107 



checked by competition with the prairie 

 grasses. Thus we have a strip of short grass 

 extending from Montana to Texas limited on 

 the west by drought and on the east by com- 

 petition with other grasses. Therefore the in- 

 crease in the annual rainfall as we go from 

 north to south in the short-grass region repre- 

 sents the additional amount of rain needed to 

 offset the increased evaporation. In Montana 

 short grasses unmixed with other grasses oc- 

 cur in districts having an annual rainfall of 

 approximately 14 inches; in Colorado in 

 regions having rainfall of 17 inches, and in 

 the Panhandle of Texas in regions having an 

 annual rainfall of 21 inches. 



"The same type of grass requires in Texas 

 7 inches more of rain a year than in Mon- 

 tana. This gives us at once a measure of the 

 additional amount of rain required for a grain 

 crop in Texas as compared with Montana. 

 * * * * A rainfall of 21 inches in the 

 Panhandle of Texas is therefore really no 

 better for growing crops than 15 inches in 

 Montana or Dakota." 



Another and still more important factor is 

 that of seasonal rainfall. The seasonal dis- 

 tribution of rainfall in the semi-arid lands is 

 divided into three types the summer rainfall 

 of the Great Plains region; the winter and 



