Volume 17, numéro 2, 2004

FR :

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.

EN :

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.

FR :

Nous présentons dans cet article l'expérience algérienne acquise dans le domaine des études de retenues collinaires depuis 1987 à ce jour. En effet, plus de 40 % des retenues collinaires ayant fait l'objet d'une étude sont aujourd'hui en exploitation et ont atteint l'objectif visé.

Avec une demande en eau croissante, l'Algérie s'est vue contrainte de mobiliser au maximum ses ressources en eau superficielles par la réalisation de petits ouvrages.

Après les échecs enregistrés dans la réalisation de tels ouvrages du fait de la négligence d'études préalables, l'administration publique concernée pris en charge sérieusement la conception des retenues collinaires sur la base d'un dossier d'études répondant à des normes techniques établies. Le manque d'expérience dans de telles études a engendré un investissement colossal dans les travaux de conception. Pour remédier à cette situation, les bureaux d'études algériens ont développé leur propre méthodologie d'étude.

Contrairement aux grands barrages, les petits barrages et retenues collinaires ont été considérés différemment vu leurs particularités. La consistance des études a été sensiblement modifiée avec comme résultat des délais de réalisation qui sont passés de 12 à 6 mois se répercutant aussi sur le coût global, qui est passé de 2 500 000 à 1 200 000 DA (50 000 à 25 000 $), sans pour autant affecter la qualité et la fiabilité de l'étude. Cette réduction a été obtenue grâce à la normalisation de certaines études et à la rationalisation des tâches, comme l'étude hydrologique, les reconnaissances géologiques et géotechniques (sondages, fouilles, essais en laboratoire), le dimensionnement hydraulique des ouvrages, le calcul de génie civil des ouvrages en béton armé et les pièces dessinées.

La réduction des délais a été obtenue également grâce à la maîtrise technique et à l'outil informatique.

Les travaux d'études ont été scindés en six missions distinctes : travaux topographiques, étude hydrologique, étude géologique et géotechnique, étude de faisabilité, étude d'exécution, et dossier d'appel d'offre. En effet l'étude hydrologique ne fait ressortir que les paramètres essentiels nécessaires à la suite des travaux d'études. L'étude géologique et géotechnique, sur la base d'une campagne de reconnaissance restreinte et d'essais en laboratoire limités, définit le type de remblai à mettre en place, les remèdes à prévoir dans le traitement des fondations, et les paramètres géotechniques rentrant dans le dimensionnement des ouvrages.

Le choix du type de remblai qui est lié à la disponibilité des matériaux de construction en quantité et en qualité, se limite à un remblai homogène ou zoné. Les détails du remblai font l'objet d'un plan d'exécution standard valable pour toutes les retenues, il suffit seulement de mettre les cotes correspondantes.

Le choix du type d'évacuateur de crues dépend essentiellement du relief. Il est préconisé des évacuateurs de crues à surface libre à entonnement frontal ou latéral. Les hauteurs des murs bajoyers ont été standardisées et des plans types de ferraillage de différentes hauteurs sont élaborés, prêts pour être adaptés à l'étude considérée. Les bassins d'amortissement qui sont préconisés sont de types USBR.

Deux types d'ouvrages de vidange et de prise d'eau ont fait l'objet d'une préparation technique et dimensionnelle, dans le but de les caler sur tous les sites de retenues collinaires, et des plans d'exécution sont élaborés et prêts à une large utilisation.

Pour mieux cerner ces études, un échantillon de plus d'une vingtaine de retenues collinaires réalisées et actuellement en exploitation, dans cinq Wilayas du Nord de l'Algérie, est présenté. La diffusion de notre expérience certes un peu courte comparativement à certains pays méditerranéens mérite d'être prise en considération et peut contribuer quelque part à améliorer la sécurité et les performances de ces ouvrages.

EN :

The objective of this paper was to summarize the information obtained from studies of small dams in Algeria from 1987 to the present With the growing consumer demand, Algeria chose to maximize its surface water resources by the construction of small dams. Following the inadequate performance of these projects, the public administration re-evaluated its responsibility for designing small dams. The lack of experience with such studies resulted in a big investment in dam design. To improve this situation, the Algerian consulting engineers developed their own calculation methodology. In contrast to big dams, small dams are unique due to their particular characteristics. The studies have become more consistent and as a result the construction delays have decreased from 12 to 6 months, thus reducing the overall cost, which passed from 2 500 000 to 1 200 000 DA ($50 000 to $25 000) without affecting the quality and the reliability of the design. These improvements have been achieved thanks to a certain normalization of the studies and a rationalization of the tasks, such as the hydrological study, the geological and geotechnical investigations (bores, excavations, laboratory tests), the hydraulic design of the construction works, the calculation of the reinforced concrete works and the drawings.

The reduction in delays was obtained thanks to technical improvements and computing tools. The working studies were divided into six distinct projects: topographical; hydrological; geological and geotechnical; feasibility; planning; and call for tenders. The hydrological study highlights only essential parameters that are required for the continuation of studies. The geological and geotechnical study, performed on the basis of data collected in a reconnaissance campaign and data from limited laboratory tests, defines the type of embankment required, considers the possible treatments for the foundations, and generates the geotechnical parameters needed for scaling the hydraulic structure.

The choice of type of embankment, which is linked at the availability of construction materials of suitable quality, is limited to either a homogeneous or zoned embankment. The details of the embankment are derived from the application of a standard execution plan, which is valid for all projects involving small dams; only the local geotechnical levels need to be specified. The choice of spill wall mainly depends on the relief. Two types of spillways are recommended: lateral with frontal entry; and lateral with lateral entry. The heights of the spillway walls are standardized, and plans for an iron framework with different heights are developed, ready to be adapted to the specific project. USBR types stilling basins are recommended. Two standardised outlet types have been selected and designed, to be applied to all the small dam sites, and the implementation plans are developed and ready for a wide use.

To better understand these studies, a sample of more than twenty small dams currently in operation, in five Wilayases in northern Algeria, is presented.

