Dr. Nicolas Flipo
Researcher MINES ParisTech | ParisTech · Centre de Géosciences
Coupled hydrological-hydrogeological models, emphasizing the importance of the stream–aquifer interface, are more and more used in hydrological sciences for pluridisciplinary studies aiming at investigating environmental issues. They are of peculiar interest for biogeochemists who try to assess the elimination of biogenic compounds in such interfaces. If ecohydrologists do consider biogeochemical processes in sediments, it remains a challenge to quantify the functioning of the hyporheic buffer which controls the fate of biogenic compounds. In this presentation, I first illustrate the importance of the sediment compartment for the in-river carbon fate in the Seine River downstream Paris urban area using the hydro-ecological model ProSe. Far from being negligible as usually expected for large rivers, the sediments contribute for 30% to the total respiration of the system (Vilmin et al. 2016). Those estimates rely on a well calibrated model, and especially sediment fluxes along the 220 km simulated river stretch (Vilmin et al. 2015). The hyporheic zone also plays a significant role in river metabolism and greenhouse gas emission. In a collaborative work, we also recently estimated that the production and emission towards the river of end products such as CO2 are driven and controlled by the connection status (gaining or loosing) and the frequency of flow reversal due to hydrological events (Newcomer et al. 2018). It is therefore crucial to develop tools and methodologies to assess water fluxes between surface and subsurface (Flipo et al. 2014).
At the regional scale, data from river discharge gauging stations and piezometer networks are usually used to calibrate models. One of the major challenges to estimate the low-frequency component of stream-aquifer exchanges is to define the aquifer recharge properly. An original 2-step calibration procedure based on a proper hydrograph separation is presented for the Seine basin (75 000 km2). Once the aquifer recharge is estimated, the influence of high frequency processes, such as in-river water level fluctuations due to hydrological events, on stream aquifer exchanges is illustrated on the Loire basin (120 000 km2) (Baratelli et al. 2016). To achieve a proper calibration of the conductance coefficient at regional scale, a relationship is established with the aquifer properties (Pryet et al. 2015).
Albeit the conductance model is calibrated at the regional scale based on the horizontal transmissivity of the underlying aquifer, such a simplified 1D vertical coefficient is more difficult to assess at the watershed scale, where stream bed heterogeneities exerts a major control on stream-aquifer exchanges. Following the holistic view of water fluxes within a hydrosystem, an innovative multi-scale sampling system was developed on a 46 km2-watershed. The system consists in distributed measurements of water pressure and temperature of the equivalent porous media (Mouhri et al. 2013). We also developed a 1D sampling system, which allows the identification of hydro-thermal properties of the hyporheic zone based on a Bayesian inversion procedure (Cucchi et al. 2017). This method provides a good estimate of the hyporheic zone which reduces the number of parameters to identify at the stream-aquifer interface. It is then possible to determine the 2D hydro-thermo properties with a screening approach. Finally based on nested modeling it is possible to upscale the related conductance value at the regional scale (Cousquer et al. 2017) and to perform a coupled simulation of the watershed, as illustrated for the sampled basin.