20 BULLETIN 1004, U. S. DEPARTMENT OF AGRICULTURE. 



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 yield 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 study 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 way as in the previous studies. 



This study was made on plat A. which is continuously cropped to 

 wheat. 



The quantity of water used from the 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 