Our experiences, although limited when compared to certain countries in the Mediterranean basin, deserve to be taken into consideration and can contribute in improvements in the security and performance of small dams.

EN :

The ongoing intensification of grazing as well as the replacement of traditional land management systems in the Maghreb has brought to the forefront the fundamental role of land-use in determining soil erosion hazard. This paper reports on erosion rates and soil hydrological characteristics of a variety of land uses in Morocco and Tunisia. The results were obtained through rainfall simulation experiments carried out in the field using a portable simulator, following the design of CERDÀ et al. (1997).

Traditional land management systems - typically involving a combination of agriculture, animal husbandry and forestry - produced the least amounts of overland flow and the lowest soil erosion rates. Over-exploitation of these systems apparently has only minor hydrological and erosional impacts. Heavily grazed, degraded "maquis" shrublands, on the other hand, produced considerable amounts of overland flow. At the plot scale of the rainfall simulation experiments (0.24 m2), the corresponding sediment loads are rather insignificant. Nevertheless, slopes where "maquis" shrublands (which generally have very compacted soils) occur upslope from more erodible soils may present a major erosion hazard.

FR :

Les zones marginales du Maghreb subissent actuellement de rapides changements au niveau des types d'usage des sols. L'augmentation de la pression démographique au cours des dernières décennies a poussé leur population à intensifier leur système traditionnel d'agro-sylvo-pastoralisme et à adopter des pratiques de gestion des sols non traditionnelles (LAOUINA, 1999). Il est possible de distinguer trois grands types de changements d'usage des sols aux causes et aux conséquences se révèlent étroitement liées menant à une augmentation de la dégradation des sols et de l'érosion :

1. La mise en culture de sols jusqu'alors considérés comme non arables (lithosols pauvres sur pentes escarpées), à l'origine uniquement consacrés aux activités de pâturage et de sylviculture. Le labour est réalisé généralement dans le sens de la pente ; il est en plus mécanisé, partout où la pente le permet.

2. Le surpâturage des zones de maquis. Il résulte d'une part de l'augmentation du nombre de têtes de bétail et, d'autre part, de la réduction de la surface réservée à l'activité de pâturage conséquente à la reconversion de ces terres en zones de culture. (MIKESELL, 1960).

3. La plantation d'espèces à croissance rapide telles que les eucalyptus et les pins. L'extension de ce type de plantation se fait aux dépens des plantations de chênes liège et des zones de maquis (NAFAA et al., 2000). Elle a pour origine une tentative d'approvisionnement en bois et en combustible des populations locales.

De ces changements d'usage de sol résultent une augmentation de l'érosion du sol et la dégradation des terres, comme décrit par LAOUINA (1990, 1998), LAOUINA et al. (1993), HAMZA (1994), et COELHO et al. (2002), menant pour les cas extrêmes à la formation de ravines et de " badlands ". La dégradation des terres peut souvent être considérée comme le résultat direct de l'abandon des pratiques traditionnelles de gestion durable (LAOUINA et al., 1993).

Cet article présente les implications, l'érosion et le ruissellement superficiel des changements d'usage de sol survenus dans les zones marginales de la région du Maghreb. Une hiérarchisation des différents niveaux de pression sur le sol sera également établie pour trois différents usages (pâturage, sylviculture et agriculture).

La méthodologie utilisée établit, pour chaque usage du sol, une description de la couverture végétale et de la litière sur des transects de 25 m. Les caractéristiques d'humidité du sol, de ruissellement superficiel et de taux d'érosion ont été déterminées par le biais d'une série d'expériences de simulation de pluie (CERDA et al., 1997). Le simulateur de pluie est constitué d'un diffuseur placé à une hauteur de 2 m capable de produire une intensité de pluie spatialement homogène de 50,5 mm/h sur une surface au sol de 1 m2. Une structure métallique circulaire délimitant une surface expérimentale de 0,24 m2 est insérée dans le sol. À l'intérieur de cette surface, une sonde de mesure de l'humidité du sol " Theta probe " est installée jusqu'à une profondeur de 6 cm. Les mesures de ruissellement superficiel et d'humidité du sol sont effectuées toutes les minutes. Cinq échantillons d'eau de ruissellement superficiel sont recueillis pour permettre la détermination de l'évolution de la charge en sédiment. Chaque simulation a une durée totale de 60 minutes.

Après chaque simulation, un échantillon de sol est prélevé afin de déterminer au laboratoire, par un granulomètre " Coulter LS Particle Size Analyser ", la texture du sol de la fraction inférieure à 2 mm. La teneur en matière organique est déterminée par calcination à 550 ºC durant 120 mn. La détermination de la résistance du sol à la pénétration et à la torsion est réalisée par 20 tests effectués autour de la parcelle à l'aide d'un pénétromètre et d'un " torvane ". L'analyse du couvert végétal indique que les peuplements d'espèces exotiques présentent des sous-strates végétales significativement plus faibles que les peuplements de chêne liège. Ceci s'explique par le labour précédant la plantation et la production de litière plus faible des peuplements d'eucalyptus en zone semi-aride.

Les zones de pâturage présentent un large éventail de couverture végétale variant de 0 à 80 %. Les résultats relatifs aux " badlands " ont été obtenus dans une zone de pâturage incluant à la fois des zones de prairie avec une couverture atteignant 80 % et des zones ravinées sans aucune végétation. On observe une diminution de la couverture herbacée avec l'augmentation de la pression de pâturage.

Au niveau des caractéristiques de résistance à la pénétration du sol, excepté pour les sites ayant subi un labour profond et les jachères qui présentent des valeurs plus faibles, tous les usages de sol présentent des valeurs supérieures à 2 kg?c m-3. Les " badlands " atteignent de manière homogène les valeurs les plus élevées. Les caractéristiques de résistance à la torsion du sol des peuplements forestiers, aux taux de matière organique élevés, présentent des valeurs inférieures à 0,25 kg?c m-2 en moyenne. Les surfaces labourées dont la structure du sol a été détruite atteignent les valeurs les plus faibles. Les zones de pâturage intensif sous couvert de chêne liège révèlent des valeurs de résistance à la torsion élévées. Les valeurs mesurées pour les " badlands " sont élevées mais très hétérogènes. Ce qui implique que pour certaines zones de " badlands " une grande fiabilité du sol en dépit d'un degré de compaction élevé.

