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Deal E.,Helmholtz Center Potsdam | Favre A.-C.,LTHE | Braun J.,Helmholtz Center Potsdam
Water Resources Research | Year: 2017

Rainfall is an important driver of erosion processes. The mean rainfall rate is often used to account for the erosive impact of a particular climate. However, for some erosion processes, erosion rate is a nonlinear function of rainfall, e.g., due to a threshold for erosion. When this is the case, it is important to take into account the full distribution of rainfall, instead of just the mean. In light of this, we have characterized the variability of daily rainfall over the Himalayan orogen using high spatial and temporal resolution rainfall data sets. We find significant variations in rainfall variability over the Himalayan orogen, with increasing rainfall variability to the west and north of the orogen. By taking into account variability of rainfall in addition to mean rainfall rate, we find a pattern of rainfall that, from a geomorphological perspective, is significantly different from mean rainfall rate alone. Using these findings, we argue that short-term rainfall variability may help explain observed short and long-term erosion rates in the Himalayan orogen. © 2017. American Geophysical Union. All Rights Reserved.

Ryom Nielsen M.,Ramboll | Hagensen T.F.,Danish Ministry of the Environment | Chalikakis K.,French National Institute for Agricultural Research | Legchenko A.,LTHE
Near Surface Geophysics | Year: 2011

In Denmark, hydrogeophysical mapping is performed intensively in accordance with the Danish Government's environmental plans of ensuring a high quality drinking water supply. Previously the magnetic resonance sounding (MRS) method has been proven to improve the hydrogeological characterization from geophysical surface measurements in Denmark. In order to implement hydraulic parameters estimated from MRS together with hydraulic parameters obtained from pumping tests, it is necessary to analyse and compare the hydraulic parameters from the two data sources. These parameters are determined very differently with uncertainties arising from very different sources and the comparison requires an analysis in each specific case with attention on the investigated volumes. Several comparisons of transmissivities from MRS and pumping tests in Denmark show that the pumping test transmissivity value corresponds to the MRS transmissivity accumulated over depth intervals determined by the screen position depth interval in the borehole. When specific determinations of these depth intervals are performed, very good correlation is obtained between transmissivities from MRS and pumping tests. Comparisons of transmissivities from MRS and pumping tests will also lead to the determination of MRS calibration coefficients. MRS calibration is essential for quantitative use of MRS transmissivities. Sixteen calibration coefficients in different survey areas have been calculated. If these calibration coefficients can be categorized according to the different Danish survey locations and hydrogeological conditions in which they are obtained, these categorized calibration coefficients could be used in future MRS surveys without the need of calibrating with pumping tests in each survey. However, only a slight tendency of increasing calibration coefficient with increasing transmissivity and increasing grain size is observed. The present study shows that in general only little dispersion is observed around an average calibration coefficient for the different Danish survey locations and hydrogeological conditions. © 2011 European Association of Geoscientists & Engineers.

Mazzilli N.,HSM | Chalikakis K.,EMMAH | Boucher M.,LTHE | Legchenko A.,LTHE | And 4 more authors.
Near Surface Geoscience 2012 | Year: 2012

This study aims to assess Magnetic Resonance Soundings (MRS) potential for the characterisation of the unsaturated zone of karst systems and thereby groundwater vulnerability assessment. The issues addressed are: (i) does water storage within the unsaturated zone of karst yield quantifiable MRS measurements ? if so, (ii) do different hydrogeological contexts yield different MRS signals ?, and (iii) what are the key factors influencing MRS signal ? The proposed methodology is based on a careful characterisation of the experimental sites, involving both hydrogeological and geomorphological analyses. MRS measurements have been performed on a total of 19 sites with varied hydrogeological and geomorphological setting. Analysis of the measured MRS signals shows that : (i) water storage within the unsaturated zone of karst does yield quantifiable MRS measurements, (ii) the first-order determinant of the MRS signals is the nature of the geological formation. These results confirm MRS applicability for the characterization of the unsaturated zone of karst aquifers and thereby vulnerability assessment. Future work will focus on the variability of the MRS response within a given geological formation.

Chalikakis K.,French National Institute for Agricultural Research | Carriere S.D.,French National Institute for Agricultural Research | Mazzilli N.,French National Institute for Agricultural Research | Danquigny C.,French National Institute for Agricultural Research | And 2 more authors.
Near Surface Geoscience 2014 - 20th European Meeting of Environmental and Engineering Geophysics | Year: 2014

Karst unsaturated zone (UZ) is recognized to play a key role in karst hydrosystems recharge and contaminant attenuation processes. Its characteristics are identified as an important factor for karst groundwater vulnerability assessment and resource management. A large scale hydrogeophysical experiment was undertaken over the last 4 years combining several surface-based geophysical methods and measuring techniques in a unique experimental site within the Fontaine de Vaucluse (FDV) hydrosystem in southern France. The aim was to apprehend the UZ structure and functioning of typical urgonian karstified mediterranean limestone. Geological structure is being well recognized by electrical, electromagnetic and seismic measurements. Magnetic Resonance Sounding (MRS) results clearly identify water presence and evidence seasonal variations within the karst UZ. Due to these results a conceptual hydrogeological model is proposed. On May-June 2014 a drilling campaign is being programmed and a cross-validation of the MRS results is also planned.

