78 



THE IRRIGATION AGE. 



when they are sometimes required to carry 

 very large volumes of water in a very short, 

 space of time. It therefore follows that 

 they must be of much larger size and ca- 

 pacity than if the carrying of all the water 

 required were evenly distributed over the 

 entire season. To determine what the ca- 

 pacity of such canals should be, it is nec- 

 essary to know the maximum floods from 

 their watersheds and what portion of the 

 flood- waters is required to fill the reservoir 

 sites they are intended to supply. 



Another point requiring special care is 

 the construction of the diversion dams to 

 take the water into such canals. The sud- 

 denness with which the floods rise and fall 

 and the large quantities of water the dams 

 are required to handle, make it necessary 

 that they should be made very safe and 

 strong. Should the diverting dam fail, 

 the water needed to fill the reservoir might 

 run to waste before the dam could be re- 

 paired, and another opportunity might not 

 occur to obtain sufficient water for filling 

 the reservoir before it would be needed. 

 Hence the importance of perfect designing 

 and construction of head works for such 

 canals. Often the elevation of storage res'- 

 ervoir supply canals is considerably above 

 sea level, so that even in warm climates 

 there may be danger of their filling up 

 with snow and ice at the time when they 

 will be most needed. It is frequently the 

 case in semi-tropical climates that consid- 

 erable snow falls and cold weather occurs 

 at high elevations just before and during 

 the time of heavy rains. This being the 

 case, canals in such places may be filled 

 with snow and ice when the water comes 

 down which they are intended to carry off. 

 The way to overcome this difficulty is by 

 the construction of snow-sheds to prevent 

 the drifting snow from filling the canal. 

 These matters will be more fully discussed 

 in the chapter on canals, but are referred 

 to in this place for the purpose of showing 

 which are the principal matters to be con- 

 sidered in making estimates of the cost of 

 bringing the water from a watershed not 

 directly tributary to a reservoir site into 

 such reservoir site. 



PROXIMITY OF RESERVOIR SITES TO LANDS 

 SUSCEPTIBLE OF IRRIGATION. 



In general, the points already discussed 

 cover everything essential to impart value 

 to a storage project. Sometimes, however, 

 reservoir sites are found so distant from 

 lands where the water can be used for 



irrigation that the cost of bringing the 

 water to the lauds, added to the cost of 

 the reservoir, is greater than the profits to 

 be realized from the enterprise will war- 

 rant. This is especially the case where an 

 artificial channel to convey the water for 

 the whole distance has to be provided. It 

 is, therefore, always a part of prudence 

 and good practice on the part of an irriga- 

 tion engineer to know what will be the 

 expense of conveying the water proposed 

 to be stored by a reservoir to the lands 

 where it is intended for use. For this 

 purpose surveys of one or more practicable 

 routes from the reservoir site to the point 

 of delivery on the land to be served should 

 be made. 



These surveys should be made with suffi- 

 cient precision to furnish the data required 

 for making thoroughly reliable estimates 

 of cost. In cases of reservoirs very dis- 

 tant from lands susceptible of irrigation, it 

 is often possible to convey the water in the 

 natural channels of rivers and streams for 

 the greater part of the way. Thus it is 

 often feasible to make a large saving in the 

 expense of constructing artificial channels 

 by utilizing those which nature has already 

 provided. It does not follow, however, 

 that all which is necessary is to find a 

 natural river or creek channel in which the 

 water will flow down to or near the place 

 required. It often occurs that the loss of 

 water in such conduits is so great as to 

 make their use impossible. This point 

 must therefore be investigated in order to 

 find what will be the percentage of loss. 

 Such examinations can either be made by 

 actual measurements at times when water 

 is flowing in the channels in question, or 

 by comparison of the conditions and na- 

 ture of the channels with others in which 

 the percentage of loss is already known. 

 It should be remembered, however, that 

 the former method is always the more cer- 

 tain and reliable, and is therefore to be 

 preferred. 



A CORRECTION. 



There was an error in the formula in the 

 second column on page 19 of the July 

 number being in line 23 of that column 

 from the top in Finkle's article on Water 

 Supplies. The error consists in making 

 the fractional number -f } - appear as a factor 

 in the formula instead of as the coefficient 

 of the factor A. The formula .will then 

 read as follows, which is the way it was 

 intended: Q=A. (pcf). ED. 



