Before proceeding to an examination of the areas in the United
States subject to the methods of dry-farming it may be well to
define somewhat more clearly the terms ordinarily used in the
description of the great territory involved in the discussion.

The states lying west of the 100th meridian are loosely spoken of as
arid, semiarid, or sub-humid states. For commercial purposes no
state wants to be classed as arid and to suffer under the handicap
of advertised aridity. The annual rainfall of these states ranges
from about 3 to over 30 inches.

In order to arrive at greater definiteness, it may be well to assign
definite rainfall values to the ordinarily used descriptive terms of
the region in question. It is proposed, therefore, that districts
receiving less than 10 inches of atmospheric precipitation annually,
be designated arid; those receiving between 10 and 20 inches,
semiarid; those receiving between 20 and 30 inches, sub-humid, and
those receiving over 30 inches, humid. It is admitted that even such
a classification is arbitrary, since aridity does not alone depend
upon the rainfall, and even under such a classification there is an
unavoidable overlapping. However, no one factor so fully represents
varying degrees of aridity as the annual precipitation, and there is
a great need for concise definitions of the terms used in describing
the parts of the country that come under dry-farming discussions. In
this volume, the terms "arid," "semiarid," "sub-humid" and "humid"
are used as above defined.

Precipitation over the dry-farm territory