WATER RESOURCES IN ARID REGIONS 95 
development, they alone may be regulated. The aims of the 
development plans of Tunisia are: (a) increase of production by 
seeking better utilization of rainfall by dry-land cultures; 
(4) among the dry cultures, the development especially of the 
tree cultures, since the root system of trees permits the exploita- 
tion of deep soil layers and consequently maximum utilization of 
soil water reserves; (c) independently of the economic means 
enumerated earlier the exploitation of all reserves usable for 
irrigation whethe: they be large or small, underground, or surface 
reservoirs. The demographic expansion of the country being 
what it is, none should be neglected. 
Water Needs: Evapotransptration 
Calculation of the evapotranspiration according to the method 
of Thornthwaite (20, 21) has been done for Tunisia by Preciozi 
(17). Because this method permits a more detailed analysis of the 
climate than others, it is of interest, for we have noted the great 
variety of conditions in arid regions. 
One of its principal applications is, perhaps, the prediction of 
water needs for irrigation. In Table 2 are the results of the 
calculation of these needs, after Preciozi, for three Tunisian 
TABLE 2 
Stations 
Tabarca Gabes Tozeur 
Annual rainfall, m/m 1029 175 89 
Climatic indices* of Thorn- B: B3 S: al EB; d a! EA! d bi 
thwaite 
Water insufficiency in July, m/m 150 166 214 
Corresponding flow, liter/second/ 0.40 0.45 0.58 
hectare 
Water insufficiency through the 407 820 1082 
year, m/m 
Corresponding flow, liter/second/ 0.13 0.26 0.34 
hectare 
¢B,, humidclimate;E, arid climate; B3, mesothermal: evapotranspiration between 
855 and 997 m/m; A!, megathermal: evapotranspiration more than 1140 m/m; Sz, _im- 
portant humidity deficit in summer; d, very important humidity deficit; a’, summer 
concentration of thermic efficiency, 48%; bi, summer concentration of thermic effi- 
ciency, 51.9%. 
