Habib Abida

Sediment yield variability in central Tunisia: a quantitative analysis of its controlling factors

Abstract This paper refers to the quantification and prediction of the sedimentation rate of 26 hillside-dam reservoirs in Central Tunisia. The objectives of the study are to develop a simple and practical methodology to identify controlling factors of sedimentation, and to propose a regionalization from the study sites. Principal component analysis (PCA) and complementary multi-dimensional statistical methods are used to relate highly variable area-specific sediment yield to hydro-morphometric, lithological, geomorphological and anthropogenic characteristics of catchments. It appears that catchment area is not the main controlling factor of sedimentation in the studied area. The overall slope index, drainage network characteristics and runoff parameters are also important in characterizing sediment yield. Applied to the annual sedimentation rate series, PCA resulted in retaining the first three principal axes, explaining 65% of the total variance. Statistical methods showed that the overall slope index, the total drainage length, the compacity index and the runoff parameters are as important for the sedimentation quantification. This allowed a graphical clustering of the study zone into three distinct groups having similar behaviours: (i) watersheds characterized by high sediment transport rates and high runoff coefficients, (ii) basins distinguished by relatively low values of both flow discharge and sediment transport rates, and (iii) watersheds with an intermediate sediment yield, especially characterized by relatively high relief. In a second step, a multiple regression model including the four characteristic catchment properties was developed, presenting a valuable tool to predict area-specific sediment yield from catchments in central Tunisia. This model shows reasonable efficiency with an absolute prediction error of 81%. Citation Ayadi, I., Abida, H., Djebbar, Y. & Mahjoub, M. R. (2010) Sediment yield variability in central Tunisia: a quantitative analysis of its controlling factors. Hydrol. Sci. J. 55(3), 446–458.

A triangular model for the generation of synthetic hyetographs

Abstract The method of “historic event” is used to generate synthetic hyetographs based on statistical analysis of precipitation data. A synthetic triangular model was developed based on rainfall data of Zioud watershed (central Tunisia) with a standard time step of one hour. A database of 2799 observed rainfall events was used to provide statistical parameters for a simple triangular-shaped hyetograph model. The developed model provides a synthetic hyetograph in dimensionless form for different storm durations (2, 3 and 4 hours). For a given season and location, the variation of the first dimensionless moment with duration was relatively small, with an average range of 13% for all the stations. The resulting dimensionless hyetographs were found to be nearly identical when they were non-dimensionalized using the rainfall depth and duration, showing some seasonal effect and insignificant effects of the rainfall duration. A good agreement between simulated and observed hyetographs was achieved based on not only visual impressions, but also statistical numerical and graphical tests.

Monte Carlo simulation-aided analytical hierarchy process (AHP) for flood susceptibility mapping in Gabes Basin (southeastern Tunisia)

Abstract Flash floods are among the most severe hazards which have disastrous environmental, human, and economic impacts. This study is interested in the characterization of flood hazard in Gabes Catchment (southeastern Tunisia), considered as an important step for flood management in the region. Analytical hierarchy process (AHP) and geographic information system are applied to delineate and characterize flood areas. A spatial database was developed based on geological map, digital elevation model, land use, and rainfall data in order to evaluate the different factors susceptible to affect flood analysis. However, the uncertainties that are associated with AHP techniques may significantly impact the results. Flood susceptibility is analyzed as a function of weights using Monte Carlo (MC) simulation and Global sensitivity analysis. AHP and MC–AHP models gave similar results. However, compared to AHP approach, MC–AHP confidence intervals (95%) of the overall scores had small overlaps. Results obtained were validated by remote sensing data for the zones that showed very high flood hazard during the extreme rainfall event of June 2014 that hit the study basin.

