Isotopic 'fingerprinting' of distinct water reservoirs in the critical zone and their exploitation by different tree species

Channel incision, surface erosion, and drainage of landscapes leads to emergent hillslopes underlain by varying degrees of soil development and weathered bedrock. Vertical and lateral differentiation by weathering leads to a structured heterogeneity of soil and rock properties that affect water pathways, moisture retention, and moisture availability to plants. Near surface evaporative losses cause seasonal isotopic enrichment of soil water, but we asked, what is the isotopic pattern of the deeper moisture and how important is it as a water source to the vegetation? Here, we report on the use of stable isotopic data to 'fingerprint' and then characterize the structure of subsurface water reservoirs that lie beneath an old-growth forested hillslope in the Eel River basin of northern California. The site, Rivendell, a 4000 m2 heavily instrumented, unchannled basin draining to Elder Creek 0.5 km upstream from the confluence with the South Fork Eel River, is composed of turbidite sequences of vertically dipping mudstone and minor sandstone. Soils are thin (0-60 cm), and are underlain by a 1-4 m thick saprolite layer, and a 3-20 m thick weathered rock zone. Annual precipitation of 1900 mm, predominantly from October to May, raises the water table from summer lows of 5-28 m below the ground to winter highs of 2.5 m below the ground, with depth to the water table increasing with upslope distance from Elder Creek. Whereas the north-facing slope has a dominant forest canopy composed of Douglas-fir (Pseudotsuga menziesii), and a mid-story of hardwood species (Quercus agrifolia, Umbellularia californica, Notholithocarpus densiflorus, Arbutus menziesii, and the riparian tree, Acer macrophyllum), the south-facing slope, which drains directly to the South Fork Eel River, is primarily hardwood. Intensive sampling over 20 months reveals subsurface moisture reservoirs in the mobile and bulk water of the soil, saprolite, and weathered rock, with persistent and distinct isotopic signatures (values). Well water is nearly temporally invariant isotopically despite a wide range of meteoric water inputs. Mobile water, which can pass very quickly to the wells, is consistently isotopically enriched compared to water bound in the soil, saprolite, or weathered rock. The degree of persistence of this isotope signal, and the clear association of water transit time to the well (e.g. more enrichment with slower transit times), suggests equilibrium fractionation of water during infiltration. Isotope analyses of root water shows that Douglas-fir trees use a narrow range of sub-surface water reservoirs, primarily from the shallow saprolite, and show little to no seasonal isotopic variance. Adjacent hardwood species used a much broader range of water sources; shallow and mobile water in the winter, shallow soil water in the early summer, and deep soil and shallow saprolite water in the late summer. These observations suggest that geomorphic processes control a subsurface organization of reservoirs that in turn affects the flow path and chemistry of runoff, the degree to which water is retained in the unsaturated zone, and the accessibility of those water reservoirs to different tree species that will in turn transpire water from different reservoirs back to the atmosphere.