2 LANDS OF THE ARID REGION OF THE UNITED STATES. 
as to indicate approximately the boundary between the Arid Region, where 
irrigation is necessary to agriculture, and the Humid Region, where the lands 
receive enough moisture from the clouds for the maturing of crops. Expe- 
rience teaches that it is not wise to depend upon rainfall where the amount 
is less than 20 inches annually, if this amount is somewhat evenly distrib- 
uted throughout the year; but if the rainfall is unevenly distributed, so that 
“rainy seasons” are produced, the question whether agriculture is possible 
“rainy season” and the 
without irrigation depends upon the time of the 
amount of its rainfall, Any unequal distribution of rain through the year, 
though the inequality be so slight as not to produce ‘rainy seasons”, affects 
agriculture either favorably or unfavorably. If the spring and summer pre- 
cipitation exceeds that of the fall and winter, a smaller amount of annual rain 
may be sufficient; but if the rainfall during the season of growing crops is less 
than the average of the same length of time during the remainder of the year, 
a greater amount of annual precipitation is necessary. In some localities in 
the western portion of the United States this unequal distribution of rainfall 
through the seasons affects agriculture favorably, and this is true imme- 
diately west of the northern portion of the line of 20 inches of rainfall, which 
extends along the plains from our northern to our southern boundary. 
The isohyetal or mean annual rainfall line of 20 inches, as indicated 
on the rain chart accompanying this report, begins on the southern bound- 
ary of the United States, about 60 miles west of Brownsville, on the Rio 
Grande del Norte, and intersects the northern boundary about 50 miles east 
of Pembina. Between these two points the line is very irregular, but in 
middle latitudes makes a general curve to the westward. On the southern 
portion of the line the rainfall is somewhat evenly distributed through the 
seasons, but along the northern portion the rainfall of spring and summer 
is greater than that of fall and winter, and hence the boundary of what has 
been called the Arid Region runs farther to the west. Again, there is 
another modifying condition, namely, that of temperature. Where the tem- 
perature is greater, more rainfall is needed; where the temperature is less, 
agriculture is successful with a smaller amount of precipitation. But geo- 
altitude and 
latitude. Along the northern portion of the line latitude is an important 
graphically this temperature is dependent upon two conditions 
