Influence of rainfall and runoff fluctuations on sediment yield : case of the Upper Tafna watershed
Watersheds in semi-arid Mediterranean areas are subject to a variety of disturbances, including rainfall variability, land clearances, with consequent effects on the erosive potential. In this context, double mass plots were used to demonstrate recent trends in suspended sediment yields in this region. The study was conducted in the Upper Tafna basin of 256 km2 located in northwest Algeria. Sediment yield was explained as a function of the annual and seasonal time series of rainfall and water contribution. As observed elsewhere in North Africa, in the 70's, rainfall began to decrease in the study area, inducing a progressive degradation of the vegetation. As a result, rainfall often occurs on dry, hard and bare soil and leads to high peak discharges with associated high sediment transport. The study revealed that, since the beginning of 1990’s, the behaviour of the seasons has changed. Even though annual precipitation has decreased, the amount of rainfall occurring during the dry season has doubled. The gradual disappearance of vegetation cover due to the recurrent drought ravaging the region over more than three decades, combined with the increase in summer storms, has led to changes in the functioning of the watershed and a more important susceptibility to erosion. For particular years, moderate annual precipitations have generated high water discharge carrying a significant sediment flux. Since the end of the 1980's, the annual mean sediment contribution was seven times greater than that in the prior period, with a 4-fold increase in the contribution of the dry season.