International audience

International audience | Karstic watersheds are highly complex hydrogeological systems that are characterized by a multiscale behaviour corresponding to the different pathways of water in these systems. The main issue of karstic spring discharge fluctuations consists in the presence and the identification of characteristic time scales in the discharge time series. To identify and characterize these dynamics, we acquired, for many years at the outlet of two karstic watersheds in South of France, discharge data at 3-mn, 30-mn and daily sampling rate. These hydrological records constitute to our knowledge the longest uninterrupted discharge time series available at these sampling rates. The analysis of the hydrological records at different levels of detail leads to a natural scale analysis of these time series in a multifractal framework. From a universal class of multifractal models based on cascade multiplicative processes, the time series first highlights two cut-off scales around 1 and 16h that correspond to distinct responses of the aquifer drainage system. Then we provide estimates of the multifractal parameters α and C 1 and the moment of divergence q D corresponding to the behaviour of karstic systems. These results constitute the first estimates of the multifractal characteristics of karstic spingflows based on 10years of high-resolution discharge time series and should lead to several improvements in rainfall-karstic springflow simulation models.

Soumis à Hydrological Processes, accepté définitivement ; en attente de publication. | International audience | Rainfall and its consequences are usually considered as the major factor trig- gering slope instabilities within clay deposits. The link between rainfall, water inltration and landslide activity may be complex and comprehensive sets of data are still necessary to understand how water inltrates in cohesive material. On unstable slopes made of ne-grained sediments, gravitational deformation generates supercial shear ruptures and tension ssures whose continuous opening could be amplied by shrinkage during dry periods. This ssure network at the surface can be very dense and strongly controls the water inltration process. However, it re- mains dicult to assess the ssure evolution at depth and its eventual relationship with shallow slip surfaces. This work presents the results of a geophysical study which aimed to characterize the ssures that are observed along the unstable clay slopes of the Trieves area (French Alps). The site was rst investigated combining geomorphological analysis, drilling, borehole logging, geotechnical tests and geo- physical experiments. This investigation evidenced the presence of at least three rupture surfaces (at 5m, 10 to 15m and 42m) with a water ow at the second one. In a second step, permanent instruments were installed (piezometers and soil water content (SWC) probes). Results indicate high water inltration velocities, down to the water table located at 2.5m depth, that are likely to be linked with sub-vertical preferential paths, such as ssures. Geophysical parameters (electrical resistivity, S-wave velocities, Rayleigh waves attenuation) were monitored. Geophysical results suggest that ssures are permanently open, that they serve as preferential inltra- tion paths, and that they can reach, at least, a depth of 2m. The methods employed in this research put forward that the system of imbricated ssures drains water from the surface down to the shallow slip surfaces at 5m and 10-5m. This work highlights the role of ssures network in water inltration in the numerous clay landslides of the Trieves area.

International audience

International audience | Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e. the conventional approach based on radionuclide and geochemical concentrations and the alternative diffuse reflectance infrared Fourier transform spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterized by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterized by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way of prioritizing the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. Copyright © 2012 John Wiley & Sons, Ltd.

In this study, the Mean Transit Time and Mixing Model Analysis methods are combined to unravel the runoff generation process of the San Francisco River basin (73.5 km2) situated on the Amazonian side of the Cordillera Real in the southernmost Andes of Ecuador. The montane basin is covered with cloud forest, sub-páramo, pasture and ferns. Nested sampling was applied for the collection of streamwater samples and discharge measurements in the main tributaries and outlet of the basin, and for the collection of soil and rock water samples. Weekly to biweekly water grab samples were taken at all stations in the period April 2007–November 2008. Hydrometric data, Mean Transit Time and Mixing Model Analysis allowed preliminary evaluation of the processes controlling the runoff in the San Francisco River basin. Results suggest that flow during dry conditions mainly consists of lateral flow through the C-horizon and cracks in the top weathered bedrock layer, and that all subcatchments have an important contribution of this deep water to runoff, no matter whether pristine or deforested. During normal to low precipitation intensities, when antecedent soil moisture conditions favour water infiltration, vertical flow paths to deeper soil horizons with subsequent lateral subsurface flow contribute most to streamflow. Under wet conditions in forested catchments, streamflow is controlled by near surface lateral flow through the organic horizon. Exceptionally, saturation excess overland flow occurs. By absence of the litter layer in pasture, streamflow under wet conditions originates from the A horizon, and overland flow.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAU | All that glitters is not gold is one of those universal truths that also applies to hydrology, and particularly to the issue of model calibration, where a glittering mathematical optimum is too often mistaken for a hydrological optimum. This commentary aims at underlining the fact that calibration difficulties have not disappeared with the advent of the latest search algorithms. While it is true that progress on the numerical front has allowed us to quasi-eradicate miscalibration issues, we still too often underestimate the remaining hydrological task: screening mathematical optima in order to identify those parameter sets which will also work sufficiently outside the calibration period.

Clay shale landscapes are sensitive to landslide, erosion, and re-sedimentation processes. In this context, tectonized clay shales have peculiar hydrological and mechanical behaviours. In order to improve our understanding of the processes involved in such systems, an experimental site has been settled in the Draix Observatory (ORE DRAIX) on a black marl hill slope of the Southern Alps (France) in the framework of the ECOU-PREF project. The site is a natural laboratory to study the evolution of clay shale landscapes at several scales (regional to µm). In this study, we aim to characterize the internal structure and fissure patterns of a stable interfluve, in order to locate preferential water flows within the system, and anticipate its evolution. This work is based on a series of mutli-disciplinary approaches including petrophysical analyses, well pulse injection tests and downhole geophysical measurements. Borehole geophysical results were combined with laboratory measurements on core plugs (permeability, density, porosity, and acoustic velocity). Optical and acoustical images have been used as an original tool for internal discontinuities characterisation and potential active flow path detection. They allowed the identification of three main lithological units interpreted as several stages of shale alteration. In addition, several main structural plane discontinuities (open fractures, schistosity planes, and fractures infilled with calcite or clays) were identified from the images and their potential transmissivity discussed with regard to piezometric and tracer measurements. In all, the integration of measurements leads to propose a simple scenario of fluid circulation and chemical alteration of the interfluve.