An isotope-enabled modeling approach to track snowmelt and groundwater contribution to runoff and root water uptake in a snow dominated mountainous catchment

The snow dominated headwaters of the Colorado River are crucial for the water supply of the south-western US. The current water crisis in the Colorado basin makes understanding runoff processes in mountainous regions more necessary than ever. We present how our observations of stable isotopes of water (²H and ¹⁸O) in the precipitation, stream-, soil-, xylem-, and groundwater at the East River in the upper Colorado River, combined with multiple hydrometric datasets since 2014 (multi-location stream gauging, groundwater levels, soil moisture snow water equivalent, and eddy-covariance fluxes), can be used to rigourously contrain and evaluate an ecohydrological modelling tool to then identify the time and location of snowmelt and groundwater subsidies to runoff and plant water use. To this end, we deployed a new version of the spatially-distributed, process-based model EcH₂O-iso, with a multi-objective model-data fusion procedure. The simulations notably underline the dominant role of snowmelt as a main driver of runoff generation, through its direct contribution to runoff peak during the late spring snowmelt, and to the groundwater recharge that eventually feeds the significant baseflow contribution in this catchment. Our analysis further explores the use of water ages and numerical tracers to better disentangle these cross-seasons carry-over of water between critical zone compartments.