and soil conditions. if the slope is not too great to carry a small stream without excessive washing, the rows are run straight down the grade from the supply ditch or flume, which occupies the crest of the highest ground: otherwise the rows are run diagonally at the angle giving the proper grade. The length of furrow that can he used depends upon the char acter of the soil and the head of the water. The more porous the soil, the larger should be the stream or the shorter the furrow. For most field and garden crops a larger stream and a shorter run are used than for fruit-trees. Late rals or supply ditches are usually taken across the slopes of the land at interval, of about forty reds. The laterals should be as nearly level as possible, so that they can be kept full and will discharge uniform amounts of water through the openings into the furrows. (6) Raised-bed irrigation, in which a raised bed is surrounded by a small ditch from which the water passes itto the soil by seepage and capillary action, is a modification of the furrow system, especially suited to rather heavy, retentive soils in which water moves readily. (7) Subir•igation, or dis tribution by means of underground pipes with suitable outlets, or from tile drains or blind ditches, from which the water can rise to the roots of plants by capillarity. The method expensive and of doubtful practicability, except on a limited scale, in greenhouse and other horti cultural work. A similar method, known as irrigation,' eonsists in opening fur rows at considerable distances apart and keeping them filled with water until the ground water rises so that it can reach the roots of plants by capillarity. The method is little used. (S) Dis tribution by means of underground pipes, stand pipes, and connections fur sprinkling is a method which is considered too expensive for use on a large scale.
The choice of a method must be determined by the amount of water available, the slope and nature of the land, the character of the crop, etc.
According to Wick:son, a method to be of value must secure the following results: "(1 ) Distribu tion of moisture evenly throughout the soil mass to as great a depth as possible. providing it does not sink beyond the reach of the plant by root extension nor beyond recovery by capillary rise; (2) economy of labor both in aggregate time and in the feasibility of operating without employ ment of extra hands: (3) economy of water in the prevention of waste by overflow or evapora tion or by rapid percolation, and in placing the water where it will do the most good; (4) leav ing the land in the best condition for attaining with least labor a slate of tilth which conserves moisture and at the same time favors thrift in the plant." Crops sown broadcast can be irri gated only by flooding or sprinkling. Flooding is also best adapted to very loose soils. The contour cheek and furrow method require least labor. The furrow method is best suited to subsequent culti vation by ho•se-power, is a matter of great importance, since thorough cultivation, to over come the compacting tendency of irrigation and to secure a soil mulch, should in all eases follow as soon after irrigation as the condition of the soil will permit. Deep and thorough preparation of the soil increases its storage capacity for water, and frequent cultivation of the surface reduces loss from evaporation. thus reducing materially the amount of irrigation required and enabling the soil to utilize to the best advantage whatever rainfall may occur.