Évaluation des transferts entre barrage et aquifères par la méthode de bilan d'une retenue en zone semi-aride. Cas d'El Haouareb en Tunisie centrale

Le barrage d'El Haouareb est situé en Tunisie centrale, dans une zone caractérisée par un climat semi-aride. Il draine le bassin versant de l'oued Merguellil d'une superficie d'environ 1 200 km2. Le but de cette recherche est d'évaluer, à partir du bilan du barrage, l'impact de la mise en place de cet ouvrage sur les ressources en eau de surface et souterraine, en vue de mieux cerner le fonctionnement de l'ensemble de ce système et faciliter ainsi sa gestion.Le bilan en eau de surface fait appel à deux termes non mesurés : l'infiltration à travers la cuvette de la retenue et les apports des oueds incomplètement contrôlés. Par une méthodologie originale, nous avons modélisé et calculé l'infiltration. Les apports journaliers au barrage, déduits du bilan ont été ensuite validés par comparaison aux débits mesurés sur une station située, plus en amont, sur le tributaire principal.Cette recherche a mis en évidence que, malgré des pertes non négligeables par évaporation (25% des apports), l'infiltration constitue, de loin, le terme le plus important du bilan de la retenue d'El Haouareb (63% des apports).Cette dernière a ainsi injecté, directement ou par nappe interposée, dans l'aquifère aval de la plaine de Kairouan plus de 9 millions de m3 par an.The El Haouareb dam and the neighbouring dams (Nebhana and Sidi Saâd) were constructed with the objective of protecting the inhabitants of the Kairouan plain from floods. This area constitutes the natural downstream region of three principal wadis of central Tunisia (Merguellil, Nebhana and Zeroud) where there have been floods in the past. The second objective of this dam is to control the recharge of the Kairouan groundwater aquifer in the absence of natural wadi flows, by diverting flows from the dam and by infiltration through the foundation and basin of the dam. In a semi-arid region characterized by very strong evaporation and plant transpiration rates (about 1700 mm per year), it is obviously of great importance to recharge the underground water systems, which are less subject to these kinds of losses and which better preserve the water resources. The last objective of this dam is to supply water to the population, especially for agricultural needs. The dam was expected to supply water to an irrigated area of about 2500 hectares in this region. During the construction of the El Haouareb dam spillway, underground flows occurred in the limestone of the El Haouareb Hill. At the beginning of the exploitation of the dam these underground flows increased. This development has affected the management plans for the dam and accordingly a monitoring program has been put in place to follow of the evolution over time of the infiltration. The purpose of this research was to evaluate, by the water balance method, the impact of the construction of this dam on surface and ground water. This research will improve the understanding of the system, and thus, facilitate management. In addition, this project will evaluate recharge rates towards the groundwater reserves of the Kairouan plain, which constitutes the natural downstream region of this system.The dam is situated at an interface of several superficial and underground hydraulic systems. For surface water, the El Haouareb dam drains the Merguellil wadi catchment whose surface area is about 1200 km2. Besides the Merguellil wadi, two of its tributaries flow directly into the El Haouareb dam (Hammam and Ben Zitoune wadis) but without any measurement of their flows. Nevertheless, two hydrometric stations were established on the Merguellil wadi, upstream of El Haouareb dam: Haffouz in 1966 and Sidi Boujdaria in 1974. The first station is still functional whereas the second one stopped working in 1989. For underground systems, the Aïn Baïdha groundwater located upstream from the dam, and the downstream groundwater of the Kairouan plain constitute the principal aquifers in relation to this dam. A third aquifer of the El Haouareb hill, composed mainly of limestone, was also identified. It receives both Aïn Baïdha groundwater and infiltrations from the El Haouareb dam, and empties towards the groundwater of the Kairouan plain. A daily database of El Haouareb dam management, collected by the agricultural ministry, was available. The data-base contained the following elements: rainfall amounts for the site; water levels in the dam; evaporation rates measured by a Colorado tub; three states of the physical characteristics of dam measured before the beginning of exploitation (1989) and during two drying times of the dam (1994 and 1997); releases from the dam for irrigation; releases from the dam to fight against siltation of the dam and to recharge the Kairouan plain aquifer. There were also rain data measured from the surrounding rain gages and hydrometric data observed at the Haffouz and Sidi Boujdaria stations. The water balance equation of this dam presents two unknowns: the infiltration from the dam and the flows that arrive in the dam by wadis. To resolve these unknowns, we proceeded by minimizing one of the two unknowns with a goal to determine the other one.As infiltration rates were inaccessible, we worked on periods without flows, chosen according to four criteria. During these periods the rain on the dam and on the surrounding rain gages must be equal to 0 mm, the flows measured at the upstream station of Haffouz must be inferior or equal to 50 L/s, and the variation of the water level in the dam must always be decreasing. Finally, the fourth criterion is that the length of these periods that must be equal or greater than 15 d. Thus we were able to calculate the infiltration flows during the periods without flows. These values were corrected by adding the low water flows, which were assumed to arrive at the dam. These flows were calculated by a linear equation established between the low water flows at the Haffouz and Sidi Boujdaria stations. The representation of this calculated infiltration according to the water level in the dam gave a cluster of points from which three simple linear adjustments were done. Thus, we obtained a model composed of three linear equations that represented the changes in infiltration as a function of time. This model showed that the infiltration flow ratecorresponding to a particular level, decreased with time as sedimentation in the dam proceeded. For the daily average level (206 meters), the infiltration rate passed from 650 L/s in 1989 to 310 L/s in 1998.Despite the scarcity of "periods without flows" between 1989 and 1991, and some uncertainties due essentially to the measurement of water levels in the dam, this model is representative of reality because it takes into account the evolution of sedimentation in the dam. The model yielded infiltration flow rates that were higher than those calculated in preceding studies. The infiltration model obtained was then applied to the entire period of study, allowing resolution of the water balance equation for the dam (which now has only one unknown): the flows of the different wadis arriving in the dam.In conclusion, this research showed that despite considerable losses by evaporation (25% of flows), infiltration (accounting for 63% of flows) constituted the more important term in the water balance of the El Haouareb dam. This dam has thus injected, directly or via the interposed limestone aquifer, more than 9 million m3 per year into the downstream groundwater of the Kairouan plain