80 REPor.T 8. A. A. Advaxckment of .Scif.nce. 



extended very far from its source, and for the remainder of its length 

 the stream is intermittent, whicli renders storage necessarv for success- 

 ful irrigation. 



Broadly speaking two methods of irrigation ai-e practised in South 

 Africa — 



Flood irrigation, and 



Perennial irrigation. 



Flood Irri<iatlov. — The use of the flood- water taken directly from 

 the natural tiow in the ri\er channel is what has been referred to as^ 

 flood irrigation. 



In order to utilise the water, which, it must be remembered, is 

 flowing in a deep channel perhaps 15 to 20 ft. or more below the 

 adjoining land, the usual practice is to construct a weir high enougli 

 to throw out the water on to the land, the surface of which is often 

 so le\el that the water spreads over it without the aid oi distributary 

 channels. 



The weir used for this purpose may be of quite a temporary 

 nature, e.if. it may be of earth, which will wash away ever}' time 

 that it comes into use. In such case it would be built rather higher 

 than the adjoining river banks so as to ensure that it should not be 

 overtopped before it had served its purpose of throwing the river 

 over the land. The success of the operation here indicated is aided 

 by the rather curious circumstance that the land in many instances 

 falls away from the river banks — as shown in fig. 1, which is a cross 

 section at rioht anc^les to the river channel. 



'f/fl»>>>/>>,,r, 



Fig. 1. 



This condition is brought about by the fact that the river more 

 often overspills the land near to its banks than that more remote, 

 and, as each overspill ineans a deposit of silt, the lands adjacent to 

 the river receive the greater deposit and gradually rise to the higher 

 level. 



A flctoding of this kind once or twice in the year would be of 

 inunense benefit to the ground, and would improve the natural growth 

 of bush or grass and greatl}' enhance the grazing value of the veld. In 

 addition tu such flood irrigation of grazing lands, the same method is 

 also successfully practised for the growing of wheat. Its success for 

 this purpose will depend upon the ri\er coming down during the right 



