2<J2 VELD-I!UKNING i:XPIiRIMENTS AT GROENKLOOF, PRETORIA. 



During- the period September 15th, 1918, to February 8th, 

 1919, in which the readings were taken, the temperature of Ej was 

 on the average, 13.7° F. during the day and 3.5° F. higher during 

 the night than D. The greatest difference during the day was 

 during the weeks ending November 24th 'and December 29th, 

 when the temperature of E^ was 19° F. higher than that of D. 

 The smallest difference was during the week ending September 

 15th, when El was only 7° F. higher than D. 



During the day, D was, on the average, 19.5° F. lower in 

 temperature than the air, while E^ was only 5.8° F. lower. 

 During the night this was reversed, as D was only 4.4° F. higher 

 than the air, while E^ was 8.1^ F. higher. 



Tliese data are graphically represented in the charts to 

 be found at the end of this paper. 



The conclusion seems to be that the soil, denuded of its 

 vegetation by burning, absorbs more heat during the day, and the 

 radiation during the night is not sufficient to lower the temperature 

 to that of soil thickly covered with vegetation, so that it is warmer 

 both during the day and night than a soil covered with vegetation. 



On the other hand, both the day and night temperatures of the 

 soil covered with vegetation show less variation, as will be seen 

 from the accompanying curves. 



Soil ■Moisture.* 



The data on the amount of soil moisture contained in the 

 soils on the various plots has not been obtained in sufficient detail 

 to draw any definite conclusions as to the effect of denuding a 

 piece of ground of its vegetation by burning. Also, the fact that 

 I had taken the rainfall records at Pretoria, and -not at Groen- 

 •' kloof, has led to an error, which is clearly seen when plotting 

 the curve showing the increase of water content in the soil with 

 increased rainfall. In one instance there was an- increase of 

 0.70 inch in the rainfall at Pretoria; but the moisture contents of 

 the soil dropped considerably, and I can only conclude that no 

 rain fell at Groenkloof during the period. 



There are two facts, however, wTiich I think may be safely 

 deduced from the scanty records, and these are : — 



I. In plots bearing no vegetation, the water content of the 

 soil rises far more after rain than on plots covered with vegetation, 

 but the water is lost again more rapidly. This can be seen from 

 the curve indicating percentage of moisture on the plots A and 

 El (both bare plots), as compared with C, D, and E„. In none 

 of the latter is there such a sudden absorption and loss of water; 

 the water content remains more uniform. 



* The analyses were kindly made for me by Dr. B. de C. Marchand, 

 of the Division of Chemistry, Pretoria. 



