This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
Care should be used to run the distributing laterals on a light grade, because water must be taken out of them for the corrugations or checks, and if they have a heavy grade it is a difficult thing to do, because it requires so many checks in the laterals. These laterals can be made with a plow and a go-devil, or a regular ditch plow. The corrugations should be run in the direction of the greatest fall.
The best method of irrigation to be followed is dependent upon the character of soil and the slope of the land. The corrugation system is suitable for land that takes up water readily and that has ten feet of uniform slope per mile (a trifle more than one inch per fifty feet) or more. Corrugations may be placed as close together as is necessary. Usually twenty inches apart is the minimum width and three feet apart the maximum. The furrows are made about four inches deep and should be made immediately after seeding or before the seed germinates. In old alfalfa fields the corrugations can be renewed when it becomes desirable with an iron corrugator that has sharp, plow-like points.
Sub-laterals are made parallel with the laterals. For corrugation or furrow irrigation in Idaho a sufficient head of water is taken from the lateral to supply about twenty corrugations. This is run into the sub-laterals, and from these distributed to the corrugations. This breaks up the head of water into small streams that can be more easily handled. A controlling device to regulate these small heads of water may be made from 1x4 inch stuff nailed together, making a box four inches square outside measurements and about three feet long. This box is placed between the lateral and sub-lateral low enough to intercept the flow of water. The sub-lateral is generally a plow furrow with the dirt thrown down hill. Dirt dams are placed in it at proper intervals (about every twenty corrugations) to force the water out to the corrugations.
It is sometimes necessary to place checks immediately below the diversion points in the lateral in order to raise the water level high enough to force the water into the corrugations. These checks are boxes or gates put in ditches or laterals through which the water is made to pass. The height of water maintained above the check is regulated by building up the opening (with narrow strips beginning at the bottom) through which the water passes. Canvas dams can also be used as checks when they are properly made. Sacks filled with dirt can also be used for diversions.
The corrugation or furrow system is the method best adapted for the watering of all root crops. Potatoes show the bad effects of even a break between the furrows and consequent flooding. For potatoes the furrows are made quite large, the rows being ridged and furrows made between the rows. Sometimes the best method for the crop and the most economical in the use of water is to irrigate only alternate furrows at each irrigation. Water should be on the ground until it is sufficiently wet, sometimes from twelve to twenty-four hours. If the soil is very mellow and readily permeable, then four to eight hours is generally sufficient. The irrigation should continue until the moisture 'subs' between the furrows until it meets. Orchards are commonly irrigated by the furrow method. Care should be taken to keep the water away from the trees, as it is found that they thrive better when the water does not touch them but percolates into the soil and reaches the roots. When all the ground between the trees is moistened the roots spread uniformly. Grain and alfalfa may be irrigated with corrugations and in this section it is the most common method. By its use water is evenly distributed over the fields, is absolutely under control all the time, and where the land is in proper shape the work is quite rapid and probably less expensive than any other method except by the border system.
The flooding system of irrigation can be used for watering grain and alfalfa. The laterals are most commonly run parallel to the slope, water being taken out from only one side of the lateral and extending to the next one. Sometimes on nearly level land they are run down the steepest slope, the irrigation water being taken out from each side and extending midway to the next lateral. Under this method the distance between laterals should not be over 200 feet and it is better to have them closer together, not over 100 feet. These laterals may be either permanent or made over each year, as in the case of grain crops.
This method is in general suitable for medium slopes, soils which do not bake, grain pasture and hay crops, and where lands are not of great value, such as meadow lands at high altitudes. One man can irrigate from two to five acres a day. As a rule the irrigation by this method is more uneven than with the other system.
Some system to care for waste water is a necessary part of an irrigated farm. Especially where the ranch is bordered by other irrigated tracts or by public roads it is essential to have an adequate system of waste ditches that will receive and care for whatever water is not used by the irrigation. It often happens that draws or coulees are so located that they will carry away the waste water and in such cases no further attention is necessary. The amount of waste water is so variable that it is seldom satisfactory for any one else to try to use it for irrigation, and because of the fluctuations in its flow is not best to waste the water into another farm ditch.
The term 'duty of water' as used in irrigation is accepted to mean the acreage of land that a certain amount of water should sufficiently irrigate. The standard measurement for running water is in cubic feet per second - that is, a running stream is measured by the number of cubic feet which pass a given point per second. In Idaho a miner's inch of water is one fiftieth of a cubic foot per second. The amounts used in irrigation are commonly spoken of in acre feet or acre inches. This is the amount covering an acre of land one foot deep or one inch deep as the case may be.
The amount of water needed to grow and mature a crop is dependent on a great variety of conditions, some of which fluctuate even on a given farm from year to year. The composition of the soils, the subsoil, the annual rainfall, the humidity of the air, temperature, the time of seeding and many other things have appreciable effects upon the amount of water necessary for a given crop for maximum results. It is a question of knowing how little instead of how much to use. The soil can retain only a certain amount, and whatever is applied more than this is lost by percolation into the subsoil and beyond the food gathering area of the roots of plants. An appreciably large per cent, of irrigation water is lost in this way. New land will take more water the first year than thereafter, and for the third year less is required than for the second. The time to apply water is when the crop needs it and not before. This is determined in alfalfa by a darkening in the green of the leaves. The moisture content of the soil should be closely watched, and by the exercise of a little good judgment no serious mistake in applying water need be made. Too little as well as too much moisture in soils injures plants. The amount of water to be put on at one application is dependent on the crop and the soil, but generally about five acre inches is sufficient for an irrigation. On sandy loam, and making allowances for some loss as waste and evaporation, this amount will wet the soil about as deep as plant roots go.
In localities where water for irrigation is scarce, fall or winter irrigation is often practised. It has a number of things to commend it besides the principal fact that if the water is not stored in the land it is lost. By using the water in this way, where there is a scarcity, additional land can be farmed and made to yield remunerative crops.
Enough moisture can often be stored in the soil during the fall or winter to grow a crop of grain or potatoes. Cultivation usually plays quite a prominent part in raising such crops. It too often happens that where water is plentiful irrigation is substituted for cultivation. Where water is scarce it is an advantage to get the seed in the ground just as early in the spring as is possible so that the crop may be shading the ground before warm weather comes on. This prevents evaporation."