Irrigation, the watering of lands by currents distributed over or near the surface, and also by temporarily flooding them. It is one of the oldest of arts, was practised by the ancient Egyptians, Arabians, Assyrians, Babylonians, and Chinese, and has always formed a part of the agriculture of the countries bordering on the Mediterranean. The valley of Mareb in Yemen, Arabia, was irrigated by waters distributed from a vast reservoir made by a dam 2 m. long and 120 ft. high, constructed by an Adite monarch of Saba, probably long before the times of Solomon. This dam was built of enormous blocks of hewn stone, and must have been a work of no mean engineering skill, as it stood and restrained the current of a large stream of 70 tributaries for about 2,000 years, when it burst with desolating effect. The canal of the Pharaohs, connecting ancient Pelusium with the Red sea, was constructed for purposes of irrigation. The plains of Oman in Arabia are watered by subterranean canals supplied by reservoirs in the mountains, and a vegetation of rare luxuriance, consisting of most of the fruits and grains of Persia and India, is produced in consequence.
The plains of Assyria and Babylonia were covered with an immense system of canals, some of them hundreds of miles in length, intended partly for irrigation and partly for navigation; and their ruins are among the interesting antiquities of those countries. Into some of these canals the water was raised by machines which consisted of rude buckets worked by oxen, in much the same manner still practised on the banks of the Tigris and Euphrates. Among the ancient Egyptians irrigation was performed with water flowing directly from the Nile, or raised out of it, or, as was often the case, from wells. The most ancient machine was the sweep, or bucket suspended from a balanced pole, such as was generally used many years ago in this country, and is still in some parts at the present time. Afterward the chain of pots, or sakiyeh, was used, as it also was by the Assyrians and Babylonians. At present water is raised from the Nile by means of Persian wheels, which differ from the chain of pots by the vessels being hung upon the periphery of a large wheel, and so adjusted as to tip over and empty their contents into a trough when they arrive at a certain height, instead of being placed upon an endless chain or rope. - In the agriculture of Italy, France, and Spain great attention is paid to irrigation.
The Romans during several centuries constructed extensive works, which are still in use. The water is carried not only over the grass lands and the rice fields, but between the ridges in the grain fields and through the vineyards round the roots of the vines. The distribution of it is controlled by a regular system. The state itself claims the waters of all the rivers of Lombardy; and in the Venetian territories all the springs and collections of rain water belong to the government. The use of the waters of the rivers is rented out at a certain price by the hour or half hour, or for so many days at certain seasons of the year. Persons are entitled to make canals through the lands of others lying between them and the river, on paying for the damage thus caused. The channels for leading the water into the lands and the parallel channels alternating with them, placed at about 6 in. lower elevation for conveying the water away, are laid out with great regularity, at distances usually of about 22 ft. between them. In summer the water is allowed to flow through them for several hours about once a week, but the flow is steadily kept up from October to April, except at the time of cutting the grass.
In northern Italy lands that can be irrigated rent for one third more than the same class of lands not thus improved. On the American continent, the ancient inhabitants of Peru were found by their Spanish conquerors in the use of the most costly works constructed for irrigating their lands. Prescott says: "Canals and aqueducts were seen crossing the low lands in all directions, and spreading over the country like a vast network, diffusing fertility and beauty around them." In the article Aqueduct the wonderful magnitude of some of these works has been noticed. The Aztecs of Mexico also made use of similar means to counteract the natural dryness of their atmosphere; and in the beautiful gardens of Iztapalapan, watered by canals and aqueducts, and moistened by the spray of fountains, was exhibited to the astonished Spaniards a perfection of horticulture at that time unknown in their own country. - Irrigation on a large scale and by the Italian method was attempted in England in the 16th century, on the estate of Babraham in Cambridgeshire; but the system was not fairly established as an important branch of agriculture until the commencement of the present century.
