Mar. i, 1924 
Movement of Water in Irrigated Soils 
625 
the limits of the classes of soil water listed above. The class of water 
with which the irrigation farmer is chiefly concerned is the one designated 
available water. 
The quantity of this available water that may be added to a soil by 
an irrigation may be determined by sampling the soil to the depth of the 
root zone when the crop shows evidence of needing irrigation and sam¬ 
pling again to the same depth as soon as possible after irrigation. The 
increase in moisture content of the soil shows how much available water 
has been added by irrigation. 
The results of such an investigation with reference to two very different 
types of soil are shown in Table IV. The figures in the table give, in 
terms of the percentage of water to the dry weight of the soil, a measure 
of the increase in the water supply of the 3-foot layer of soil due to an 
irrigation. These percentage figures may be converted into their equiv¬ 
alents in terms of inches of water for each foot of soil and thus express 
the volume of water in a more concrete way than the percentage state¬ 
ment does. The volume relations of the soil and the water it contains 
are shown in figure 2, which is based on Table IV. In converting per¬ 
centages of water to inches of water it was assumed that the clay soil had 
a density of 85 pounds per cubic foot and the sandy soil a density of 82 
pounds per cubic foot. 
Table IV .—Quantity of water in the soil to the depth of 9 feet before and after irrigation 
at two locations on each of two kinds of soil 
i 
! 
1 
i 
i 
Quantity erf water expressed as a percentage erf the 
dry weight of the soil. 
| 
Depth. 
1 * # i 
Newlands Experiment j 
Prosser Experiment 
| Farm, clay soil. ! 
! ! 
Farm, sandy soil. 
Location i. J 
Location 2. j 
Location 1. 
Location 2. 
First foot: 
1 
i 
Before irrigation. 
M -7 i 
11. 0 
3-2 
4 - 5 
After irrigation. 
Second foot: 
j 22. 6 j 
j ; 
32. 5 
10. 8 
I 5 - 2 
Before irrigation. 
21. 4 
19. 6 
5 - 4 
5-3 
After irrigation. 
; 21 .4 
3 1 - 8 
11. 8 
19. 0 
Third foot: 
Before irrigation. 
21.4 
* 7 - 7 
4 - 2 
5 - 4 
After irrigation. 
2 r, 4 
2 3 - 8 
14* 9 
• 12. 7 
In figure 2 each column of cubes represents a column of soil to the 
depth of 3 feet, each cube representing 1 foot of soil. The quantity of 
water in each foot of soil before irrigation is represented by the shaded 
portion of the cube, and the quantity of water added by irrigation is 
shown by the black portion. In the first column, on the left, the soil 
contained before irrigation 9.4 inches of water to the depth of 3 feet, 
the second column 7.9 inches, the third column 1.91 inches, and the 
fourth contained 2.4 inches. 
The soil represented by column 1 in figure 2 was not in crop when 
the samples were taken. It was a bare spot in a field of alfalfa where 
the soil did not take water readily. The irrigation which followed the 
sampling added only 1.2 inches of water to the soil and that increase was 
