Sustainable water management in coastal irrigated plains.
Physical and probabilistic modelling
Application to the Cap Bon, Tunisia

Abstract
The project has been conducted by the Center for HYdrogeology of the university of Neuchâtel (CHYN), the “Institut National Agronomique de Tunisie” (INAT) and the “Commissariat Régional de Développement Agricole” (CRDA Nabeul). The objectives of the research were to develop through the case study of the Korba aquifer (Cape Bon, Tunisia), field investigations and modeling techniques to monitor, analyze and forecast the behavior of coastal aquifers with the final aim of sustainable management of fresh water in coastal areas.

The Cape Bon is located 50 km South-East of Tunis. It is one of the most productive agricultural areas in Tunisia. In the same time, it is suffering from scarce water resources and salinization of the groundwater due to seawater intrusion in the coastal aquifers. The local water management authority developed alternative sources of freshwater by constructing dams and surface water irrigation systems. However, because of the large variability of the climatic conditions, farmers still need to exploit the groundwater resources. The World Bank is noticing that productivity is still increasing while the available resource is already fully used. They expect major water problems in the next years.

To improve the understanding of the groundwater situation in the area today, data acquisition, field investigations and data analysis were carried out in a first step. Field data related to the exploitation, the piezometry, the chemistry and the salinity were collected and integrated in a Geographical Information System data base. The data collected during August 2004 confirmed the worsening of the state of the groundwater in the region. Near the Korba city, the zone of water table below sea level (leading to sweater inland flow) increased and reached, in the centre of the piezometric depression, -12 m.a.s.l. Vertical logging of the electric conductivity showed that many boreholes situated in less than 2.5 km of the coast showed an increase of the salinity, in surface as well as in depth, compared to the values measured in 1996. Later in 2005, a geochemical analysis showed that seawater intrusion is the major source of salinization in the Korba aquifer, except in some local zones where salinization is due to fertilizers use and salt recycling.

In parallel, a deterministic estimation of all the regional groundwater balance terms was carried out. The natural recharge was estimated by the Thornthwaite-Mather method to 40 mm/year. This value was then validated by groundwater numerical modelling. No precise information on aquifer exploitation rates is recorded by the local authority concerning either the total extraction rates or their time evolution. Two approaches were then applied to estimate those exploitation volumes. The first was based on advanced remote sensing technology. It consists to identify the regions which are irrigated and to estimate, through an energy balance equation, the amount of water that evaporates. For this purpose the S-SEBI algorithm (Roerink and al, 2000) and the equation of Penman-Montheith were applied. This method allowed to estimate the groundwater exploitation around 20 106 m3/year. This amount is lower than the volume of about 50 106 m3/year given by the local authority; this discrepancy may be attributed to the parameters that are considered in the S-SEBI algorithm or the method used by the local authority. Based on these results, it has been decided to model the uncertainty on these values in a stochastic framework.

To evaluate and minimize the range of uncertainty in the estimation of the exploitation rates, we used a multivariate statistical analysis based on direct and secondary information including: pumping rates measured in 1996 in a sub area of the domain, aquifer geometry and physical parameters, surface water irrigation records and field measurements. Stochastic maps of exploitation rates were then produced using geostatistical technique. The impact of this uncertainty on the seawater intrusion was evaluated with Monte Carlo simulations, based on a 3D density-dependent variably saturated groundwater flow and miscible salt transport model. To circumvent the large computing time required to run multiple 3D simulations, the numerical model was run on the GRID infrastructure developed by the EGEE project (Enabling Grid for E-Science in Europe). The Monte-Carlo simulations showed that uncertain pumping rates led to a zone of 20.4 km2 where the groundwater heads and concentrations are not known with accuracy. Additional uncertainty due to the variability of the recharge and the heterogeneity of the aquifer are currently investigated in a follow-up project.

The Cape Bon, whose capital is Nabeul (www.nabeul.net), counts 700.000 inhabitants, 31% of them are agricultural workers. It is one of the most productive agricultural areas in Tunisia and suffers from scarce water resources and salinization of the groundwater due to seawater intrusion in the coastal aquifers.

 
Contacts:
 
In Tunisia 
Prof. Jamila Tarhouni
Département de Génie Rural, Eaux et Forêts
Institut National Agronomique de Tunisie
43 Avenue Charles Nicolle
1082 Cité Mahrajène, TUNISIE
Phone/Fax +216 71 287 110/ 799 311
Email: tarhouni.jamila@inat.agrinet.tn
 
In Switzerland 
Jaouher Kerrou
Centre for Hydrogeology
University of Neuchâtel
11 Rue Emile Argand
CH-2009 Neuchâtel, SWITZERLAND
Phone/Fax: +32 718 26 77/ 26 03
Email: jaouher.kerrou@unine.ch
Website: www.unine.ch/chyn
 
Prof. Philippe Renard
Centre for Hydrogeology
University of Neuchâtel
11 Rue Emile Argand
CH-2009 Neuchâtel, SWITZERLAND
Phone/Fax: +32 718 26 90/ 26 03
Email: philippe.renard@unine.ch
Website: www.unine.ch/chyn