which is the great Canon of the Colorado.  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 possessed 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 influenced 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 and down to the seacoast, semiarid to sub-humid conditions are again found.

Seasonal distribution of rainfall

It is doubtless true that the total annual precipitation is the chief factor in determining the success of dry-farming.  However, the distribution of the rainfall throughout the year is also of great importance, and should be known by the farmer.  A small rainfall, coming at the most desirable season, will have greater crop-producing power than a very much larger rainfall poorly distributed.  Moreover, the methods of tillage to be employed where most of the precipitation comes in winter must be considerably different from those used where the bulk of the precipitation comes in the summer.  The successful dry-farmer must know the average annual precipitation, and also the average seasonal distribution of the rainfall, over the land which he intends to dry-farm before he can safely choose his cultural methods.

With reference to the monthly distribution of the precipitation over the dry-farm territory of the United States, Henry of the United States Weather Bureau recognizes five distinct types; namely:  (1) Pacific, (2) Sub-Pacific, (3) Arizona, (4) the Northern Rocky Mountain and Eastern Foothills, and (5) the Plains Type:—­

"The Pacific Type.—­This type is found in all of the territory west of the Cascade and Sierra Nevada ranges, and also obtains in a fringe of country to the eastward of the mountain summits.  The distinguishing characteristic of the Pacific type is a wet season, extending from October to March, and a practically rainless summer, except in northern California and parts of Oregon and Washington.  About half of the yearly precipitation comes in the months of December, January, and February, the remaining half being distributed throughout the seven months—­September, October, November, March, April, May, and June.”