Analyse fréquentielle du développement des ravins dans le semi-aride tunisien

L'érosion ravinante constitue un risque important dans les pays méditerranéens, notamment au centre de la Tunisie en raison des conditions climatiques et des activités humaines. La présente étude a pour objectif de prédire le processus de ravinement dans le bassin versant Merguellil, un bassin représentatif du semi-aride tunisien. La méthode employée pour calculer la probabilité d'apparition des ravins repose sur l'utilisation d'un environnement SIG. Elle se base sur la méthode de rapport des fréquences (RF), en utilisant les séparateurs à vaste marge (SVM) pour extraire les ravins de l'image multispectrale Sentinel, ainsi que 16 des facteurs responsables de l'érosion ravinante. Les résultats montrent que plus de 20 % de la superficie totale du bassin est vulnérable, pouvant augmenter le réseau actuel de ravins de plus de 155 km. La probabilité d'occurrence de ravins augmente en fonction de la pente topographique, en particulier en présence de roches avec une texture de sol peu évoluée. Plus encore, l'utilisation des terres agricoles peut également influencer la formation de ravins, notamment pour les zones cultivées et les parcours. L'étude montre que la cartographie de la sensibilité à la formation des ravins peut être automatisée en combinant l'inventaire des ravins et le rapport des fréquences, ce qui permet de produire des cartes utiles pour la gestion et la conservation des terres. Mots clés: Télédétection, érosion ravinane, probabilité d’occurrence, semi-aride tunisien, analyse fréquentielle, facteurs contributif

Effects of Water Scarcity on the Performances of the Agricultural Sector and Adaptation Strategies in Tunisia

The chapter aims to develop a regionally disaggregated agricultural supply model for Tunisia in order to investigate the potential effects of increasing water scarcity on the performances of the agricultural sector in the country, and the structural adaptation strategies needed to face such a challenge. A set of scenarios combining future water availability, water use efficiency, and increasing producer prices were simulated using the developed model. Results show that the agricultural sector in Tunisia, particularly the agricultural employment, would be negatively affected in case of decreasing irrigation water availability, and mostly affected regions would be the north east, central west, and southern areas. However, it is always possible to mitigate such effects through a combination of structural adjustments (changing land use in different regions), enhanced water use efficiency, and support of producer prices. The model also provides recommendations regarding specific crops that should be promoted in specific regions in order to maintain an agricultural sector with high added value in Tunisia

A participatory approach for adapting river basins to climate change

Climate change is expected to reduce water availability in the Mediterranean region and water management needs to adapt to future conditions. The aims of this study were (1) to develop a participatory approach for identifying and evaluating management options for river basin climate adaptation and (2) to apply and evaluate the approach in four case-study river basins across the Mediterranean. As part of the approach, a diverse group of stakeholders joined a series of workshops and consultations in four river basins located in Cyprus, Slovenia, Spain and Tunisia. In each river basin, stakeholders expressed their views on challenges in their river basins, as well as options to tackle these challenges. We used the information on challenges, as well as the factors contributing to these challenges to develop a fuzzy cognitive map for each basin. These maps were converted into mathematical models and were used to assess the impact of a total of 102 suggested management options for the four river basins. We linked the options and their estimated impacts with a multi-criteria analysis to identify the most preferred options. The approach was positively evaluated by the participating stakeholders and allowed the link of stakeholders' knowledge and perceptions about their river basin with their preferences for options to adapt the management of their river basins to future conditions

Water Erosion Modelling using Fractal Rainfall Disaggregation, a Study in Semiarid Tunisia

In the Tunisian semiarid area, water erosion processes have led to negative economic and environmental consequences in a context of limited water resources. To characterize and predict these degradation phenomena, a comprehensive high-resolution data base on erosive rainfall, together with siltation records for 28 small reservoirs were analyzed. The studied small reservoir network displays a general life-span of about 14 years. The average soil loss is 14.5 tonnes/ha/year. The complex relationship between the erosive rainfall events and the annual soil loss rates can be explained by two important factors. The first factor is related to the soil “degradation cycle”. It determines the soil particle delivery potential of the catchment. The second factor corresponds to the “degradation front”. The latter presents a north-western/south-eastern direction. To investigate rainfall disaggregation possibilities, a regionalization of fine time-scale and daily rainfall was undertaken. The results showed that the spatial properties are typically non-isotropic. Clustering showed that two different homogeneous rainfall subgroups are closely related to the predominant convective and frontal rainfall types. The comparison illustrated important similarities between the maximum 15-min and the daily rainfall data. The above findings were an incentive to explore the multiplicative properties of a 4-year rainfall time series. The time series showed scaling behavior for time scales up to 100 min that coincides with the most active erosion process time scale. Moreover, the temporal structure of rainfall was reproduced using a disaggregation model (Olsson, 1998). The observed and generated rainfall time series displayed several similar characteristics. This allowed the reproduction of erosivity for erosive rainfall events longer than 45 minutes. The erosion modeling was performed using the USLE/GIS approach. Maps of observed and generated spatial erosion distribution were combined with the Masson’s and Wischmeier-Smith’s erosion limit intervals. According to the above, the potential of rainfall scaling-based approaches to predict water erosion levels in semiarid areas seems promising. Using this approach may help soil and water authorities in semiarid areas to better manage soil erosion problems