Furrow-irrigation is usually employed in watering trees and vines. some localities, however, basin or pool irrigation is practised. Where water is especially scanty and corre spondingly high-priced, the supply is conducted in cement-lined ditches and by wooden flumes as near as possible to the trees and vines and is then turned out into the furrows ploughed around or near as possible to the trees and vines. The water issuing from small aper tures in the side of the wooden box falls into the furrows and is immediately conducted to the vicinity of the trees. Care is usually taken that the water shall not actually touch the tree trunks and that it reaches the extremities of the roots to encourage these to spread out ward. After the water has traversed the fur rows to the lower end of the orchard, the supply is cut off and the ground is tilled as soon as the surface dries sufficiently.
Sub-irrigation.-- Attempts have been made conduct the water beneath the surface im mediately to the roots of the trees, thus pre venting waste by evaporation from the sur face of the ground. Few devices have been successful, owing to the fact that the roots of the trees rapidly seek and enter the openings from which the water issues, or, surrounding the pipe by a dense network, cut off the supply. Porous clay tiling has been laid through or chards and also iron pipes perforated so as to furnish supply of water along their length. In orchards where sub-irrigation has been un successful because of roots stopping-up minute openings beneath the surface, the system has been reconstructed and water has been brought to the surface at or near each tree by means of small hydrants. Vertical pipes are placed at short intervals leading to the level of the graimd, and in these are small iron gates or shutters so arranged that the flow can be cut off in the buried pipe and forced to rise and overflow the surface of the ground near each tree. For annual or root crops sub irrigation Las been successfully practised by the use of tile or of small metal pipes partly open at the bottom, allowing a small amount of water to escape. These are laid 12 inches or more beneath the surface and are connected with lines of tile or clay pipes leading from the reservoir or source of supply. As the crops are removed each year and the ground cultivated, the roots have no opportunity to stop up the pipes. The term sub-irrigation is occasionally applied to conditions occurring in nature where water percolates freely beneath the ground for a considerable distance suf ficiently near the surface to supply the need of crops. Where the sub-soil transmits water freely, irrigation ditches may sub-irrigate large tracts of country without rendering them marshy. Thus farms may obtain an ample supply of water from ditches half a mile or more away without the necessity of distribut ing small streams over the surface. In the San Joaquin Valley, Cal., vineyards in cer tain localities are thus maintained in good condition, although water has not been visibly applied for many years.
Quantity of Water.— The amount of water required for raising crops varies according to the character of the soil. The plants themselves need a certain minimum supply, but a far larger quantity is required to saturate the surrounding soil to such a degree that the vitalizing proc esses can continue. Agricultural investigators
have found by direct measurements that from 300 to 500 pounds of water or even more are required for each pound of dry matter pro duced. When the ground is first irrigated a large quantity of water is sometimes required to saturate the subsoil. The water turned upon the surface and absorbed during the first year of two has frequently been equivalent to an amount sufficient to cover the ground to a depth of 10 or 20 feet, and in many cases an amount equal to a depth of five feet or more per annum has been thus employed for several years. Grad ually however the dry soil is filled. The pio neers of irrigation usually apply too much water to their fields, often to their disad vantage.
The quantity of water used in irrigation is usually stated in one of two ways: (1) In terms of (tenth of •••nt^r on the surface; (2) in quantities of flowing water through the irri gating season In the humid regions the rain fall is usually from three to four inches per month during the crop season. In the arid re gion, where the sunlight is more continuous, and the evaporation greater, there should be for the ordinary crops at least enough water dur ing the growing season to cover the ground from four to six inches in depth each month or from a third to half of an acre-foot. The second method of stating the quantities neces sary for irrigation is of convemence when con sidering a stream upon which there is no stor age. It is estimated that one cubic foot per second, flowing through an irrigating season of 90 days, will irrigate 100 acres. One second foot will cover an acre nearly two feet deep during 24 hours, and in 90 days it will cover 180 acres one foot deep, or 100 acres to a depth of 1.8 feet, or 21.6 inches. This is equivalent to a depth of water of a little over seven inches per month during the season of 90 days or about one and three-quarters acre feet. Suc cessive years of deficient water supply notably in southern California have served to prove that, with careful cultivation, crops, orchards and vineyards can be maintained by using very small quantities of water. In some cases an amount not exceeding six inches in depth was applied during the year, this being conducted directly to the plants and the grounds kept carefully tilled and free from weeds. As esti mated by various water companies in southern California, one miner's inch of water will irri gate from 5 to 10 acres, the miner's inch equal ing 12,960 gallons in 24 hours, or almost exactly 0.02 second-foot, this being the amount deliv ered under a four-inch head, measured from the centre of the opening. Under this assump tion one second-foot should irrigate from 250 to 500 acres. If it is assumed that one miner's inch is allowed for 10 acres, or one second-foot for 500 acres, this quantity of water flowing from May to October, inclusive, will cover the ground to a depth of a little over seven-tenths of a foot. With alfalfa flooding is practised, using upwards of half an acre foot to an acre; with small grains the water in less quantity is run in furrows; while with orchards the water is sometimes applied directly to each tree, or in furrows, four or five being plowed in paral lel lines between two rows of trees.