WATER RESOURCES IN ARID REGIONS 103 
It certainly would be too costly to maintain many hydro- 
metric stations. A rational program of study should be based on 
the following thoughts: 
1. To establish a general pluviometric network much more 
dense than the present one, chiefly in mountainous areas. 
2. To equip for complete observations a small number of basins 
of limited extent in order to make possible a detailed study. 
3. To equip these basins with a sufficiently dense network of 
self-recording rain gages. 
4. To analyze for these basins the elements of the flow of each 
individual flood. 
Thus, for the basins studied, one could obtain increasingly 
exact correlations between runoff and meteorological conditions. 
The comparison of these basin prototypes would allow differentia- 
tion of the effects of physiographical factors. One could then come 
to know the rates of flow of other basins from the information 
furnished by the posts of the general pluviometric network and 
from the topographical, geological, and pedological elements. 
In passing let us call attention to a difficulty peculiar to hydro- 
metric measurements. Since the water courses are dry for long 
periods, the devices for taking the water level (floaters or mano- 
metric capsules) risk being blocked by sand. Equipment that is 
well adapted to these conditions should be sought. 
Erosion 
We will not close the section on surface waters without giving 
some consideration to erosion and the silt flow of streams in arid 
regions. 
The annual average erosion, calculated in tons of sediment 
per square kilometer carried away by the waters, reaches im- 
portant amounts in the arid Mediterranean regions that have the 
most rainfall (say about 500 mm). It decreases in going toward 
the desert areas (23), but in all places the great floods carry 
waters heavily loaded with sediment, which causes all kinds of 
difficulties. 
We have even less information on erosion than on runoff. 
The program of studies sketched above should be completed by 
measures of sediment load and salinity of water. It is even more 
