IRRIGATION 207 



to be irrigated. Fortier says that less than one-third of the water 

 diverted from the streams is actually used by the crops. 



The Duty of Water. " The duty of water," a term long since 

 coined, means the quantity of water needed to mature crops. It 

 may be expressed in various ways. Sometimes the duty of water 

 is expressed as the number of pounds of water required to produce 

 one pound of the dry matter of the crop; under other conditions, 

 as the depth of water over the field required during the growing 

 season to produce the crop. 



More commonly, however, the duty of water is expressed as the 

 number of acres that may be irrigated by a definite quantity of 

 water, say a second-foot, flowing continuously through the growing 

 season. 6 A second-foot of water means that a cubic foot of water is 



Fio. 119. Chains for puddling the mud of canals to prevent seepage. 



delivered each second and may be easily reduced to acre-feet or acre- 

 inches, since at this rate an acre-inch will be delivered each hour. 



The absolute duty of water is the total amount that the crop 

 receives by irrigation, by rainfall, and that contained in the soil. 

 It is expressed as acre-inches. The net duty of water is the amount 

 actually delivered to the farmer through his head-gate. 



One second-foot serves to irrigate from '2~> to over 300 acres 

 during the growing season. An average is from 7. 1 ) to 100 acres. Tf 

 the acreage irrigated by a second-foot is small, the duty of water is 

 low, while if the acreage is large the duty is high. 



The duty of water varies with several factors: (1) The rainfall 

 varies in irrigated regions from almost nothing to .10 or 10 inches. 

 The acreage irrigated bv a second-foot will necessarilv vary with 

 the rainfall. (?) Soils that are quite porous will require more 

 water for the crop than the less pervious ones, since much will 

 be lost by percolation. Even hardpan soils require more water than 



