104 THE FUTURE OF ARID LANDS 
necessary than for runoff that there be a study of particular and 
discontinuous phenomena. The intensity of rainfall 1s an even 
more determining factor for erosion than for runoff, and statistics 
of intensity should be established for periods less than or equal 
to a season, because of the evolution of the vegetation in the 
course of the year. 
The more one moves toward desert regions, the more sediment 
tends to be at the bottom of the streams and not in suspension. 
Now everyone knows how difficult it 1s to evaluate flow at a 
depth. It would seem that for deserts the topographical method 
is capable of giving good results. Desert streams flow in general 
only over a certain length of their course, and a topographical 
survey would suffice to determine the solid deposit of a flood. 
For reasons of economy this can be done only in strictly limited 
cases. 
Underground Waters 
The rain divides itself after soil saturation into surface and 
underground waters in a proportion difficult to determine, save in 
extreme cases: very pervious limestone formations or quite 
impervious basins, where the excess of rain goes almost entirely 
to one of these two fractions. Particularly striking examples of 
these extreme cases can be found in the Hydrological Year Book 
of Israel. In the course of the hydrological cycle, there are some- 
times transfers from one fraction to the other. In general, the 
individuality of underground water bodies in arid zones is more 
sharply defined to the degree that the deep aquifers are more 
important than the shallow ones. 
W ater-Bearing Formations 
The permanent and autocthonous phreatic aquifers yield 
important resources in semi-arid regions, but increasingly less 
important ones as one progresses toward drier regions, because 
these aquifers are subject to a more and more intense evaporation, 
involving great soil depth. In very dry regions, the existing 
phreatic water levels are generally fed by the underflow of great 
streams, or by a resurgence of deep underground water. In 
