Page:The New International Encyclopædia 1st ed. v. 10.djvu/899

IRRIGATION. 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 check, 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 This method is little used except for orchard, vineyard, garden, and rice irrigation, being largely superseded for other purposes by the contour check method.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) Depressed bed method, in which the ditches are carried 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 check system, and is used in the growing of vegetables and small fruits. It is best suited to porous soils, which require frequent 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. (5) 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 be used depends upon the character 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. Laterals or supply ditches are usually taken across the slopes of the land at intervals of about forty rods. 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 into 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) Subirrigation, or distribution 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 is expensive and of doubtful practicability, except on a limited scale, in greenhouse and other horticultural work. A similar method, known as ‘underflow irrigation,’ consists in opening furrows 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. (8) Distribution by means of underground pipes, standpipes, and connections for 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 Wickson, a method to be of value must secure the following results: “(1) Distribution 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 employment of extra hands; (3) economy of water in the prevention of waste by overflow or evaporation or by rapid percolation, and in placing the water where it will do the most good; (4) leaving the land in the best condition for attaining with least labor a state of tilth which conserves moisture and at the same time favors thrift in the plant.” Crops sown broadcast can be irrigated only by flooding or sprinkling. Flooding is also best adapted to very loose soils. The contour check and furrow method require least labor. The furrow method is best suited to subsequent cultivation by horse-power, which is a matter of great importance, since thorough cultivation, to overcome the compacting tendency of irrigation and to secure a soil mulch, should in all cases 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.

. The amount of water required to irrigate different crops under varying conditions is a matter of fundamental importance, but has never been very accurately determined, and there is no very accurate simple means of judging when a soil needs irrigation. The term duty of water is commonly used to express the number of acres which a given quantity of water will adequately irrigate and is best stated in acre-feet or acre-inches, which are the