94 THE FUTURE OF ARID LANDS 
artificial nucleation and atomic processes, whose effects can be 
considered a part of the first plan. A new field of study of varia- 
bility is opening. This will become more complicated because the 
causes of natural variability will be less clear (14). 
Since the artificial modification of rainfall is beyond the bounds 
of this study, we make only one comment. The present studies of 
the processes of nucleation aim above all at increasing rainfall 
when the conditions favorable to natural precipitation have 
already been met. These processes permit the making of one 
general reservation. If they are effective, they risk, in fact, 
increasing the variability of the climate, and their application 
should consequently be considered under such conditions that 
the processes will contribute not only to an increase of precipita- 
tion but also to an increase in water reserves. 
Water Resources 
The water resources of a region, other than the rain absorbed 
directly by the soil, are the portion ot the rainfall stored in under- 
ground aquifers and the portion which runs off and which can be 
stored in the soil by dispersion or in surface reservoirs. All these 
fractions are interconnected. For convenience of discussion, how- 
ever, we shall look at them successively. But first it is necessary 
to call attention to several fundamental characteristics of the 
hydrology of arid regions. 
General Water Balance 
Tunisia receives per year: 32.5 billion cubic meters of rain 
water and o.§ billion brought in from neighboring regions, of 
which 2 billions return to the sea, and less than 1 billion passes 
by the underground aquifers to be used for irrigation or uselessly 
evaporated—all this for a population of 3,500,000. 
The fraction evaporated and recoverable for dry culture 
amounts to some 30 billion cubic meters. It represents therefore a 
much greater volume than that which is recoverable for irrigation. 
But the importance of irrigation cannot be measured in cubic 
meters of water. The fractions usable for irrigation are, in fact, 
put to much better and efficient agricultural use, and, in countries 
where the variability of the climate is the main obstacle to their 
