Agricultural Water Management: Proceedings of a Workshop in Tunisia (2007)

Sihem Benabdallah

Professor, Laboratory of Geochemistry, Physics, and Chemistry of Water,

Institut National de Recherche Scientifique et Technique (INRST), Tunisia

Tunisia is located in North Africa, on the border of the Mediterranean. Covering 162,155 km², it is characterized by a temperate climate in the north, with mild rainy winters and hot summers, and a desert type of climate in the south. It has a population of 9.84 million growing at a rate of 1.08% per year (INS, 2003). About 65% of the population is urban, residing primarily on the coast. Tunisia is classified among the least water resources endowed countries in the Mediterranean basin.

The history of Tunisia reveals how the scarcity of water resource forced its inhabitants to deal with its unequal distribution within the country. As early as 130 B.C., the Roman Emperor Adrian constructed a temple of water and a huge aqueduct to transfer water over 123 kilometers from a spring located in the region of Zaghouan to the city of Carthage. In the early eighth century, the Arabic Dynasty of Aghlabides transferred groundwater and stored it in big basins to supply the new founded town of Kairouan. This concern for water still persists since it is required for development in all social and economic sectors.

This paper is intended to provide an overview of the current water situation in Tunisia by presenting its water potential with a focus on its regime and spatial variability, water demand trends for the different economic sectors, the choices made to manage the water balance deficits, and the various problems and challenges faced in managing this resource.

Surface water resources in Tunisia are characterized by problems of quantity and quality. These resources are limited because of the semi-arid to arid climate found in most of the country, with episodic droughts, and a natural deterioration of water quality because of the salty types of rocks found within the country.

Tunisia receives on average 230 mm/year of rainfall; that is 36 billion cubic meters (bcm) of rainfall. However, this volume varies between 11bcm during a drought year and 90 bcm during a very wet year.

The variability of the climate under the Mediterranean influence in the north and under the Saharan influence in the south make rainfall at the same time scarce and unequally distributed in space and time. The annual precipitation is on average 594 mm in the north, 289 mm in the center and only about 150 mm in the south. The ratio between the highest observed values and the lowest observed values of precipitation vary from 4.4 in the north to 15.8 in the south,

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

production of drinking water, then approximately 36% of groundwater resources are not suited for these two sectors which are in increasing demand.

Another phenomenon, which highly affects the quality of water, is drought. In periods of drought, the salinity of the water stored in shallow aquifers can reach 3.5 g/l due to overdraft as resources are drawn down for both drinking and irrigation.

Water resources in Tunisia are estimated at 4700 Mcm including 650 Mcm of nonrenewable resources or 13.8% of the total water resources. Groundwater resources represent 42.5% of the total potential. Thus, the per capita endowment is at about 450 Mcm per year. This ratio will reach 315 Mcm per capita per year in 2030. This ratio is higher in other Mediterranean countries such as Morocco with a ratio of about 1083 or Algeria, with a ratio of 655.

The main water uses are irrigation, tourism, industry and drinking water, totaling a combined annual volume of 2528 Mcm.

The demand for irrigation is 2115 Mcm, or 84% of the total allowances, making agriculture by far the largest water consumer. For this reason, considerable efforts have been made through several reforms to set prices for water, to develop water saving programs and to encourage water saving technologies by establishing financial incentives and by penalizing waste, to actively involve users in water management through setting collective interest groups, and finally to value non conventional water by reusing the treated sewage water.

In addition, the demand for water for the domestic, tourist and industrial needs continues to increase. Drinking water demand was estimated at 290 Mcm in 1996 and is projected to reach 491 Mcm by the year 2030. The water demand for industry is also projected to almost double between 1996 and 2030 going from 104 Mcm to 203 Mcm.

With 1298 km of mostly sandy shorelines, the tourist sector has seen a tremendous increase over the last decade. The number of hotel beds increased from about 150,000 in 1996 to 214,319 for the year 2002 within 777 hotels (ONTT; 2004). Water demand in this sector was estimated at 19 Mcm in 1996 and is projected to reach 41 Mcm in 2030.

Further, there is a significant spatial variability in water demand. Water consumption is greatest on the coastal areas whereas water resources are primarily found in the north and eastern interior of the country. Nearly two thirds of the population is located on the low plains of the Mediterranean coast. One third of the manufacturing industries are located around Tunis, with the remaining being spread between the north and the southern part of the coastline. The richest and most fertile soils are located in the north where the coastal plains and valleys are used to produce wheat, barley, tomatoes, vegetables and grapes and in the Cap Bon peninsula, which produces oranges.

This situation requires the transfer of water from centers of production; mainly from the northern part where the surface water resources are available to the centers of demand, mainly located in the coastline.

Meeting the increasing demand for water while accounting for the space and time imbalance of water quantity and quality makes water scarcity one of the most urgent problems of the country.

From the information given above, it can be easily derived that the water balance will be negative in the near future. The projected volumes for the year 2030 show a strong water stress warning by comparing exploitable water volume of 2732 Mcm to the water demand volume of 2760 Mcm. The use of the reclaimed water becomes necessary to fill the water deficit of 389Mcm.

