D/H isotope ratios in the global hydrologic cycle constrain the partitioning of global terrestrial water fluxes.

Abstract:

Deuterium to hydrogen (D/H) ratios in Earth’s hydrologic cycle have long served as important tracers of biosphere-atmosphere exchange, yet the global HDO budget remains poorly constrained because of uncertainties in the isotopic compositions of continental evapotranspiration and runoff. Through synthesis of ship-based observation of marine water vapor collected from the world oceans, we bias-correct satellite retrievals of HDO and H2O concentrations from the Tropospheric Emissions Spectrometer to resolve the global HDO budget. This budget provides a global baseline for geochemically enabled Earth system models, demonstrates patterns in entrainment of moisture into the marine surface layer, and constrains the isotopic composition of continental output fluxes critical for global ecohydrologic investigations. Based on the composition global continental runoff and evapotranspiration, we quantify the magnitude of hydrologic flux sub-components of transpiration, soil evaporation, surface water evaporation, and interception that are consistent with the global flux isotope ratios. We also investigate the hydrologic connectivity of bound, plant available soil waters with more mobile surface waters. Our results indicate that globally, transpiration is 64 ± 13% (mean ± 1 standard deviation) of evapotranspiration, and 65 ± 26% of evaporation originates from soils and not surface waters. We estimate that 38 ± 28% of surface water is derived from the plant-accessed soil water pool. This limited connectivity between soil and surface waters fundamentally structures the physical and biogeochemical interactions of water transiting through catchments.