Pascal Viennot

Multi-model comparison of a major flood in the groundwater-fed basin of the Somme River (France)

The Somme River Basin is located above a chalk aquifer and the discharge of the somme River is highly influenced by groundwater inflow (90% of river discharge is baseflow). In 2001, the Somme River Basin suffered from a major flood causing damages estimated to 100 million euro (Deneux and Martin , 2001). The purpose of the present research is to evaluate the ability of four hydrologic models to reproduce flood events in the Somme River Basin over an 18-year period, by comparison with observed river discharge and piezometric level as well as satellite-derived extents of flooded area. The models used differ in their computation of surface water budget and in their representation of saturated and unsaturated zones. One model needed structural modification to be able to accurately simulate the riverflows of the Somme river. The models obtained fair to good simulations of the observed piezometric levels, but they all overestimate the piezometric level after flooding, possibly because of a simplistic representation of deep unsaturated flow. Models differ in their annual partition of the infiltration of water within the root zone (mostly driven by simulated evapotranspiration), but these differences are attenuated by water transfers within the saturated and unsaturated zone. As a consequence, the inter-model dispersion of the computed annual baseflow is reduced. The aquifer overflow areas simulated during flooding compare well with local data and satellite images. The models showed that this overflow occurs almost every year in the same areas (in floodplain), and that Correspondence to: F. Habets florence.habets@mines-paristech.fr the flooding of 2001 was characterized by an increase in the quantity of the overflow and not much by a spreading of the overflow areas. Inconsistencies between river discharge and piezometric levels suggest that further investigation are needed to estimate the relative influence of unsaturated and saturated zones on the hydrodynamics of the Somme River Basin.

Estimating the Marginal Social Value of Agriculturally Driven Nitrate Concentrations in an Aquifer: A Combined Theoretical-Applied Approach

We combine a theoretical model and a quantitative modeling chain based on a bio-economic model and a hydrological model in order to assess the marginal damage related to the nitrate concentration in an aquifer. The fundamental concept is to take the steady state level resulting from a social planner’s optimization program as the target level of nitrate concentration. The interest of doing this is three-fold: (i) we characterize the social value of damage related to the targeted nitrate concentration; which (ii) leads us to design the optimal path consistent with the target; and (iii) we can in turn assess welfare losses arising when the tax path deviates from the optimal one.