266 SOILS 



of other field crops which do not grasp so much 

 soil with their roots. 



The volume of water needed to irrigate an acre 

 in one year may be roughly stated as from 30,000 

 to 60,000 gallons, which is equivalent to one miner's 

 inch for 5 to 10 acres or one second foot for 250 to 

 500 acres. It is commonly considered that about 

 the maximum duty of a miner's inch of water is 

 4 to 6 acres of vegetables and small fruits and 5 to 10 

 acres of orchard fruits. This means that the 

 ground will be covered from 8 to 16 inches deep in a 

 growing season from May to October. The 

 average for most arid regions is about 12 inches 

 per year. In parts of southern California 30 inches 

 are sometimes used. The volume of water used 

 in sewage irrigation is always much more than 

 this. 



The amount of water needed is always dependent 

 upon the amount of rainfall and is an addition to 

 it. The more rainfall the less irrigation, for irri- 



fation should supplement the natural supply, 

 ince the rainfall in a section under irrigation may 

 range from almost nothing to 20 or more inches, and 

 varies from year to year, the difficulty of establish- 

 ing any definite standard is increased. The season 

 of the year when this rainfall comes has a marked 

 influence upon the quantity of water needed in 

 irrigation. If it comes in summer it is less valuable, 

 as a rule, than winter rainfall. 



Levelling the Land. Arid lands that are 

 irrigated are usually nearly level and are 

 covered with sagebrush. They often have 

 slight irregularities due to the drifting of the 

 light soil. Virgin land must be prepared for 

 irrigation by removing the sagebrush and level- 

 ling the surface. The brush is usually grubbed 



