WATER RESOURCES IN ARID REGIONS 101 
The coefficients 4 and a naturally vary with the temperature 
and the distribution of the rainfall in the course of the year. 
By referring to the study of the basins of South Africa (24), 
it can be verified that regions of summer rainfall yield less runoft 
than regions of winter rainfall. 
The available data on runoff are very scanty in arid regions 
with rainfall less than 400 m/m. For lack of other elements, one 
can thus try to use a formula of this type which represents rather 
well the relationship of rainfall to runoff in Tunisia with a = 3 
and 4 between 0.25 and 0.4; but this can in no way hide the at 
present gross insufficiency of the basic data. 
The high value of the coefficient a brings out the interest in the 
mountainous parts of watersheds, which not only are more watered 
but also have steeper slopes and a lower water retention than the 
plains. 
Runoff Conditions 
The very notion of average runoff leaves room for the same 
observations as the average rainfall. It would be of little use to 
know an annual average rate of runoff without knowing its 
distribution and variability. The runoff depends on many other 
factors besides the annual rainfall. 
In humid regions, the runoff is relatively continuous for long 
periods. In arid regions, the flow presents continuity only if it 
has been regularized upstream of the measuring point by under- 
ground reservoirs. We reserve the study of this last case until the 
section on underground water. In general, the phenomenon is 
discontinuous. 
If the rain falls on a saturated soil, there will be runoff, but 
there can be runoff even if the soil is not saturated, if the intensity 
of the rainfall is fairly great. Thus one can classify floods into 
saturation floods and intensity floods. 
Saturation of the soil by the rainfall becomes rarer in proportion 
as the aridity of the region increases. However, even in the deserts 
there can be times when the soil is saturated. This comes from the 
fact that the rains, while rare, can be abundant, as indicated by 
Table 3, which gives the annual hundred-year rainfall by extra- 
