USE OF WATER BY SPRING WHEAT OX GREAT PLAINS. 
27 
the others. Even at these stations the number of years was far too 
small for a reliable correlation to be made, but it -was thought that 
any close relation that existed would be shown. 
At Edgelev the results for 10 years on plat A were used. The 
correlation between the total water used and the yield of ^ain was 
0.90^:0.04. Since the other studies made showed little difference 
in relations between the yield of grain and the total weight, only the 
one correlation was made. Figure 9 shows the use of water and the 
yield of the plat each year. In this figure the axis of ordinates rep- 
resents the water used in inches and the axis of abscissas the yield 
in bushels. 
It can be easily seen that the points established in "the different 
years group them^selyes along a line that does not haye its origin at 
zero. The use of seyeral inches of water seems to be necessary before 
20 
Fig. 9. — Diagram showing the yield of wheat and use of water on plat A at Edgeley, 
N. Dak., for 10 years. The line i.s drawn from the linear-regression equation M= 
8.16 + 0.20 e. 
any yield is produced. After enough water to produce a yield has 
been used there appears to be a linear relation between the water 
and the yield. The starting point and slope of a line representing 
the relation of water use and yield were determined by the linear- 
regression formula : 
Determined in this way. the resulting linear-regression* equation 
was M equals 8.16 plus 0.20 e. 
The application of this equation is made as follows: Eight and 
sixteen one-hundredths inches represents the quantity of water neces- 
sary before a 3'ield of grain is secured. Starting from that point, 
each succeeding increase of 0.20 inch of water should result in an 
increase of 1 bushel in yield. The line in Figure 9 is the line es- 
tablished by this equation. 
It is not contended that 8.16 inches represents the quantity of 
water necessary before a yield is secured. It is simply a point de- 
termined by 10 years' results that indicate the ayerage zero point 
