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."
The irrigating season lasts about three months. While the crops are
maturing during part of May, June and July, they will receive two or
three waterings, and in early September the hay lands are again watered
in order to start the growth of grass before the frosts.
In case all the surplus water of a given stream is stored, the duty of
that stream will be increased by the amount of water now flowing to
waste during the remaining nine months, and as a portion of this time
is the flood period, owing to the melting of the snows in the mountains
and to the spring rains, this storage water will increase the duty of
the stream at least five-fold; that is, five times as many acres may be
irrigated by the stream as at present, provided that storage capacity
can be found for all of its waste waters.
In considering the duty of a stream it must be remembered that there is
a great loss of water by seepage through the sides of a canal and
evaporation from its surface, between the headworks and the irrigated
lands, this loss may amount to from 25 to 35 per cent., according to
climate, soil, and the length and cross-section of the canal.