The exploitation index of renewable resources, the degree to which renewable natural water is exploited, and the vulnerability of the country as regards cyclical shortages is estimated at 57 % for the period of 1990 to 1997 (Blue Plan, 2000). Above 50%, this index reveals high pressure on renewable fresh water resources and indicates the need for rationalizing the management of water uses and demands. It highlights the need for Tunisia to adjust policy on water availability and demand.

Faced with a water deficit with respect to the mobilized resources, the strategic choice was to move from increasing water production to demand management by financial means, water pricing, new techniques, legal and institutional mechanisms.

The Tunisian national strategy centers around three major points:

1. The management of the demand: a question of preserving the resource, ensuring economic efficiency, and preserving social equity by a good water distribution

2. The integrated management of the resources: the use of groundwater during periods of drought, the recharge of groundwater to face problems of overdraft and degradation, and the use of treated waste water and brackish water

3. Resource and environmental protection: quantitative conservation through reinforcement and improvement of water capture and storage, qualitative conservation of water resources and ecosystems through pollution reduction, monitoring, and cost evaluation

With regard to demand management, some reduction targets were set as objectives for the year 2010: 30% for agriculture by improving of the systems of irrigation, replacing some of the hydraulic structures and modernization of the distribution network; 20% for industry by recycling, improving the production processes and the introduction of clean technologies; and 27% for drinking water by the use of modern equipment.

With regard to integrated water management and protection of the resource, the goals were as follows:

• Make use of reclaimed water in the agriculture and industrial sectors

• Evaluate groundwater recharge potential

• Desalinate brackish groundwater for drinking

• Promote the use of agriculture species tolerant to salinity and hydraulic stress

• Protect water bodies from pollution

In order to ensure equality of water distribution, improve drinking water quality and set up a short and long term strategy for water storage, transfer and economical management of the various interconnected catchments, the Ministry of Agriculture invested in devising a digital master plan to simulate and compare water resource needs for the different sectors to the available water for different time horizons. The model is based on the introduction of projected cases of production and usages. Thus, the simulation can take into account several criterion to determine the water balance: (1) the demand development perspective by considering several trends for each sector of demand, (2) the initial storage in dams and the available resources in groundwater, (3) the type of the hydrologic year defined by the manager as being average dry or wet, (4) the possibility of mixing water with different salinity coming from different water bodies, (5) planning horizon and time step simulation.

The almost 10,000 types of simulated scenarios lead to the development of regional water balance maps indicating regional shortages and deficits. Further, this tool was used as a decision support system to plan for the construction of the remaining dams and the additional mobilization infrastructures in order to prevent water shortages until the year 2030.

Tunisia will shortly be confronted with the problem of a water deficit between its consumptive uses and its water productivity. Conscious of this problem, managers and policy makers are giving particular attention to improvements in water supply and mobilization programs, to enhancements in water productivity with an integrated management perspective, to promotion of treated sewage water and its development, and to controlling the different economic sectors of water demand and protecting the available resources from losses and pollution.

The transfer of water, intended to correct spatial distribution variations and to reduce the gap between resources and demand by giving priority to drinking water, will reach its limits by the year 2030. The rate of mobilized water will reach 95% in the year 2011.

To ensure sustainable socio-economic development, the current action plans need to be continued and others need to be initiated. Several general challenges need to be faced:

• More attention is needed to develop integrated water and quality management approaches taking into account both surface and groundwater and through the use of adapted modeling and sophisticated decision making tools. It is not a question any more of taking into account water production and its quality in the two compartments independently

(surface water, groundwater), but rather the quantitative and qualitative aspects of each of these compartments should be integrated. Conjunctive management of groundwater and surface water is being used in dealing with urban water demand in drought periods. This experience should also be extended to the wet years and applied to agricultural water demand.

• Given the specific characteristics of rainfall and its periodicity, the development of new ways of flood management, through appropriate water and soil conservation or recharge techniques, will allow for additional storage. Real time management could also add to a better management scheme and additional resources.

• The use of reclaimed water will contribute significantly to the Tunisian water balance. The development of this source of water requires a specific action plan. The planning and management of this resource will be based on real demand rather than on a bulk volume of available water. Pilot projects in different activity sectors such as agriculture and industry are still needed to assess the technical, economic and financial feasibility. Applied research is needed to further study the potential negative impacts on the environment and public health.

• Agriculture is the largest water consumptive center of use. Therefore, any opportunity for water conservation in this sector will help preserve significant water resources. Reducing the distribution losses within the irrigation and drinking network is among the priorities of the water saving program. Yet, more attention should be paid to the local soil and plant characteristics. Understanding the vulnerability of soils and plant to salinity and to hydraulic stress could translate into improvement in soil moisture conditions, adequate drainage systems design, reduction in water intake and optimal farm environment.

• Protecting the hydraulic structures and water reservoirs from sediment buildup will be a continuing challenge. A ten year water and conservation program was established with the objective of recovering additional water resources and protecting large reservoirs from rapid silting and losing their valuable storage capacity. The Tunisian program is based on the construction of 1000 hill-dams, 200 mountain dams and the reforestation in the watershed behind the major reservoirs.

• Technical research should play an important part in these pressing issues: finding the appropriate agriculture techniques to confront water scarcity and poor water quality; evaluating the real potential and vulnerability of aquifers and their possibility of recharge; finding alternative sources of water storage and supply; and defining the real potential and limits of reclaimed water.

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