WATER RESOURCES IN ARID REGIONS 99 
limited by the increase in salinity which accompanies each 
utilization. Its increase is especially great where evaporation 
is stronger. Therefore in arid regions one must study the salt 
cycle as well as the water cycle. 
The recovery of water used in towns poses salinity problems. 
Waters distributed for public use are charged with salt before 
passing into the sewers by the following process: evaporation in 
the course of urban utilization, consumption of salt by the 
inhabitants, industrial waste products, and penetration into the 
sewers of water coming down from saline water strata, if the 
sewers are not watertight. 
For Tunis, starting with drinking water having a saline content 
of around 500 m/g per liter, we hope to arrive at sewage water 
having a salinity of less than 1,500 m/g and usable for irrigation. 
Surface Waters 
Stream flow in arid regions is subject to evaporation which 
excludes in many cases any possibility of utilization by surface 
reservoirs when the annual rainfall is less than 300 m/m. More- 
over, these reservoirs would have to operate on interannual regu- 
lation over many years because of the irregularity of the runoff. 
This irregularity is illustrated in Figure 1, where at the bottom is 
given the distribution of the runoff of two Tunisian water courses 
whose basins receive an annual rainfall of the order of 500 m/m. 
and for which thirty years’ observations were plotted. 
However, the study of runoff in the arid zone is fundamental: 
(a) because the runoff waters can be stored up in the soil; (4) 
because they contribute to the recharge of underground aquifers 
for a more and more important part as one goes toward drier 
regions. 
Recovery of Runoff Waters 
We shall return to point (2) when we study the underground 
reserves. For the moment let us consider point (2). Runoff waters 
can be recovered in the soil, either by practicing retentive means 
where the rain falls, 1.e., at the origin of runoff, or by letting the 