Concernant le ruissellement superficiel, les peuplements de chêne liège présentent des valeurs faibles, inférieures à 20%. L'augmentation de la pression de pâturage entraîne un accroissement significatif du ruissellement qui dépasse 50 % de la pluie incidente pour certains " badlands ". Les plantations d'eucalyptus présentent des taux de ruissellement supérieurs aux peuplements de chênes liège. La pratique du labour augmente la quantité de ruissellement, cependant le labour suivant les courbes de niveau semble une technique conservatrice contrairement au labour dans le sens de la pente.

Les taux d'érosion les plus faibles ont mis en évidence pour les peuplements de chêne liège. La garrigue et les " badlands " présentent des taux d'érosion à peine plus élevés malgré des ruissellements importants. Les zones de pâturage intensif sous couvert de chêne liège montrent une érosion très importante. Dans les plantations d'eucalyptus, les quantités de sédiments produites sont réduites de moitié par rapport à la production de sédiments dans les zones de surpâturage. Ce sont les zones labourées qui présentent les taux d'érosion les plus élevés. En effet, cette pratique élimine le couvert végétal et la litière et détruit la structure et la cohérence des sols. La pratique du labour dans le sens de la pente sur des versants très pentus.

En conclusion, pour chaque type d'usage du sol étudié, un indice de risque d'érosion des sols a été attribué. Les plantations d'eucalyptus ont été classées comme l'usage de sol ayant les conséquences négatives les plus faibles en matière de conservation des sols et de l'eau. Par ailleurs elles permettent de réduire la surexploitation des peuplements de chênes liège. Ces plantations représentent donc une alternative tout à fait acceptable.

Les zones labourées présentent les taux d'érosion des sols les plus élevés. Cependant, le sol étant travaillé régulièrement, elles ne présentent pas les structures classiques des sols dégradés par l'érosion tels que les ravines ou les " badlands ". Cependant, l'érosion est évidente et pourrait se traduire par une baisse de fertilité des sols. Pour résoudre ce problème, le recours à des associations d'agriculteurs ou au remembrement foncier permettant l'emploi de techniques plus conservatrices serait nécessaire.

Le surpâturage est souvent associé à des formes très marquées d'érosion. Malgré la très bonne cohérence des sols, les capacités d'infiltration réduites qui augmentent le ruissellement superficiel, et entraînant la formation de ravines et de " badlands ". Une solution à ce problème serait d'augmenter la surface de pâturage, soit par l'élargissement des surfaces de végétation naturelle, soit par le maintien d'une surface agricole en jachère plus importante, permettant au sol de se régénérer et d'alimenter le bétail.

FR :

En raison du développement de l'industrie légumière, les retenues collinaires à vocation d'irrigation se multiplient dans le département du Morbihan (Bretagne, France). Même si de tels aménagements hydrauliques ont une influence individuelle faible, leur multiplication et leur concentration géographique peuvent avoir des conséquences dommageables sur l'hydrologie de surface. Nous proposons d'utiliser une fonction de transfert de type Hydrogramme Unitaire Géomorphologique, basée sur l'identification de la fonction aire normalisée du bassin versant contribuant effectivement à l'écoulement et sur l'estimation d'une vitesse moyenne d'écoulement dans les thalwegs. Le déterminisme de cette fonction de transfert permet de semi-distribuer l'afflux pluviométrique. Il nous permet en outre de proposer une méthodologie de simulation explicite de l'effet agrégé des retenues collinaires, connaissant leur mode de fonctionnement hydraulique local. Puis, pour le bassin versant pilote de l'Yvel (302 km2), nous étudions a priori l'impact de l'implantation de retenues collinaires selon un scénario géographique de développement préférentiel et pour différents degrés d'intensification.

EN :

In response to the increasing demand from the food industry for fresh produce, many farms located in the administrative county of Morbihan (Brittany, France) have intensified their vegetable crop production (e.g. peas, beans, spinach) due to their high value. To obtain the best yields and to respect contracts signed with industrial partners, irrigation has been widely used by the farming community to support this production. Since 1989, 500 to 600 ha/year of irrigated farmland have been developed with financial support from state, regional and county agencies. The official purpose of this financing program is to irrigate 50% of the fields cultivated for vegetable crops. Water use for irrigation was estimated to be 6 million m3 in 1999. The demand for irrigation coincides with the period of low water levels in the stream. Thus, there is strong competition with drinking water demands and ecological flow requirements. Moreover, the potential use of groundwater to fulfill these demands is not possible. One of the solutions is to build small headwater dams to store winter flows and use this resource during the growing season. In order to help local public authorities assess their strategy, we need to develop a robust quantification method to study the impact of this development.

The 302 km2 Yvel watershed is the site of a highly developed region of small headwater dams because the geological substratum is composed mainly of schist with low groundwater reserves, precipitation is less than the regional average, and the industrial demand for vegetable crops is high. For all these reasons, this watershed is particularly vulnerable. Moreover, from a regional policy point of view, the downstream part of the watershed is located within the Morbihan county whereas the upstream part is located within the Côtes d'Armor and Ille et Vilaine counties, which do not provide any financial support for irrigation development. The current concentration of headwater irrigation dams reflects these regional development incentives as dams are mostly built in the downstream part of the watershed.

To study the impact of small headwater dams on watershed hydrology, we propose a conceptual tool built around a geomorphological instantaneous unit hydrograph (GIUH) modelling approach. The GIUH structure, which is based on the effective watershed normalized area function and an estimated mean flow velocity, allows for the explicit accounting of dams and, and thus, can be used for a priori assessment studies. It can also be used in a semi-distributed perspective, where spatial discretization is determined from rainfall variabilities instead of gauging station constraints. These two points allowed us to study three regional irrigation development scenarios: construction of 100, 200 and 300 future headwater irrigation dams. Using two observed rainfall events, we showed that the simulated hydrograph shape at the watershed outlet can be significantly modified. In conclusion, in situ observations and simulation results suggest that : (1) headwater dams do not have any impact when reservoirs are full - lamination is locally observed and thus, does not have any impact on the watershed hydrograph; (2) a spatially homogeneous implementation of headwater dams modifies the watershed hydrograph according to a contributing watershed area ratio; similar behaviour is observed for low water level periods; and (3) localized implementation can be studied through modelling with enough precision to give relevant information to decision makers.

