236 IRRIGATION PRACTICE 



employed in water conservation vary with the seasonal 

 distribution of the rainfall. The rainfall varies not only 

 from place to place, but varies also in its seasonal distribu- 

 tion. Thus, on the Pacific seaboard, west of the Sierra 

 Nevadas and the Cascades, the wet season extends from 

 October to March, with a practically rainless summer. 

 This is the Pacific type of distribution. Under the sub- 

 Pacific type, which extends over eastern Washington, 

 Nevada and Utah, the maximum rainfall is shifted toward 

 the early spring, but the summers are still quite rainless. 

 Under the Arizona type, fully developed in Arizona and 

 New Mexico, about 35 per cent of all the rain falls in July 

 and August, and May and June are generally the rainless 

 months. Under the Rocky Mountain foothills type, 

 most rain falls from April to June. Finally, under the 

 Plains type, embracing the larger part of the Dakotas, 

 Nebraska, Oklahoma and Texas, the heaviest rains come 

 during May to July, when crops are growing. Moving 

 eastward from the Pacific Coast to the Great Plains, the 

 major rains are shifted from midwinter to midsummer. 

 Moreover, in many places show lies long on the ground 

 in the winter, while in other places there is no snowfall. 

 The methods of conserving the rainfall in the soil must be 

 varied somewhat with regard to the prevailing type of 

 precipitation. Whether rains come in winter or summer 

 they must be stored in the soil for some time. Especially 

 where the rains come in the winter, they must be held in 

 the soil until the growing season arrives. 



146. Storing water in the soil. Well-plowed soil is in 

 a good condition to store water from the rains and snows. 

 In the western United States, where the summers and 

 falls are somewhat rainless, and the chief precipitation 

 comes in winter or early spring, fall plowing is commonly 



