Transport and Water Age Dynamics in Soils: A Comparative Study of Spatially Integrated and Spatially Explicit Models

We address transport by transit times (i.e., the age of water parcels leaving a storage as discharge, deep loss, or evapotranspiration) in subvertical soil systems, key to our understanding of water quality in catchments and streams. While the use of field and lysimeter observations to constrain and validate modeling approaches is generally accepted, different views exist on the relative ranges of applicability of spatially integrated or spatially explicit approaches. This study specifically illustrates how one class of spatially integrated models of transport, based on StorAge Selection (SAS) functions, fares with respect to spatially explicit hydrologic models in a case where detailed tracer data from experimental lysimeters exist and for which both approaches are viable. Data from two lysimeters experiments that differ in atmospheric conditions, size of the installation, tracer type, soil texture, and vegetation are used to contrast results from two transport models: tran-SAS (space implicit) and HYDRUS-1D (space explicit). Results suggest that although the two lysimeters are characterized by different transit time distributions, their underlying transport mechanisms are similar and represented well by both models. The comparison between the two models results in robust estimates of the transport timescales and clearly shows that percolation fluxes at the bottom of a lysimeter tend to drain the relatively old components of the soil storage. We conclude that the convergence of the approaches in a geomorphologically simple and data-rich problem supports extensive uses of the spatially integrated approach in cases where the scale of the problem, or subgrid parameterization needs, may limit the applicability of detailed modeling approaches.