Geochemical behaviour of the aerosol sampled in a suburban zone of Sfax City (Tunisia)

This work examines the geochemistry of the soluble elements (Ca2+, Mg2+, Fe2+ + Fe3+, Na+, K+, NH4+, Cl−, NO3−, SO42− and PO43−) present in the aerosol collected from a suburban site in Sfax City (Tunisia), which is frequently subjected to the urban plume. The descriptive study showed the influence of enrichment sources and meteorological factors. Complementary statistical approaches based on principal component analysis and multiple linear regression showed that the sampled aerosol has three different origins: crustal, marine and anthropogenic. The corresponding components are respectively made of (Ca2+, Mg2+, Fe2+ + Fe3+, K+ and PO43−), (Cl− and Na+) and (SO42−, NH4+ and NO3−). The contribution of the first two components is shown to depend on seasonal effects caused by two antagonistic circulations of continental and marine winds. The contribution of the anthropogenic component (SO42−, NH4+ and NO3−) is shown to be related to the effect of dominant north-easterly winds, which carry the pollution of Sfax City to the study site. Statistical approaches related to the behaviour of these anthropogenic compounds, in the presence of secondary aerosol precursors (SO2, NO2), proved their dependence on gas/particle conversion processes, especially those of neutralisation.

Seasonal evolutions of ozone (O3) and its nitrogen precursors (NO, NO2) in urban Sfax (Tunisia)

Seasonal evolution of ozone (O3) and its nitrogen precursors (NO, NO2) in downtown Sfax (Tunisia) was monitored. Nitrogen oxides are shown to be closely related to local vehicle sources. Seasonal ozone levels, however, are shown to be dependent on regional meteorological conditions. High ozone levels are due to the effect of anticyclones and stratosphere intrusions (cut-off lows). Low levels are associated with cyclonic conditions of small vertical range of motion. Other than these particular conditions, ozone levels are shown to be relatively higher in fall and winter seasons, characterised by a very steady atmosphere. Overall, the examined meteorological conditions, the ozone concentrations observed in downtown Sfax are characterised by clear day/night cycles, which can be explained by the significant ventilation of the region.

Drought intensity and spatial variability in Gabes Watershed, south-eastern Tunisia

Abstract Chronological series of monthly and annual precipitation data recorded in Gabes Watershed, south-eastern Tunisia, were analyzed. The study is based on the standardized precipitation index (SPI) values, computed for 10 rainfall stations over the period 1987–2012, which corresponds to an observatory period of 25 hydrologic years (from September to August). The results obtained show a great variability in SPI values. The historical evolution of the SPI made it possible to define the periods of excess and deficit, corresponding to wet and dry periods respectively. The wet years were found to be 1989–1990, 1995–1996 and 2006–2007 while the dry years were 1987–1988, 1996–1997, 2000–2001, 2001–2002, 2007–2008, 2008–2009 and 2009–2010. This clearly shows alternating wet and dry periods, but with drought episodes taking prevalence over rainy fronts throughout the study period. Indeed, a high tendency towards a drop in precipitation and important sequences of drought were observed. Spatial variability of drought throughout Gabes Watershed was examined by geostatistical analysis of SPI, as drought and rainfall distribution vary with latitude, longitude, topography and proximity to the Mediterranean Sea. The results obtained showed that, compared to coastal and southern areas, drought was observed to be more important in the West and the North of Gabes Watershed. The SPI showed that moderate droughts are generally more frequent than severe or extreme droughts in most of the Watershed.

Olive oil Mill Wastewater properties change during infiltration through clay soils

Olive oil Mill Wastewaters (OMW), are stored in clay basins in the region of Sfax (Tunisia). Field permeability measurements showed that the clay is not completely impermeable and its structure can be modified by the infiltration of OMW. Analysis of chemical composition of soil revealed the presence of phenolic compounds resulting from the percolation of OMW. An infiltration filled with clay and submerged with OMW at a height of 1.5 m was used to study the composition change of the support as OMW percolates through the clay. Compared to the raw OMW, infiltrated effluent showed a decease of its polluting load.

Correction to: Spatial and temporal variability of rainfall: case of Bizerte-Ichkeul Basin (Northern Tunisia)

The article which was recently published contained errors in Table 1. The values of coefficient of skewness (Cs) and coefficient of kurtosis (Ck) are not in their proper locations. Given in this article is the corrected table. The original article has been corrected.