USE OF WATER BY SPRING WHEAT ON GREAT PLAINS. 31 



inches for wheat, depending upon the season and the locality. Yields 

 have a direct relation to the quantity of water consumed above this 

 minimum. In general, the quantity required as a minimum to any 

 production as well as the quantity above this minimum required for 

 each unit increment of yield increases with change of location from 

 north to south. 



The study now made shows the incessant and heavy demand of the 

 crop for water from the time it commences growth until it matures. 

 The yield depends upon this demand being met uninterruptedly. 

 When the water to meet this demand is not available in the soil, the 

 rate of use is necessarily decreased. Such decrease always compro- 

 mises the yield. Fortunately for the success of agriculture, death 

 of the plant from a shortage of water is a very indefinite and long- 

 delayed occurrence. When field crops suffer distress, they sacrifice 

 vegetative growth to seed production and only cease their efforts 

 after they have exhausted all available water and produced more or 

 less seed. Under conditions where the water supply is normally 

 insufficient or very seldom more than sufficient to meet the possible 

 maximum demands of the crop under full possible development, it 

 is consequently the normal condition for the available soil water to 

 be exhausted when the crop matures. If death of the crop were as 

 sudden a phenomenon as the exhaustion of soil water, the percentage 

 of crop failures would be much higher than it is. 



Abbreviated column headings used in the tables and repeated in 

 the text have distinguished between water used from the soil and 

 watter supplied by precipitation. Actually, of course, the entire 

 use is -from the soil, and the division is between the quantity already 

 in the soil at the beginning of the period studied and the precipita- 

 tion by which this quantity is replenished from time to time. The 

 available water in the soil is the reservoir from which the crop 

 draws. Its quantity may fluctuate from time to time as the precipi- 

 tation is greater or less than the current needs of the crop. So long 

 as it is not depleted to the point of exhaustion the crop appears able 

 to maintain its normal rate of use indicated by an uninterrupted 

 curve. When depletion reaches the critical point marking the divi- 

 sion between available and nonavailable water, the rate of use is 

 interrupted and the crop suffers. Precipitation replenishes the 

 supply, and the use of water is resumed. A sufficient^ detailed study 

 of this condition would show intermittently low and high rates of 

 use. In the present study these are averaged together for consider- 

 able periods of time. The result of this is to show a low rate of use 

 for the period. When such a condition exists early in the season the 

 vegetative growth is either reduced or not developed, and the rate 

 of use when water is available is thus reduced. This is the condition 

 reflected in the curves that show a low rate of use and a small quan- 

 tity of water used. This is the condition referred to by such state- 

 ments in the text as " The crop suffered from drought throughout the 

 season." 



Maximum possibilities can be realized only when the water sup- 

 ply is at all times adequate to permit use at an uninterrupted rate. 

 The present determinations of this rate show that at the northern 

 stations the wheat crop in the field requires an inch of water every 

 five or six days. At the southern stations the demand rises to an 

 inch of water every four or even every three days. 



