WATER RESOURCES IN ARID REGIONS 111 
this case than in that of sandy formations to determine the ge- 
ometry of the formations. The points of emergence of the water 
are better localized and observation of them permits easier study 
of the water balance. 
Conclusion 
The arid zone is characterized as much by the variability of 
its climate as by lack of water. To enable a maximum population 
to live in an arid region, there must be maximum exploitation 
of all the resources, and compensation for the variations in climate 
which could not be compensated within that region, by means 
of exchanges with vaster and vaster territories, according to the 
degree of compensation necessary. Thus one must study global 
variations in climate over territories of greater and greater extent. 
In order to know the ensemble of the resources, there is need of 
longer observations than in the humid regions. This gives special 
interest to archaeological and historical research and increases the 
importance of international exchanges of information. 
The networks of pluviometric and hydrological observations 
are generally not extensive enough, especially in mountain and 
desert areas. Instruments of observation need improvement in 
order to be adapted to arid zone conditions: rain gages capable 
of functioning automatically and withstanding sand winds; 
gages fitted for streams which are dry part of the year; devices 
for measuring sediment flow at the bottom of the streams. 
For underground waters, the prolonged time of the observa- 
tions requires great precision in the measurement of flow and 
water levels, which presupposes a good leveling survey, providing 
for quick detection of often slow fluctuations. 
It is very important to determine the natural ground water 
discharge that can be salvaged. In this respect there is a lack of 
information on the losses from underground aquifers by evapora- 
tion in saline soils and by halophyte vegetation. 
Hydraulic resources are characterized by the discontinuity of 
their supply. We believe it necessary to study hydrological 
phenomena individually: rain, runoff, erosion, recharge and dis- 
charge. 
