106 Irrigation and Drainage 



with the other fact for a side-light turned upon it, 

 that 6 inches of water, when the crop can have it to 

 use to the best advantage, is enough to produce 20 

 bushels of wheat to the acre, we can see its outlines 

 with sufficient clearness to feel sure that more study 

 in the field would give us its full solution. As the 

 matter now stands, the case is sufficiently clear that 

 we may not conclude, because 9 to 12 inches of rain 

 in California has produced abundant crops of wheat, 

 that a similar rainfall in the sub -humid belt ought 

 to produce like results. It should be sufficiently 

 evident, also, that even with the best modes of till- 

 age we can hope to adopt, there will still be much 

 more water required per pound of dry matter pro- 

 duced all through the sub -humid region, than is de- 

 manded under the conditions of the lower San Joa- 

 quin valley. 



The same principles make it very clear, also, that a 

 judicious application of water by the methods of irri- 

 gation, in many humid climates, is certain to be at- 

 tended by marked increase in the yield. 



FREQUENCY AND LENGTH OF PERIODS OF 

 DROUGHT 



In humid and sub -humid regions, it is the frequent recur- 

 rence of periods of small or no rainfall, especially if they occur 

 at the time when the crop is approaching or has reached the 

 fruiting stage, that, more than anything else, makes extremely- 

 careful and thorough tillage, or else supplementary irrigation, 

 indispensable, if large yields are to be realized. 



In our repeated trials in the field cylinders here in Wiscon- 



