The third condition involves the application of the sewage to a steep slope, and this may be treated in either one of two ways. The sewage may be led to the top of the hill and allowed to flow, for a short distance only, over the surface on which, presumably, grass is to be grown. If the length of the furrow is more than about a dozen feet, the flowing stream acquires enough velocity to wash the surface and to form gullies. To prevent this, a secondary ditch or small bank is thrown up to arrest the flow. The water is led out again from behind this ditch or bank at intervals, to repeat the process further down the hill (see Fig. [48]). If the slope of the ground is moderate, so that there is no tendency of the water to form gullies, the water may be let out of the ditch at intervals and allowed to distribute itself over the field, as shown in Fig. [49]. The water thus overflowing should be collected in a drain at the lower end of the slope, and will be found suitably purified for discharge into any running stream not used for drinking purposes. The occasional use of a shovel or hoe may be needed to change the flow of the water over the field if it is found that any tendency exists for definite channels to be formed.
Fig. 49.—Distribution of Sewage on a Hillside of Moderate Slope.
In order to plant vegetables on such a hill, small furrows may be made along the hill and laid out with great care so that the flow of sewage in the furrows shall be only at a slow velocity, so slow that the soil can absorb the moisture as the water passes along. By zigzagging this furrow back and forth down the hillside, vegetation on the hill will receive the benefit of the water, and if any of the sewage succeeds in reaching the bottom of the hill, it will be so purified that it may be safely discharged into any depression or watercourse there found.
In the case of orchards, irrigation is practised by flooding the ground around the tree, being careful, however, to throw up a mound of earth around the tree so that no water comes within two feet of the tree itself. Fig. [50] shows a Western method of forming square beds, each bed about twenty feet on a side, with one tree at the centre. Furrows are also used to distribute the water, a common practice followed being to being to have the furrow always under the extreme edge of the foliage, thus discharging the water in the vicinity of the tender rootlets of the tree. Usually the furrow system is carried only in one direction, so that the application of water by this method is not so complete as by the flooding method. But for small volumes of water constantly applied, it is probably more satisfactory. Fig. [51] shows a grain field irrigated by the furrow method.
Fig. 50.—Square Beds for Orchards According to Some Western Practice.
In all cases where irrigation is practised, stress is laid by those experienced in the matter on the necessity of cultivation of the soil in connection with the irrigation. Apparently, there is a tendency for the surface layers, with the application of water, to cake or crust over the lower strata, thus depriving the soil of the necessary air. In order to break up this crust, the soil must be continually worked, either by a hoe or rake or some sort of horse cultivator. Where the ground is shaded, as in the case of land covered with grass or grain, the tendency to crust is not so marked, but on cultivated land where root crops are grown the cultivator must be used regularly after each irrigation. Where the sewage is carried onto the field in furrows, the soil in the furrow should be hoed at frequent intervals, not only to break up the crust which deprives the soil of the air, but in order to open the particles of soil for the reception of the irrigating-water.
Fig. 51.—Grain Field in Spring, in Process of Irrigation.