Comparative calibration of 1D+2D and 3D hydrogeological watershed models

In this work, we study the calibration of the parameters of a hydrogeological watershed model by comparing a 1D+2D approach that combines unsaturated 1D columns and a saturated 2D model, with a full 3D approach. In a first step, a heterogeneous permeability field is estimated by an inversion procedure for each model (2D saturated and 3D unsaturated). The fields obtained are similar but the calculation time is obviously much higher in the case of the 3D model: the 2D model seems therefore sufficient and more efficient to evaluate permeability fields using piezometric measurements in the case of vertically homogeneous aquifers. The second step focuses on the calibration of the hydraulic parameters by adjusting the hydraulic heights either derived from a 1D+2D reference model at several fictitious points distributed over the entire domain, or measured in a dozen real piezometers. Both approaches provide a good fit to the piezometric measurements, but the parameter values differ significantly: the van Genuchten alpha coefficient is unrealistic in the 1D+2D approach, reflecting a poorer consideration of the modeling unsaturated zone, while the porosity value is higher in the 3D approach, which can probably be remedied by developing a more suitable cost function.