Relationship between small dam efficiency and gully erodibility of the lithologic formations covering their watershed

Soil conservation strategy in the semi-arid areas of central Tunisia has focused on the construction of numerous hill reservoirs. However, limited rainfall combined with severe soil erosion is jeopardizing the usefulness of hill reservoirs such that there is an urgent need to undertake further studies into the location and efficiency of small dams. The paper reports investigations into the relationship between location of the dam and the underlying lithology of the catchment areas with the dam efficiency. A gully erodibility factor has been proposed for nine groups of lithological formations identified and a watershed lithologic number was computed as the product of seven watersheds for which 1:50,000 scale lithological maps were available. A significant linear relationship was determined between the watershed lithologic number and the efficiency of the small dams, indicating that an accurate lithological survey prior to small dam planning can be used to indicate the dam's likely efficiency and to plan any necessary soil and water conservation works in the catchment areas.