Suspended sediment transport in a semiarid watershed, Wadi Abd, Algeria (1973 1995)

SummaryA quantification of the fine sediment budget of a wadi (dryland river) in NW Algeria is presented for a period of 22 hydrological years (1973-1995). The climate is Mediterranean over the Wadi Abd basin (2480 km ²), the mean annual precipitation is 250 mm and the mean annual discharge is 1.0 m ³ s ^-1 at the gauging station. Regression relationships between water discharge Q and suspended sediment concentration C are calculated from 1432 paired measurements in the Wadi Abd, leading to power-law equations of the type C = a Q^b. The variability of coefficients a and b, calculated for 138 floods and flood stages, is analyzed. The median value of b is 0.757, indicating that C is almost proportional to Q^3/4. Given that the ( a, b) pairs are correctly aligned ( r² = 0.578), the coefficients a and b are not independent. Regression relationships between daily Q and daily suspended sediment concentration and discharge Q_s are calculated from 702 input data. The performances of these regression relationships are shown to be equivalent, leading to over-estimations of 20-25% of the suspended sediment flux. The non-biased C- Q sediment rating curve is used to extrapolate a time series of C measurements, and thus to analyze the long-term patterns in suspended sediment transport. Average sediment wash-down (136 t km ^-2 yr ^-1) is similar to the mean global value. The ratio of sediment wash-down to the river water discharge is 10.7 × 10 ⁶ t km ³, 20 times greater than the average ratio in the Earth's eastern hemisphere, and illustrates the highly erosive power of wadis. Variability is shown to be significant at the seasonal scale (CV = 89%) and higher at the interannual scale (CV = 139%). The fine sediment flux mainly occurs in autumn (48.4%) and spring (32.7%). Although precipitation decreased, it was more irregular from one year to another over the period 1985-1995 than during the period 1973-1985, and the Wadi Abd, which was a perennial river, became intermittent in the late 1980s. This increasing irregularity is accompanied by: an amplification of the variations of discharge, an increase in the average discharge of approximately 20% during the second period, and a higher and more irregular suspended particulate flux. The mean annual suspended sediment yield is shown to be highly correlated with the standard deviation of mean daily discharge calculated per year ( r² = 0.989). The highly significant interannual variability points to the difficulty of defining a suitable period to calculate a reference value for sediment budgets. It also emphasizes the absolute necessity of continuing a series of measurements over longer time periods to study fluctuations in the context of climate change.