The water well below the pump may be bored, or a perforated well pipe may be driven; or several well points may be connected. The kind of well must depend upon the condition of the earth and the nature of the water supply. Driven wells are more successful when water is found in a stratum of coarse gravel.
Before buying irrigation machinery, it is a good plan to test the water supply by temporary means. Any good farm pump may be hitched to a gasoline engine to determine if the water supply is lasting or not. Permanent pumping machinery should deliver the water on high ground. A main irrigation ditch may be run across the upper end of the field. This ditch should hold the water high enough so it may be tapped at convenient places to run through the corrugations to reach the roots of the plants to be benefited. There are different systems of irrigation designed to fit different soils. Corrugations are the cheapest and the most satisfactory when soils are loose enough to permit the water to soak into the soil sideways, as well as to sink down. The water should penetrate the soil on both sides of the corrugations for distances of several inches. Corrugations should be straight and true and just far enough apart so the irrigation water will soak across and meet between. Some soils will wash or gully out if the fall is too rapid. In such cases it may be necessary to terrace the land by following the natural contour around the ridges so the water may flow gently. Where the fall is very slight, that is, where the ground is so nearly level that it slopes away less than six inches in a hundred feet, it becomes necessary to prepare the land by building checks and borders to confine the water for a certain length of time. Then it is let out into the next check. In the check and border system the check bank on the lower side has an opening which is closed during the soaking period with a canvas dam. When the canvas is lifted the water flows through and fills the next check. This system is more expensive, and it requires more knowledge of irrigation to get it started, and it is not likely to prove satisfactory in the East.
For fruits and vegetables, what is known as the furrow system of irrigation is the most practical. An orchard is irrigated by plowing furrows on each side of each row of trees. The water is turned into these furrows and it runs across the orchard like so many little rivulets. Potatoes are irrigated on the same plan by running water through between the rows after the potatoes have been ridged by a double shovel-plow. This plan also works well with strawberries. After the land is prepared for irrigation, the expense of supplying water to a fruit orchard, strawberry patch or potato field is very little compared with the increase in yield. In fact, there are seasons when one irrigation will save the crop and produce an abundant yield, when otherwise it would have been almost a total loss.
Overhead Spray Irrigation.—The most satisfactory garden irrigation is the overhead spray system. Posts are set ten feet apart in rows 50 feet apart. Water pipes are laid on the tops of the posts and held loosely in position by large staples. These water pipes are perforated by drilling a line of small holes about three feet apart in a straight line along one side of the pipe. The holes are tapped and small brass nozzles are screwed in. The overhead pipes are connected with standpipes at the highest place, generally at the ends of the rows. The pipe-lines are loosely coupled to the standpipes to permit them to roll partly around to direct the hundreds of spray nozzles as needed.
Figure 127.—Overhead Irrigation. Diagram showing the arrangement of pipes for irrigating one acre of land. The pipes are supported on posts six feet high.
Six feet high is sufficient to throw a fine mist or spray twenty-five feet, which is far enough to meet the spray from the next row, so the ground will be completely covered. To do this the pipes are rolled from one side to the other, through a 90 degree arc to throw the spray on both sides. The pipes usually are laid with a grade which follows down the slope of the land. A fall of one foot in fifty is sufficient. Water is always admitted at the upper end of each pipe-line to flow down by gravity, assisted by tank pressure. A pressure of about forty pounds is needed to produce a fine spray, and to send it across to meet the opposite jets. The little brass nozzles are drilled with about a one-eighth inch hollow. But the jet opening is small, about No. 20 W. G. This gives a wire-drawn stream that quickly vaporizes when it meets the resistance of the atmosphere. When properly installed a fine misty rain is created, which quickly takes the same temperature as the air, and settles so gently that the most delicate plants are not injured.
Quantity of Water to Use.—Good judgment is necessary in applying water to crops in regard to quantity, as well as the time of making application. Generally speaking, it is better to wait until the crop really needs moisture. When the pump is started give the crop plenty with the expectation that one irrigation will be sufficient. Much depends upon the amount of moisture in the soil; also the kind of crop and weather conditions enter into the problem. On sandy land that is very dry where drainage is good, water may be permitted to run in the corrugations for several days until the ground is thoroughly soaked. When potatoes are forming, or clover is putting down its big root system, a great deal of water is needed. Irrigation sufficient to make two inches of rainfall may be used to advantage for such crops under ordinary farming conditions. It is necessary after each irrigation to break the soil crust by cultivation to prevent evaporation. This is just as important after irrigation as it is after a rain shower. Also any little pockets that hold water must be carefully drained out, otherwise the crop will be injured by standing water. We are not supposed to have such pockets on land that has been prepared for irrigation.