Vincent Marc

Investigation of the hydrological processes using chemical and isotopic tracers in a small Mediterranean forested catchment during autumn recharge

Abstract Flood generation processes are studied in a small mountainous Mediterranean catchment using natural chemical and isotopic water tracing. The study takes place during the autumn of 1996 for three successive floods of uneven magnitude. The flow resumption following the summer period is a prime opportunity to establish the impact of the soil moisture status on the catchment response. A dynamic scheme of the catchment behaviour and mechanisms is obtained. The combined use of different tracers highlights the speed of the water transfer from the hillslope to the catchment outlet as well as the predominant contribution of short residence time water (current water and previous rain water). Study results show that subsurface drainage through the soil macroporosity is one of the dominant processes in the formation of streamflow generation. This is in keeping with the soils high permeability. Hydrograph separations demonstrate that pre-event water is the major contributor to streamflow. However, with time, a rise in the proportion of direct rainwater is observed, thereby highlighting the impact of the spread of soil saturation on the speed of water transfer and on the expansion of the contributing areas. Thus, for the last flood event (13/15-10), almost a third of the streamwater volume comes from the event rainfall and 50% from the first autumn rain event (19-9). The contributing areas reached 14% of the total catchment surface. A δ 18 O simulation in shallow subsurface runoff is carried out, at the slope scale, with the use of a simple model. The results demonstrate that this subsurface runoff came from the mixing of the rain events successive fractions without involving pre-existent water stored in the upper soil layer. For this autumnal flow resumption, the catchment mechanisms can be explained by a two-reservoir system, each geochemically distinct from the other. One is a surface reservoir primarily containing rainwater and the other is a subsurface reservoir made up of pre-event water from earlier rainfall. The contribution of an underground long residence time water is negligible.

The role of forest cover on streamflow down sub-Mediterranean mountain watersheds: a modelling approach

The impact of forest cover on streamflow has been studied in sub-Mediterranean mountain watersheds using the daily time step conceptual rainfall/runoff model GRHUM. Certain characteristics (pertaining to the soil, vegetation or atmosphere) are used to compute model parameters that describe the behaviour of the various interfaces with respect to water exchange by evapotranspiration. This model enables the simulation of runoff and soil water content evolution. A snow function has been added to the initial version of the model to simulate the hydrological behaviour during the snow melt periods. This model has been applied to the Mont-Lozere experimental watersheds, for which hydro-meteorological data have been available since 1981. A site where a gradual felling of the forest was prescribed between 1987 and 1989 (the Latte watershed) is compared to an unexploited forest site (the Sapine watershed), used as reference. The period covered by the study of the Latte site has been divided into three parts, on the basis of changes in land cover. For both watersheds, the model produced good results over all three periods. It is also possible to identify the role of forests on hydrological behaviour by simulating discharge from the Latte watershed during the felling period using adjusted pre- and post-felling model parameters. This exercise has shown that felling induces a significant increase in high flow rates. A similar approach conducted for the Sapine reference watershed made it possible to evaluate the validity of our model predictions (accurate to within 10% of true values); the model bias being mostly due to climatic differences between the three simulation periods. Taking into account this model bias, results suggest that felling may increase runoff by 10%.

Hydrogeological features of a highly fractured rock-slab

In many geological contexts, the hydrogeological features of highly fractured rock slabs can drive slope instability processes. This is the case of San Leo (northern Apennines of Italy), where groundwater processes were recognized as predisposing factor for the last large-scale rock fall that took place the 27th of February 2014. In the present work, the hydrogeological features of the San Leo slab were analyzed by means of spring-discharge analysis, piezometric monitoring data and slug tests. The maximum spring yield, the depletion coefficient and the hydraulic conductivity values were estimated. Time-series analyses were used to better understand the groundwater behavior within the rock slab and to estimate its response to rainfall regime. Moreover, the calculation of the groundwater flow direction and modulus in the unsaturated zone was performed. Results highlight the presence of two systems, which are probably related to the characteristics of different joint sets (aperture, spacing); the hydraulic conductivity values were shown to vary with depth. A first general interpretation of the aquifer behavior is given, which is related to the main structural elements of the slab.

Groundwater—Surface waters interactions at slope and catchment scales: implications for landsliding in clay-rich slopes

Understanding water infiltration and transfer in soft-clay shales slopes is an important scientific issue, especially for landsliding. Geochemical investigations are carried out at the Super-Sauze and Draix-Laval landslides, both developed in the Callovo-Oxfordian black marls, with the objective to define the origin of the groundwater. In situ investigations, soil leaching experiments and geochemical modeling are combined to identify the boundaries of the hydrological systems. At Super-Sauze, the observations indicate that an external water flow occurs in the upper part of the landslide at the contact between the weathered black marls and the overlying formations, or at the landslide basement through a fault network. Such external origin of water is not observed at the local scale of the Draix-Laval landslide but is detected at the catchment scale with the influence of deep waters in the streamwater quality of low river flows. Hydrogeological conceptual models are proposed emphasizing the role of the interactions between local (slope) and regional (catchment) flow systems. The observations suggest that this situation is a common case in the Alpine area. Expected consequences of the regional flows on slope stability are discussed in term of rise of pore water pressures and physicochemical weathering of the clay shales.

Sprinkling Tests to Understand Hydrological Behaviour of Mudslide

The unsaturated zone buffers precipitation and controls groundwater recharges. Quantification of groundwater recharges is important for the improvement of hydrogeomorphological hazard analysis. The importance of fast preferential flow is recognized in literature, but its quantification remains difficult.

Improving the Hydrogeological Conceptual Model of the Sidi Merzoug-Sbiba Aquifer System (North-West of Tunisia) Using Hydrochemistry and Isotopic Tools

The aim of the study is to improve the knowledge of the hydrogeological systems in the Sidi Merzoug Sbiba Basin (Northwestern Tunisia), using chemical and isotopic tools. Three major aquifers have been identified in this semi-arid region by previous hydrogeological studies: the Cretaceous, the Miocene and the Plio-Quaternary aquifers. Its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions. Given the heterogeneity of the multilayer aquifer system in a complex fractured zone, an hydrochemical and environmental isotope (2H, 18O,3H and14C) data were used to caracterised the groundwater flow and provide valuable information about the geochemical processes controlling groundwater quality and the circulation patterns of the different groundwater bodies. Three major processes control the chemical composition: i) dissolution of carbonate minerals, ii) cation exchange reactions and iii) evaporation process. Stable isotopes indicate that most groundwater samples originate from infiltration of modern precipitation. A significant infiltration before evaporation takes place, indicating a major recharge directly from Cretaceous and Miocene formations of surrounding mountains and infiltration of surface water in the El Breck and Sbiba rivers. Downstream, the isotopic signature of evaporated water clearly indicates a recharge from rivers, irrigation fields or Sbiba dam. Tritium and 14C contents confirm the existence of modern groundwaters in the South-Western border and in the North -Eastern part of the basin and confirm the stratification of the system.