ON DRY-FARMING INVESTIGATIONS IN THE UNITED STATES. 265 
Rainfall. 
Tt has become customary to use the average annual rainfall as a 
measure of the relative value of different areas for dry-farming pur- 
poses. Since the water-supply is usually the primary limiting factor, 
the annual rainfall must of course be emphasised. All who are 
engaged in dry-farming investigations recognise, however, the severe 
limitations of this classification. The seasonal distribution and thie 
character of the torrential or in the form of 
numerous light showers, or occurring as steady, 
more important than the total annual rainfall in determining the pro- 
ductivity of a dry-farming region. The uncertainty of the rainfall 
should also be considered whenever sufficient statistical evidence is 
available. 
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Fro. 1.—Chart showing the monthly distribution of the rainfall at representative 
stations in the Great Plains, Intermountain, and Pacific coast regions. The 
length of the black lines in each diagram represents the monthly precipitation 
at that place, beginning with January on the left. The scale in inches given 
on the right of each diagram can be used to find the actual amount of the 
monthly rainfall. For example, the average monthly rainfall at Bismarck, 
N. Dak., for June is seen to be 3} inches, while for July it is only a little more 
than 2 inches. It will be noted that in the Pacific coast region the rain comes 
principally at the beginning and end of the year, that is, in the winter; in the 
Intermountain districts during the winter and spring months ; and in the Great 
Plains during the summer months. 
Rainfall is not the only factor of importance, however. We shall 
refer later to the desirability of knowing the seasonal evaporation as 
measured from freely exposed tanks, which affords a summation of 
those factors which determine the rate of transpiration. The maximum 
temperatures and the wind velocity are also important factors. For 
an adequate comparison of widely separated dry-farming areas, a know- 
ledge at least of the annual rainfall, its seasonal distribution, the 
