TIME OF IRRIGATION 183 



rapid; and this rapid growth, or increase in weight, con- 

 tinues during the whole time of early flowering. When 

 seed-formation begins, the rate of growth diminishes; 

 and after the seeds have been formed, it is even smaller 

 than in the earlier stages. 



The water transpired by crops is generally, though 

 not always, hi proportion to the rate of growth. Water 

 lost by evaporation from the soil increases and decreases 

 largely in the same proportion, because the time, tem- 

 perature and other conditions that determine the rate of 

 plant-growth also determine the rate of direct evaporation. 



Such a coincident variation would mean that little 

 water needs be applied in the earlier periods of plant- 

 growth, but that, as the rate of growth increases, the rate 

 of adding water must be increased until the period of 

 seed-formation approaches, when the supply may again 

 be diminished. In practice, it is exceedingly difficult even 

 to approximate this ideal system of irrigation, for the 

 stream flow in most localities decreases rapidly from early 

 spring until the time of maximum water needs is reached. 

 At that time of high requirements and low supply it is 

 difficult for the farmer to supply his crops with the best 

 quantity of water at the right time. Under reservoir con- 

 ditions, the ideal requirements are more nearly met. 

 Nevertheless, under any conditions, the farmer must 

 attempt, as nearly as may be possible, to give his crops 

 most water at the time when the crops need water most. 

 This time, in turn, depends on the crops grown under the 

 system. By a wise diversity of crops, a small stream hi 

 early or late summer may be made to served a large area 

 well. 



115. Time of irrigating short-season crops. Wheat 

 and the other small grains, peas, beans and similar short- 



