Water Oxygen Isotope Thermometer in Headwaters: Indicating Groundwater Recharge and Peatland Water Dependence on Air Temperature

Stable isotopes in precipitation, which fractionate with air temperature, serve as an isotope thermometer and offer valuable insights into hydrological processes. This study explores the application of this concept in a small mountainous watershed in the Pyrenees (France), where isotopic data from precipitation and streamflow were monitored over 8 years. In precipitation, a clear positive correlation between stable isotopic values in precipitation waters and air temperature was observed. In stream water, isotopic variability is primarily driven by the seasonal mixes of warm summer and cold winter waters (which transfers can be delayed by the ground), with a lesser influence from snowmelt fractionation. The annual averages isotopic composition of stream water reflect the air temperature at the time of precipitation, showing a bias toward winter conditions-periods of high streamflow-resulting in lower isotope-inferred temperatures compared to annual averages air temperature. These isotope-inferred temperatures follow linear relationships with key hydrological variables, including groundwater recharge and peatland water table depth. Notably, a 1°C increase in air temperature at the time of precipitation, corresponds to a 40% reduction in groundwater recharge efficiency and a 3 cm drop in peatland water table depth. The method identified to correct this bias provides a framework for understanding the impacts of climate variability on water resources, useful for paleohydrology and hydrogeology, particularly in mountainous regions where evapotranspiration and fractionation are limited. The isotope-inferred temperature records initial hydrological conditions and clarifies the link between upstream weather conditions and downstream hydrolo(geo) gical compartments along the atmosphere-surface-groundwater continuum of the critical zone for a better understanding of the water cycle.