FR :

Dans le cadre d'une collaboration entre le CENID-RASPA (Centro National de Investigación Disciplinaria - Relación Agua, Suelo, Planta y Atmosfera, México) et l'IRD (anciennement ORSTOM, France), sept petites retenues du ranch Atotonilco, ranch d'élevage extensif situé au Nord-Mexique dans une zone semi-aride montagneuse, ont été équipées pour l'enregistrement en continu des niveaux d'eau (1996-1998). Des pluviomètres et pluviographes ont été installés sur leurs bassins versants (0,15 à 4,10 km2) et répartis sur toute la surface du ranch (450 km2). Par ailleurs, une soixantaine de petits barrages équipent ce ranch. En période pluvieuse ils complètent l'alimentation en eau du bétail, assurée principalement par des forages. L'objectif général de l'étude menée par le CENID-RASPA est d'analyser quel est et quel pourrait être le rôle de ces petits barrages dans l'alimentation en eau du ranch.

À l'échelle des bassins versants, l'analyse des relations entre la hauteur de pluie et la lame ruisselée montre que la prise en compte de l'intensité de pluie est nécessaire pour expliquer le ruissellement. Dans ce but, un modèle hydrologique global conceptuel a été défini utilisant les notions :

- d'intensité de pluie limite du ruissellement (IL),

- d'indice des pluies antérieures (IK)

- de pluie utile (PUIL), somme des pluies dont l'intensité, pour une même averse, dépasse la valeur limite IL.

Les averses sans ruissellement et avec un faible ruissellement servent à définir la relation entre IL et IK. Deux paramètres définissent les relations hydro-pluviométriques : le coefficient de croissance parabolique E et le paramètre de position F, associé à l'origine des ruissellements. Le paramètre de position F varie en fonction de l'indice des pluies antérieures et tend à s'annuler pour une forte valeur de IK. Le paramètre E varie peu pour un même bassin, sauf lorsque deux averses sont espacées de moins de 24 heures et lorsque le couvert végétal herbeux est très dégradé. Dans ces deux cas, le coefficient E est nettement plus élevé, parfois trop élevé pour l'utilisation d'une relation parabolique.

Les relations ainsi définies ont été utilisées pour calculer de longues séries d'apports journaliers dans les petits barrages du ranch Atotonilco (ESTRADA AVALOS, 1999).

EN :

In a co-operative programme between CENID RASPA (Centro National de Investigación Disciplinaria - Relación Agua, Suelo, Planta y Atmosfera, México) and IRD (formerly ORSTOM, France), seven small dams were equipped for hydrological measurements in an extensive breeding system, the ranch Atotonilco, located in a semi-arid area of northern Mexico. The main aim of the equipment was to follow the surface water in the water supply system of the ranch (ESTRADA AVALOS, 1999). In order to analyse the role of the seven small dams, it was necessary to understand the hydrological conditions of their filling and emptying. Also, manual and automatic rain gauges were installed on small experimental watersheds (0.15 to 5 km2) and on the ranch (450 km2), and in addition evaporation pans and a floating pan were placed on the banks of a few dams and in a reservoir (THIEBAUX, 2000).

The aim of this paper is to present the overall conceptual hydrological model used to calculate runoff in the small catchments in the Atotonilco ranch, considering that it was necessary to utilise the rainfall intensities to explain the surface runoff. Hydrological observations were made from 1996 to 1998 on the Atotonilco ranch. Spatial analysis of the rainfall demonstrated the existence of several types of storms: those with a small extension (17 km2), with a middle extension (160 km2) or with a great extension (more than 450 km2). The average distance between two isohyets with a deviation of 10 mm varied from 2 to 3 km. In this semi-arid area, obtaining rainfall measurements of good precision requires a dense rain gauge network. On the small catchment scale it was necessary to put three or four rain gauges in each river basin.

On the scale of small catchments, the analysis of the relationship between the amount of rainfall and surface runoff showed that rainfall intensity explained very clearly the surface runoff. This led us to the construction of a lumped model using the following criteria:

- rainfall intensity limit of surface runoff (IL), with a time step of 30 min,

- forward rainfall index (IK),

- useful rainfall (PUIL), sum of the depths of rainfall with intensities higher than the limit value of IL.

Experimental measures without runoff and with surface runoff have been used to describe the variation of the rainfall intensity limit of the surface runoff (IL), which decreases exponentially with increasing forward rainfall index (IK). This index IK is defined as the sum of the previous rainfall andthe previous index, which decreases in an exponential manner with the time interval ?t expressed in days (and partial days) between two successive showers (LINSLEY et al., 1949). The interval dt, which defines the rainfall intensity, depends on the time of concentration in the small catchments. On the Atotonilco ranch, the correlations between rainfall intensities for different intervals and surface runoff demonstrated that a 30-min interval provided the best results. However, we did not obtain significant correlations between surface runoff and different values of the exponential decreasing coefficient of IL as a function of IK (a).

Considering the results obtained by ESTRADA AVALOS (1999) on the scale of experimental plots (60 m2), we have chosen the value a ?=0.1 day-1. This value increases IK during the rainy season when increasing soil moisture and grass cover growth, but these two factors have opposite effects on the surface runoff.Two parameters define the quadratic relationships between the useful depth of rainfall and the surface runoff: the parabolic increasing coefficient (E) and the position coefficient (F). It is possible to determine the position coefficient F with the value of the useful rainfall (PUIL₀) which gives a null value to the depth of surface runoff.

