NoTKs UN TiiiuiATioN i.v Caim: Colow. 83 



('(Uiafs. - The fanners' ciiiiiils do not often run to any great size, 

 and those built without professi(«ial assistance often leave much to be 

 desired in regard to regtUarity of grade. 



Xo large canals have yet been constructed, and the o)i]y machines 

 usetl for their construction have been the familiar plough and scraper 

 with cattle as motor power. For shallow furrows in soft ground these 

 imi)lenients do good service, and a few pence per cubic yard would be 

 an ordinary price for the work. Where rock is met with hand drill- 

 ing and blasting would be the method adopted, and tlie excavation 

 might cost perhaps 5s. per cubic yard. Between these two classes of 

 ground would be found all varieties, and the price for work would 

 generally var}' between the two extremes given. If any ver}' large 

 scheme were carried out with canals running into deep cutting, doubt- 

 less some excavating machinery would be reijuii-ed for speed and 

 economy. 



Much of the ground is very pervious, and gives rather heavy per- 

 colation losses unless some preventive measures aie adopted. Evapora- 

 tion losses are also considerable, but the greater portion of this will be 

 on the irrigable lands, and not in the canal. The actual evaporation 

 from the canal itself under ordinary conditions would probably not 

 exceed one ten-thousandth of the flow of water in the canal. Whereas 

 if the water fiowed over the hot surface of the land on a bright day 

 a ver}'^ high percentage of the whole might be lost. 



Silt. — The water of many of the South African rivers, as, e.j/., the 

 Orange, Zak and Great Fish, carries much silt, and will, if the canal 

 gradients are properly adjusted, (juickly render a furrow water-tight 

 even in porous ground. To this end the gradient should be so flat as 

 to promote a slight deposit, and at the same time not so flat as to 

 endanger the silting up of the canal. 



Apart from consideration of the condition of the canal, it is 

 generally desirable, on account of the highly fertile nature of the 

 silt, to cause the water to carry it on to the land. Even in very 

 moderate floods the percentage of silt carried is considei'able, and in 

 high floods as much as 5 per cent, in bulk of the water has been 

 recorded, the measurement being made after settlement of the silt to 

 the consistency of thick mud. 



The Vaal River, although its water is seldom (piite clear, does n<)t 

 carry nearly so much silt as the Orange and some other rivers. 



Storage of Water. — Owing to the intermittent nature of most of 

 the streams of the country, irrigation can generally be practised on a 

 large scale only by the aid of storage reservoirs. There are in the 

 country a large number of dams made by the farmers which are mostly 

 of small proportions, and hardly come under the category of irrigation 

 reservoirs, being intended either for the watering of stock or for the 

 irrigation of very limited areas. They are usually of small height, and 

 placed either in a valley having only small drainage area, or so placed 

 that they can be fed by a furrow from some stream having a larger 

 catchment, so that the supply is partly under contr(^l. These dams 

 iire usually well made, but notwithstanding that a large percentage of 



p2 



