CHAPTER VII.

LOCATION AND PLANS OF FARM DRAINS.

To secure efficiency and economy in the construction of farm drains the work should be planned, and the location of the drains decided upon over the entire area that may need draining, in accordance with a definite and well-matured system, in which every condition that may influence the results has been fully considered and provided for. When but part of the work can be done in a single season, the advantages of a complete plan for the drainage of all lands that can discharge water at a common outlet, before any drains are made, must be obvious, as each line of tiles laid will then form a consistent link in the general system, and the losses that are likely to arise from a change of plan in the progress of the work will be avoided. There are certain principles to be kept in mind in planning a system of drainage that it may be well to notice before discussing other details.

Direction of Drains. In the first place, all drains should run directly down the slope, in the line of steepest descent, in order to secure the greatest efficiency in the discharge of water, in connection with the widest distance between the drains that can be made, and at the same time secure thorough drainage over the entire area to be drained. Any considerable variation from this rule should only be made for good and sufficient reasons, to secure other advantages that fully compensate for any faults that may arise in deviating from the most direct course.

It will readily be seen that when parallel drains are laid directly across the slope, a drain can receive no water from the space immediately below it, and that it must receive water from the whole width of the space between it and the next drain above. Moreover, when the slope of the field is considerable, these transverse drains allow water to escape at the joints of the tiles and wet the soil of the space below them, and thus add to the duty of the next drain. Many instances have come under my observation, where water from springs has escaped from drains laid across the slope, and saturated land which before was comparatively free from drainage water, the drains only serving to transfer the springs from one locality to another.

On the other hand, when drains run directly down the slope, they receive water from but one-half of the space between adjacent drains; impervious strata that bring water to the surface to form springs, are cut across; the water table is uniformly lowered; and the flow of water from one drain to another does not take place. The drains can then be laid at wider intervals, and the cost of thorough draining materially diminished. Parallel drains at equal distances are desirable, but when the slope of the field is not uniformly in the same direction they cannot be so made, and at the same time run directly down the slope in the line of the most rapid fall. Good judgment will then be required to secure a happy mean between the conflicting requirements, that will give the best results, but, as a general rule, the line of greatest descent should be the dominant factor in determining the location of the drains.

Main Drains.-A sufficient outlet must be secured for the main drain, and it should then be laid in the lowest ground, without any abrupt changes in fall, to check the flow of water passing through it, and it may be necessary to lay it at a greater depth from the sur

face in some places, to secure the desired uniformity in its slope or rate of fall, and, if possible, there should be an increase in fall towards the outlet. When the fall in

the upper course of a drain is considerable, and but a slight fall can be secured in its lower course, a larger tile will be required where the fall is diminished, to carry the water received from above, and prevent it from being forced out at the joints by the pressure from the head of water in the upper course of the drain, and thus undermining and displacing the tiles.

If the valley through which the main is to be laid is broad and nearly level from side to side, a sub-main should be laid on each side of it, near, the foot of the slope, to avoid the rapid decrease in the fall of the lateral drains, that would be made if they were continued to the middle of the valley, and the space between the sub-mains may then be drained by laterals of smaller tiles. When a change in the direction of a main, or submain, is necessary, it should be made gradually, or with a gentle curve, as abrupt angles check the current of water and materially diminish the capacity of the drain. This fact should be kept in mind in all cases, but in the upper course of laterals, laid with two inch tiles, this is not as important, as they are not as likely to run full.

Depth of Drains. It is important that the depth at which drains are to be laid should be decided upon before laying out, or determining their location in the field. Those who have had no experience in draining land are liable to fall into the error of laying the tiles too near the surface, from mistaken notions of economy. Practically the depth of retentive soils, as we have seen, is limited by the surface of the water table, and the drains should, therefore, be laid at sufficient depth to secure a free range of root distribution throughout the largest mass of soil that can be made available, with reasonable economy in construction.

The roots of nearly all of our cultivated crops penetrate the soil, under favorable conditions, to the depth of, at least, four feet, and this may safely be recommended as a desirable depth for laterals, while the mains, if possible, should be laid at least their own diameter deeper. There can be no doubt that drains four feet in depth have a number of advantages over those that are shallower, that must more than compensate for a considerable increase in cost, but it does not follow, however, that the draining of a field to the depth of four feet is necessarily more expensive than draining to the depth of three feet.

On the ground of efficiency, it appears that when heavy rainfalls occur after a season of drouth, the discharge of water begins sooner and continues longer; a larger mass of soil, with its supplies of nutritive materials, is made available for growing crops by the processes of metabolism; a wider range of root distribution is secured; and there is an increased capacity for holding capillary water for the purposes of vegetation in time of drouths. The extreme climatic conditions of excessive rainfall and intense drouth are, therefore, more completely corrected, and a greater uniformity in productiveness may reasonably be expected. The item of economy in the construction of four-foot drains will be considered in the next paragraph.

Distance Between Drains.-No absolute rule can be laid down as to the proper distance between drains, to secure the best results at the least expense. Good judgment in the application of general principles will be found the best guide in each particular case. The conditions that have an influence in determining the most desirable distance between drains are, the depth at which they are laid, the character of the soil, and the amount of rainfall that is likely to occur in single showers, or within a few days, which is of greater importance than the annual rainfall.

In order to secure the same efficiency in removing water from the soil, drains but three feet deep must be laid nearer together than when they are four feet deep, and the expense of draining a given area may, therefore, be less with the deeper drains, as the cost of digging the additional foot in depth of the four-foot drains will be compensated for by a saving in tiles, and in the number of ditches that are required. On the score of economy, as well as efficiency, the four-foot drains will undoubtedly prove most satisfactory. Mr. Parkes' table 21, (page 114), may be profitably studied in this connection.

The character of the soil should be carefully studied, and its behavior, as the drains are laid, should be closely observed. In the most retentive soils, when the drains are four feet deep, it will seldom be necessary to make the distance between them less than twenty-five or thirty feet, and in many soils, that need draining, a distance of fifty to sixty feet may give satisfactory results. The amount of rainfall should be considered, in connection with other conditions, as it may be of assistance, in some cases, in deciding upon the most desirable distance. The depth of drains is, however, a more important factor in preventing injury to crops from excessive rainfall than the distance between them.

Map of the System of Drainage. In all cases it will be desirable to make a map of the field, or the area to be drained, on which the location and depth of every drain is accurately recorded. The general details of the nap should be in black ink, and the proposed drains laid down with dotted red lines. As fast as the drains are finished the dotted line can readily be changed to a continuous red line, and a record may thus be conveniently kept of the progress of the work. When the work is not all done in a single season, the importance of an accurate map of the drains already made, as a means of definitely locating them, in order to form junc