38 BULLETIX 1340, IT. S. DEPARTMENT OF AGRICULTURE 
be diverted and applied to arable lands. Many of the results of ex- 
periments herein reported show that profitable yields of many of 
the standard crops can be produced with relatively small quantities 
of water. To be most effective, this water should "be applied at the 
stage of growth when it will be of greatest benefit, but without 
storage this practice is not always possible and too often the farm- 
er's choice lies between an early spring irrigation or none at all. 
SEASONAL NET WATER REQUIREMENTS OF THE ARABLE LANDS 
OF EACH SUBDIVISION OF THE GREAT BASIN 
By way of conclusion, an estimate has been made, based on the 
data available, of the seasonal net water requirements of the crops 
produced and that may be produced in the Basin. This estimate 
is intended to apply mainly to the lands to be reclaimed by irriga- 
tion. In order to take cognizance of the varying conditions which 
affect water requirements and at the same time recognize geographic 
position and similarity of climate, products, and types of farming, 
the Basin has been separated into 12 subdivisions by placing in the 
same subdivision as far as practicable all of the contiguous arable 
lands requiring similar quantities of water for profitable crop pro- 
duction. (See fig. 15.) 
The net requirements of any farm or other tract of land represent 
the quantity of water (expressed in acre-feet per acre) which is 
needed in any one crop-growing season. This quantity is exclusive 
of all transmission and other losses which may occur between the 
source of supply and the margin of the farm, and contemplates the 
recovery by pumping of water lost by deep percolation in the upper 
lands. 
In computing the acreage of irrigable lands on which the water 
is to be used no deductions have been made for unirrigated portions. 
Such portions are made up of the spaces occupied by lanes and roads, 
building sites, corrals, fences, ditches, and land which for one reason 
or another is not irrigated every year. These nonirrigated portions 
amount, as a rule, to about 25 per cent of the total irrigable area. 
Accordingly the transmission losses which occur in conveying water 
to a farm may be offset by the reduction of the area on which the 
water is applied. So, too, if the transmission losses do not exceed 
25 per cent of the total quantity admitted through the intake, the 
intake requirements at the river will not exceed the net requirements 
on the farms. In other words, the reduction in the quantity of water 
needed, owing to the difference between the gross and net areas, 
compensates, in a measure, for the loss of water in conveyance. 
The net water requirements as given in Table 6 for each division 
of territory are likewise based on the character and amount of the 
supplementary rainfall during the growing period, the character 
of the soil and subsoil, the kind of crops raised, the adoption of suit- 
able methods of applying water, the proper preparation of the land 
surface, and the exclusion of all run-off and a reasonably small, deep 
percolation loss. • 
It may be stated that in making these estimates the furnishing 
of sufficient water for maximum yields has not been the main object 
sought. The results of experimentation show that the yields of 
many crops can be increased by the application of excessive (man- 
