The physical basis for ecohydrologic separation: the roles of soil hydraulics and climate

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 isotopic differences, illustrating how mobile and immobile domains interact with local conditions to physically result in subsurface separations. These results show how different critical-zone solute fluxes can be generated by representing contrasting transport dynamics in distinct domains across a range of soils and climate conditions.