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Climate variability, non-optimal water resources management and increase of human pressure are three possible factors influencing runoff and water resources in semi-arid Algerian catchments, such as the Tafna catchment (western Algeria). This catchment is subject to a semi-arid climate. In addition to rainfall impact, we examine the influence of other factors, such as geology, topography and human activity, on the runoff variability in the Tafna catchment. For this purpose, to complement the rainfall and discharge series analysis, we considered baseflow and the baseflow index (BFI). The analysis was conducted for five sub-catchments of the Tafna basin, where daily rainfall and discharge series were available for the 1976-2006 period. In the analysis, we distinguished two domains defined from the topography: a high-altitude area and a plain area. According to their geological setting, the hydrological regime is different. High-altitude catchments have a larger baseflow and BFI than catchments in the plain. This may be related to the differences in lithology, as high-altitude catchments have a large area of karstic geological formations. No significant trends or step changes were found in the annual rainfall for the 1976-2006 period. We found a decrease in the mean annual discharge of two high-altitude discharge stations (significant at the 1% and 10% levels, respectively). A decreasing trend was also found for two high-altitude catchments and a plain catchment (significant at both the 1% and 5% level). The BFI decrease is only significant for one high-altitude catchment. This is evidence of a decrease in storage within high-altitude catchments. Runoff change in the Tafna catchment could therefore be related to other factors, such as human impact, rather than to rainfall change.