12 BULLETIN 1004, IT. S. DEPARTMENT OF AGRICULTURE. 



To determine this point the rate of the use of water for the period 

 when the moisture content of the soil neared depletion was com- 

 pared with the rate of use of water for the two periods preceding. 

 The conditions necessary for obtaining data on this point were as 

 follows: (1) All three periods must have been within the period 

 of rapid growth, (2) the last period must have ended before the soil 

 moisture was depleted, and (3) the samples must have been taken 

 deep enough to include all soil in which a change in water content 

 took place. 



The number of years in which data on this point could be obtained 

 was small, and the experimental error due to the short lengths of the 

 periods was large. In order to obtain data for the consecutive 

 periods within the period of rapid growth, the three periods selected 

 were necessarily short. Data were obtained for an average of about 

 two years at each station. 



It was found that when the results of all stations were averaged 

 the use of water for each period was nearly the same. It can not be 

 said that anything conclusive was shown by the data, however. 

 The experimental error in the rate of use per day was so large that 

 it obscured any small differences there might have been between the 

 different periods. No consistency in water use in the different 

 periods is shown either between the different stations or the different 

 years at the same station. 



The conclusion drawn from this study is that though there may be 

 a slight decrease in the rate of use of water as the soil becomes drier, 

 such decrease is too small to be measured by the means employed. 

 The wheat crop can exhaust nearly all the available water without 

 any serious reduction in its rate of use. 



RATE OF THE USE OF WATER DURING THE GROWING SEASON. 



Under the second preceding heading the rate of the use of water 

 under certain conditions and for a part of the growing season was 

 considered. The conditions were such as to permit or require the 

 use of water at maximum rates per day. Under the present heading 

 the rate of the use of water for the entire growing season under 

 conditions that obtain for the entire season will be considered. 



The use of water was determined, in terms of inches used, for 

 each period between successive samples, beginning with the one near- 

 est seeding time and ending with the one at or near harvest. The 

 same method was used as in the preceding study. While the primary 

 calculation was in terms of use for each period, the material so 

 obtained was prepared for use by successive additions of the water 

 from period to period. The sum at any sampling date represents 

 the total quantity of water used by the crop from the first date of 

 sampling until the date in question. The successive sums from date 

 to date show the cumulative use of water during the growing season 

 from zero at the beginning to the total quantity at harvest time. 



The data so assembled were further prepared for study by charting 

 on cross-section paper with calendar dates as abscissa and quantities 

 of water used as ordinates. The first sampling is indicated by a 

 point on the zero line of ordinates at the given date. The second 

 sampling is indicated by a point the abscissa of which is the date 

 and the ordinate of which is the quantity of water used to that date. 



