128 



THE ESSENTIALS OF AGRICULTURE 



over 2,000,000 acres are now in dry farms. Here the common 

 practice is to sow one half of the farm to grain and to allow the 

 other half to lie in fallow. In the Great Plains region a 

 larger seasonal rainfall is required for successful farming than 

 in most of the Rocky Mountain states, due to a longer growing 

 season and a higher rate of evaporation. 



155. Rainfall and other climatic conditions. The manner 

 in which the water is distributed through the different seasons 

 of the year is important as affecting farming under dry-land 

 conditions. On the Pacific coast the rain falls almost en- 

 tirely during the winter. 

 In the Intermountain 

 region the greatest pre- 

 cipitation occurs during 

 the late winter and early 

 spring. In the south- 

 ern part of the area, 

 particularly in Arizona, 

 the rainfall is divided 

 into two periods. The 

 first rainy season occurs 

 during the winter, and 

 the second, which is 

 larger, during July and 

 August. In the Great Plains region the rainfall occurs during 

 the late spring and early summer, and the winter is usually 

 very dry. 



The rate at which evaporation takes place has a marked in- 

 fluence upon the amount of rainfall required for successful dry 

 farming. In the southern part of the dry-land area, where the 

 summers are warm, ^and particularly in the southern part of the 

 Great Plains region, where hot, dry winds accompany the warm 

 weather, the rate of evaporation is much higher than in the 

 northern part of the region, where the weather is cooler and the air 

 more humid. The evaporation from a free water-surface at Garden 

 City, Kansas, is twice as great as at Williston, North Dakota. 



FIG. 55. Autumn tillage to prevent blowing 

 and to hold moisture 