Using the observations collected from 1996-1998, we demonstrated that the position coefficient F decreased linearly with an increasing forward rainfall index IK. In addition, the parabolic increasing coefficient E was constant in the same basin, except when the storms were spaced less than 24 h apart and when the grass cover was low at the beginning of the rainfall season (1996). In these two cases, the runoff aptitude of the watersheds was clearly greater resulting in a higher E coefficient, which was sometimes too high for a parabolic relationship. It would be better to extrapolate above the limit value of PUIL taking into consideration that the additional runoff is equal to the additional useful rainfall.

In order to validate the calibration of this model we used the criterion of NASH and SUTCLIFFE (1970). This criterion was calculated only for the year of 1996 but for two scenarios: the first scenario considered the parameter F as a constant and the second scenario allowed for a linear decrease of this parameter with the forward rainfall index IK. The results demonstrated that it was always better to use, for the position parameter F, a linearly decreasing relationship with the index IK.

The relationships defined for the seven experimental catchments in the Atotonilco ranch were used to calculate long daily chronic inflows into the little dams (ESTRADA AVALOS, 1999). The hydrological parameters were also mixed with the natural characteristics of the small catchments in order to perform a principal components analysis of the relationships between hydrological parameters and natural criteria. Rules for the transposition of the parameters of this quadratic model on the Atotonilco ranch were established using the results of the analysis.

FR :

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.

EN :

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.

FR :

L'article qui est proposé s'inscrit dans le cadre de la recherche de systèmes de gestion de l'eau qui soient en mesure de satisfaire une demande alimentaire croissante dans un contexte de rareté de l'eau, tout en respectant les exigences de l'environnement. Il développe la phase 1 d'une démarche méthodologique visant l'élaboration d'un plan de gestion des eaux d'un lac collinaire en zone semi-aride tunisienne pour un développement agricole durable. Cette phase correspond à l'utilisation d'un modèle de prévision du ruissellement et des sédiments par averse. Le modèle mathématique à paramètres physiques distribués KINEROS est utilisé et appliqué sur un petit bassin versant de 135,35 ha pour 9 averses enregistrées durant les deux années 1997 et 1998 en affectant aux différents paramètres du modèle des valeurs réalistes basées sur les résultats expérimentaux collectés dans la littérature.

L'application de ce type de modèle nécessite la connaissance de l'occupation du sol à un instant donné. Pour ce faire, une carte d'aménagement agricole pour l'ensemble du bassin versant a été établie correspondant à la résultante de deux cartes élaborées ; la carte culturale et la carte d'aménagement antiérosif.

L'analyse des résultats obtenus a montré que le modèle KINEROS peut être utilisé comme outil d'aide à la gestion des terres et des sols en zone semi-aride tunisienne à relief accidenté.

EN :

The process of erosion-sedimentation is initiated by rainfall and runoff from hill slopes and channels. Runoff is considered as a transport vector, which is related to agricultural management systems and land use. To simulate and predict the behaviour of ungauged watersheds, distributed parameter models are of great interest. Among the different developed models, conceptual and physically-based models are interesting to investigate, specifically under semi-arid conditions. As reported by many authors, these models allow reliable evaluation of the hydrologic responses to land use changes and thus can be used for elaboration of water and soil conservation plans. The accuracy of erosion and sedimentation results is related to the quality of the hydrologic component. An appropriate way of studying soil erosion is through the formulation of the fundamental transport equations of water and sediment using the kinematic wave simplification. In this context, the KINEROS model is used. This paper deals with water management systems, which are able to satisfy an increasing food supply requirement within a context of water scarcity while respecting environmental requirements. It develops the first phase of a methodological approach for the establishment of a management plan for water stored in a small dam for a sustainable agricultural management.

This phase was elaborated and applied to the M'Richet El Anze watershed in a Tunisian semi-arid area using the following approach. A study of the hydrological and sedimentological performances of the KINEROS model using a set of hydrological and sedimentological data measured at a hydrologic station for a sub-catchment of 40 ha of the M'Richet El Anze watershed during the period September 1994 - March 1996. Flow has been measured since 1994 at a hydrometric station with a rectangular channel (9.3 m T 1.2 m T 1.5 m in height, with an average slope of 2.5%). A limnigraph (OTT X) and a triangular flume, situated at the end of the channel, were used to measure the flow rate with a time step of 10 min. Sediment load was measured by taking manual samples with 1 L bottles during flow events at non-regular times. Sediment deposited in the artificial channel was weighed after the runoff event. Rainfall was measured with a time step of 5 min using two gauges situated within the watershed. Input data files were elaborated using : (1) a topographic map for watershed discretisation, slope and size planes; (2) soil sampling for soil characteristics and parameters; and (3) land surface and vegetation status maps of the watershed and photos on parcels corresponding to different periods of each year for land use and surface information parameters.

The KINEROS model was used to predict runoff and sediment loads for a small dam in the M'Richet El Anze watershed using an elaborate agricultural management map, which is the product of a crop map and a water and soil conservation map. Elaboration of the crop map was based on three criteria : soil texture, depth and rock cover, whereas the water and soil conservation management map was based on two criteria: land slope and soil rock cover. The rainfall data used for the runoff and sediment simulation were measured for the period 1997-1998 and only rainfall-generated runoff at the hydrologic station was considered. Characterization of rainfall erosivity was applied for different rainstorms, using the empirical erosivity index EI. The maximum erosivity was obtained for the 1998-09-25 rainstorm (39.4 K.J.mm/m2.h). Results showed that autumnal rainfalls were the most aggressive events affecting soil erosion in the Tunisian semi-arid region, especially those in September and October. We also noted an important rainfall erosivity index for June with an orographic rainstorm (30.1K.J.mm/m2.h). Related to these results, runoff seems to be related to rainstorm erosivity and soil moisture. Erosivity can affect soil structure while soil moisture involves an infiltration component, and both affect soil hydraulic conductivity. In this context, analyzing runoff-rainstorm events within a watershed and relating them to the rainfall erosivity index allowed runoff modeling, which can be done with more results. As developed by the author in 1988 for a small watershed in a Tunisian semi-arid region, a boundary intensity (IL) can be identified that is related to an antecedent soil moisture index (IPA). Related to the same work, runoff generation is defined when a pounded rainfall (PI) is anticipated. The difference between total and pounded rainfall correspond to excess rainfall. The PI is related to antecedent soil moisture index (IPA) and the boundary intensity (IL). These results showed the importance of the hydrologic component of the model and especially infiltration modeling.

