262 ENGINEERING ON THE FARM 



Duty. Duty of water is usually understood to mean the 

 relation between a given quantity of water and the area of 

 land which it serves. Gross duty is the quantity delivered 

 at the reservoir or at the pumps and net duty is the quantity 

 delivered by a supply ditch to the field. There is always 

 considerable waste due to evaporation and seepage between 

 the source of the water supply and the crop irrigated. The 

 gross duty may be several times the net duty. For example, 

 for each acre irrigated there might be liberated at the reser- 

 voir 5 acre-feet, which would be gross duty, but by the time 

 the water was actually distributed there might be only suffi- 

 cient to furnish 2}4 acre-feet, one-half of the water being 

 lost in transmission. Water is said to have a low duty when 

 there is a large quantity leaving the supply point in compari- 

 son with that which reaches the crop. It has a high duty 

 when the quantity reaching the crop is nearly the same as 

 that which leaves the supply point. 



Water supply. Sources of water for irrigation vary with 

 the locality. For large systems in arid country dams 

 (Figs. 218-221) are constructed and the water in flowing 

 streams is impounded in large reservoirs. Where streams 

 are not available, underground strata are tapped by wells 

 and the water is pumped at the point where it is to be used. 

 In plains country, where other sources are not available, 

 dams are constructed across ravines and valleys and the 

 water from the melting snows and winter rains is impounded 

 and used for irrigating small tracts, or as winter and spring 

 irrigation on areas of pasture lands. In humid regions water- 

 bearing strata near the surface of the ground are developed 

 by means of wells for spray irrigation. Lakes and small 

 streams, when available, are used for the same purpose. 

 Where the water supply is limited, as in a well, some 

 economical method of placing the water on the land is 

 essential. In arid country great expense is incurred for lining 

 supply ditches to prevent waste of water in passage from 

 the supply point to the distributing point. 