Lebel T.,LTHE | Parker D.J.,University of Leeds | Flamant C.,French National Center for Scientific Research | Bourles B.,LEGOS | And 9 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2010

AMMA - the African Monsoon Multidisciplinary Analysis - is the biggest programme of research into environment and climate ever attempted in Africa. AMMA has involved a comprehensive field experiment bringing together ocean, land and atmospheric measurements, on time-scales ranging from hourly and daily variability up to the changes in seasonal activity over a number of years. Many of the publications in this special issue make use of subsets of the AMMA measurements, collected from a diverse set of sensors. As a general introduction to the special issue, this paper provides a comprehensive overview of the AMMA observational programme, and summarises the scientific strategy which has defined the field deployment. The relationship between the existing observational monitoring networks of the region and the new sensors deployed for AMMA, and for the future, is described. Making use of regional and sub-regional maps, the main groups of sensors are described in terms of their deployment periods and their spatial co-ordination. The key linkages between different groups of measurements are also outlined, in terms of the strategy for their combined use and in terms of their interdependence. Some brief summaries of conditions sampled during the three years of the AMMA Extended Observing Period are also given. © 2009 Royal Meteorological Society and Crown Copyright.

Mahe G.,Mohammed V University | Lienou G.,University of Yaounde I | Bamba F.,University of Sfax | Paturel J.E.,CNRS HydroSciences Montpellier Laboratory | And 6 more authors.
IAHS-AISH Publication | Year: 2011

The Niger River basin covers 1.2 million km2 and extends over nine countries in West Africa. Its average discharge to the sea is about 6000 m3 s-1. The main upstream flood is produced during the 6-7 months of the tropical rainy season. The hydrological regime has been modified due to climatic and anthropogenic changes. There are only a few major dams on the River Niger, and future development plans will modify its regime and the flooded areas. The goal of this study is to provide a comprehensive overview of the hydrological changes of the Niger regime in its different sub-basins, in order to assess the impacts of future changes. The time series at Koulikoro and for the upstream basins show a high interannual flow variability since 1907, and a strong decrease since 1970. The runoff deficit in the Bani River after 1970 is greater than the rainfall deficit, due to a decrease in the groundwater level. NOAA images for 1990-2000 indicate that the flooded surfaces have decreased in the Inner Delta. The Sahelian flood from rivers in Burkina-Faso and Niger, and from the Sokoto River in Nigeria, has increased due to land degradation, despite the reduced rainfall. Now, more than half of the water of the Lower Niger comes from the Benue River. For the Sahelian basins, the increase in runoff can be closely related to the deforestation, but the dramatic decrease of the groundwater levels seems to be related to climate change. In the future, large changes will come from the proposed dams, which will reduce the flood peaks and the flooded surface areas, but will also have positive effects on the management of water resources. © 2011 IAHS Press.

Hamon M.,Mercator Ocean | Beuvier J.,Mercator Ocean | Beuvier J.,Meteo - France | Somot S.,Meteo - France | And 10 more authors.
Ocean Science | Year: 2016

The French research community in the Mediterranean Sea modeling and the French operational ocean forecasting center Mercator Océan have gathered their skill and expertise in physical oceanography, ocean modeling, atmospheric forcings and data assimilation to carry out a MEDiterranean sea ReanalYsiS (MEDRYS) at high resolution for the period 1992-2013. The ocean model used is NEMOMED12, a Mediterranean configuration of NEMO with a 1=12° (∼7 km) horizontal resolution and 75 vertical z levels with partial steps. At the surface, it is forced by a new atmospheric-forcing data set (ALDERA), coming from a dynamical downscaling of the ERA-Interim atmospheric reanalysis by the regional climate model ALADIN-Climate with a 12 km horizontal and 3 h temporal resolutions. This configuration is used to carry a 34-year hindcast simulation over the period 1979-2013 (NM12-FREE), which is the initial state of the reanalysis in October 1992. MEDRYS uses the existing Mercator Océan data assimilation system SAM2 that is based on a reduced-order Kalman filter with a threedimensional (3-D) multivariate modal decomposition of the forecast error. Altimeter data, satellite sea surface temperature (SST) and temperature and salinity vertical profiles are jointly assimilated. This paper describes the configuration we used to perform MEDRYS. We then validate the skills of the data assimilation system. It is shown that the data assimilation restores a good average temperature and salinity at intermediate layers compared to the hindcast. No particular biases are identified in the bottom layers. However, the reanalysis shows slight positive biases of 0.02 psu and 0.15 °C above 150m depth. In the validation stage, it is also shown that the assimilation allows one to better reproduce water, heat and salt transports through the Strait of Gibraltar. Finally, the ability of the reanalysis to represent the sea surface high-frequency variability is shown. © 2016 Author(s).

