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.
SOIL.
The soil along the stream bottoms at a slight elevation above their beds is usually a heavy, black, clayey loam, and though rich and fertile is soon clogged by water, and then in drying, cakes on the surface, killing the young plants. On this account the irrigators seldom water these bottom lands until after the crop has acquired a healthy growth, preferring to trust to the early rains to force the young sprouts above the surface, rather than run the risk of its crusting and thus preventing them from breaking through.
These bottom lands though really the poorest for irrigating, are nearly the only lands now cultivated, because of the greater ease and cheapness of supplying them with water. From two to three tons of hay and from 35 to 50 bushels of grain per acre are raised even on these inferior soils.
The best, and by far the more abundant agricultural lands, are the "bench lands," these are situated high above the stream beds and the soil is usually a warm open, rich, sandy-loam, several feet in depth and usually underlain by a deep bed of gravel. Though in irrigating, this soil at first requires more water, it will, owing to its excellent natural drainage, last for all time and will neither clog with water nor cake on the surface.
It is these bench lands which will be rendered irrigable by government aid and surveys, though to develop them will require large amounts of capital; still, they are so extensive in area that the work can generally be conducted on a grand and economical scale.
DUTY OF WATER.
From the meagre information now obtainable it is probable that in average soils and for the staple hay, grain and vegetable crops in Montana, about one cubic foot of water per second, flowing during the irrigating season, will be sufficient for 100 acres; this quantity is known as the "duty of water."