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 1 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 2 J 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 

 in 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 determina- 

 tion of this faotor 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 



