DRAINAGE OF LAND. Agricultural or field drainage consists in freeing the soil from stagnant and superfluous water by means of surface or underground channels. Surface drainage is usually effected by ploughing the land into convex ridges off which the water runs into intervening furrows and is conveyed into ditches.
The presence of stagnant water in the soil is the source of many evils. Wet pasture land produces only the coarser grasses, and many subaquatic plants and mosses, which are of little or no value for feeding ; its herbage is late in spring, and fails early in autumn ; the animals grazed upon it are unduly liable to disease, and sheep, especially, to foot-rot and liver-rot. In the case of arable land the crops are poor and moisture-loving weeds flourish. Tillage operations on such land are easily interrupted by rain, and the period is limited in which they can be prosecuted at all; the compactness and toughness of the soil render each operation more arduous, and its repetition more necessary than in the case of dry land. The best seed time is often missed, and this usually proves the prelude to a scanty crop, or to a late and disastrous harvest. The cultivation of root crops, which require the soil to be wrought to a deep and free tilth, either becomes altogether impracticable and must be abandoned for the safe but costly bare fallow, or is carried out with great labour and hazard ; and the crop, when grown, can neither be removed from the ground, nor consumed upon it by sheep without damage to the soil by "poaching." The Water-table.—The roots of plants require both air and warmth. A deep stratum through which water can percolate, but in which it can never stagnate, is therefore necessary. A water logged soil is impenetrable by air, and owing to the continuous process of evaporation and radiation, its temperature is much be low that of drained soil. The surface of the water in the super saturated soil is known as the "water-table" and is exemplified in water standing in a well. Above the "water-table" the water is held by capillarity, and the percentage of water held decreases as we approach the surface, where there may be perfect dryness. Water will rise in clay by capillarity to a height of Soin. ; in sand to 22in. Draining reduces the "surface tension" of the capillary water by removal of the excess, but the "water-table" may be many feet below.
In setting about the draining of a field, or farm, or estate, the first point is to secure a proper outfall. The lines of the receiving drains must next be determined, and then* the direction of the parallel drains. The former must occupy the lowest part of the natural hollows, and the latter must run in the line of the greatest slope of the ground. In the case of flat land, where a fall is obtained chiefly by increasing the depth of the drains at their lower ends, these lines may be disposed in any direction that is found convenient ; but in undulating ground a single field may require several distinct sets of drains lying at different angles, so as to suit its several slopes.
The depth and distance apart of the parallel drains have been the subject of much controversy. At one time a minimum depth of 4 feet was considered necessary in all cases. This was fre quently proved unsatisfactory because the distance apart was too great. It has been laid down as a general rule in Britain that in clay soils the drains should be 16 to 24ft., in medium soils 26 to 3 2 f t. and in light soils 4o to 6o feet apart, assuming that the depth in all cases is 4 feet.
The desirability of drawing stagnant water from the land was apparent to the earliest cultivators of the land, but the methods of so doing were primitive. Surface drainage by the ridge and furrow and open ditches were practically the only means adopted. Walter Blith in the middle of the 17th century advocated the removal of the excess of stagnant water on or near the surface by mears of channels filled with faggots or stones, but his teach ing was little regarded. In the latter half of the i8th century Joseph Elkington, a Warwickshire farmer, invented a plan of draining sloping ground that is drowned by the outbursting of springs. When the higher-lying portion of such land is porous, rain falling upon it sinks down until it is arrested by clay or other impervious matter, which causes it again to issue at the surface and wet the lower-lying ground. Elkington showed that by cutting a deep drain through the clay, aided when necessary by wells or auger holes, the subjacent bed of sand or gravel in which a body of water is pent up by the clay, as in a vessel, might be tapped and the water conveyed in the covered drain to the nearest ditch or stream. In the circumstances to which it is applicable, and in the hands of skilful drainers, Elkington's plan, known as "sink hole drainage," by bringing into play the natural drainage fur nished by porous strata, was often successful.
The sound principles promulgated by Smith were speedily adopted and extensively carried into practice. The great labour and cost incurred in procuring stones in adequate quantities, and the difficulty of carting them in wet seasons, soon led to the sub stitution of "tiles," and soles of burnt earthenware. The limited supply and high price of these tiles for a time impeded the prog ress of the new system of draining; but the invention of tile making machines removed this impediment, and gave a stimulus to this fundamental agricultural improvement. The substitution of cylindrical pipes for the original horse-shoe tiles has still further lowered the cost and increased the efficiency and per manency of drainage works.
