52 



Land Planning Report 



The map shows that droughts, as thus defined, are 

 comparatively infrequent east of the Mississippi River, 

 and, as a rule, become increasingly frequent westward 

 from the Mississippi Valley. That is, areas having com- 

 paratively Ught rainfall are subject to the most fre- 

 quent variation from normal, especially with regard to 

 deficient amounts. 



The lowest frequencies are shown for the vSouth- 

 eastern States, the eastern Ohio Valley, the Northeast, 

 and the Lake region. In these areas considerable sec- 

 tions show as low as 2 to 5 percent, or during the active 

 growing season rainfall is less than two-thirds of nor- 

 mal only once in 20 years or more. In fact, in most 

 places east of the Mississippi River, droughts of this 

 character occur less frequently than once in 10 years, 

 on the average. 



In the Great Plains States they are more frequent, 

 becoming, as a rule, increasingly prevalent with de- 

 crease in the normal rainfall. Here the percentages 

 range mostly from 10 to 20. However, there are sev- 

 eral considerable areas, especially in north-central Ne- 

 braska and southern South Dakota, southwestern and 

 extreme eastern Montana, the eastern half of Wyoming, 



northern Oklahoma and extreme southern Kansas, and 

 the Panhandle of Texas, which show comparatively 

 infrequent droughts. On the other hand, there is a sec- 

 tion comprising western Texas and southern New Mex- 

 ico which shows much greater drought frequency, while 

 smaller areas in different parts of the Plains States have 

 comparatively large percentages. In the Great Basin 

 and the far Southwest droughts are more frequent than 

 in any other areas shown on the chart. Here the per- 

 centages in most places range up to 25, or higher, show- 

 ing an average of 1 drought year in 3 or 4. The largest 

 percentages appear in the far Southwest, especially in 

 southern Nevada and southwestern Arizona wliere they 

 run as high as 40 to 50. 



It is important to bear in mind the fact that relative 

 frequency of drought recurrence is much more serious 

 in areas of low average rainfall than in regions of liigh 

 average rainfall, for in the latter the accumulated re- 

 serve of water, both above ground and underground, is 

 much more ample. ^ 



■ For extended information and maps on rainfall conditions, ttie reader is referred 

 to pt. ni, National Resources Board Report of the Water Planning Committee, 

 December 1934. 