Lee J.H.,Polytechnic of Milan | Lee J.H.,CNRS Center for the Study of the Biosphere from Space | Pellarin T.,LTHE | Kerr Y.H.,CNRS Center for the Study of the Biosphere from Space
Agricultural and Forest Meteorology | Year: 2014

The application of Soil-Vegetation-Atmosphere-Transfer (SVAT) scheme into the estimation of soil moisture profile in semi-arid regions is largely constrained by a scarcity of spatially distributed soil and hydraulic property information. Especially, on a large scale in very dry and sandy soils or other extreme conditions, it is difficult to accurately map soil and hydraulic properties with soil maps-based Pedo-Transfer Functions (PTFs), because PTFs are usually semi-empirically defined for specific sites. One strategy to overcome this limitation is to employ satellite data for a purpose of calibration. This paper provides an operational framework of inverting the SVAT soil hydraulic variables from the deterministic ensemble Kalman filter (DEnKF) analysis of Soil Moisture and Ocean Salinity (SMOS) surface soil moisture product. This inverse calibration was first verified with the Analyses Multidisciplinaires de la Mousson Africaine (AMMA) super site data representative of a single grid cell (0.25°) of satellite data. At this local scale, the results demonstrated that the mis-estimation problems of soil surface variable C1 and equilibrium soil moisture θgeq were successfully solved after calibration, demonstrating a better agreement with the field measurement of soil moisture profile than the SMOS product and un-calibrated SVAT scheme using soil maps-based PTFs. On the meso scale, the calibrated SVAT scheme using inverted surface variables appropriately captured a non-linear relationship between surface and root zone soil moisture by showing a typical soil moisture profile in dry climates, where dry surface soil moisture is spatially consistent with rainfall events, but wet root zone soil moisture shows low correlations with surface soil moisture distributions and rainfall events. In contrast, the un-calibrated SVAT scheme using soil maps-based PTFs significantly overestimated surface soil moisture and rainfall effect. This approach suggests several operational merits in that there is no need to heavily rely on empirically defined PTFs or recalibrate land surface parameters for different land surface conditions, and this can be applied even when parameter measurements are unavailable or highly uncertain. © 2013.

Guyard H.,LTHE | Clement R.,IRSTEA | Oxarango L.,LTHE
Near Surface Geoscience 2012 | Year: 2012

Surface electrical resistivity tomography (ERT) has been used to monitor resistivity variation during infiltration process including leachate injection in Municipal Solid Waste (MSW) experiments with a relative success. Today, ERT inversion has been improved by some advanced procedures, but the impact of quadripoles acquisition time compared to the grown up rate of the affected infiltration surface is often evicted This paper highlight, by a classical numerical modelling approach, that distortions induced by the measurement may occur during ERT monitoring infiltration. This study was done on hypothetical resistivity model and on resistivity model derived from hydrodynamical modelling of a leachate injection experiment. We show that, even with advanced inversion procedure, some distortions on bulb geometry and resistivity artefact can exist. Even if complementary studies are necessary to characterize distortions created in various flow configurations, this study put forward that the ERT data acquisition sequence and electrode array should be carefully chosen to minimize distortions when studying very fast infiltration processes, as observed in leachate recirculating MSW landfills.

Hauet A.,Électricité de France | Buannic G.,Électricité de France | Antoine G.,Électricité de France | Jodeau M.,Électricité de France | And 2 more authors.
Proceedings of the International Conference on Fluvial Hydraulics, RIVER FLOW 2014 | Year: 2014

The Arc River (French Alps) undergoes consistent suspended sediment transport, therefore hydroelectric facilities are affected by high rates of sedimentation. To clarify suspended sediment transport in this river a tracing experiment has been set in March 2013. Flow discharge was near the minimum instream flow. A known volume of fluorescent tracer (Rhodamine Water Tracer) was injected (Dirac-type pulse) in the river and monitored over 7 km downstream using immerged field fluorometers located in 5 measurement sites. On every site, measurements show a clear pattern of tracer transport which shape evolves from upstream to downstream sites, showing the advection-dispersion process. Additional monitoring of the flow (with water level and surface radar velocity measurements) was done during the tracer experiment. Additionally, a 1D advection-dispersion and hydrodynamic numerical model (Mascaret Courlis, part of Telemac Mascaret System) was compared with in-situ measurements. Constant dispersion coefficients are tried out as well as different formulae depending on hydraulic parameters. The numerical model well reproduces the data. However, approximation due to the one dimensional approach limited the whole comparison with field data. Using, the measurements on both sides of the river bed, a discussion of the mixing dynamic is proposed. © 2014 Taylor & Francis Group, London.

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