USE OF WATER BY SPRING WHEAT ON 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 Edgeley the results for 10 years on plat A were used. The 

 correlation between the total water used and the yield of grain 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 themselves along a line that does not have its origin at 

 zero. The use of several inches of water seems to be necessary before 



20 

 /8 

 /6 

 /4 





. — ___ Q 



o 



2 4 6 8 /O /2 /4 /6 /8 20 22 24 26 28 30 32 34 36 



£(/S/¥£lS 



Fig. 9. — Diagram showing the yield of wheat and use of water on plat A at Edgeley, 

 N. Dak., for 10 years. The line is 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 : 



M = y— (-y/xy <rx + <ry x) + -yfxy <r% + cry e. 



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 yield 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 average zero point 



