9. -I INFLUENCE OF FOUEST.S ON NATURAL 

 WATER-SUPPLY. 



By C. I). H. Hhaink, A.M.I.C.E. 



Ill the ile\elopaieiit of arid and semi-arid countries the question 

 of a permanent water-supply, either for towns, industries or irrigation, 

 offers many importtint and interesting problems to the civil engineer 

 ixud economist. Such regions often possess large stretches of valuable 

 and fertile soil, but are distinguished b}' a marked absence of luxuriant 

 forms of natui'al growth ; the soil lies bare and exposed ; there is no 

 vegetation to shade it from tlie sun ; there is nothing to prevent the 

 run-off of the rainfall, and there is no covering to afford shelter from 

 the dessicating action of winds, or prevent the erosion by floods. 

 These conditions exist in many parts of the world, tind are charac- 

 teristic of vast areas in South Africa. There, however, the natural 

 conditions are rendered still worse, and hundreds of square miles 

 of surface are further denuded of covering, by the pernicious system 

 of grass burning The result of this is that an increased water-supply 

 is the crying need of practically all the country south of the Zambesi 

 and west of the Drakensberg. It is not only a pressing necessity for 

 the agricultural farmer, but also for the stock-breeder ; for a very large 

 percentage of the total import trade of Soutli Africa is made up of 

 food-stuffs and frozen meat. In the spring of 1906, pasturage in the 

 Transvaal was so poor that farmers in some places were killing off the 

 newly born lambs in order to save the lives of the ewes. 



There are many causes that contribute to this unsatisfactory 

 condition, and one of the greatest factors is the want of a permanent 

 water-supply. On the tablelands of South Africa rain falls during 

 four to six of the summer months, and the precipitation during the 

 rest of the year is practically nil. Although the average annual rain- 

 fall in many localities is (juite as great as that in London, the precipi- 

 tation is irregular and often very violent. For a great distance 

 iKH'thwards, and for a considerable distance inland, fi'om the lower 

 reaches of the Olifants River the climate is one of the driest in the 

 world.* At Walfisch Bay the ainiual rainfall is not (piite one-third of 

 an inch. The extraordinary aridity of this extensive region is caused 



Iiiolies. 



* liaiufail ill 1»()4 at (Jaries 1 -SO 



,, ,, Fraserlnujj; l{oa<I ... ... ... ... JTO 



,, ,, (ianiKipan 192 



,, 1903at Piiii(;e .\lbert }\()a<l ()-7!) 



,, ,, Port Nollotli 1-8.-) 



in!>2at Port Nolloth r4.') 



,, Pella l-;^0 



In tlie )ii)itli-easteni 'J'rausvaal tlio rainfall often o.\<'ee<ls 70 inches ],o,v 

 annum. 



