USE OF WATER BY SPRING WHEAT, OX GREAT PLAINS. 
19 
to come to maturity aii"! make a low yield through timely additions of moisture 
t" the soil through rains. This tyi)e of season may occur in any portion of the 
Plains, but is more frequent in the southern than in the northern part. 
To these might be added the very exceptional year of extreme 
drought in which the crop does not germinate or make any growth. 
Each of these several types of seasons can be distinguished in the 
• urves fonned by the rate of the use of water during the season. 
Other conclusions drawn from the study of these data as a whole are 
as follows : 
There is a loss of water fi'om the soil before the crop has made growth 
enough to make its own demands heavy. This loss is small at the northern 
stations and increasingly gi'eater farther south. It represents largely evapora- 
tion from the soil and is greater at the southern stations because of their 
iiiL'her temperatures and lower humidity*. 
Rapid growth of the wheat crop accompanied by a higher rate of use of 
water begins at approximately the same time each year at a given location. 
This date is earlier at the southern than at the northern stations, owing to 
I lie earlier development of the crop. 
From the time the crop commences rapid growth until harvest the rate of 
tlie use of water continues nearly uniform as long as the crop does not suffer 
from drought. This is contrary to the general opinion, but this point was evi- 
dent in all of the cumulative losses charted that were capable of yielding evi- 
dence on this point. In nearly every case where there was no suffering for 
water at any time during the season, the points established by the different 
dates of sampling between the time the crop commenced rapid growth and 
harvest lay in an approximately straight line. 
The quantity of water used during periods of the same length was nearly 
the same, no matter whether the use was largely the precipitation for that 
period or whether it represented water .stored in the soil. This indicates 
that the precipitation falling upon the soil during the growing season is as 
effective as if it were all added to the soil at a depth great enough to be 
lieyond the reach of evaporation. The conclusion that all precipitation is as 
effective as if the amount of the precipitation were added to the quantity of 
water in the soil is open to criticism because of the fact that showers, particu- 
larly small ones falling on a dry soil, do not penetrate to a depth great enough 
to be available to crop roots. This is no doubt true, but. on the other hand, 
these showers are accompanied by lower temperatures and increased humidity. 
While none of the water falling in a small shower may be used by crops, the 
occurrence of the shower and its accompanying phenomena reduces the 
demand for water by the crop until the net result is somewhere near what 
it would have been had there been no shower but a quantity of water equiva- 
lent to it added dhectly to the soil. 
The rate of the u.se of water by the crop when in full growth and neither 
suffering for lack of water nor previously compromised by a lack of water is 
determined by the environment due to geographical location, the environment 
due to season, and the extent of vegetative growth that constitutes it. This is 
the phase of the question that has been considered under the heading " Daily 
rate of the use of water while the crop is growing rapidly." 
Any shortage of available water in the soil is accompanied by one of two 
phenomena in crop behavior and water use. If the crop is near maturity at 
the time the shortage occurs, a forced ripening takes place. The rate of water 
use may not be decrea.sed noticeably during the last period, but ripening takes 
place at the time the soil moisture is exhausted. If drought begins to affect 
the crop earlier in the season, there is in addition to a forced ripening a period 
in which the crop either wilts or fires. During this period very little water is 
used, because the soil moisture is nearly exhausted. The crop may mature 
grain, but the yield is always seriously reduced. 
The final yield of the wheat crop is determined more by the length of time it 
uses water rapidly than by the rate of use. In other words, the length of the 
line from the approximate time the crop commences to use water rapidly to the 
point where the water is exhausted and the rate of use becomes slower is 
nearly proportional to the yield. A high yield of wheat usually means not only 
a great quantity of water used during the season but a late harvest date as 
