102 THE FUTURE OF ARID LANDS 
TABLE 3 
Axncal Daily Rainfall 
Stations R et Be Maximum 100-Year Length of Observation 
ania’, ™M Observed, Rainfall, 
(1) (2)mm (3) mm 
Ain Draham 15 gy 171 230 (1) Normals: 1g01—-s50 
Tunis 420 144 200 (2) Totality of the observa- 
tion from 1890 to 1945 
Motmatas 239 160 250 (3) Extrapolation observation 
period, 1901-45 
Gabes 175 103 150 
Tozeur 89 63 58 
Adrar 17 33 (4) (4) Period, 1926-50 
polation according to Galton Gibrat’s method from the observa- 
tions covering the period 1901-45. 
On the other hand, there is not sufficient information on the 
water-retaining capacity of arid soils. There is in Tunisia a great 
difference in this regard between regions whose rainfall is greater 
than 200 mm and regions where it is less. In the region of Oued 
Kebir, the annual rainfall is around 500 mm, the soil is saturated 
when the rainfall approximately attains 100 mm. In the Sfax 
region, the annual rainfall reaches 200 mm. The soil depth 1s 
sometimes very great, and evaporation can reach a great depth, 
which can therefore bring an even more important deficit in 
saturation. On the contrary, in the regions of rainfall of less than 
200 mm and especially in the deserts, the retentive capacity can 
be very weak because of the slowing down, by lack of water, of 
the processes of soil formation. In fact, over vast areas the rocks 
are bare. In any event the quantity of rainfall is so reduced that 
it generally falls on non-saturated terrain and yet rarely do 
streams remain several years without flow, even in the middle of 
the desert. So it must be admitted that a flood takes place either 
because the soil is saturated within limited regions of the basin 
or because the chief factor in the flow is the intensity of precipita- 
tion. 
Dubief estimates that in central Sahara there is a flood when 
the rate of the rainfall exceeds 5 mm with an intensity greater 
than 0.5 mm. per minute. 
