USE OF WATER BY SPRING WHEAT ON GREAT PLAINS. 33 



tion and insufficiency of water supply later may result in firing and 

 forced maturity. Failure of the water supply near the end of the 

 growth period may result in forced maturity without marked re- 

 duction in the rate of use before maturity. Type curves showing 

 the rate of use for the entire season are presented for representa- 

 tive years and localities. 



The daily rate of use of water by a wheat crop is dependent upon 

 climatic environment and the quantity of vegetation that composes 

 it. As climatic environment is determined by geographical location, 

 the rate of use of water by a unit quantity of crop is consequently pri- 

 marily dependent upon geographical location and secondarily upon 

 seasonal conditions. Within the limits that these overlap, either 

 one or the other may dominate. 



The normal rate of the use of water by the wheat crop during its 

 period of rapid growth has been determined to average from about 

 0.15 to about 0.20 inch per day at stations in the northern Great 

 Plains, the rate depending upon the quantity of crop as influenced by 

 cultural practices. This rate is increased from 50 to 60 per cent at 

 stations in the southern Great Plains. 



The total consumption of water by the crop shows a high degree 

 of correlation with the yield produced by it. A high yield necessi- 

 tates a high use of water. Of 93 crops studied in this connection only 

 one yield of more than 20 bushels per acre was produced from the 

 use of less than 11.5 inches of water. On the other hand, a high use 

 of water may not necessarily result in a high yield, because of failure 

 of the water supply toward the end of the growing season, or because 

 unfavorable conditions other than water supply may intervene to 

 prevent the potential yield from being realized. This lowers the 

 correlation. 



The data studied indicate that for any locality within the Great 

 Plains the relation between the yield and the water consumed during 

 the growth of the crop is a linear one, but the origin of the line is not 

 zero. The consumption of a certain minimum quantity of water is 

 necessary before any yield above zero is realized. Each unit increase 

 of water consumed above this minimum appears to result in a unit 

 increment of yield. Both the minimum quantity required and the 

 quantity required for each unit increment of yield depend upon the 

 climatic environment of the season and in the average upon the 

 geographical location. The minimum quantity of water necessary 

 to the harvest of any grain has varied from about 4 inches at the 

 northern stations to as high as 10 inches at the southern stations. 

 Tentative formulas are presented for Edgeley, N. Dak., and North 

 Platte, Nebr., by which to calculate the yield from the water con- 

 sumption. 



The quantity of available water that may be stored in the soil is 

 not sufficient of itself to meet the needs of the crop. The quantity 

 of available water in the soil can be quickly determined at any time. 

 Knowing the daily rate of use and the time necessary to mature the 

 crop or the total quantity of water required for a given production, 

 the probable yield from a given precipitation during the remainder 

 of the growing season or the precipitation necessary to produce a 

 given yield can be calculated. This method of forecasting yields has 

 obvious limitations, but it affords a better basis of approximation 

 than has heretofore been available. 



