IMPORTANT QUESTIONS RELATING TO IRRIGABLE LANDS. 85 
AREA OF IRRIGABLE LAND SOMETIMES NOT LIMITED BY WATER SUPPLY. 
While, as a general fact, the area of arable land is greater than the area 
of irrigable land, by reason of the insufficient supply of water, yet in con- 
sidering limited tracts it may often be found that the supply of water is so 
great that only a part of it can be used thereon. In such cases the area of 
irrigable land is limited by the extent to which the water can be used by 
proper engineering skill. This is true in considering some portions of Utah, 
where the waters of the Green and Colorado cannot all be used within that 
territory. Eventually these surplus waters will be used in southern Cali- 
fornia. 
METHOD OF DETERMINING THE SUPPLY OF WATER. 
To determine the amount of irrigable land in Utah, it was necessary 
to consider the supply; that is, to determine the amount of water flowing 
in the several streams. Again, this quantity is variable in each stream from 
season to season and from year to year. The irrigable season is but a 
small portion of the year. To utilize the entire annual discharge of the 
water, it would be necessary to hold the surplus flowing in the non-growing 
season in reservoirs, and even by this method the whole amount could not 
be utilized, as a great quantity would be lost by evaporation. As the utili- 
zation of the water by reservoirs will be to a great extent postponed for 
many years, the question of immediate practical importance is resolved 
into a consideration of the amount of water that the streams will afford 
during the irrigating season. But in the earlier part of the season the flow 
in most of the streams in this western region is great, and it steadily 
diminishes to the end of the summer. Earlier in the season there is more 
water, while for the average of crops the greater amount is needed later. 
The practical capacity of a stream will then be determined by its flow 
at the time when that is least in comparison with the demands of the grow- 
ing crops. This will be called the critical period, and the volume of water 
of the critical period will determine the capacity of the stream. The criti- 
eal period will vary in different parts of the region from the latter part of 
June until the first part of August. For the purposes of this discussion it 
was only necessary to determine the flow of the water during the critical 
