168 F. H. Neivell — Arid Regions of the United States. 



The limits are not easy to place, for they depend upon climatic 

 forces which vary in intensity from year to year — that is to say, 

 in aay given locality within the arid regions there may not be 

 for several successive years sufficient moisture for maturing 

 crops of grain, while in the following year rain occurring at the 

 right time may enable a farmer to produce a heavy crop. Thus 

 in the latter year these arid regions might be considered as re- 

 duced in size, to be again increased as drought follows drought. 

 It is necessary, therefore, to assume certain arbitrary boundaries 

 based upon considerations of general success or failure of ordi- 

 nary agricultural operations in so far as they are dependent 

 upon rainfall. 



For the eastern boundary it is convenient to assume the one 

 hundredth meridian west of Greenwich, although, as a matter 

 of fact, " dry" farming has been. successfully carried on as far 

 west as the one hundred and fifth meridian or even beyond. 

 The western boundary is more irregular, owing to a wide differ- 

 ence in the topography of the country which lies between the 

 well-defined arid and humid areas near the Pacific coast. 



As laid down by Powell* on the maps of the Geological Survey, 

 the southwestern boundary of the arid region is the Pacific 

 ocean up to a point on the coast of California north of Monterey 

 bay. From here the line turns inward across the valley of the 

 San Joaquin, then, excluding the bay counties, follows northward 

 along the western foothills of the Sierra Nevadas and the eastern 

 slopes of the Cascade range of Oregon and Washington, in which 

 latter state it turns eastward, excluding from the arid regions the 

 northeastern portions of Washington and Idaho. These lines, 

 as originall}^ drawn, were based largely upon the assumption that 

 twenty inches of annual rainfall were necessary for farming opera- 

 tions, but were modified, however, by considerations of the sea- 

 sonal distribution.f The lines thus laid down, although they 

 may be criticised from various standpoints, are sufficiently exact 

 for any general discussion, and are, perhaps, more useful than 

 others drawn with greater nicety and attempting to reach higher 

 precision. 



*J. W. Powell : Second annual report of the irrigation survey, in Elev- 

 enth Annual Eeport of the United States Geological Survey, part 2, irri- 

 gation, Washington, 1891. 



t Lands of the Arid Eegions of the United States, J. W. Powell, Wash- 

 ington, 1879, p. P> et seq. 



