Variability and Predictability 
of Water Supply 
FRANK DIXEY 
Colonial Geological Surveys, 
London, United Kingdom 
This paper is concerned essentially with the variability and 
predictability of water supply in the drier parts of certain of the 
British Overseas Territories and Commonwealth countries, 
mainly in Africa. A great deal of experience in these matters has 
been gained in these territories, and in recent years much coor- 
dinated investigation and development of water supplies has 
been achieved. 
Common Features 
The drier parts of these territories are not all strictly arid in 
the sense of the definition adopted by UNESCO (17). The basis 
for the division employed in this definition is the system developed 
by Thornthwaite (26), which uses an index based upon the ade- 
quacy of precipitation in relation to the needs of plants, so that 
precipitation and temperature data and various other factors 
are employed. This is described on pages 74-80. 
Data for the application of Thornthwaite’s system are not 
ordinarily available, although in some territories, as in Tan- 
ganyika, attempts have recently been made to obtain them locally. 
Consequently, the territories to be discussed may be described 
in general terms as arid or semi-arid, corresponding respectively 
to the desert and steppe of many authors. Culturally, the arid 
areas are those in which the rainfall on a given piece of land 1s 
not adequate for crop production; in the semi-arid lands, rainfall 
121 
