Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion

There is increasing interest in the use of multiple measurement types, including indirect (geophysical) methods, to constrain hydrologic interpretations. To date, most examples integrating geophysical measurements in hydrology have followed a three-step, uncoupled inverse approach. This approach begins with independent geophysical inversion to infer the spatial and/or temporal distribution of a geophysical property (e.g. electrical conductivity). The geophysical property is then converted to a hydrologic property (e.g. water content) through a petrophysical relation. The inferred hydrologic property is then used either independently or together with direct hydrologic observations to constrain a hydrologic inversion. We present an alternative approach, coupled inversion, which relies on direct coupling of hydrologic models and geophysical models during inversion. We compare the abilities of coupled and uncoupled inversion using a synthetic example where surface-based electrical conductivity surveys are used to monitor one-dimensional infiltration and redistribution

Vadose zone model-data fusion: State of the art and future challenges

Models are quantitative formulations of assumptions regarding key physical processes, their mathematical representations, and site-specific relevant properties at a particular scale of analysis. Models are fused with data in a two-way process that uses information contained in observational data to refine models and the context provided by models to improve information extraction from observational data. This process of model-data fusion leads to improved understanding of hydrological processes by providing improved estimates of parameters, fluxes, and states of the vadose zone system of interest, as well as of the associated uncertainties of these values. Notwithstanding recent progress, there are still numerous challenges associated with model-data fusion, including: (i) dealing with the increasing complexity of models, (ii) considering new and typically indirect measurements, and (iii) quantifying uncertainty. This special section presents nine contributions that address the state of the art of model-data fusion

Near-surface solute redistribution during evaporation

We present results from solute transport experiments in an evaporating composite porous medium consisting of a cylindrical inner core with coarse sand that was surrounded by a mantle with fine sand. Small volumes of dye and salt tracer were applied at the surface of the fine material of the evaporating column. The pressure head at the bottom boundary was kept constant using a hanging water table ensuring liquid phase continuity to top surface in both fine and coarse material, whereby the latter was hydraulically less conductive at these pressure conditions. Contrary to the expectation that solute accumulation at an evaporating surface is proportional to local cumulative evaporation, high concentration spots developed at the surface of the coarse material, for which IR surface temperature measurements did not indicate higher evaporation fluxes. 3D unsaturated flow and transport simulations and a second tracer experiment monitored with magnetic resonance imaging (MRI) demonstrated that preferential upward water flux in the fine sand deeper in the column and near-surface lateral water flow from the fine into the coarse sand in combination with a downward diffusive flux are responsible for the local solute accumulation. We propose that at the wet regions of a soil surface, solute accumulation is largely decoupled from local evaporation fluxes and strongly governed by relative differences of the hydraulic conductivities. The possible formation of high solute concentration spots at the surface of coarser regions usually representing preferential flow pathways during strong precipitation may have an accelerating effect on the leaching of solutes. Citation: Bechtold, M., S. Haber-Pohlmeier, J. Vanderborght, A. Pohlmeier, T. P. A. Ferre, and H. Vereecken (2011), Near-surface solute redistribution during evaporation, Geophys. Res. Lett., 38, L17404, doi:10.1029/2011GL048147