20 BULLETIN 1004, U. S. DEPARTMENT OF AGEICULTURE. 
well. In general, years in which high yields have been obtained show rates of 
use of water above the average, but the rate does not seem to be proportional 
to the size of the crop. 
In considering the problem of water use, the ability of the wheat 
crop to adapt itself to the moisture conditions of the soil must be 
emphasized. Unless there is a great excess of precipitation over 
normal the crop will make growth enough or continue to grow long 
enough to utilize all the available moisture in the soil. In years 
when the growth is continued a long time the jdeld is always high 
unless adverse conditions, such as hail or rust, injure the crop. If at 
any stage during its growth wheat suffers from drought it seems to 
spend its last energy in producing seed, and if there is any possi- 
bility of doing so it will mature some grain even at the expense of 
its vegetative growth. In the extensive experiments with wheat in 
the Great Plains the available soil moisture has been entirely ex- 
hausted at harvest in 90 per cent of the cases studied. 
QUANTITY OF WATER USED DURING THE GROWING SEASON. 
The preceding studies have been concerned with the rate of the 
use of water. Under the present heading consideration will be 
given to the total quantity of water used during the growth of the 
crop under conditions that have actually existed at the points of 
experimentation during the several years for which data are 
available. 
The total quantity differs in some cases from that reached at the 
final determination in the immediately preceding study in that the 
period may be somewhat shorter. That studj^ began with the first 
determination in the spring, which was usually at seeding time, but 
may have been earlier. This served a useful purpose in fixing the 
rate of use. or loss as it might more properly be regarded, from the 
soil alone before the crop started growth. This period is an un- 
certain and variable one and likely to be much longer at the southern 
than at the northern stations. For the present study of the total 
quantity of water used by the crop it was considered that more com- 
parable data would be obtained by limiting the period to the time 
the ground was occupied by the crop. The quantity of water used 
has. therefore, been calculated from a determination of soil moisture 
made about the time the wheat came up to a determination at har- 
vest. In each case the determination selected as the first was the 
one made nearest to the time that the wheat came up. Its date 
varied from about April 15 to May 1 at the southern stations to 
about May 1 to May 10 at the northern stations. 
To obtain the total loss of water during the growth of the crop 
the loss of water from the soil between the time the crop came up 
and harvest was determined and to this quantity was added the 
precipitation for the same period. The loss of water from the soil 
was determined in the same waj^ as in the previous studies. 
This study was made on plat A, which is continuously cropped to 
wheat. 
The quantity of water used from tlie soil during the growing sea- 
son, the precipitation for that period, and the total use of water for 
the growing season are shown in the first three figure columns of 
Table 3. The next three columns give the yield of grain per acre in 
