Irrigation, signifies the watering, or artificial flooding of land.
The principal object in irrigating the soil, is a proper supply of water ; and, if that fluid abound with mud, the land will be more permanently improved, than by conducting a clear spring over its surface : hence it is certain, that if the liquor collected in farm-yards, the Washings of different sinks, ditches, and the drains of the contiguous fields, could be conveyed into the common stream, .the water would acquire a more fertilizing property.
But, in order to manage this important branch of rural economy with success, it will be requisite previously to ascertain, whether the stream will admit of a temporary wear or dam being constructed across it; whether the water can be raised to such a height as to overflow a particular spot, without injuring the contiguous land; and whether it can be drawn off with, the same facility as it is con-ducfed on the soil. If these objects can be attained, the process of irrigation may be undertaken in the following manner :
When the descent cannot be determined with tolerable precision by the eye, an accurate level of the ground must be taken, and the most elevated part compared with the height of the stream to be em-ployed. The instrument used for this purpose, is called a water-level; and, as the common machines are but imperfectly adapted to the purpose, we have subjoined the following representation of a simple implement for taking levels, contrived by Dr. James Anderson ; as it has been recommended to the notice of the public, by the Board of Agriculture.
It consists of two legs of common deal, A and B, about twelve feet in length, which are joined together at the top, and connected below by a cross bar, as above delineated. From the angle at the top, a plummet C, is suspended by a small cord; which, when a mark is made in the middle of the connecting bar, and the two legs are perfectly straight, will strike such mark, so that the level may be easily ascertained.
In the practical use of this instrument, Dr. Anderson directs a wooden pin, on which one of the legs of the frame may rest, to be driven into the ground at the level of the water, where the irrigation is intended to commence. The other leg is then to be brought round, till it touch the ground, on a level with the top of such pin, when another is to be driven in. After the level has in this manner been perfectly adjusted, the last mentioned pin is to be employed as a rest for one foot, and the other turned about, till the level is found in the same manner. Thus, the precise direction which the stream should take will at once be discovered, without any trouble, or incurring any additional expence by digging through heights or filling up cavities.
The level being taken as directed, a wide ditch is to be cut as near to the dam as possible, that the water may be conveyed to the highest part of the meadow ; the sides and banks of the ditch being uniformly kept at an equal height, and elevated about three inches above the surface. Where the meadow is extensive, and the soil uneven, it will be necessary to have three works or dams in different directions, each of which should be five feet in width, if too meadow contain fifteen acres ; and the highest part be the most distant from the stream. A ditch, ten feet wide, and three in depth, will, in general, be fully adequate to overflow ten acres of land; and if there be three works or dams in a meadow, and flood-hatches, of flood-gates, be placed at the mouth of each, when the water is not sufficient to irrigate the whole soil at once, it may be performed at three different periods ; by taking out one of the hatches or gates for the space of ten days, at the expiration of which it is to' be let down, and the other two taken up alternately for a similar period : thus each division will receive a proper share of water in its turn; and derive from it equal benefit.
Where the fall of the meadow renders it practicable, the bottom of the first work should be made as deep as the bed of the river j because the water, in proportion to its depth, will carry a larger or smaller quantity of mud with it; and consequently fertilize the soil in a more or less effectual manner. Small ditches, or troughs, ought likewise to be cut from the works at right angles, about 12 yards apart from each other, and their breadth should be adequate to the distance to which the water is to be conveyed : thus a trough two feet in width, and one foot deep, will irrigate a surface twelve yards, wide, and forty feet in length.
It will, however, be requisite to provide the ditches with occasional flood-gates or sluices, especially when the water is rapid, in order to keep it sufficiently high to flow through the perforations in the gates, or over the sides. Between every second trough, a drain is to be cut at equal distances in a parallel direction, and of a proper depth for receiving all the water which overflows the adjacent lands, and conducting it to the principal drain with such rapidity, as to keep the whole stream in constant motion. For, if it be suffered to stagnate, it will be productive of the worst consequences; as the turf would become rotten, the soil be soaked without being ameliorated, and the land produce only coarse grass, rushes, or other aquatic weeds.
