HYDROLOGIC PRINCIPLES 25 



tion — are somewhat more extensive than the areas in which 

 the annual precipitation is less than 20 inches. These areas are 

 shown on Fig. 3. Also shown on this map are the areas of moist 

 climates in which there is a summer water deficiency in aver- 

 age years — and in which therefore as a general agricultural 

 practice it may be desirable to supplement precipitation by 

 some form of irrigation. 



The Delineation of Arid and Humid Regions 



The areas in which precipitation is normally less than 20 

 inches a year (Fig. 1) are also the areas which generate the least 

 stream flow (Fig. 2). Annual potential evapotranspiration 

 (computed on the basis of temperature) exceeds the precipita- 

 tion over a somewhat larger area, and Thornthwaite shows 

 that in addition there are humid areas where water is deficient 

 in most summers (Fig. 3). None of those maps is complete; 

 many of the western mountain ranges are not distinguished 

 from the surrounding arid region because they are too small 

 to show on these maps or because there are no data on pre- 

 cipitation or stream flow to indicate the climatic conditions. 

 Several humid "islands" that appear on the map showing run- 

 off are not included on the other maps because meteorological 

 data are lacking for the higher parts of most mountain ranges. 



All three maps confirm the existence of a large area of per- 

 ennial water deficiency covering most of the western half of 

 the country. Throughout this area the precipitation is ordi- 

 narily insufficient for agriculture, although border zones with 

 semiarid climate may be successful dry-farming areas in some 

 years. Generally, however, cultivated crops require irrigation, 

 and the major sources of water for irrigation — whether from 

 streams, springs, or wells — are the humid mountain ranges. 

 Water for municipal and industrial use in arid and semiarid 

 regions is also obtained from the surplus produced in these 

 humid areas. Thus the water resources of the mountain ranges 

 are of critical importance to the economy of the West. 



These maps portray average conditions, and the chance that 



