What Ecohydrologic Separation Is and Where We Can Go With It

The ecohydrologic separation hypothesis challenged assumptions of translatory flow through the rooting zone. However, studies aiming to test ecohydrologic separation have largely done so with insufficient discussion of infiltration and rooting zone recharge processes and instead have mostly focused on either isotopic differences between stream water and plant water or the presence of fractionated isotope ratios in plant water. Based on extensively observed heterogeneities in soils and watersheds, we posit that differences in isotopic compositions of water in plants, streams, and other subsurface pools are expected in most scenarios. Interpretation of those plant-and-stream water isotopic differences is important, but diagnosing the role of any specific process is typically confounded by the diversity of potential mechanisms contributing to those isotopic differences. Thus, we should progress from simply describing their occurrences and refocus the discussion of ecohydrologic separation on how heterogeneous infiltration and root uptake processes lead to such differences. Consequently, we outline areas where plant and soil-water stable isotope data may be useful for advancing our understanding and representation of soil-water transport and plant-water recharge.