The drainage of land is an obligation falling on the owners, being part of the permanent equipment of a farm. If the farm is let the tenant is entitled under the Agricultural Holdings Acts to carry out any necessary drainage operations himself and to claim compensation on quitting the holding provided he has be fore executing the work notified the landlord and given him the opportunity of doing it himself. A very large amount of capital has been expended by landown ers on the drainage of land and special legislation was passed in the last century to enable them to obtain loans for the purpose and charge the cost on their property on a sinking fund basis. In recent years, especially since the World War, little has been done and although no trust worthy statistics are available there is little doubt that much agricultural land is deteriorating and its productivity being re duced by lack of effective field drainage. This takes no account of the necessity in many districts for arterial drainage, which is the subject of another article (see DRAINAGE, ARTERIAL). (X.) Drainage of land by means of small open ditches was probably practised by individual owners in the seaboard States at an early date, but there are no records of such work prior to the i9th cen tury. The first recorded use of tiles for underdraining farm lands was in 1835. Co-operative reclamation began in 185o, when Congress passed the swamp act. Under this law such of the public lands as were too wet to cultivate were granted to the States in which they lay with the understanding that the States were to drain and sell them. The real era of land drainage began about 1885 with the invention of the dipper dredge, which made it possible to construct large ditches at small. cost. The invention of machines for making both clay and concrete tiles in large quantities at a reasonable cost made underdrainage practicable and profitable.
The census of 193o showed that of the 1,903,215,36o ac. of land in the United States, 986,7 71,016 ac. were in farms. Of the latter area, 44,523,685 ac. were reported as having been provided with underdrainage by the individual farm owners, and 18,173, 703 ac. as in need of underdrainage. The census also showed that 84,408,093 ac. of land had been included in operating drainage enterprises. More than 88% of this drainage work was done after 1890. Drainage enterprises in most instances merely provide outlet facilities for the tile drains of the individual land owners. Un doubtedly most of the 44,523,685 ac. mentioned are included in drainage enterprises.
To facilitate land reclamation on a large scale, practically all the States have passed drainage laws under which a majority of land owners or the owners of a major portion of the land pro posed to be reclaimed can organize a drainage district and compel the minority owners to join them in paying for the improvement. These districts have corporate powers and are managed by com missioners. Funds are raised by special assessments, proportional to the benefits received. The assessments are paid in from ten to I5 annual instalments, and are a lien against the land until paid. Serial bonds are sold to meet the construction costs and are retired annually as the assessments are paid.
Along the flood plains of large streams, each of the districts may construct (I) levees to keep out the flood waters, (2) ditches to convey the run-off to the pumping station, (3) a pumping station to pump the interior drainage water over the levee, and sometimes (4) a diversion ditch at the foot of the high ground to intercept the drainage from the high ground and to carry it around the lower end of the district. The tile underdrains are laid by the individual land owners.
Along smaller streams, drainage involves channel cleaning and straightening and the construction of small ditch or large tile laterals to serve as outlets for the small tile underdrains. The drainage of large upland areas is accomplished by one or more main ditches and as many lateral ditches as are necessary. The laterals are generally spaced at either half mile or mile intervals, so that no land in the district will be more than half a mile from an outlet. Drainage districts range in size from small tile districts of a few hundred acres to large outlet districts of 5oo,000 acres.
Underdrainage is effected by systems of parallel tile drains laid at a customary depth of 21 to 4 ft. and spaced 6o to 15o ft. apart. The present practice is to use 4- or 5-in. tiles for these laterals.
In 1922, the bureau of public roads, U.S. Department of Agri culture, made a compilation of the unreclaimed swamp, over flowed, and tidal marsh lands in the United States, and found 95 million ac. of such lands divided as follows : Swamp and periodically swamp lands, 56-i million ac. ; periodically overflowed lands, 30i million ac.; and tidal marsh lands, 7i million acres.
See G. W. Pickels, Drainage and Flood-Control Engineering (1925); C. G. Elliott, Engineering for Land Drainage (1919) ; W. L. Powers and T. A. H. Teeter, Land Drainage (1932) ; B. A. Etcheverry, Land Drainage and Flood Control (1936). (G. W. P.)