Update of a regional groundwater model using an ensemble-based approach

A groundwater model of the Tarn-et-Garonne department in southern France was updated and recalibrated using an ensemble-based approach to support short- to medium-term groundwater level forecasting. The model uses the MARTHE finite-difference groundwater modeling software (developed by the BRGM) to simulate groundwater flow, stream-aquifer interactions and groundwater and surface water withdrawals in a Quaternary alluvial aquifer system.  The model was originally developed to support local groundwater management and define allowable groundwater abstraction volumes. It was later coupled with the SURFEX land surface model (developed by the CNRM) and integrated into Aqui-FR, a French hydrometeorological modeling platform that provides groundwater level forecasts at a national scale. The model was last calibrated using data through 2015 and a trial-and-error approach. The update incorporated ten additional years of groundwater level, stream flow and pumping data, along with the latest recharge and surface runoff estimates generated by the SURFEX model. History matching then was performed using an iterative ensemble smoother to generate an ensemble of posterior parameter realizations that honor both expert knowledge and observed groundwater levels and stream flows. Using the posterior parameter ensemble, the uncertainty in various predictions of interest, including groundwater levels and standardized piezometric level indices, was evaluated at multiple locations across the study area. All analyses were implemented using a fully scripted workflow to facilitate future model updates and deployment of the workflow in other areas.