The extent to which soil hydraulics can explain ecohydrological separation

The degree of water mixing in the critical zone is under intense debate. Field measurements of isotope ratios indicate varying degrees of separation between pools of water that supply streams and vegetation. The exact physical mechanisms behind ecohydrologic separation are unknown, but local conditions such as soil heterogeneities likely influence the extent of mixing and separation of subsurface water pools. Using a well-established soil physics model, we simulated if isotopic separations occur within 650 distinct configurations of soil properties, climatologies, and mobile/immobile soil-water domains. Simulations demonstrated separations in isotope ratios between storage and drainage waters during periods of high precipitation, soil water content, and drainage. Separations grew with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Across soil types and climates, lower saturated hydraulic conductivity and higher rainfall rates amplified separations. These results illustrate how separate domains of mobile and immobile water can physically result in subsurface separation of water pools. We show how different critical-zone solute fluxes can be generated by contrasting transport dynamics in distinct domains across a range of soils and climate conditions.

Impact/Purpose

Contaminants moving through the soil profile represent a major pathway that contaminants enter groundwater and surface water.  Our ability to predict contaminant transport depends on accurate simulation of transport through the soil, particularly the unsaturated zone known as the vadose zone.  Previous research has shown that soil water mixing in the vadose zone is much more complex than most models depict, with pool separation between water moving rapidly and water held tightly in the soil matrix.  Here, we use a well-established soil physics model Hydrus 1D to test if the dual porosity mode in the model is sufficient to simulate the observed separation across 650 distinct configurations of soil properties, climatologies, and mobile/immobile soil-water domains.  This work is vital to improving modeling of contaminant transport within soils.

Citation

Finkenbiner, C., S. Good, J. Renee Brooks, S. Allen, AND S. Sasidharan. The extent to which soil hydraulics can explain ecohydrological separation. Nature Publishing Group, London, UK, 13:6492, (2022). [DOI: 10.1038/s41467-022-34215-7]

DOI: The extent to which soil hydraulics can explain ecohydrological separation