Dual-Isotope Eddy Covariance Measurements: Insights and Challenges in Ecosystem Water Flux Measurements Over Winter Wheat in Central Germany

Dual-isotope eddy covariance measurements offer a novel approach for studying water fluxes in ecosystems, providing detailed insights into evapotranspiration (ET) and its components, evaporation (E) and transpiration (T). During the 2024 growing season, a dual-isotope eddy covariance system was deployed over a winter wheat cropland in central Germany, integrating a Los Gatos Research (LGR) Water Vapor Isotope Analyzer with a conventional eddy covariance setup. This system continuously measured isotopic fluxes (δD and δ¹⁸O) alongside water vapor, carbon dioxide, and energy fluxes at high temporal resolution. These measurements were supplemented by soil water profiles, biometeorological observations, and vegetation indices.

The isotopic flux data revealed diurnal and seasonal dynamics of water vapor isotopes, linked to environmental drivers such as vapor pressure deficit, soil moisture, and crop phenology. Preliminary results show a diurnal cycle of isotope fluxes of ET, characterized by isotopic enrichment during the middle of the day, with δ¹⁸O_ET reaching -12‰ and δD_ET reaching -110‰ (both against VSMOW). The results suggest that transpiration dominates ET during peak growth stages, while evaporation increases following precipitation events or during early crop development.

Key challenges include correcting for high-frequency dampening effects and addressing the analyzer’s sensitivity to water vapor concentration under different conditions, particularly during low-flux periods. Despite these challenges, dual-isotope techniques give valuable insights into crop water use strategies and responses to environmental drivers, offer the opportunity for isotope-based flux partitioning, and give a unique dataset for validating isotope-enabled land surface models.