USE OF WATER BY SPRING WHEAT ON GREAT PLAINS. 7 



introduction of irrelevant data in a correlation of the rate of the 

 use of water with the yield. 



There remained 53 years in which the daily rate of the use of 

 water could be determined under the conditions imposed. These are 

 distributed among 14 stations " located in the Great Plains from 

 Huntley, Mont., and Williston, N. Dak., on the north, to Amarillo, 

 Tex., on the south. Two of these stations — Ardmore, S. Dak., and 

 Garden City, Kans. — furnished data for only a single year each, 

 while at Edgeley, N. Dak., a measure of the rate was determined for 

 10 years. 



The rate was determined separately for two cultural methods at 

 each station. The one designated plat A is continuously spring 

 plowed and cropped to wheat. The one designated plat C or D is 

 alternately summer fallow and wheat. This method occupies two 

 plats, so there is available for study each year a crop of wheat grow- 

 ing on land bare summer tilled the previous year. 



In the average of conditions this method contains more water in 

 the soil at the time the crop commences rapid growth, supports a 

 heavier growth of vegetation, and produces a larger crop than the 

 plat continuously cropped, but exceptions are to be noted when one 

 or more of these conditions are reversed. 



The results of this study are presented in Table 1. This gives the 

 station, the year, the elates of the soil-moisture determinations mark- 

 ing the beginning and end of the period, the average daily precipita- 

 tion during the period, the average daily evaporation from a free 

 water surface during the period, the daily rate of water use and 

 yield of plat A, and the daily rate of water use and yield of plat 

 C or D. 



The daily rate of water use has been charted with calendar dates 

 as abscissa and total use as ordinates. The slope of the curve then 

 indicates the rate of use. This chart is not presented, because a con- 

 siderable part of its value for study lies in the identification of each 

 line with the year and station it represents. It is impracticable to 

 attempt this on the scale to which it necessarily would be reduced in 

 publication. Such charting shows that the rate of use is not deter- 

 mined by the dates within which or the part of the period of rapid 

 growth during which the rate was determined. This phase of the 

 subject is more fully developed in the study of the rate of the use 

 of water during the season. In this it is shown that after beginning 

 rapid growth, about the time tillering is completed, the rate of use 

 remains fairly constant until harvest if not interrupted by failure of 

 the water supply or by some destructive or inhibiting agency. 



These determinations afford opportunity for a study of the rela- 

 tive importance of water from the soil and of precipitation in deter- 

 mining the rate of use and for the examination of the reasons for 

 combining the two to obtain this rate. 



The material has been studied in various ways. Charting the two 

 quantities and the total which they make up shows very clearly that 

 they are complementary. Under given conditions a given crop re- 

 quires a certain quantity of water. If this is supplied by precipita- 

 tion the quantity of water in the soil will not be reduced, and if 

 more than the required quantity is supplied it will be increased; 

 but if the rainfall is not sufficient to meet the demand the available 