Thus, accuracy of erosion data is related to the precision of the hydrological results. The following can be concluded from the application of the KINEROS model:

1. The KINEROS model can be used for predicting runoff from ungauged watersheds and for evaluating future land use master-plans for Tunisian semi-arid high lands ;

2. Best runoff simulations were obtained for rainstorms that lasted less than one hour, with high intensity and a total rain of more than 20 mm ;

3. The accuracy of erosion results was related to the hydrological component and precision of parameters assessment ;

4. For runoff forecasts in the small dam of M'Richet El Anze, the small amount and the irregularity of runoff prompt the use of collected water for extra irrigation of crops and vegetation tolerant to water stress ;

5. Development of annual crops would increase the efficiency of autumnal runoff ;

6. Irrigation duty duration from the small dam was related to the extent of water erosion and sediment deposition, which were related to runoff and topography ;

7. The developed methodology used for the application of the KINEROS model to predict the impact of water and soil conservation management on a Tunisian semi-arid watershed without calibration is interesting, especially because it allows the elaboration of distributed soil parameters and crop maps, which can be used as a decision support system for erosion control on hill slopes, keeping dams located downstream in good working order.

FR :

En climat méditerranéen, les ressources en eau des bassins sédimentaires sont rares. Pour limiter les écoulement de surface et recharger les nappes souterraines, des lacs collinaires ont été construits sous une pluviométrie annuelle comprise entre 250 et 500 mm. Des échantillons d'eau ont été prélevés dans le bassin versant d'El Gouazine, situé en Tunisie centrale, pour mieux expliciter le fonctionnement hydrochimique et la recharge de la nappe phréatique d'un lac collinaire très filtrant. Les eaux ont été échantillonnées à deux périodes distinctes (retenue presque asséchée et retenue pleine), en amont de la retenue dans le bassin versant et en aval dans l'aquifère alluvial.

Trois faciès chimiques (bicarbonaté calcique, chloruré sodique et sulfaté calcique) caractérisent les eaux de nappe, l'eau de la retenue étant sulfatée calcique.

La roche-mère et la nappe phréatique sont chimiquement très dépendantes. Les isotopes stables de l'eau montre que la nappe amont est peu profonde et d'origine météorique. L'eau de la retenue se mélange avec les eaux souterraines en conservant un caractère météorique en période d'écoulement et en acquérant un caractère évaporé en période d'assèchement. La nappe alluviale aval est alimentée par les eaux mélangées de la retenue. L'altération d'un affleurement gréseux forme un aquifère en rive gauche du lac expliquant en partie les pertes par infiltration. Les principaux processus géochimiques, qui se produisent au cours de l'écoulement de la nappe dans les formations superficielles, permettent une compréhension partielle du fonctionnement hydrochimique de la retenue et de son bassin versant.

EN :

In a Mediterranean climate, water resources are scarce in sedimentary basins. In Tunisia, as in other semiarid countries, the revival of traditional floodwater harvesting, such as hill reservoirs, can provide water resources for the development of agriculture and agroforestry as well as reduce the use of other valuable water resources such as groundwater. Since the early 1990s, more than 600 hill reservoirs were built within the 250-500 mm range of mean annual rainfall. Most of them can limit water loss by runoff and enhance groundwater recharge. The El Gouazine reservoir in Central Tunisia was chosen within the European Union sponsored project Hydromed (1997-2001) because its groundwater balance is highly negative, ranging annually from -25,408 m3 in 1999-2000 (Tunisian hydrological year, conventionally starting in September and ending in August) to -273,435 m3 in 1995-1996, thus suggesting an important water loss by infiltration. The goal of the present paper consists in studying the hydrochemical behaviour of the watershed to improve the understanding of alluvial groundwater recharge below the hill reservoir.

Water sampling was carried out in May 1998 when the reservoir was almost empty (14 samples) and in March 1999 when it was full (21 samples). Surface waters were collected within the open water surface of the reservoir, in a small upstream pond created during road construction, in a temporary affluent river, in a dam seepage and in the downstream riverbed. Underground water samples were collected from three well locations located downstream from the hill reservoir, from 10 upstream well locations and from two downstream pit locations. All the samples were immediately filtered on site.

Dissolved oxygen content, temperature, pH, electrical conductivity at 25°C (EC) and alkalinity were measured in the field before filtration. The concentrations of major cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺) and anions (Cl-, SO₄ ^2- and HCO₃ -) were determined by ion chromatography. The aqueous silica (SiO₂) concentration was measured by ICP-AES. Total alkalinity was measured by titration with 0.1N HCl (in the field) and 0.02N H2SO4 (in the laboratory). The ratio of water stable isotopes was measured with a mass spectrometer and expressed in d-values, the deviations in parts per thousands (‰) from the International Standard V-SMOW.

The total dissolved solids (TDS) of the surface waters ranged from 0.65 to 6.0 g L-1 in the dry period (DP) and from 0.25 to 5.7 g L-1 in the flow period (FP). Reservoir water was less mineralised in the FP than in the DP with an ion concentration factor of 2.6. The pH was nearly neutral and tended to be higher in reservoir water (10.1 in the DP and 8.6 in the FP). The silica concentration of the reservoir water was much lower in the DP and in the FP as well.

The TDS of the ground waters, located within the watershed, varied from 0.6 to 6.2 g L-1 in the DP and from 0.6 to 4.9 g L-1 in the FP. The pH values were mainly neutral. In the DP, total alkalinity ranged from 254 to 529 mg L-1 as well as in the FP. Silica concentrations demonstrated relatively low variation ranging from 22 to 27 mg L-1 in the DP and from 15 to 27 mg L-1 in the FP.

