Volume 4 Article informations The impact of the dimensions of jessour on the destruction of the small hydraulic units (Jessour) in the Mounts of Matmata Ines Gasmi a,* , Nissaf Karbout a , Mohamed Moussa a Abstract Southeast of Tunisia is characterized by an arid Mediterranean climate, where the mean annual rainfall is between 100- 200mm. Rainfalls in this area are torrential and stormy which promotes water erosion. Agricultural production in the dry lands is generally based on rain fed agriculture, which is in its turn based on rainwater harvesting techniques such as "jessour". Moreover, serious efforts to exploit the runoff water for rain fed agriculture and reduce water erosion in mountainous areas have given a lot more interest to the technique of jessour. However, despite their significant morphological roles, the jessour known by their fragility that can be easily destroyed in case of an excess water retained behind dams. The risk of destruction has been carefully evaluated at all the small hydraulic structures in the micro watershed (El-Jouabit to Toujène, Mareth) in Matmata Mountains. The collected data were used to develop different types of maps, which show that in most cases, the disproportion between surface of retention, height of retention and catchment area surface causes overflowing during exceptional rainfall events and after that the destruction of small hydraulic units. The analysis of these parameters show, that there is an advanced disproportion between the surface of retention and the surface of the catchment area, which causes, an important imbalance between, the volume of flowing water and the retention capacity, which is the principal cause of overflow. Therefore, the evolution of the number of the destroyed small hydraulic units, is simulated, by the establishment of many scenarios for different depth of runoff water blade that range between 2 mm and 50 mm. These simulations show, that the units with large dimensions are more resistant to runoff volumes hence a small increase of water blade does not have a marked impact on the number of overflowing units. Thus, the units located in the upstream are the most risky (steep slope and disproportion between surface of retention, height of retention and surface of the catchment area). The micro watershed study helped to develop a database for the solution for the risks of water erosion phenomenon that can be applied for the entire chain of Matmata. Key words: Dry land, Rainwater harvesting, Soil/water conservation system, Typical jessour, Tunisia