362 Report S.A.A. Advancement of Science. 



10. Large Works. 



These will usually take the form of large storage reservoirs, 

 constructed on streams with fairly large catchments, and of moderate- 

 sized canals led from rivers, with fairly good perennial flow. The 

 expense of the construction of such works will generally be so great 

 that they will be out of the reach of Water Boards, as it is not 

 likely that the State will feel justified in advancing to them loans 

 of the necessary magnitude. Their construction will also involve 

 high professional skill, and their maintenance, considerable adminis- 

 trative ability, both of which can best be supplied by the State. 



The direct financial returns from such works will probably for 

 many years be small, as development by irrigation is slow, and, even 

 when full development is attained, will not be so great as to tempt 

 commercial enterprise to embark on such schemes. The State can, 

 however, profitably undertake them ; it can afford to wait for the 

 complete development of its projects, and it can be content with 

 moderate direct returns, which will be accompanied by indirect 

 returns, from which it will also reap benefit. Moreover, for these 

 reasons the State can be a more liberal owner than can a financial 

 corporation, and the irrigators will thus gain increased advantage 

 by being placed under an administration which has for its main 

 object, not high profits, but the general advancement and prosperity 

 of the agriculturist, which wull lead to those of the whole community. 

 The State is justified in looking upon sound irrigation schemes as 

 public works for the development of its territories, just as much as 

 are roads and public buildings, from which no direct return is 

 received. 



The advantages of large, compared with small, storage reservoirs 

 are their cheaper rate of storage ; they are the only ones which can 

 take full advantage of the run-off from large catchments. They are 

 also the only ones economical on them, seeing that, for any given 

 catchment, the size and cost of the waste weir are independent of 

 those of the dam embankment, while the cost of the outlet and the 

 puddle trench do not vary greatly with the general scale of the 

 project. The mean depth of large reservoirs is much greater than 

 that of small ones, and thus in the former the proportionate loss by 

 evaporation is much less than it is in the latter. This consideration 

 is of much importance in South Africa, where the quantity of such 

 loss will probably be equivalent to an amount of storage equal to 

 the mean area of a reservoir multiplied by a depth of 4 feet. 

 Further, a small reservoir on a large catchment will silt up much 

 more rapidly than a large one on it, for both will intercept very 

 nearly the same amount of deposit. 



The advantages of large canals from large rivers are that these 

 works also utilise more" fully the natural asset of water available, and 

 that these rivers have a greater and more certain perennial flow, 

 especially in bad seasons, than have small rivers. As the larger 

 rivers naturally have considerable floods and usually run in deep 

 troughs, to secure immunity from damage, the canals from them must 

 be rapidly led out of the flood margin and on to the country. Thus,. 



