illustrating the temporal irregularity and variability of rainfall. The decade beginning in 1990 had 4 dry years, one very wet year (1995 -96), three relatively wet years and two average rainy years. In the southern part of the country, this decade was rather a dry one.
Rainfall information has been collected and stored since 1900. Over the last decade, records show that Tunisia experienced 12 important flood periods alternated with 17 dry periods. Droughts appear two to three times every 10 years and can last two, three or even four successive years.
Surface water resources are estimated at 2700 million cubic meters (Mcm) distributed per year over three natural areas distinguished by their climatic and hydrological conditions as well as by rather homogeneous geomorphologic and geological aspects.
The north provides relatively regular contributions evaluated to 2190 Mcm, thus representing 82% of the total surface water potential while covering only 16% of the country. The center part, covering 22% of the area, is characterized by irregular resources. It provides 12% of the total surface water potential. The southern part of the country which accounts for approximately 62% of the total area is the poorest in surface water, providing very irregular resources evaluated at 190 Mcm which represents 6% of the country’s total potential of water.
The quality of surface water, evaluated by its degree of salinity, varies according to the origin of the resource. Considering that a salinity of less than 1.5g/l is acceptable, then approximately 72% of the surface resources may be considered of good quality. Water quality also varies across the country with 82% of the water resources in the north considered good quality, 48% of that in the center and only 3% in the south.
These inequalities in quantities and quality make water management more difficult and explain the need to transfer surface water from the north to the Sahel and the south in order to improve the drinking water supply and insure equity between regions.
The groundwater resource is estimated at 2000 Mcm, confined within 212 shallow aquifers containing 719 Mcm and 267 deep aquifers. It is estimated that 650 Mcm of this resource, located mainly in the south, is nonrenewable.
Like surface water, groundwater is characterized by unequal allocation and variable quality in terms of salinity. Groundwater is distributed as follows:
The north has 55% of the shallow groundwater resources and only 18% of the deep groundwater resources
The center provides 30% of the shallow resources and 24% of the deep resources
The south provides 15% of the shallow resources and has 58% of the deep resources.
Good quality groundwater is found in only 8% of shallow water and 20% of deep aquifers. If one accepts that salty water up to 3g/l can be used in the agricultural sector and for the