USE OF WATER BY SPRIN^G WHEAT ON GREAT PLAINS. 11 
that a rate of use of water that represents a maximum at a northern 
station is only an average rate at a southern station preclude the pos- 
sibility of this correlation being close. 
To determine whether a closer correlation existed if the difference 
between the stations could be eliminated, a second study was made. 
In this second study the average rate of the use of water for each 
station and the average yield of each station were used as bases for 
computing departures. Determined in this manner, a correlation of 
0.4 with a probable error of ±0.08 was shown for plat A. This 
is much closer than the previous one and shows clearly that a much 
closer correlation between water use and yield exists at the individual 
station than is shown for the "Great Plains as a whole. This corre- 
lation is marked enough to show that a relation between the rate 
of water use and yield actually exists, though it does not indicate 
how close the relationship may be. 
It is probable that a closer correlation exists between the poten- 
tial yields and the rate of the use of water. In other words, if all 
the crops from which these data were obtained could have been car- 
ried through to harvest without suffering from drought, the ones 
showing the higher rate of water use would have produced the higher 
yields. This is, of course, a matter of opinion and can not be sub- 
jected to any mathematical test. It is worthy of note, however, that 
years in which hea\'}" straw growth has been noted almost invari- 
ably have been above the average in the rate of water use. 
An attempt was made to correlate the rate of water use with the 
evaporation, as controlled water-requirement studies have shown that 
evaporation is one of the strongest factors in determining the quan- 
tity of water needed by the crop. 
It was found that while, in general, stations with a higher evapo- 
ration showed the greatest rate of use of water, little relation was 
shown between evaporation and the rate of water use for different 
years at the same station. 
This was accounted for by the fact that the water lost from a 
plat in a season of low evaporation may exceed that lost in one with 
a high evaporation. The water lost from the wheat itself is un- 
doubtedly higher in years with high evaporation. This has been 
conclusively proved by investigations of the water requirements of 
crops. The water lost from the soil, however, may be much smaller. 
As a rule, years having a high evaporation are dry, and the quan-^ 
tity of surface moisture or moisture near enough to the surface to 
be affected by evaporation is small. Low seasonal evaporation is 
usually accompanied by frequent rains. ^^Tien these occur there is 
a great deal of water that is subject to loss by evaporation from the 
surface. In spite of the fact that the rate of evaporation from a 
free water surface is low. considerably more moisture may be lost 
from the soil by evaporation in years of this type than in years 
having a high rate of evaporation. 
RATE OF THE USE OF WATER AS THE SOIL MOISTURE 
APPROACHES DEPLETION. 
This study was made for the purpose of finding whether the rate 
at which the wheat crop is able to use water from the soil decreases 
as the water content of the soil nears depletion. 
