Programme de recherche sur les lacs collinaires dans les zones semi-arides du pourtour méditerranéen

Un lac collinaire est une retenue créée par un petit barrage en terre. Les lacs collinaires contiennent quelques dizaines de milliers à 1 million de m3 d'eau recueillie sur des bassins versants d'une superficie de quelques hectares à quelques km2. Ils s'intègrent de façon naturelle dans le paysage en ne créant pas de nuisance particulière. Ils sont aptes à réguler les flux hydriques et donc susceptibles de maintenir les populations en place en leur assurant de réelles possibilités de développement.HYDROMED est un programme de recherche mené dans quatre pays (Liban, Maroc, Syrie et Tunisie), engagés dans une politique de construction de petits barrages en complément des réalisations de grande hydraulique.L'objectif du projet HYDROMED a été d'étudier l'aménagement " lac collinaire" et ses impacts sur l'environnement proche et sur les sociétés rurales riveraines. Outre les opérations de coordination et de formation, quatre opérations de recherches ont été menées- Synthèse des travaux existants et choix des sites pour les expérimentations,- Eau - Sol - Environnement,- Agronomie - Impacts sociaux et économiques,- Pérennité du lac collinaire et son intégration dans le développement durable des régions marginales.A hill reservoir is a small lake resulting from a small dam. Hill reservoirs contain fifty thousand to one million m3 of runoff water from watersheds whose areas range from a few hectares to several km2. They are integrated in a natural way within the landscape and do not create any particular pollution problems. They regulate water flow and thus help to maintain populations in place, ensuring for them real possibilities of development. Their construction has several objectives:- protection of downstream infrastructures against floods and erosion ;- assuring availability of water at several places in the landscape for domestic needs, watering cattle, micro-irrigation, and water harvesting for refilling shallow groundwater reserves ;- development of new economic activities: irrigation, breeding, fishing and fish farming, tourism, and improvement of the environment (creation of oases, afforestation, etc.).The HYDROMED research project was undertaken in four countries (Lebanon, Morocco, Syria and Tunisia) where a strong policy favouring the building of small dam is going on in parallel to the construction of large dams.The goal of the HYDROMED project is to assess the impact of these hydraulic dams on the local environment and on lakeside rural communities.In addition to a training program and co-ordination activities, four projects were carried out:- Synthesis of existent data from each country and a choice of test sites for relevant experimentation ; - Characterization of the water and soil environment ;- Examination of agronomy, agricultural economy and the social management of water ;- Sustainability of the hill reservoir and its integration into sustainable development of marginal regions.After an analysis and synthesis of these projects in each country, seven test sites were chosen and a multidisciplinary approach was adopted.All small reservoirs at the test sites were equipped with a water level gauge, an evaporation pan, and two stations for automatic data collection. One station was connected to a tipping bucket rain gauge (0.5 mm rainwater), whereas the other was connected to probes that measured water and air temperatures and water levels with 1 cm accuracy. The spillways were shaped in such a way that discharge could be estimated. Since the beginning of the project, annual records of all observations made from September to August at these test sites have been published. A computerized bank of hydrological data was set up. The parameters describing the watersheds were also recorded in a similar data bank. Maps of different watersheds were stored using a geographical information system (GIS). The main objective of this work was to build a hydrological model suitable for semiarid Mediterranean catchments with hill reservoirs, with two specific objectives: (i) rainfall-runoff simulation and (ii) simulation of reservoir storage capacity and probability of failure. This model, called HYDROMED (RAGAB et al., 2001 a, b, c), provided the project with an accurate understanding of the water resource and allowed the evaluation of water availability for different uses.The bathymetry of each reservoir was measured at least once every two hydrological years, and was compared with a fine resolution land survey, making it possible to determine the rate of siltation in the pond, and to establish depth/volume and depth/surface curves. At the same time, studies on sheet erosion and gully erosion were carried out in the watersheds. Solid transport varied from 1.8 t/ha/an on a small forested catchment with soil conservation management (El Gouazine in Tunisia) to 50 t/ha/an on a cultivated catchment with marl in the Rif foothills (Morocco). A model was applied to compute the solid transport for each flood (ALBERGEL et al. 2003). It showed that solid transport is not a linear function of time and noted, for example, that 3 floods were responsible for 50% of the sedimentation that occurred in the Kamech dam (Tunisia) between the years 1994 and 2002.Water samples were collected seasonally during the project. Electrical conductivity (E.C.) and pH were measured and the concentrations in the major ions were determined (RAHAINGOMANANA, 1998). The geochemical characterization of the water in hill reservoirs showed three major groups: calcium sulphate water types, calcium bicarbonate dominated waters and sodium chloride dominated waters. The observations made during different hydrological periods aided in the understanding of the present geochemical evolution of water and confirmed the importance of reservoir hydrology in this evolution. Simulations with the Expreso model (RIEU et al., 1997) were used to estimate the risk of water quality deterioration due to evaporation.An agronomic survey was carried out in Tunisia on water requirements for the main cropping systems and for different irrigation systems (traditional, sprinkler or drip irrigation) on two semi-arid sites. At the catchment scale, measurements of energy balance showed a very strong water demand throughout the year. Values of potential evapotranspiration were always greater than 4 mm/day at Kamech (Cap Bon Peninsula) and greater than 5 mm/day at El Gouazine (Central Tunisia). During winter, high potential evapotranspiration values are due to strong winds. Micro-meteorological studies on rainfed agriculture allowed the estimation of actual evapotranspiration fluxes for hard wheat at both sites. The observed actual evapotranspiration of hard wheat (2.3 mm/day at Kamech and 1.9 mm/day at El Gouazine from mid-February to mid-May) was always less than potential evapotranspiration. Micro-meteorological measurements on a drip irrigated tomato crop showed an actual evapotranspiration rate of 6 to 7 mm/day in July for an irrigation height of 9 to 10 mm/day. Thermic stresses were observed in summer time when air temperatures exceeded 45°C (Vacher and Mougou, in HYDROMED 2001).Economic, social and environmental studies were made using surveys with the farmers. Access to water and social organizations were studied as well as the division of responsibilities among state, collectivities and the individual.The use of water in the hill reservoirs appeared to be limited, but variable from one lake to the other. On the Kamech site in Cap Bon the dam was equipped with 4 fuel water pumps. Intensive cultivation of drip-irrigated tomatoes and peppers (10 ha) coexists with traditional irrigated gardens.There is little to no exploitation of hill reservoirs that are far from the markets. Among other uses of the reservoir water, cattle watering and domestic needs were also important.In all semi-arid Mediterranean regions, ovine breeding is an important income for farmers