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Ruissellement et éronsion hydrique en milieu méditerranéen vertique : approche expérimentale et modélisation

Abstract : As one of the major types of land degradation, soil erosion by water induced large-scale environmental deterioration and declines in land productivity, especially in the Mediterranean area. If water erosion factors are now well known, the complex hierarchy of erosion processes over a wide range of spatial and temporal scales still needs to be studied. Shrink–swell soils that are widespread under Mediterranean climate, imply additional changes in terms of hydrological and erosive responses in relation to the changing soil water conditions. In this context, this study aims to better understand the processes and factors affecting the lateral transfers of water and sediments in an agricultural catchment dominated by vertisols under a range of spatial and temporal scales. A detailed monitoring investigation was conducted in the Kamech catchment (ORE OMERE, Tunisia) that includes continuous runoff and suspended sediment load measurement between 2005 and 2012 at the outlet of a four hydrological gauging stations: 1/ a plot (1.32 ha), 2/ a gully (0.17 ha) in which drains a 1.20 ha plot; 3/ a micro-catchment (15.2 ha) integrating the two previous stations and 4/ the oued station (175 ha) located in the main drain just upstream the Kamech reservoir. Data analysis showed that topsoil cracks appeared to seriously affect the seasonal dynamics of water and sediment delivery whatever the considered spatial scale. A similar time lag in the seasonality between water and sediments delivery was observed: although the runoff rates were globally low during the presence of topsoil cracks in autumn, most sediment transport occurred during this period because of very high sediment concentrations. Mean annual runoff proved to slightly increase with the scale area with 95 mm/year for the plot, 105 mm/ year for the gully and 120mm/year for the micro-catchment station. Increase of area with low infiltration capacity when moving from plot to micro-catchment has been identified as a major explanation of this increase. A simple linear model combining a single rate of topsoil erosion (17 t ha-1 yr-1) and a single rate of gully erosion (80 t ha-1 yr-1) -weighted by their respective surface area- has successfully reproduced the differences observed in the mean annual erosion rates between the spatial scales. The sediment fingerprinting method applied on sediments collected during four flood events at the different stations showed that the apportionment of the processes responsible for the transfer of sediments was relatively stable during the year. The application of the Morel-Seytoux infiltration model has allowed reproducing runoff measured at the parcel outlet for most of the events, whatever the presence of cracks or not. However, the evolution of the two parameters in this model (the saturated hydraulic conductivity and the storage-suction factor) as a function of the presence of the cracks, of the soil surface conditions and of the degree of humidity still need to be modelled. This work allows concluding that the Kamech catchment is characterized by a very high hydro-sedimentary connectivity and a predominance of topsoil erosion processes and that the implementation of erosion control measures should incentivize farming conservation practices focusing especially on the autumn period. This work also highlights the crucial role of the cracks on the catchment runoff and sediment dynamics in Mediterranean vertisol context and the need to better understand and model both the runoff and soil erosion processes associated with cracking soils environment.
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  • HAL Id : tel-01568046, version 1

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Nesrine Inoubli. Ruissellement et éronsion hydrique en milieu méditerranéen vertique : approche expérimentale et modélisation. Sciences agricoles. Montpellier SupAgro; Institut national agronomique de Tunisie, 2016. Français. ⟨NNT : 2016NSAM0040⟩. ⟨tel-01568046⟩

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