It has been stated by Major J. W. Powell, that in a general way the
line between the humid and arid regions, or the amount of precipitation
below which irrigation becomes necessary for the cultivation of crops,
is from 24 to 28 inches per annum. This of course depends largely on
the distribution of the rainfall, the proportion falling during the
growing season, the humidity of the atmosphere, the character of the
soil, etc.
The average annual precipitation in Montana is 14.92 inches, while the
total average precipitation during the growing season is but 5.23
inches; from these considerations alone it is evident that the State
lies wholly in the arid region.
This statement is further born out by the fact that no native farmer
will settle a ranch or undertake to raise any kind of crops without
facilities for irrigating, since experience has taught them all, that,
though there may occasionally be an exceptionally wet season in which
they can raise good crops without artificial aid, still, the years when
crops depending wholly upon rain-fall for their moisture would be
entirely lost, are so frequent as to render farming without irrigation
very hazardous and unprofitable.