USE OF WATER BY SPRING WHEAT ON GREAT PLAINS. 19 



to come to maturity and make a low yield through timely additions of moisture 

 to the soil through rains. This type 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 

 curves formed 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 from 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 greater farther south. It represents largely evapora- 

 tion from the soil and is greater at the southern stations because of their 

 higher 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 

 the earlier development of the crop. 



From the time the crop commences rapid growth until harvest the rate of 

 the 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 

 beyond 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 directly to the soil. 



The rate of the use 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 decreased 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 



