The isotopic evolution of a raindrop through the critical zone

Abstract:

The use of stable isotopes of water (O18 and D) to determine the pathway of water through a hilslope, and source water for vegetation generally assumes that isotopic composition is conserved through the shallow evaporative front. Frequent sampling through the entire critical zone at Rivendell, a 32° hillslope in a mixed conifer forest, reveals a structured heterogeneity in the isotopic composition of subsurface water. We demonstrate that the moisture held in the soil, saprolite, and weathered bedrock is isotopically light relative to both the average meteoric water, and to the mobile water in the shallow subsurface and saturated zone. Weathered argillite, the dominant rock type, retains moisture that is isotopically more negative than neighboring sandstone. These differences in isotopic composition are persistent, suggesting subsurface fractionation and/or filtration processes. Different species of vegetation collocated on the same hillslope use different subsurface reservoirs. Throughout the year, Douglas-fir xylem water occupies a region of dual isotope space that differs from hardwoods (madrone, live oak, and tanoak) Whereas Douglas-firs use non-evaporatively enriched, deep bulk soil moisture, and unsaturated zone rock moisture throughout year, hardwoods switch their source water from shallow mobile water, to bulk soil moisture, to unsaturated zone rock moisture depending on subsurface water availability.
Furthermore, Douglas-fir roots transport water that is more negative than collocated madrone roots. At no time do trees use groundwater. Collectively, these discoveries suggest that a deep and frequent sampling campaign is required to capture the structured heterogeneity in the critical zone, as well as the species-specific and seasonal variability of vegetative water use.