260 Transactions of the South African Philosophical Society. 



nearly constant, the amount of moisture supplied to the soil must 

 just balance the quantity lost, e.g., the quantity actually added has a 

 zero value. Should these two processes not exactly balance one 

 another, a tendency will be established which will in time become 

 intensified, so that after a certain period the physical aspect of the 

 country will be entirely altered. 



It is, I consider, only in the years of abnormal rainfall that the 

 additions more than balance the losses, and that the average -level 

 of the ground water is actually raised. 



What exact effect a period of very low rainfall has is not quite so 

 clear, but possibly the losses may not increase in the ratio expected. 

 It must not be lost sight of that the present abundance of under- 

 ground water may in part represent the result of gradual accumula- 

 tion during the former period of higher rainfall. 



Amount available. — So far yet we have not taken into considera- 

 tion the disturbing effect of the artificial removal of water as a 

 consequence of human occupation. 



We may take the average depth at which water is found on 

 sinking a well in this area at 50 feet ; some wells in the district are 

 over 100 feet in depth, and there is one example as much as 

 140 feet deep. 



A certain number of wells have had to be deepened from time to 

 time, as the water was removed from the rock and as the supplies 

 had to be drawn from points further distant. The practical limiting 

 depth, however, may be taken at 150 feet. 



The whole of the moisture contained in any given volume of 

 saturated soil cannot be entirely withdrawn ; experiments show that 

 even after several years of draining about 20 per cent, is still 

 retained. After wells have served their purpose, boreholes may be 

 employed to drain the lower levels, but it must be remembered that 

 the pore-space in rocks diminishes as we descend, so that the 

 advantage gained in depth will finally be set off by the diminished 

 porosity of the water-bearing material. 



The movement of water underground towards wells and boreholes 

 is slow, and varies considerably for different materials ; for average 

 sandy soils and pressure gradients the velocity * of flow is from 

 1 to 2 miles per annum. The rate decreases very rapidly with 

 diminished porosity, and in decomposed rock the velocity must be 

 extremely small indeed. 



It may therefore happen that a well will show a gradual falling 



* C. Slichter, " Motions of Underground Waters." United States Geological 

 Survey, Water Supply and Irrigation Papers, No. 67, p. 26, 1902. 



