While the volume of well water used in irrigation is small compared to that obtained by gravity from flowing streams, yet, as re gards value, it may be said that some of the most important sources of irrigation supply are utilized through pumping. The force of flowing water itself is frequently employed to bring water up to the level of the irrigable land, the bucket-wheel having been utilized from the earliest historical times up to the present. An important source of power for pumping water is the wind. Over the broad valleys and plains of the arid region the wind movement is almost continuous for days and weeks. It is a comparatively simple and in expensive operation to sink a well into the water and erect a windmill, attaching this to a suitable pump. A windmill once erected on the plains is operated day and night by the wind, bringing to the surface a small but con tinuous supply of water. This small stream if turned out on the soil would flow a short distance, then disappear into the thirsty ground, so that irrigation directly from a windmill is usually impracticable. To overcome this diffi culty it has been found necessary to provide small storage reservoirs or tanks, built of earth, wood or metal, to hold the water until it has accumulated to a volume sufficient to permit a stream of considerable size to be taken out for irrigation. Such a stream, flow ing rapidly over the surface, will penetrate to a distance and cover an area much greater than is possible with the small flow delivered by an ordinary pump. One disadvantage con nected with the use of windmills is that most of them are constructed to operate only in moderate winds. As the strength of the wind increases, the wheel begins to revolve, increas ing in efficiency until the velocity of the wind is about 8 or 10 miles an hour. At greater speed the mills are usually so constructed that the efficiency decreases rapidly as the wind becomes more powerful. When it approaches a gale, the mill stops completely.
Although there are in use large numbers of windmills in pumping water for irrigation of small tracts, the aggregate area is small com pared with the extent of lands watered by more powerful devices, such as those made possible by the development of hydro-electric power. Within the past decade much attention has been given to this matter, particularly in con nection with the use of power developed for municipal and manufacturing purposes and which is available for farm use at seasons or times of day when not needed for the principal industry. It is possible at such times to obtain power at low rates and to utilize it in pumping water for agricultural purposes.
Application of The methods of irrigation practised in various parts of the United States differ according to the climatic conditions and soil, and especially with the early habits or training of the irrigatori. Water is applied to the irrigated field usually in one of three ways — by flooding, by furrows and by sub-irrigation.
Flooding.—This is done by the check sys tem or by wild flooding. By the latter process the irrigator turns the water from a lateral or distributing irrigation ditch over a level field and completely submerges it. Perfectly level fields are, however, comparatively rare, and the first step in primitive agriculture by irriga tion has been to build a low ridge around two or three sides of a slightly sloping field, so that the water is held in ponds. These low banks
are commonly known as levees or checks. In construction they are frequently laid out at right angles or more often following the con tour of the ground dividing the land into a number of compartments. Water is turned from the irrigation ditch into the highest of these compartments, and when the ground is flooded the bank of the lower side is cut or a small sluice-way opened, and the water passes into the next field, and so on until each in turn is watered.
Furrows.— Irrigation in checks has grad ually decreased in relative importance owing to the expense of leveling and leveeing the ground. With experience the irrigator has be come able to apply water to crops which are cultivated in furrows without resorting to such expensive means. The furrows are plowed in such a direction that the water when turned into them from the lateral ditches will flow freely down them without washing away the soil. When the water has completely filled the furrows, and has reached the lowest points, the little streams are cut off and turned into another set of furrows. The methods of doing this differ. Sometimes the irrigator simply cuts the bank of the distributing ditch with a shovel and then closes the opening after sufficient water has escaped. A more sys tematic method is employed in California. Water is carried to the upper end of the fur rows in a small box-flume with openings about one inch square in the side. These openings are closed by shutters and a number can be opened at once, permitting a certain quantity of water to escape into each furrow. The slope given the furrows determines to a certain extent the amount of water received by the soil. If the fall is very gentle, the water moves slowly and a large portion is absorbed while the furrow is being filled. If steep, the water quickly passes to a lower end and the ground does not absorb so much. When the entire field has been watered the furrows are usually plowed out and a thin layer of the top soil stirred to make an open, porous covering or mulch, preventing excessive evaporation and allowing the air to enter the ground. Without such cultivation a hard crust may be formed. The loosening of this crust breaks the capil lary connection with the moisture beneath and thus lessens the loss of water. For irrigating small grain the fields, brought to a uniform surface, are thoroughly cultivated, and after the grain has been sown, small parallel lines are made similar to furrows, but smaller and nearer together. These tiny channels are made either by a suitable drag or by a roller upon which are projections so arranged as to make small grooves in the soil. These are made in the direction of the desired slope, so that the water can flow down the marks through the grain as it would in furrows through a cornfield. The rapidly growing grain shades the surface and prevents the for mation of crust, rendering subsequent cultiva tion unnecessary. In order to cause the water to spread from the lateral ditches into the fur rows through the ground, use is made of a canvas dam or tappoon — a small sheet of metal of such shape as to fit across the ditch. This can be forced into the soft earth, making a small darn and causing the water to back up and overflow the field of grain.