DISTRIBUTION OF ANNUAL RAINFALL. 



11 



eastern corner of North Dakota and passing southward through 

 central South Dakota, central Nebraska, western Kansas, and the 

 Panhandle of Texas, passes through points in these States where the 

 normal annual rainfall is 20 inches. The average annual rainfall of 

 the country lying to the east of this line is in general more than 20 

 inches and to the west of this line less than 20 inches. This line, 

 representing points having a normal annual rainfall of 20 inches, is 

 about 4° of longitude, or approximately 200 miles, farther west 

 in Texas than in North Dakota. 



The line representing an average annual rainfall of 15 inches ex- 

 tends west in North Dakota nearly to the western boundary, then 

 bends eastward and passes through the center of South Dakota, then 

 runs westward through the southwestern corner of Wyoming, and 

 making a detour in the mountainous region of Colorado passes 

 through the eastern part of that State and southward through the 

 central portion of New Mexico. The country lying between this 

 line of 15 inches annual rainfall and the line of 20 inches annual 

 rainfall to the east has, then, an annual rainfall of 15 to 20 inches, 

 the rainfall becoming greater as the 20-inch line is approached. A 

 second region in which the annual rainfall is between 15 inches and 

 20 inches is shown in central Montana and northwestern Wyoming. 

 A third region having this rainfall is to be found in northwestern 

 Wyoming and central Idaho and northeastern Oregon. Similar 

 areas exist in northern Utah, in the mountainous part of Arizona, 

 and in northeastern California. 



The line representing 10 inches of annual rainfall passes north 

 and west through the central part of California and through extreme 

 western Nevada, southeastern Oregon, and western Idaho, then 

 bends sharply southward, making a great loop in the State of Utah, 

 then runs through southwestern Wyoming, extreme western and 

 southwestern Colorado, northwestern New Mexico, and northern 

 and western Arizona. The unshaded area lying within this boundary 

 has an annual rainfall of less than 10 inches. This area as a whole 

 has not sufficient rainfall to make dry farming profitable, as it is 

 carried on at the present time. The conditions are much more 

 severe in the southern part of this area, owing to the higher evapora- 

 tion. It is possible that grain may eventually be produced along 

 the extreme northern border, particularly where other conditions are 

 especially favorable. 



In southern Washington a second region is to be found in which 

 the annual rainfall is less than 10 inches. The evaporation in this 

 region is much lower than in the sections just mentioned. It has 

 been found possible to produce wheat successfully within the borders 

 of this region on an annual rainfall of about 9 inches, using the 

 method of alternate cropping and summer tillage. 



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