Variabilité spatio-temporelle des apports liquide et solide en zone semi-aride. Cas du bassin versant de l'oued Mina (nord-ouest algérien)

Les zones méditerranéennes et semi - arides sont caractérisées par des régimes pluviométriques irréguliers dans le temps et dans l'espace. L'érosion hydrique des bassins versants et la sédimentation des cours d'eau constituent des phénomènes complexes très répandus qui posent de nombreux problèmes aux ingénieurs chargés des projets de mise en valeur. En raison de son ampleur, cette érosion constitue une contrainte majeure au développement de l'agriculture et à la promotion des activités rurales en Algérie. Une approche statistique de quantification des apports solides est développée dans cette étude. Les données utilisées sont recueillies au niveau de cinq sous-bassins du bassin versant de l'Oued Mina drainés par les stations hydrométriques, en amont du barrage de Sidi M'hamed Ben Aouda. La période d'observation s'étale sur 22 ans (1973 à 1995). Il existe une grande variabilité inter-annuelle et intrannuelle des apports liquides et solides. En plus de cette variation temporelle, il a été remarqué une variabilité spatiale des apports solides et liquides.Les valeurs maximales du transport solide sont enregistrées au début d'automne et à la fin du printemps. Le transport solide d'automne reste le plus élevé, dépassant significativement les autres saisons. Cette variabilité trouve son explication dans la pauvreté du couvert végétal durant cette saison et la nature agressive des pluies d'automne. La charge spécifique du bassin versant de l'oued Haddad est de l'ordre de 212 t/km2/an. Le bassin versant de l'oued Taht a une charge spécifique inférieure à celle du bassin de l'oued Haddad (Ds=191 t/km2/an). Les bassins versants de l'oued el Abd présentent des charges spécifiques de l'ordre de 117 t/km2/an à Ain El Hamara et de 65 t/km2/an à Takhmaret. La charge spécifique de l'oued Mina au barrage de Sidi M'hamed Ben Aouda est de 160 t/km2/an.Des modèles mathématiques liant l'apport solide au coefficient d'écoulement ont été proposés pour les cinq sous-bassins drainés par les stations hydrométriques.Mediterranean and semi - arid zones are characterized by an aggressive climate and by irregular and violent rainfall events. Water-induced erosion in river basins and river sedimentation constitute a complex phenomenon, which poses many problems for the engineers responsible for development projects. This phenomenon constitutes a major constraint limiting agricultural development in Algeria. We propose a statistical approach to quantify the solid yield at different gauging sites.The data were collected at five sites in the Oued Mina basin (upstream of the Sidi Me hamed Ben Aouda dam). The observation period covered 22 years, from 1973 to 1995. The results showed a high inter-annual and annual variability of the liquid discharge and solid yields; the coefficient of variation exceeded 60% in all cases. In addition to this temporal variation, we noted a variability of the solid yields and liquid discharge among the different sites. Analysis of the monthly solid yields shows that the autumn season contributes a large proportion of the annual solid yield (more than 40%) for the different basins. Tranport of solids in the Oued Mina and its effluents is highly variable in space and time. This variability is due primarily to the pluviometric regime, the vegetation cover and the lithology. In general, the seasonal variations of the solid contributions follow those of the liquid flow. The maximum values of the solid discharges, for all basins, are observed at the beginning of autumn and the end of spring. Generally, the monthly minimal solid yields are recorded in summer. This season is characterized by low rainfall and consequently a weak surface flow. However, the minimal solid yields do not necessarily correspond to the month characterized by the lowest liquid discharge. The autumn contributes 31% to the annual liquid flow and 51% to the annual solid yields of the Oued Mina area at Wadi El Abtal station; corresponding values are (26 %, 33 %), (33%, 53 %), (29%, 46%) and (36%, 48 %) at the stations of Sidi A.E.K Djillali, Ain Hamara, Kef Mehboula and Takhmaret, respectively. The autumn solid transport remains highest, significantly exceeding the other seasons. This variability is explained by the variation in vegetation cover (bare ground) during the year and the aggressive nature of the autumn rains. Because of these two factors, the first autumn flood transports considerable quantities of sediments after a long dry season characterized by high temperatures and destruction of the soil aggregates by these temperatures. The energy produced by the first rains and the low degree of vegetative cover rate in this season also contribute to this increase in autumn. The Oued Haddad basin presents a high specific degradation compared to the other basins (Ds=212 t/km2/an). This impoverishment of the soil is very visible at this site. It is due to factors favouring erosion : steep topography and discontinuous vegetative cover, which protects the ground poorly. The Oued That basin, which has a topography favouring surface flow and abundant rains, has a specific degradation lower than the Oued Haddad basin (Ds=191 t/km2/an). The basins of Oued El Abd present a specific degradation about 117 t/km2/an at Ain El Hamara station and about 65 t/km2/an at Takhmaret station. The Oued Mina at Sidi M'hamed Ben Aouda dam gives a specific degradation about 396 t/km2/an.The runoff coefficient explains a large part of the variance of the solid yield (more than 60%). This coefficient depends intrinsically on lithology, geology, vegetative cover, topography and the hydrological and pluviometric regimes. By the introduction of this parameter, we sought to explain solid transport by the explanatory variables of this phenomenon. The result was very encouraging, considering the percentage of the explained variance (more than 60%). Following this analysis, mathematical models are proposed, connecting the solid yield to the runoff coefficient. The determinations coefficients are equal to : 0.79; 0.61; 0.64; 0.78; 0.65 for the stations of oued Al Abtal, Sidi Abdelkader Djillali, Ain Hamara, Kef Mehboula and Takhmaret, respectively

