Observability limits in isotope ecohydrology

Stable isotopes of water link precipitation, soil water, xylem water, streamflow and groundwater in a common tracer space, but the path from measurement to hydrological interpretation contains several steps. Evaporation, mixing and storage can alter the isotope signal; extraction and spectroscopy affect its measurement; and evaporation-line back-projection is then used to infer candidate source waters. Using the cryogenic vacuum extraction (CVE) debate and published Evaristo source data, we illustrate how correction magnitude and reconstruction geometry jointly affect whether reconstructed plant-source water is closer to reconstructed soil-source water or groundwater. We frame this as an observability problem: whether the measurement-reconstruction chain preserves enough information to support a specific hydrological interpretation.