Relation entre l'efficience des lacs collinaires et la sensibilité au ravinement de leur bassin versant en Tunisie centrale

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Five hundreds hill lakes were constructed in semi arid areas of central Tunisia. A research project on some hill dams were conducted between Tunisian research institutions and The French Institute of Research for Development since 1994 to identify the hydrologic characteristics of their watersheds. The low rainfall and the occurrence of highly erosive storms in these areas have conducted us to define a coefficient of efficiency of any hill lake as the ratio of the volume of runoff and the volume of sediment provided to the lake by a unit area of this watershed in any time span period. For the five year research duration, this hill lake efficiency coefficient was found to vary from 5.4 to 80.4 for five studied hill lakes. In a previous research we have found that this coefficient is related to the watershed lithologic number which defines the succeptibility to gully erosion of the lithologic formations covering the watershed. We have found in this research that others factors may contribute to explain the hill lake efficiency. This may be due to the predominence of gully erosion over rill and splash erosion. We have estimated the rill and splash erosion from the watersheds of Fidh Ali and Fidh Ben Naceur hill lakes using the non point source hydrologic model ANSWERS. These estimations were compared to the actual total sediment provided to the lakes to determine the ratio of gully erosion in the overwhole sediment load. These ratio were 46 % and 65 % for Fidh Ali and Fidh Ben Naceur hill lakes watersheds, respectively.