Recent changes in climate, hydrology and sediment load in the Wadi Abd, Algeria (1970–2010)

Here we investigate the changes of temperature, precipitation, river runoff and sediment transport in the Wadi Abd in northwest Algeria over a time series of 40 hydrological years (1970&ndash;2010). Temperature increased and precipitation decreased with the reduction in rainfall being relatively higher during the rainy season. A shift towards an earlier onset of first rains during summer was also found with cascading effects on hydrology (hydrological regimes, vegetation, etc.) and thus on erosion and sediment yield. During the 1980s, the flow regime shifted from perennial to intermittent with an amplification of the variations of discharge and a modification of the sediment regime with higher and more irregular suspended particulate flux. Sediment flux was shown to almost double every decade from the 1970s to the 2000s. The sediment regime shifted from two equivalent seasons of sediment yield (spring and fall) to a single major season regime. In the 2000s, autumn produced over 4 times more sediment than spring. The enhanced scatter of the <i>C</i>&ndash;<i>Q</i> pairs denotes an increase of hysteresis phenomena in the Wadi Abd that is probably related to the change in the hydrologic regime. At the end of the period, due to irregularity of the discharge, the ability of a rating curve to derive suspended sediment concentration from river discharge was poor

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

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 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 (r(2) = 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 x 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 (r(2) = 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

Soil erodibility mapping using the RUSLE model to prioritize erosion control in the Wadi Sahouat basin, North-West of Algeria

Soil losses must be quantified over watersheds in order to set up protection measures against erosion. The main objective of this paper is to quantify and to map soil losses in the Wadi Sahouat basin (2140 km(2)) in the north-west of Algeria, using the Revised Universal Soil Loss Equation (RUSLE) model assisted by a Geographic Information System (GIS) and remote sensing. The Model Builder of the GIS allowed the automation of the different operations for establishing thematic layers of the model parameters: the erosivity factor (R), the erodibility factor (K), the topographic factor (LS), the crop management factor (C), and the conservation support practice factor (P). The average annual soil loss rate in the Wadi Sahouat basin ranges from 0 to 255 t ha(-1) year(-1), maximum values being observed over steep slopes of more than 25% and between 600 and 1000 m elevations. 3.4% of the basin is classified as highly susceptible to erosion, 4.9% with a medium risk, and 91.6% at a low risk. Google Earth reveals a clear conformity with the degree of zones to erosion sensitivity. Based on the soil loss map, 32 subbasins were classified into three categories by priority of intervention: high, moderate, and low. This priority is available to sustain a management plan against sediment filling of the Ouizert damat the basin outlet. The method enhancing the RUSLE model and confrontation with Google Earth can be easily adapted to other watersheds