56 BULLETIN 339, U. S. DEPARTMENT OF AGRICULTURE. 
PROPER DUTY OF WATER FOR IDAHO. 
The investigations have determined the factors which must be 
‘taken into consideration in determining the proper duty for an entire 
irrigation project.- The first factor is the proper duty of water at 
the land. The results of the investigation indicate that under con- 
ditions similar to those obtaining in Idaho on a normal project with 
medium clay loam should furnish sufficient water so that 2 acre- 
feet can be retained upon each and every irrigated acre during the 
season; that this quantity should be delivered under a rotation 
system in heads of such sizes that economical use can be secured; 
and that where a project is devoted one half to grain and the other 
half to alfalfa or other crops requiring a similar volume of water, 
18.7 per cent of this 2 acre-feet should be delivered during May, 28.3 
per cent during June, 32.8 per cent during July, 17.2 per cent during 
August, and 2 per cent during the first half of September, there 
being but little need for irrigation during the month of April and 
practically none after the middle of September, provided stock 
water or that used for domestic purposes is not taken into considera- 
tion. It has been shown that the: farmer must receive approxi- 
mately 2} feet of water per acre at the farm in order for him to retain 
2 feet per acre upon the land. Where projects consist in whole or in 
part of porous soils or of soils with porous subsoils lying closer to the 
surface than 6 feet, more than 24 feet per acre should be delivered 
to the consumers, the quantity required being largely dependent 
upon the porosity of the soil. 
A conservative estimate of the transmission losses should be made 
im advance for each project, in either cubic feet per square foot of 
wetted area or per cent per mile, from the quantity required of canals 
to furnish sufficient water to all parts of a project. A normal proj- 
ect will be found to lose from 20 to 40 per cent of all water diverted 
before the water can be delivered to the farmer. 
After the duty of water at the land, the size of the project, and the 
probable transmission losses have been determined, the net area 
which may be irrigated may be readily found. It then becomes 
necessary to calculate the extent of waste or nonirrigated land from 
all causes which will finally be contained in the project. This, added | 
to the net area which can be irrigated, will give the gross area of the 
project. The extensive survey that was made for the determima- 
tion of this factor seems to indicate that 90 per cent of a normal 
project will be irrigated every year after the project is fully developed. 
The investigation has demonstrated the adequacy of 2 feet per 
acre for diversified crops on the better class of soils, but it requires 
careful husbandry to render this quantity adequate, and it seems 
evident that but few projects will ever exist, with conditions similar 
