184 ClASSiFICAMON of the ttrBOC LAiTDS. 



largely by experience, so-called, and has usually been greater than is 

 necessary. Frequently the lower limit has been determined in dry 

 seasons when the supply of water was meager. Therefore the duty 

 of water is susceptible of a very flexible interpretation. On each 

 irrigation project there is a generally accepted idea of the amount 

 necessary, and whether a larger or smaller amount than this is de- 

 livered by the canals is rarely determined. As a rule, however, the 

 discriminating engineer can, by examining all the evidence, fix upon 

 an approximate standard that may be considered reasonable, and 

 unless that standard approaches too closely the total amount of water 

 available adjustment is comparatively easy. Determination of the 

 duty of water is, after all, an agricultural problem, some crops re- 

 quiring more than others on the same land and different kinds of 

 soils requiring different amounts for the same crops. Having deter- 

 mined this amount, the engineer then measures or estimates the 

 losses which occur in transmission through canals and other conduits 

 from the source of supply to the cultivated fields, for duty of water 

 is, unless otherwise specified, referred to the field itself. This 

 amount is sometimes called the net duty. To the amount required 

 for actual application to the crops must be added that lost by seep- 

 age and by evaporation, and the farther the supply from the place 

 of utilization the greater the amount which must be allowed for such 

 losses. It is necessary also to consider losses in the storage reser- 

 voirs, especially the loss due to evaporation. Having summed up 

 all these items and having previously determined the total supply 

 available, the engineer can then determine how much is available for 

 new lands. To this quantity is applied the duty previously deter- 

 mined, allowance being made for the prospective losses in the con- 

 duits that will carry the water to the new land. The result of this 

 computation will show the amount of new land that can be irrigated. 



CSABACTES OY I.AN1} TO BE IBBXQA.TED. 



The elements in the investigation of the character of the lands to 

 be irrigated include topography, geology, character and depth of 

 soil, and general drainage conditions. These are partly engineering 

 Rnd partly agricultural factors, but the engineer must take them all 

 into consideration, because it is apparent that, with a given prospec- 

 tive cost of irrigation, a project would be feasible if the soil and 

 physical conditions were of one kind and not feasible if those condi- 

 tions were of another kind. The climate is another essential factor, 

 and a study must also be made of the kind of crops adapted to the 

 particular locality. It is apparent that a greater investment can be 

 made for irrigation in a region adapted to citrus fruits than in one 

 where, on account of climate, soil, or altitude, hay is the principal 

 possible commodity. The determination of these points by the engi- 



