Changement climatique et ressources en eau : tendances, fluctuations et projections pour un cas d'étude de l'eau potable en Tunisie

Climate change and water resources: trends, fluctuations and projections for a case study of potable water in Tunisia

Unité de Recherche Chimie des Matériaux et de l'Environnement UR11ES25, Institut Supérieur des Sciences Biologiques Appliquées de Tunis, Université de Tunis El Manar, Tunis, Tunisie.

¹ m.hassen.b@gmail.com
² semiacherif@yahoo.fr

Résumé

Les répercussions du changement climatique sur l'eau sont nombreuses. Cet article analyse les tendances et les fluctuations observées au niveau des trois principales étapes de l'alimentation en eau potable : le captage, le traitement et la distribution, par une quantification des effets du changement climatique sur chacune de ces étapes. Au niveau des barrages, plusieurs scénarii de projection ont montré que la longévité des ressources en eau est drastiquement réduite par l'augmentation des volumes d'évaporation : les réserves d'eau douce de surface seront épuisées dans moins d'un siècle, dans le meilleur des cas. De plus, la turbidité des eaux étudiées s'est amplifiée de 90 à 340 UTN en six ans, subissant de plein fouet les pics de pluviométrie alors que la variabilité de la température suit celle de la demande en eau potable (coefficient de corrélation 0,85). Le changement climatique représente ainsi une contrainte et un défi pour les services d'exploitation et de distribution d'eau potable.

Abstract

Climate change has multiple impacts on water sector. This paper analyzes the observed trends and fluctuations related to the drinking water field in Tunisia through its three main steps: capture, treatment and distribution, by a quantification of the effects of climate change on each of these steps. At dams, several scenarii have shown through projections that the water resources longevity is drastically reduced by the increase in the evaporated water volumes. Based on the observed evolutions, three scenarii were adopted: the first with the same evaporated volumes as in the year preceding the projection (0% of additional evaporation), the second with 3% of additional evaporation (a rate observed from 1970 to 2008) and the third with 6% of additional evaporation (observed currently, in 2013). Once these scenarii applied to  water volumes of two dams (Kasseb and Beni Metir dams), they showed that, compared to the 0% evaporation scenario, 3% of additional evaporation will dry up the  water reserves 4 years earlier. With 6% of additional evaporation, the situation deteriorates further: 27 years in advance for Kasseb dam and 22 years for Beni Metir dam. In the best case (first scenario), surface freshwater reserves will run out in less than a century. Moreover, turbidity increased from 90 to 340 NTU in six years: 90 NTU in 2007 to 170 NTU in 2009, then from 195 NTU in 2010 to 340 NTU in 2012 corresponding to 2009, 2011 and 2012 floods. Water quality is highly affected by the rainfall peaks while the water demand fluctuations follow the temperature changes (correlation coefficient 0.85). Indeed, data clustering method applied to consumed water volumes classified the months into two groups: one containing low temperature months linked to a low potable water demand and a second group containing high temperature months linked to a high potable water demand. Besides, this method showed that over the years there is an increase in the length of high temperature periods, these phenomena affecting more months over time. Thus, the climate change is a constraint and a challenge for the potable water utilities that should consider the direct effects of climate change (on capture, treatment and distribution processes) as well as indirect effects such as pipes breakages, flooding pipes and pumping stations, and destabilization of large infrastructure like tanks or dams.