From upstream to downstream of the hill reservoir, the mineralisation of groundwater decreased, suggesting that an upstream mineralised groundwater flow is diluted by a weakly mineralised reservoir water.

Three groundwater types can be distinguished in relation to the bedrock (limestone, marl, gypsiferous marl, gypsiferous mudstone, sandstone). The first type, weakly mineralised, was represented by three wells from the limestone outcrop. Bicarbonate, which ranged from 45 to 52% mmolc L-1 in the DP, and calcium, which was nearly 50% mmolc L-1, were the major ions and result from limestone weathering. The second type of groundwater included four wells, located in the marly lowlands between the limestone outcrops, and was characterised by a lower concentration of calcium and bicarbonate (30-40% mmolc L-1) coupled with a higher concentration of magnesium (20-30% mmolc L-1). Sodium was the major cation in most wells (33-43% mmolc L-1) whereas chloride was the dominant anion (45-67% mmolc L-1). In the lower part of the basin, the third type of groundwater was draining gypsiferous deposits and was dominated by calcium and sulphate ions. The reservoir water belongs to this type.

Most underground waters originated from infiltrating precipitation that was not subject to surface or subsurface alteration of its isotopic composition. Groundwater located in the limestone outcrop was less enriched in stable isotopes.

In the DP, reservoir water showed 2H and 18O enrichment, which is typical for water that has been subjected to surface evaporation. However, reservoir water is weakly mineralised, suggesting that the reservoir was an open system with more mineralised groundwater entering the reservoir and a mixed reservoir water downstream outflowing by infiltration. Downstream groundwater was weakly enriched and less mineralised than upstream groundwater. Reservoir water, which was permanently mixed, tends to be meteoric in the FP and evaporated in the DP. Physical and pedological clues indicate that the reservoir was leaking. A sandy layer (over 70% sand), situated on the left side embankment and in the sediment of the reservoir, was nearly 1.5 m thick with a bottom elevation above the reservoir bottom ranging from 3 to 5 m. The layer forms an aquifer resulting from the weathering of the sandstone outcrop and was connected to the downstream alluvial aquifer.

The high permeability of the sandy layer partly explained the high water loss of the reservoir. Flowing through clayey materials, which contain variable amounts of easily soluble minerals, such as gypsum, and which are less permeable, the alluvial groundwater was strongly mineralised. The alluvial aquifer was supplied by shallow groundwater stored in limestone aquifers resulting in a strong decrease of the concentration. The limestone aquifers were highly porous and very transmissive. They can accumulate a high water content and rapidly recharge or discharge. The meteoric water collected in the reservoir also decreases the groundwater ion concentration leading to the same effect as the limestone groundwater.

FR :

Dans le milieu aride et semi-aride tunisien, les aménagements de conservation des eaux et du sol jouent un rôle important dans la collecte et le stockage sur les versants des eaux de ruissellement. Cependant l'impact de ces aménagements sur les écoulements reste mal connu. Pour évaluer l'impact de banquettes à rétention totale à l'échelle d'un bassin versant situé au centre de la Tunisie, en zone semi-aride, nous nous proposons dans cet article d'utiliser le modèle géomorphologique H2U, fonction de transfert basée sur la répartition des chemins de l'eau à la surface du bassin. Pour la reconstitution des crues, ce modèle a été couplé à une fonction de production qui définit la pluie nette (lame ruisselée) à partir de la hauteur précipitée sur le bassin versant.

Entre juillet 96 et juillet 97, le bassin versant d'El Gouazine (18,1 km2) a été aménagé en banquettes à rétention totale. La longueur moyenne de ces banquettes est d'environ 100 m pour une hauteur moyenne de 1,50 m. L'écartement moyen entre les banquettes est de 25 m. La superficie aménagée sur le bassin versant est de 783 hectares, soit 43 % de sa superficie. Cet aménagement a intéressé principalement les terres de culture et une partie des parcours dégradés transformés à cette occasion en terres de culture. Dès lors, sur ce bassin, les eaux de ruissellement sont interceptées par ces levées de terre et elles n'atteignent l'oued principal qu'après avoir rempli les fossés créés en amont des banquettes.

Avant l'aménagement en banquettes, le coefficient de ruissellement global du bassin versant était de l'ordre de 7 à 8 % pour les hauteurs de pluie inférieures à 20 mm et compris entre 20 et 30% pour les hauteurs de pluie supérieures à 20 mm. Les pluies d'automne (septembre - octobre) présentaient les coefficients de ruissellement les plus forts car elles sont caractérisées par des intensités très élevées et les sols ne sont pas encore couverts par les végétations naturelles et cultivées. Ils présentent alors des croûtes de battance qui limitent l'infiltration des eaux de ruissellement. Le calage du modèle H2U (fonctions de transfert et de production associée) sur 12 crues avant aménagement a fourni une pluie d'imbibition initiale de 10 mm, une intensité limite pour l'apparition de ruissellement de 3,6 mm.h-1, un coefficient de ruissellement efficace de 42 % et un temps moyen de parcours de l'ordre de 40 minutes. Le modèle a été par la suite validé pour la crue du 20 septembre 1995. Les critères d'ajustement sont bons pour le calage et pour la validation.

Le critère de Nash appliqué aux débits ruisselés est de 0,62 à 0,96 pour le calage et de 0,96 pour la validation.