Where the meadows are cold, flat, and swampy, the width of the bed, that is, of the intermediate space between the trough and drain, ought never to exceed six yards. In such cases, the land cannot be too much intersected, especially when there is an abundant supply of water. The fall of the bed in every meadow ought to be in the proportion of one inch to each foot; for a rapid current always contributes to produce fine and sweet herbage; but the water ought never to flow more than two inches, nor less than one inch deep, except during the summer months.
Such is the method of irrigation practised in Gloucestershire, and likewise, with very few variations, in the counties of Wilts, Dorset, Cambridge, Hants, etc. Its advantages, indeed, are so important that, we trust, no rational agriculturist will hesitate to adopt the practice of floating land throughout Britain, in every situation where a command of water can be obtained.-Common meadows are not only enriched, but those of a swampy nature are consolidated by means of the mud conveyed on them. They are also protected from the effects of frost by the flowing water, or by the ice when it is frozen ', hence the roots of grass remain unaffected by the cold, and excellent crops are thus produced so early, as to be of infinite service for spring food, before the natural grass appears.
By irrigation, good pasturage may be procured in the beginning of March; and, if the season be mild, much earlier. This crop is particularly excellent for feeding such cattle as have been hardly wintered; and so great are the benefits attending the flooding of lands, that the farmers of Gloucestershire are enabled to commence the making of cheese, at least tone month earlier than those who do not possess the same opportunity. The utility of watering land is still farther evinced by this circumstance, that from the great forwardness of grass, the feeding between the months of March and May is worth one guinea per acre ; in June one acre will yield two tons of hay; and the after-grass may always be estimated at twenty shil-. lings, whether the summer be wet or dry.- the expence of irrigation is computed at from 31. to 61. per acre.
Land may be floated at any period of the year. In the months of December and January, the chief care consists in keeping the soil sheltered by the water from the. severity of frosty nights. It will, however, be necessary to expose the surface to the air every ten days, or fortnight, during the winter, by laying it as dry as possible for a few days : and to discontinue the flooding, whenever the land is covered with a sheet of ice.
In February, greater attention is required; for, if the water be suffered to flow over the meadow for several days in succession, a white scum will be generated, which is very pernicious to the grass; and, if the water be drawn off, and the land exposed to a severe frosty-night, without being previously dried for a whole day, the greater part of the tender plants will be totally destroyed.
In the beginning of March, the grass on well-flooded meadows will, in general, be so forward as to furnish abundant pasturage; when the water should be drained for the space of a week, that the soil may become firm, before heavy cattle be allowed to graze on such land : these, however, if the season be cold, ought to be supplied with a little hay every night, during the first week.
In the month of April, the grass may be eaten down closely; but no cattle should be turned in later; as otherwise the crop of hay will be much impaired ; the grass become soft or woolly; and consequently its value be considerably diminished.
In the beginning of May, the water is again thrown over the soil for a few days, in order to moisten it ; but the practice ought on no account to be continued during the summer; for it has an unfavourable effect: on the after-math; and produces in sheep fed on it, the disorder called the rot.
Many other advantages arise from irrigation; but, as our limits admit only of giving an outline, we refer those readers who are desirous to acquire farther information relative to this interesting branch of rural economy, to Mr. G. Boswell's Treatise on Watering Meadows, etc. (8vo. 2s. 6d. 1780), in which the necessary implements, and terms employed in irrigation, are explained and illustrated with engravings; as likewise to Mr. Wright's Art of. floating Land, etc. (2d edit. 8vo. Hatchard, 1799, pp. 95, 3s. 6d.) ; where the subject is perspicuously treated, and objections are ably answered.