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



the work was started. The wide range of the curve of every rela- 

 tion that is plotted may reveal a fundamental principle that could 

 not have been suspected from a single group of the data and makes 

 it possible to test severely hypotheses to which its study leads. 



The present study is the first that has been presented in which the 

 data are used to develop a general problem other than one dealing 

 primarily with methods of culture and the results as measured in 

 terms of resulting yield. While it develops effectively the problem it 

 undertakes, it is by no means exhaustive and suggests and invites 

 study of more questions than it endeavors to dispose of. It is only 

 suggestive of the possibilities of such data. 



While the importance of an adequate supply of water has long been 

 recognized, the dependence of yield upon an uninterrupted supply 

 has perhaps never before been so well established. Determination of 

 the daily rate of the use of water and the dependence of yield upon 

 the maintenance of this rate lays a foundation for more exact predic- 

 tion of yields than are possible without such knowledge. In the daily 

 rate of the use of water is to be found a reason for the difference in 

 effectiveness of rainfall in different sections of the Great Plains. 

 The writers have done enough checking of results outside the Great 

 Plains to be satisfied that in the same factor is to be found a basis 

 of comparison and explanation of the results in other sections and 

 under other types of rainfall. 



E. C. Chilcott, 

 Agriculturist in Charge. 



STATEMENT OF THE PROBLEM. 



Dry farming is practiced only in regions where the water avail- 

 able to crops is a factor of prime importance. As the initial water 

 supply stored in the soil and the quantity that may be supplied by 

 rains during the growth period are limited and susceptible of meas- 

 urement, it is of fundamental importance in any study of the sub- 

 ject to know the daily rate of the use of water by the growing crop 

 and the total consumption by the crop during its life period. In 

 investigational work it has appeared especially important to deter- 

 mine the actual rate of use in the field in order to properly evaluate 

 quantities of water in terms of the length of time thej^ would meet 

 the requirements of the crop. 



Other investigators have made exhaustive physiological investiga- 

 tions of the water requirements of crops directed chiefly to the 

 measurement of the water actually transpired by the crop plants 

 alone. 



In the field the crop has an initial supply measured by the avail- 

 able water stored in the soil within the zone of depletion by the crop. 

 This is replenished or added to from time to time by the precipitation. 

 The water is used by the crop and by the weeds that accompany it 

 and is lost by direct evaporation and very seldom under dry-farm- 

 ing conditions by leaching. A portion of the precipitation may be 

 lost by run-off. 



In the field it is not possible to separate the water actually used 

 by the crop in the sense of passing through the tissues of the plants 

 comprising it from that lost by other means. If it were possible, 

 it does not necessarily follow that it would be desirable. The water 



