What is the Stable Isotope Moisture Record Generated in Soils and Shallow Saprolite Across a Seasonal Wet-up and Dry-down Cycle in a Mediterranean Climate?

Abstract

In a Mediterranean climate, subsurface moisture cycles through a winter wet-up followed by an extended period of summer dry-down during which evaporation, root uptake, and some progressive drainage reduce plant-available moisture to an end-of-summer residual level. The offset between the timing of precipitation supply and evaporative demand presents an ideal opportunity to document the isotopic evolution of plant-available and residual moisture. Key questions include: 1) Is plant-available moisture at the start of the dry season biased towards early winter rains, end of wet season rains, or some predictable integral of the entire wet season rain, 2) What controls the isotopic evolution of the residual moisture (does it exchange with successive rains or form a multi-year stable value), and 3) Other than evaporative enrichment, are there other fractionation processes influencing moisture isotope evolution in the subsurface? These questions are central to interpreting plant-water source and use, transit and residence time of water, and the controls on the isotopic signature of runoff waters. Here we report the results of 15 successive sampling campaigns at five locations across an intensively monitored hillslope in the Eel River Critical Zone Observatory (northern California Coast Range) over a one-year period. The site is mantled by an old-growth mixed evergreen forest of Douglas fir (Pseudotsuga menziesii) and various hardwoods. Profiles to depths of 1.5 m penetrated through variably thick soil into underlying saprolite developed in the argillitic bedrock. The five sampling locations were located near active sampling of tree cores and monitoring of moisture and isotopes down to the ground water (~17 m below the surface). The samples have been collected during the current second year of a severe drought, but the rainfall this year of ~ 1000 mm still greatly exceeded the typical plant-available soil moisture content at the start of the dry season of 80 mm, and runoff in the winter and summer was generated from the recharged groundwater. Results to date include the observation that late summer 2020 sampling documented large variability in successive 10 cm sample profiles in the residual moisture deuterium concentration. This may help explain the variability found in successive tree core samples in individual trees.