INCREASE OF IRRIGABLE AREA BY THE STORAGE OF WATER.
Within the Arid Region great deposits of gold, silver, iron, coal, and many other minerals are found, and the rapid development of these mining industries will demand pari passu a rapid development of agriculture. Thus all the lands that can be irrigated will be required for agricultural products necessary to supply the local market created by the mines. For this purpose the waters of the non-growing season will be stored, that they may be used in the growing season.
There are two methods of storing the waste waters. Reservoirs may be constructed near the sources of the streams and the waters held in the upper valleys, or the water may be run from the canals into ponds within or adjacent to the district where irrigation is practiced. This latter method will be employed first. It is already employed to some extent where local interests demand and favorable opportunities are afforded. In general, the opportunities for ponding water in this way are infrequent, as the depressions where ponds can easily be made are liable to be so low that the waters cannot be taken from them to the adjacent lands, but occasionally very favorable sites for such ponds may be found. This is especially true near the mountains where alluvial cones have been formed at the debouchure of the streams from the mountain cañons. Just at the foot of the mountains are many places where ancient glaciation has left the general surface with many depressions favorable to ponding.
Ponding in the lower region is somewhat wasteful of water, as the evaporation is greater than above, and the pond being more or less shallow a greater proportional surface for evaporation is presented. This wastage is apparent when it is remembered that the evaporation in an arid climate may be from 60 to 80 inches annually, or even greater.
Much of the waste water comes down in the spring when the streams are high and before the growing crops demand a great supply. When this water is stored the loss by evaporation will be small.
The greater storage of water must come from the construction of great reservoirs in the highlands where lateral valleys may be dammed and the main streams conducted into them by canals. On most streams favorable sites for such water works can be found. This subject cannot be discussed at any length in a general way, from the fact that each stream presents problems peculiar to itself.
It cannot be very definitely stated to what extent irrigation can be increased by the storage of water. The rainfall is much greater in the mountain than in the valley districts. Much of this precipitation in the mountain districts falls as snow. The great snow banks are the reservoirs which hold the water for the growing seasons. Then the streams are at flood tide; many go dry after the snows have been melted by the midsummer sun; hence they supply during the irrigating time much more water than during the remainder of the year. During the fall and winter the streams are small; in late spring and early summer they are very large. A day’s flow at flood time is greater than a month’s flow at low water time. During the first part of the irrigating season less water is needed, but during that same time the supply is greatest. The chief increase will come from the storage of this excess of water in the early part of the irrigating season. The amount to be stored will then be great, and the time of this storage will be so short that it will be but little diminished by evaporation. The waters of the fall and winter are so small in amount that they will not furnish a great supply, and the time for their storage will be so great that much will be lost by evaporation. The increase by storage will eventually be important, and it would be wise to anticipate the time when it will be needed by reserving sites for principal reservoirs and larger ponds.