KELATION OF MONTHLY RAINFALL TO FAEM PKACTICE. 13 



months are very dry. In fact, about three-fourths of the total rain- 

 fall of the year at these stations occurs during the six summer months, 

 from April to September, inclusive u 



In the Pacific coast region, on the other hand, the rainfall occurs 

 almost entirely during the winter months, and it will be seen from 

 the chart that the summer months are very dry. The rainfall dis- 

 tribution in the Pacific coast region is, then, exactly opposite to 

 that found in the Great Plains. In the Intermountain region the 

 maximum precipitation occurs during the late winter and spring 

 months. Thus there is here a distribution intermediate between 

 that of the winter precipitation of the Pacific slope and the summer 

 precipitation of the Great Plains. 



The monthly distribution of the rainfall over the western portion of 

 the United States is presented diagrammatically on the map shown 

 as figure 1, which has been adapted from Bulletin N of the United 

 States Weather Bureau. The change from a summer rainfall in the 

 Great Plains to a winter rainfall on the Pacific coast is very marked, 

 and the intermediate types are shown in the intervening country. 

 In Arizona we find that the rainfall is divided into two periods during 

 a year, a rainy season occurring during the winter months and a 

 second rainy season in July and August, the latter being the greater. 



THE RELATION OF MONTHLY DISTRIBUTION OF RAINFALL TO 



FARM PRACTICE. 



In regions where the annual rainfall is low the monthly distribu- 

 tion of the rainfall determines to a large extent the farm practice of 

 the region. A comparison of the systems of dry farming employed 

 in the Great Basin and in the Great Plains will make this clear. In 

 Utah and in a large part of the Great Basin country the precipitation 

 comes largely in the winter and early spring. In consequence, spring 

 wheat is not a successful crop in this region for two reasons: (1) The 

 land can not be fitted for sowing until late in the season, owing to the 

 spring rains, and (2) the driest part of the season occurs just when 

 the spring- wheat crop would be maturing. On the other hand, the 

 conditions in this region are almost ideal for growing winter wheat 

 on summer- tilled land. The land is plowed in the fall so as to be 

 left in the best condition for holding the snow and the winter and 

 spring rains. It is then kept well tilled during the summer and con- 

 tains sufficient moisture to start the crop in the fall. If a fall rain 

 occurs about seeding time conditions are especially favorable. The 

 fall-sown crop is then able to take advantage of the winter and spring 

 precipitation and matures before the driest part of the season is 

 reached. Clean summer fallowing (summer tillage) is considered 

 absolutely necessary in this region in order to be sure of a successful 



188 



