Hydrological budget of Lake Chad : assessment of lake-groundwater interaction by coupling Bayesian approach and chemical budget

International audienceEstimation of lake-groundwater interactions is a crucial step to constrain water balance of lacustrine and aquifersystems. Located in the Sahel, the Lake Chad is at the center of an endorheic basin of 2,5.106 km2. One of themost remarkable features of this terminal lake is that, despite the semi-arid context and high evaporation rates of thearea, its waters are fresh. It is proposed in the literature that the solutes are evacuated in the underlying quaternaryaquifer bearing witness to the importance of surface water and groundwater exchanges for the chemical regulationof the lake. The water balance of this system is still not fully understood. The respective roles of evaporation versusinfiltration into the quaternary aquifer are particularly under constrained.To assess lake-groundwater flows, we used the previous conceptual hydrological model of the lake Chad proposedby Bader et al. (Hydrological Sciences Journal, 2011). This model involves six parameters including infiltrationrate. A probabilistic inversion of parameters, based on an exploration of the parameters space through a Metropolisalgorithm (a Monte Carlo Markov Chain method), allows the construction of an a posteriori Probability DensityFunction of each parameter yielding to the best fits between observed lake levels and simulated. Then, a chemicalbudget of a conservative element, such as chloride, is introduced in the water balance model using the optimalparameters resulting from the Bayesian inverse approach.The model simulates lake level and chloride concentration variations of lake Chad from 1956 up to 2008. Simulated lake levels are in overall agreement with the observations, with a Nash-Sutcliffe efficiency coefficient above0.94 for all sets of parameters retained. The infiltration value, obtained by such probabilistic inversion approach,accounts for 120±20 mm/yr, representing 5% of the total outputs of the lake. However, simulated chloride concentrations are overestimated in comparison to the scarce measurements available over that period. As an example,the mean chloride concentration measured in the southern pool on a basis of our synthesis of existing chemicaldata since the 1970’s is approximately three time lower than the computed mean concentration. This may be dueto either the non-representativeness of our chemical dataset or overestimation of the evaporation rate that is fixedto 2000 mm/yr in our model.This study tackles the quantification of the lake water flows to the quaternary aquifer system and the associateduncertainties from a probabilistic point of view. This is an essential step to improve predictions of groundwaterresources in the Lake Chad Basin under climate change

Obstacles, levers and impacts of organic farming development in Camargue

We are presenting an analysis of the obstacles and levers for the development of organic cropping systems in Camargue, documented with a multicriteria analysis of scenarios of organic farming (OF) development. This communication is built using results from on-farm agronomic monitoring, stakeholders and farmers’ interviews and the use of models for integrated assessment of scenarios. At the farm level, the obstacles are related to identification of profitable cropping systems and rotations that include enough rice, conversion being therefore risky as impacting financial management and requiring a labour reorganisation. At the regional level, the constraints are related to the absence of advisory services for technical issues, and to the lack of coordination among the different stakeholders, to the low incentive of the public policies to convert, and to a relatively opaque organization of the supply chains. At the regional level, we analysed the consequences of two scenarios related to OF development on criteria such as the rice surface area, the quantity and quality of water, energy consumption or the employment generated. These results are expected to contribute to the definition of an action plan about OF development by the local stakeholders

Supercontinuum generation from 1.35 to 1.7 mu m by nanosecond pumping near the second zero-dispersion wavelength of a microstructured fiber

International audienceWe experimentally study a new regime for supercontinuum (SC) generation in the nanosecond pulsed regime using a nucrostructured optical fiber with two zero-dispersion wavelengths (ZDWs). Pumping at 1535 nm around the second ZDW yields a nearly flat SC over 1350-1700 nm. The interplay between the effects of modulation instability and stimulated Raman scattering are described through simple phase-matching relations

11th Applied isotope geochemistery conference AIG-11

36Cl measurements in groundwaters of the deep confined aquifer of the Lake Chad Basin (LCB) were performed in order to constrain groundwater geochemical ages and residence times. Twenty-seven wells were sampled in Nigeria, Niger and Chad in the southern parts of the large (700 000 km2) multilayered aquifer of the LCB. 36Cl/Cl values range between 11±1.10-15 to 148±8.10-15 at/at. The highest ratios are observed near the recharge zone of the Nigerian part of the Continental Terminal aquifer, while the lowest ones are found in wells located near the southern fringe of the present-day lake Chad. Chloride concentrations are low (below 100 mg/l) and not correlated to the 36Cl/Cl values, indicating negligible dissolution of evaporites in most samples. Reliable 36Cl ages can be calculated along the different flow paths investigated, suggesting residence times of the deep groundwaters larger than 300 000 years. These results are consistent with new AMS-14C data below the detection limit but are in contradiction with previous 14C data obtained in the area

Les nematicides et la microflore des sols

National audienc

Effects of pesticides on the soil microflora

International audienc