THE IRRIGATION AGE. 



Irrigation of Alfalfa 



By SAMUEL FORTIER, 



Chief of Irrigation Investigations, Office of Experiment Stations. 

 U. S. Department of Agriculture. 



FARM DITCHES. 



The capacity and, to some extent, the location of 

 farm ditches depend chiefly on the method of applying 



SECTION A8 



B PLAN 



Fig. 22 A Border Gate Used in Imperial Valley. 



water. In the border method the supply ditch is usually 

 large and so located as to convey a sufficient volume of 

 water to the head of each land. In Imperial Valley in 



Fig. 24 Concrete Gate Used in Yolo County, Cal. 



California these head ditches, as they are called, have a 

 bottom width of 6 feet and a surface width of 12 to 14 

 feet. In building a ditch of this size a strip 6 feet wide 

 on the center line of the ditch is plowed 6 inches deep. 

 Then parallel strips, also 6 

 feet wide, are plowed 8 feet 

 distant from it. Scraper 

 teams then cross and recross 

 these, taking dirt from the 

 plowed strips and dumping 

 it on the unplowed spaces to 

 form the banks. The banks 

 when completed are about 2 

 feet above the natural surface 

 of the ground, and the bot- 

 tom of the ditch is 6 to 10 

 inches below it. When it is 

 deemed best not to create 

 a depression at the outer toe 



of each embankment, the borrowed dirt is taken from 

 the high parts of the adjacent land. 



The water required for each land is withdrawn from 

 the head ditch through a border gate. These are usually 

 made of wood. Figure 22 shows the type of border gate 

 used by F. N. Chaplin, of Holtville, in Imperial Valley. 

 It requires 49 feet b. m. of redwood, which, at $42 per 

 thousand, makes the lumber cost $2.06. The hardware, 

 carpentry and setting increase the cost to about $3.25. 

 If it is assumed that 22 gates are needed for a 40-acre 

 tract the cost per acre for the border gates is $1.79. A 

 cheaper border gate is shown in figure 23, which repre- 

 sents the kind used on an alfalfa tract 

 at Sunset City, Cal. In some locali- 

 ties concrete is being substituted for 

 wood, and figure 24 shows a border 

 gate of this material, quite generally 

 used for the irrigation of alfalfa in 

 Yolo county, Cal. 



In the check method of irrigation 

 the volumes used do not differ ma- 

 terially from those required to flood 

 the lands in the border method, and 

 the feed ditch for the checks cor- 

 responds in size and capacity to that 

 of the head ditch for borders. Cross 

 sections of common forms of supply 

 ditches are shown in figures 25 and 

 26. The carrying capacities of these 

 ditches under different grades are 

 given in the accompanying table. 



In flooding land from field laterals 

 two kinds of channels are needed. The 

 larger ones convey the water to the 

 highest corners of the fields and along 

 one or two borders of each field; the 

 smaller distribute the water over the 

 field. In this method of applying 



water smaller streams are used than in either the check 

 or the border method. Except on large farms the stream 

 seldom exceeds 3 cubic feet per second, and is usually 

 between 2 and 3 cubic feet. On ordinary grades only a 

 small channel is needed for this volume. Such channels 

 are made by plowing first a strip as -wide as the surface 

 of the ditch is to be when full and removing the loose 

 dirt by one of several designs of A crowders, two of 

 which are shown in figures 27 and 28. One of the best 

 implements for making field laterals is a 14 or 16-inch 

 lister plow on a sulky frame. Figures 29 and 30 show 

 cross sections of lateral ditches made in this way, while 

 figure 31 represents a common type of supply ditch. The 

 effect which grade has upon such channels is shown in 

 the accompanying table, giving discharges of these ditches, 

 with various grades. 



Subirrigation of Alfalfa Fields. 



As a general thing, alfalfa is irrigated from the sur- 

 face downward by one of the methods previously de- 

 scribed. There, is, however, a small percentage of alfalfa 

 lands, probably not more than 5 per cent of the total, 

 which is irrigated from below. Frequently the seepage 

 water from porous, earthen ditches and the waste water 

 from irrigated areas pass through the subsoil of lower 

 fields sufficiently near the surface to subirrigate them. In 

 other places these seepage waters collect at the lower 

 levels and raise the ground water near enough the 

 surface to supply the plants with the needed moisture. 

 It is questionable if alfalfa growers should place much de- 

 pendence on this mode of supplying moisture to the plant. 



Fig. 23 Border Gate Used Near Sunset City, Cal. 



What is gained in not having to irrigate is usually more 

 than lost in damage done to both soil and crop by the rise 



