36 DRY-FARMING 
North and east of the Great Basin is the Columbia 
River Basin characterized by basaltic rolling plains 
and broken mountain country. To the west, the 
floor of the Great Basin is lifted up into the region 
of eternal snow by the Sierra Nevada Mountains, 
which north of Nevada are known as the Cascades. 
On the west, the Sierra Nevadas slope gently, through 
intervening valleys and minor mountain. ranges, 
into the Pacific Ocean. It would be difficult to 
imagine a more diversified topography than is pos- 
sessed by the dry-farm territory of the United States. 
Uniform climatic conditions are not to be expected 
over such a broken country. The chief determining 
factors of climate — latitude, relative distribution 
of land and water, elevation, prevailing winds — 
swing between such large extremes that of necessity 
the climatic conditions of different sections are widely 
divergent. Dry-farming is so intimately related 
to climate that the typical climatic variations must 
be pointed out. 
The total annual precipitation is directly influ- 
enced by the land topography, especially by the 
great mountain ranges. .On the east of the Rocky 
Mountains is the sub-humid district, which receives 
from 20 to 30 inches of rainfall annually; over the 
Rockies themselves, semiarid conditions prevail; 
in the Great Basin, hemmed in by the Rockies on the 
east and the Sierra Nevadas on the west, more arid 
conditions predominate; to the west, over the Sierras 
