VARIABILITY AND PREDICTABILITY OF WATER SUPPLY 123 
hides and skins. These conditions, mainly sparse population and 
low economic prospects, have in the past afforded but little incen- 
tive toward the acquisition of basic hydrological data, and still 
do to a large extent. 
Arid regions are, as a rule, better adapted for quantitative 
hydrological studies than humid regions, and many of the avail- 
able methods of estimating ground water supplies have been 
developed in arid regions. A wider range of methods is none the 
less available in developed than in underdeveloped areas. A num- 
ber of methods, however, do not depend at all on development, 
and much successful quantitative work has been done in areas 
that were virtually undeveloped (1g). The Chott Chergui 
scheme of northern Algeria provides a good example of this (12). 
There is one important difference between arid and semi-arid 
areas which has been emphasized by Shotton (25). In those 
fringe areas—southern Palestine and parts of Jordan are good 
examples, as well as the semi-arid parts of Africa—where nature 
has provided at some period of the year an adequate rainfall and 
yet turns the countries to arid desert at other times, there is 
every incentive to search for underground water and to use it to 
balance the irregularities of the rainfall. The search may even 
be long and difficult and the results must always conform to the 
law that more water cannot be taken from the ground than 
soaks into it; but, subject to these limitations, there is a future for 
parts of the semi-desert earth which most of the true desert 
cannot hope to share. 
Climatic Variations 
The question of variations of climate in respect to arid lands 
often arises, particularly as to whether a given territory is drying 
up or not. There are, of course, cycles or variations of widely 
different amplitude, whether of 11, 30, several hundreds, or many 
thousands of years, and, over a given period of a few years, it is 
usually not possible to say whether an increasing aridity is part 
of a progressive change in that direction or merely the downward 
curve of a lesser cycle. In the levels of certain of the great lakes of 
central Africa a periodic rise and fall has been observed which 
