THE DOMAIN FOR DRY FARMING 45 



With an annual rainfall of 15 to 18 inches, it is practic- 

 able to get two grain crops in three years. With a rain- 

 fall of 20 inches it is practicable to secure good crops 

 every year from a properly adjusted rotation, on the 

 assumption that a cultivated crop is one of the factors of 

 the same. In the southern portions of the dry belt con- 

 siderably more precipitation would be called for to secure 

 these results, because of the higher evaporation. 



The time at which the precipitation falls has an im- 

 portant bearing on the crops that may be grown. In the 

 Plains region about 50 per cent, of the precipitation comes 

 during the period of greatest growth. This means that 

 the adaptation for spring cereals is relatively high. It 

 also favors the successful maturing of winter grains 

 where these have come through the winter successfully 

 In these areas, however, there may be difficulty in start- 

 ing winter grains because of the lack of moisture. But 

 since in this area the rains almost practically cease with 

 July, it is important that the spring crops grown shall 

 be of such species -and varieties as mature early. In 

 much of the Inter-mountain area the precipitation comes 

 mainly in the late autumn and early spring months. The 

 same is true of much of the Great Basin country. Be- 

 cause of this the adaptation is highest for autumn-sown 

 grains, as these mature early and before the drought 

 and greatest heat of summer is ushered in. They also 

 get the full benefit of the winter rains. In areas where 

 the highest precipitation occurs in the summer and au- 

 tumn months, as in Oklahoma, Texas and New Mexico, it 

 will be in order to grow late maturing cereals and such 

 crops as corn and the sorghums. 



The influence exercised on crop production by evapo- 

 ration is very material. It increases (1) with increase in 

 the dryness of the atmosphere ; (2) increase in the sum- 

 mer heat; (3) increase in the dryness and velocity of the 

 winds, and (4) with decrease in the humidity. It de- 



