Improvement of the solute transfer in a conceptual unsaturated zone scheme: a case study of the Seine River basin

Abstract

Abstract:For predicting the evolution of solute concentrations in groundwater and testing the impact of remediation policies, a coupling between the agronomical model STICS and the hydrogeological model MODCOU was implemented. When applied to the Seine River basin, this model accurately represents the temporal evolution of average nitrate concentrations in the aquifer, but with large local errors.We propose an improvement to the simple unsaturated zone (UZ) scheme NonsatSW used in STICS-MODCOU. The modifications are based on a comparison with the mechanistic model Metis considered as a reference as it solves Richards' equation. A more realistic saturation profile and a varying percolation rate are integrated in NonsatSW. This new model, named NonsatVG, is assessed by comparing it with NonsatSW and Metis. In an ideal case, NonsatVG generates a solute transfer and a dispersion closer to that of Metis than of NonsatSW. In real cases, without additional calibration, NonsatVG and Metis simulate better the average transfer velocities of the observed nitrate profiles.Furthermore, modifications in NonsatVG give a direct relationship between the depth of the water table and the saturation profile. We obtain, therefore, as in Metis, an evolution of the solute transfer velocity depending on the piezometric level. These dynamics are not simulated in NonsatSW.Despite a modified water transfer through the UZ, NonsatVG is also as valid as NonsatSW in the modelling of water transfer to the saturated zone.Finally, an application to the Seine basin shows that solute transfer velocities are lower with NonsatVG than with NonsatSW, but are in better agreement with literature.

Influence of multidecadal variability on high and low flows: the case of the Seine basin

“…The exchanges between groundwater and rivers are computed daily based on the gradient head between the aquifer and the river, and a transfer coefficient that accounts for the river bed characteristic (see Vergnes and Habets, 2018, for details. The water transfers in the unsaturated zone are improved (Philippe et al, 2011), which may be important considering that the unsaturated zone can reach 60 m in the Seine basin, implying a delay in the groundwater recharge. Finally, Aqui-FR uses a six-layer representation of the aquifers (Viennot et al, 2009; http://www.geosciences.…”

Assessing the water balance of the Upper Rhine Graben hydrosystem

“…However, these discrepancies may be linked to the fact that the models were 72 limited to the alluvial plain, and used simple methods to assess the subsurface flows from the 73 In this section, the models are presented briefly, with a special focus on MODCOU which is 192 mainly used in this study. The direct infiltration of water from effective rainfall through the unsaturated zone is 205 simulated with a Nash cascade (Philippe et al, 2010). 206…”

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

“…Estimates areτ = 0.02 y −1 andβ = 20 which means that around 2 % of the water and solute in the aquifer are renewed each year, and that there is a 20-year time lag before the pollution reaches the free part of the Chalk aquifer (Philippe et al, 2011).…”