PKECIPITATION. 



11 



and Nebraska lie west of this line. To include this territory seems 

 manifestly unjust and misleading, if it does not make the term semi- 

 arid" actually meaningless. It is impossible to fix a positive and 

 definite line on the one side, of which we shall say the country is humid 

 and on the other semiarid, or, as some prefer to say, ^^subhumid," for 

 there is no sudden dropping off in precipitation, but a fairly uniform 

 decrease from east to west across the two States. As generally used, 

 the term refers to a country receiving an average of between 10 and 

 20 inches of melted snow and rain annually, but in determining aridity 

 or humidity evaporation is of equal importance with precipitation. 

 In southern Texas much more than 20 inches of precipitation may 

 be required to make a humid country, but 20 inches of rainfall in the 

 Red River region of North Dakota makes a distinctly humid climate. 

 With reference to Kansas and Nebraska the writer prefers to consider 

 the western limit of 20 inches average annual precipitation as the 

 eastern limit of the semiarid region, although in southern Kansas this 

 limit may be too far west and in some other places too far east. So 

 far as the records for Kansas and Nebraska now show, this line in 

 most ^places lies 20 to 30 miles west of the one hundredth meridian. 



The accompanying map (fig. 1) shows the region to which this dis- 

 cussion is intended to apply and the average annual precipitation as 

 shown by records of the Weather Bureau. 



CLIMATE. , 



The climate of the Great Plains region has been thoroughly dis- 

 cussed by several able writers and for that reason it seems unnecessary 

 to give more than a brief summary here. It is a region peculiarly 

 subject to high winds, driving storms, and sudden changes in tempera- 

 ture. The light is intense and the air usually very dry. At least in 

 a large proportion of it hail is of frequent occurrence and does much 

 damage to crops. The native flora and even the soiP attest the gen- 

 eral dryness. To the careful student of nature these tell a story of 

 perennial dryness over which the myth of changing climate could have 

 no appeal. 



PRECIPITATION. 



All plants for proper development require a reasonable supply of 

 plant food in available form, favorable temperature, an adequate 

 supply of moisture, and an abundance of sunshine. Given a fertile 

 soil, the yield of the crop depends upon the relative distribution of 

 heat, moisture, and light throughout the season. But a chain is no 

 stronger than its weakest link. Given favorable conditions with 

 respect to all the foregoing except one, that one becomes the limiting 

 factor of success — the all-important question. In most of the Great 



1 Bulletin 55, Bureau of Soils. 



215 



