ON THE EFFECT OF VEGETATION ON THE 

 RAINFALL OF SOUTH AFRICA. 



By H. Pealing. M.Sc. 



This is a subject which has been hotly debated. On the 

 one hand a vast mass of evidence has been put forward that 

 large tracts in South Africa are drying up, pointing to a 

 diminishing rainfall ; on the other hand, it is maintained that 

 the moisture which is precipitated in South Africa comes from 

 the Atlantic and Indian Oceans, and is propelled there by forces 

 which operate outside the continent, and at great distances from 

 it, and therefore on an average the rainfall must lemain invari- 

 able from decade to decade.* 



The writer proposes to examine a little more closely the 

 latter statement. The evidence regarding the desiccation of 

 many large tracts of South Africa is so overwhelming that few 

 dispute the fact. 



South Africa may be divided into three rainfall areas — 



(a) The winter rainfall area. 



(b) The all-the-year-round rainfall area. 



(c) The summer rainfall area. 



The last area is the most important, and includes Natal, the 

 Transvaal, etc. 



The source ai supply for the rainfall for this area is the 

 Indian Ocean. The winter rainfall is derived chiefly from the 

 Atlantic Ocean. 



The writer hopes to shew that the amount of the summer 

 rainfall in a district far from the coast is dependent to a large 

 extent on the character and quantity of the vegetation of the 

 intervening coimtry. 



The rain gauge is used to ascertain the amount of the rainfall. 

 This instrument is at best a crude one, and many circumstances 

 make its readings unreliable. The first difficulty with regard to 

 the rain gauge is that of location. It is usual to place a rain 

 gauge three to six feet from the ground, and at some distance 

 from trees and other sources of o'bstruction. When this procedure 

 is adopted the readings are disturbed and rendered unreliable 

 because of the presence of eddies-circling round the instru- 

 ment which are produced by the wind, which usually 

 has a high velocity when rain is falling. These eddies 

 give a corresponding motion to the raindrops falling into 

 them, and these drops consequently receive an outward 

 deflecting motion, and many which would otherwise fall 

 into the rain gauge fall outside of it. The following 

 experiment, an account of which is contained in Milham's 

 " Meteorology." will make this clear : Two similar rain gauges 

 were taken ; one was placed in the open three feet above the 

 level of the ground, and the other was placed about 200 

 feet above the ground as nmch in the open air as possible. It 

 was found " that the lower rain gauge caught nearly twice as 



* Erosion and Rainfall : Senate Committee's Conclusion. 1914. 