Après l'aménagement, les pluies enregistrées n'ont engendré qu'un faible ruissellement : un coefficient de ruissellement compris entre 1 à 3% pour les pluies de 30 à 50 mm, un coefficient de ruissellement de 9% pour une pluie de 80 mm (24 septembre 1998). L'utilisation du modèle H2U nous a permis d'évaluer l'impact des banquettes en comparant directement les crues observées avec aménagement et les crues reconstituées par le modèle sans aménagement. Ainsi la pluie du 24 septembre 1998, de fréquence décennale, a-t-elle engendré un ruissellement 4 fois plus faible avec l'aménagement en banquettes, un débit maximum 8 fois plus faible et un temps de réponse 4 fois plus fort. Malgré son extension limitée à 43 % de la surface du bassin d'El Gouazine, l'aménagement anti-érosif en banquettes joue donc un rôle très important sur la rétention des eaux de ruissellement, au point de limiter considérablement, de 50 à 80 %, les apports dans la retenue du petit barrage collinaire. Il convient donc de trouver, pour chaque bassin versant de la dorsale tunisienne, une solution optimale à l'aménagement des terres cultivées sur les versants tout en conservant des apports suffisants aux lacs collinaires pour subvenir aux besoins en eau des cultures irriguées. Pour caler le modèle H2U et sa fonction de production sur des bassins versants aménagés, une meilleure analyse du fonctionnement hydrologique d'un système de banquettes en cascade semble donc nécessaire.

EN :

In arid and semiarid Tunisian regions, water and soil conservation is an important way to decrease erosion and to collect runoff in hill-slope catchments.

However, few studies have focused on the impacts of runoff on water harvesting techniques. This paper deals with the hydrological effects of contour ridges in the El Gouazine catchment located in central Tunisia.

Hydrological observations were used to calibrate a conceptual rainfall-runoff model. The H2U hydrological model was used for this purpose (CUDENNEC, 2000; DUCHESNE et al., 1997). This model is built around a production function that defines the net storm rainfall (portion of rainfall during a storm that reaches a stream channel as direct runoff) from the gross rainfall (observed rainfall in the catchment) and a transfer function based on the most complete possible definition of the surface drainage system (CUDENNEC, 2000).

The runoff production function defined by NASRI et al. (2001) was used for the El Gouazine catchment. Observed rainfall during 5-min time steps was used in the model.

The 18.1 km2 El Gouazine catchment is located 15 km south of the town Oueslatia. With an annual average rainfall of 411 mm and a potential evapotranspiration of 1680 mm, the climate can be regarded as semiarid. The main annual rainfall is usually received between September and January. All runoff from the catchment is collected in a small artificial lake, created in 1990. Its initial storage capacity was 233,370 m3. Water level observations in the lake were used to estimate total runoff from the catchment. The altitude of the drainage area varied from 575 m in the south to 375 m in the north at its outlet. The land surface of the catchment is partitioned into the following uses: 40% cereal production alternating with fallow land; 33% pasture, shrub-land (carob trees, lentisk), and Esparto grass steppe; 20% Aleppo pine forest (both dense and degraded forest); 6% arboriculture (olive trees, almonds); and 1% lake and dyke area.

Between July 1996 and July 1997, soil contour ridges covering 43% of the total area were constructed in the El Gouazine hillside catchment. The length of each ridge is typically about 100 m with an average height of 1.5 m and an average perpendicular distance between ridges corresponding to about 25 m.

Construction of the contour ridges significantly changed the hydrological conditions in the catchment.

Before contour ridges, average global runoff coefficients were 20-30% for autumn rainfalls (september to october) and 4-10% for rainfalls in winter and spring (November to May). Rainfalls during autumn were usually characterized by high intensity and soils were usually compacted with an impermeable surface crust. From November to February, soils usually become more permeable due to agricultural activities (e.g., ploughing) and during March to May infiltration is increased due to increasing crop cover.During this period, before introducing contours ridges in the El Gouazine catchment, the conceptual rainfall-runoff model based on surface topography and the drainage network was calibrated to reproduce observed runoff satisfactorily. Geomorphologic data (topography and drainage system characteristics) for the catchment were extracted with the geographic information software ERDAS IMAGINE of ESRI (Environmental Systems Research Institute, Inc.). The procedure allows extracting the actual probability density function (p d f) of watercourse lengths in the network (L) for the catchment, the catchment Strahler order (n), and the average length of the watercourse in the network L. These parameters are used in a gamma law type transfer function called H2U (unit hydrograph). For this study, in order to be in the usual situation of flood designing practitioners, we used topographical maps with a scale of 1:50000, which are the commonly available maps in Tunisia, and digital grids with a 10 m resolution. Due to the semiarid landscape and relief, there is no ambiguity in identifying the drainage networks.

Simulated runoffs using both the actual and theoretical pdfs were compared.

The theoretical functions give better results, especially with respect to peak flow. This is probably due to smoothing of the drainage system represented by the theoretical density function. The empirical function calculated from maps probably includes errors and uncertainties, which are smoothed in the theoretical function. For both calibration and validation events, results showed an excellent agreement between observed and simulated runoff volumes as well as peak flows. For the Nash criteria, results varied from case to case (from 0.62 to 0.96).

After introduction of the contour ridges, runoff coefficients changed to below 10%. Similarly, inflow to a reservoir at the outlet of the catchment decreased by 50-80%. Observed erosion virtually disappeared after introduction of the contour ridges. Similarly, peak discharge was reduced 60-90%. The changes made in the semiarid catchment will allow agriculture to more efficiently use existing water resources. In practical terms it means that pasture lands can be diverted into cultivated areas (e.g., cereals, olive trees, and almond trees).

However, it also means that most surface water infiltrates and eventually replenishes the groundwater, thus leaving downstream areas with almost no surface runoff. Soil contour ridges covering 43% of the El Gouazine catchment area increased the total runoff storage capacity by approximately 258 000 m3. Consequently, contour ridges may be said to present an alternative to the construction of small hillside reservoirs. The introduction of contour ridges therefore has dramatic consequences for downstream runoff conditions. To improve runoff management for both upstream and downstream conditions, it may be necessary to a) simulate runoff effects using a conceptual rainfall-runoff model and b) allow a certain part of the catchment to deviate a part of the runoff to the downstream natural watercourse.

A conceptual rainfall-runoff based on surface topography and the drainage network was seen to reproduce observed runoff accurately. By applying the model for runoff simulation with and without contour ridges a more detailed comparison of the changed runoff conditions could be made. The detailed hydraulic function of water harvesting techniques is still to a major extent unknown. A detailed study of the hydrological function of contour ridges in a cascade system is necessary to calibrate hydrological models such as H2U and their associated production function on hill-slope catchments with soil surface management.