The stable isotope signature of continental runoff

Recent international programmes such as the IAEA coordinated research project on 'Isotope tracing of hydrological processes in large river basins' have increased scientific awareness of the potential value of incorporating isotope tracers in large-scale water cycling studies to trace water origin and residence times, snowmelt processes, surface-groundwater exchange, evaporation-transpiration partitioning, precipitation variability, and climate/land use changes. Although isotope techniques have been widely tested and are operationally applied at the small catchment-scale, theoretical development and testing of isotope-mass balance approaches at the continental scale are still in progress. This paper proposes a theoretical basis for predicting the long-term (interannual) signatures of oxygen-18 and deuterium in continental runoff applicable for describing global isotope variability in the mean flux-weighted discharge of large rivers. For long time periods, the mass and isotope balances for the oceans and continents are given. Changes in atmospheric moisture are expected due to admixture of evaporated and transpired moisture to air masses over the continents. Oceanic moisture in coastal areas is shown as which becomes subsequently modified as it moves across the continent to inland areas (ranging from to). The approximate isotope compositions of components. Long-term differences in precipitation arise from Rayleigh-type fractionation of atmospheric moisture and precipitation during rainout over the continents. Transpired moisture is expected to be similar to precipitation or groundwater recharge for a given location, whereas evaporated moisture will be isotopically depleted (plotting on or above the MWL) and river discharge will tend to be isotopically enriched (plotting on or below the MWL). Shown are specific hydroclimate forcings on the 2H-18O isotopic composition including those related to long-term precipitation input signals (temperature, latitude, altitude, distance from ocean source), seasonal signals (temperature, monsoon cycles, moisture sources, glacial melt), bias in recharge due to selection of high-precipitation or thaw-season events, and seasonal oscillations in evaporative enrichment from the river or contributing sources (soil water, lakes, reservoirs, wetlands, etc.). In addition to monitoring of volumetric fluxes, the characterization of long-term and seasonal isotope signals is expected to provide additional insight into hydroclimatic changes in each hydrologic regime. In some cases the primary water balance signals in precipitation may be more preserved in river discharge whereas in arid or seasonally arid zones the selection and evaporation signals may make the differences more pronounced