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



This rapid consumption of water, considered in connection with 

 the normal rainfall, shows why it is so unusual for water to accu- 

 mulate during this period and why the water in storage at its be- 

 ginning is so generally exhausted. To furnish a surplus in ad- 

 dition to meeting the needs of the crop, a rainfall much greater than 

 the normal would be required. Of all the years and stations studied 

 there has been only one year at one station when the water content of 

 the soil at harvest time was higher than at the commencement of 

 rapid growth. 



Knowledge of the rate of use and of the quantity of water used 

 i& of fundamental importance in forecasting at any time during the 

 period of growth the probable production from the rainfall that 

 is to be expected, or the quantity of water that must be received to 

 realize a given production, or the quantity that will result in very 

 small yields or complete failure. 



After certain preliminary investigations and determinations on 

 a soil it is possible to determine quickly, within 24 to 48 hours, the 

 quantity of available water that it contains. Knowing the ap- 

 proximate daily rate of use, it is a simple calculation to determine 

 how long a period of drought can be endured without damage. It 

 is equally easy to calculate the rate at which water must be supplied 

 in order to bring the crop to maturity* without damage or to calcu- 

 late the probable results of any given rate or quantity of rainfall. 

 The probabilities of a given rainfall and the probable range of 

 rainfall for most points are known or may be easily calculated. Pre- 

 cipitation, of course, can not be foretold except as to its probabilities 

 and the limits that are likely to encompass it. 



Practical application of such forecasting has been made at a 

 number of the field stations of the Office of Dry-Land Agriculture 

 Investigations by members of the office staff for several years with 

 very gratifying results. Such predictions maj^ even be advanced 

 to the preceding fall, but the longer the period involved the greater 

 is the chance of unfavorable conditions being corrected. 



CONCLUSIONS. 



The crop has a supply of water measured by the available water 

 in the soil at the time growth begins plus the precipitation during 

 the period of growth. Under dry-farming conditions the two com- 

 bined are seldom more than sufficient to meet the needs of the crop. 



For any period during the growth of the crop the rate at which 

 water is used is practically independent of whether it is obtained 

 by reducing the water content of the soil or is supplied by pre- 

 cipitation during the period. 



Water is lost from the soil before seeding or before the crop com- 

 mences growth, and at an increasingly rapid rate as vegetation 

 develops until, at about the time tillering is completed or just be- 

 fore the crop begins to shoot, it reaches a rate that is maintained 

 practically constant until maturity, provided there is always water 

 available to maintain this rate. 



Maximum yields are dependent upon the uninterrupted main- 

 tenance of this rate throughout the period of growth. Yields are 

 compromised to the extent that the normal rate is interrupted or 

 reduced. If the water supply is short in the early season, vegeta- 

 tive growth may be reduced and the rate continue low. Interrup- 



