As to water supply, the desert belongs in two regions, arid and
semi-arid. West of the Missouri and extending to the foot of the Rocky
Mountains, is a vast area of foothills and gently sloping, grassy
prairies, which constitute a large part of the semi-arid or sub-humid
region of the United States. This broad belt, stretching from Mexico
northward into Canada, has no clear-cut boundary separating it from
the humid region on the east or the arid region on the west, owing to
the variance of the mean annual precipitation in many localities. A
convenient and easily marked line for the eastern boundary of the arid
region is one closely following the hundred and third meridian. On the
north it bends away from the meridian toward the west, and on the south
tends eastward north of the Rio Grande. On the west the arid region
extends to the Pacific Coast in extreme southern California, but from
Monterey north there is a narrow belt of semi-arid and humid country
bordering the Pacific Ocean. The Coast Range is humid, especially in
Oregon and Washington, where the rainfall is as heavy as in any other
part of the United States.
[Illustration: FORTY BUSHELS OF WHEAT PER ACRE