Suspended sediment transport in a semiarid watershed, Wadi Abd, Algeria (1973-1995)
Résumé
A 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 km2), the mean annual precipitation is 250 mm and the mean annual discharge is 1.0 m3 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 Qb. 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 Q3/4. 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 Qs 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 × 106 t km3, 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.