FARM IRRIGATION 239 



in an arid or semi-arid region without the aid of 

 irrigation, is notably and increasingly successful. 

 Dry farming is discussed in Chapter V. Thus the 

 farmer of the semi-arid Palouse region, in eastern 

 Washington and eastern Oregon, is able to grow 

 much larger crops of wheat than the farmer in other 

 regions having me same amount of rainfall, because 

 most of the ram falls in winter and early spring, and 

 is practically all absorbed by the soil as the weather 

 is cool and there is little evaporation; whereas, if 

 a large portion of it fell in summer the loss by 

 evaporation would be great. Then again, the soil 

 of tnis region a deep oasaltic ash is remarkably 

 retentive of moisture, drying out very slowly and 

 giving up its moisture gradually to crops during 

 the almost cloudless summer. 



This single illustration will emphasise sufficiently 

 the importance of these points in their relation to 

 irrigation; that the need of supplying more water 

 to a soil in order to make it produce profitable crops 

 depends not only upon the actual amount of rain, 

 but also upon the time when it falls, upon the reten- 

 tiveness 01 the soil, and upon the skill of the farmer 

 in making the fullest use of the natural supply of 

 water. Ten inches of rainfall in one section may 

 be equal to sixteen inches in another, so far as its 

 crop-producing capacity is concerned. 



IRRIGATION IN HUMID REGIONS 



In the United States irrigation is resorted to 

 chiefly as a means of correcting an absolute defi- 

 ciency of moisture. It is an arid and semi-arid 

 farming practice and is confined mainly to regions 

 having less than twenty inches of rainfall. But 

 there nas been much interest in irrigation in the 



