VARIABILITY AND PREDICTABILITY OF WATER SUPPLY 274 
gradually diminish. For this reason some observers, such as 
Wayland (28), in the Kalahari, consider that such boreholes are 
dependent upon “‘fossil” water, which accumulated in a past 
humid period; others, such as Bosazza (1), consider that small 
reservoirs underground in the Orange Free State and the southern 
Kalahari are undoubtedly recharged over a matter of years but 
can be depleted in a few months with excessive pumping. Martin 
has expressed the view that in southwest Africa the recharge of 
aquifers on any large scale takes place only during periods of 
exceptional precipitation which occur approximately every 
ten years. 
By means of lysimeters it has been shown that in some cases 
in arid countries only 2% of precipitation has penetrated more 
than 4 feet below the surface. In southern Africa it has been 
recorded that borehole water supplies are less difficult in rela- 
tively dry areas with little vegetation than in moister areas 
with denser vegetation (22). 
The United States Geological Survey has recently completed a 
paper on the qualitative aspects of the relation of soil structure 
to infiltration and unsaturated flow of water above the water 
table, and quantitative studies of this important subject are in 
hand. 
Infiltration is much affected by salts, including the nitrate 
and ammonium fertilizers; low permeability is common in regions 
where alkaline salts are present in soil or irrigation water, and 
is due to base exchange taking place during infiltration. The 
acidification of waters tends to increase the rate of infiltration 
(Qn). 
Water Quality 
The ground waters of arid lands are normally highly mineral- 
ized. Frequently the clay fraction in wells and rocks seems to be 
a principal factor in inducing salinity; in southern Mozambique, 
for example, saline waters have been found to be almost invari- 
ably associated with beds of clay and very clayey sands, whereas 
fresh waters were almost invariably associated with sandstone or 
grit (1). Experience with the deep alluvia of the Nyasa-Shire 
