VARIABILITY AND PREDICTABILITY OF WATER SUPPLY 125 
area was ideal for prolonged research on the lines of the Jonkers 
Hoek and Cathkin Peak Forestry Research Stations. The effects 
of gum and other plantations as against indigenous tree and bush 
cover, and again of tree cover in general as against grasslands, 
could be studied with great profit in this region. 
This question of rainfall is highly pertinent, too, to the outlook 
of the Sahara; some writers believe it to be decreasing, but the 
evidence is not conclusive. Probably there has been no significant 
change in the northern Sahara since Roman times. Apparent 
jes cacti can be traced to human rather than climatic causes, 
particularly overuse of the native vegetation, overdraft of ground 
water, and past raiding. There is widespread belief that the 
southern Sahara is advancing, but here again the evidence as to 
climatic change is conflicting (18). 
Tixeront (pages go-g2) ascribes the disappearance of the 
ancient agricultural civilization of the arid lands of northern 
Africa, comparable with that of the arid lands of the Near East 
and of India, to human rather than to climatic factors. He records 
that in the recent development of Tunisia full use has been made 
of archaeological and historic records relating to Roman and Arab 
cisterns, wells, and other works which when reconstructed and 
restored appear to function much as originally, and certain 
springs in this region also seem to be flowing as well as in Roman 
days. 
The Search for Water 
On the question of the search for water in arid lands, one must 
pay tribute to the extreme efficiency and thoroughness with 
which the earlier inhabitants sought out and exploited temporary 
surface supplies in scarcely perceptible hollows and at the foot 
of rock catchments, as well as ground waters in shallow well 
fields. It is amazing to see the great herds of the larger and smaller 
stock that can be maintained from these sources. Even with the 
aid of modern techniques it is often very difficult to find such 
occurrences of water which are not already being exploited or 
have been used in the past, and frequently the only advance we 
can make is to prove, by drilling, deeper supplies that were 
