336 IRRIGATION PRACTICE 



at the intake of the large lateral. It is the duty for the 

 whole system and is of primary interest to the engineers 

 who design irrigation systems. In transit from the head 

 of the main canal to the farm where the water is applied 

 to the crops, large volumes of water are lost by evaporation 

 and seepage, and the duty of water for the system does 

 not at all represent the actual water requirement of the 

 crops grown under the system. The net duty is therefore 

 of prime value to the farmer whose chief interest is in the 

 water actually received by him at his farm. 



199. Determination of duty of water difficult. The 

 duty of water under any irrigation system is always diffi- 

 cult to determine. The soil, climate, methods and time 

 of application and many other factors do much to increase 

 or decrease the area that may be served by a given quan- 

 tity of water. The reservoirs and canals themselves, 

 whether lined or unlined, whether passing over gravelly 

 strata or clay beds, determine in large degree the gross 

 duty under the system. After all such factors have been 

 taken into consideration, there remains, as a disturbing 

 factor, the law that the more water is added to a crop, the 

 smaller the yield to the unit of water. This law of increas- 

 ing water-cost brings always to the front the question of 

 whether much water shall be used to obtain the largest 

 possible yield an acre, or whether moderate quantities 

 shall be used to obtain the largest yield from each acre- 

 foot of water. There is a depth of water for each set of 

 land, crop and water conditions, which produces the 

 greatest profits. When water is added above or below this 

 point the profitableness decreases. This point of optimum 

 duty will, as our knowledge increases, be determined for 

 different crops and irrigation projects. 



An example will illustrate what is meant by this point 