Of late years what are called water meadows have become a common feature in some of the best cultivated counties, especially in Wilts, Devon, Somerset, and Gloucester, and also in the southern part of Scotland. Some peculiar methods have been introduced, as that of irrigating with currents of liquid manure; and the sewerage of Edinburgh is distributed on the same principle with the most beneficial results over the meadows that lie below the level of the city. The grass grown upon the meadows thus watered has to be cut about once a month from April to November, and it is described as remarkably tender and succulent, admirably adapted as a milk-producing food for cows. With some exceptions the general practice is not to leave the water standing upon the lands; but taking it from a. running stream (which should be tapped if practicable far enough above the meadow for the water to flow in from the bottom of the current, where it is most charged with sediment), it is conveyed in a main channel around the further margin of the meadow, and numerous side branches lead off in nearly parallel lines into its central portions, each tapering to a point.
These are commonly interlocked by others proceeding in the opposite direction from the main channel, on the lower side of its circuit, as it passes back to the river. The second set, being at a lower level than the first, serve as drains, conveying the water that overflows from the first to the main channel, which in the latter part of its course is no longer a feeder but a drain. The water is allowed to flow through this system as often as may be desirable, care being taken that it shall not lie at rest at any time, the effect of which is found to be a tendency to cause the growth of a coarse grass. This method is called bed-work irrigation, from the ground being laid out in nearly regular beds by the channels. It is applicable only to tolerably level land. By another method, called catch work, irrigation is conveniently applied to uneven ground. One set of channels follow the contour lines of the ground, each retaining its own level. These are crossed nearly at right angles by numerous other small channels, which are fed at their upper ends by the main gutter, and the water is directed by stops of clods of earth into the level side channels, which are filled as may be desired.
The laying out of the work and management of the operation, so as to distribute the water uniformly, in the proper quantities, and at the right times, require good judgment, close attention, and much experience. The irrigating season in England is the colder portion of the year, commencing in October or November, and terminating in March or April. The letting the water on or off during frosty weather is to be avoided, as a crust of ice may root out the grass as it thaws. As nearly as may be, with reference to this danger, the water is allowed to flow through the channels for two or three weeks at a time, and is then drawn completely off, so that the ground may become as thoroughly dry as possible. In this condition it is left for live or six days, when if there is no fear of freezing the operation is repeated; and so on through the winter. When the grass begins to vegetate, the periods of irrigation should be shortened, and cease entirely when it is sufficiently forward to make good pasture. The effect of this practice is often very striking; the grass is brought forward very early in the spring. After feeding off one crop or mowing the grass, the land is sometimes again irrigated for a short time to great advantage.
A second crop is ready to be cut by the time the first has ripened on the dry meadows. Three or four crops of grass are thus obtained in each season. - But the perfection of irrigation is when it is combined with thorough under-draining. The water flowing in brings with it in solution and suspension various mineral and organic substances suitable for the food of plants. By evaporation and by various chemical reactions the soluble ingredients may be set free, when they become entangled with the other foreign matters in the grass and in the soil beneath, both of which act as filters. Thus the finely comminuted sediments and the soluble salts are equally distributed among the rootlets, and these are refreshed by the new supplies furnished by each repetition of the process. By the drains the excess of moisture is soon removed, stagnation, so injurious to vegetation, is prevented, and the elements that feed the plants below the surface are kept in a similar condition of healthy renewal with those of the air circulating among the branches and adding to the vegetable growth by assimilation going on through the leaves.
The benefits derived from the process vary of course with the quality of the ingredients brought in by the water, according as these are more or less suited to the requirements of the soil and of the crop. The hard water, charged with carbonate of lime, which it has gathered in flowing through a limestone region, brings a valuable fertilizing ingredient to silicious soils deficient in lime; and the clayey sediment washed out of alluvial bottoms is spread with the most beneficial effect over loose sandy soils. Sometimes organic liquid manures, such as the drainage of farmyards and teachings of compost heaps, are supplied to the soil by being mingled with the water used in irrigating; but the principal object of irrigation is to supply moisture, as it is always easy to add manure in a solid form. - Much attention is now given to the subject of irrigation in that portion of the United States lying between the Mississippi river and the Rocky mountains; so that a vast region, some of which was long known as the Great American desert, bids fair in time to be for the most part brought under fair cultivation. The Mormons in Utah by means of irrigation render their barren country fertile.
The general plan with them, and also in California, is to lead the water in canals from the rivers or the mountains, and allow it to flow over the fields, either through small channels made in the soil, or over the even surface.