Spatially distributed tracer-aided modelling to explore DOC dynamics, hot spots and hot moments in a tropical mountain catchment

Tracer-aided rainfall-runoff modelling is a promising tool for understanding catchment hydrology, particularly when tracers provide information about coupled hydrological-biogeochemical processes. Such models allow for predicting the quality and quantity of water under changing climatic and anthropogenic conditions. Here, we present the Spatially-distributed Tracer-Aided Rainfall-Runoff model with a coupled biogeochemical reactive tracer module (STARR-DOC) to simulate dissolved organic carbon (DOC) dynamics and sources. The STARR-DOC model was developed and tested for a humid high Andean ecosystem (páramo) using high-resolution hourly DOC and hydrometeorological data to simulate hourly discharge and DOC at a fine spatial (10 × 10 m) resolution. Overall, the model was able to acceptably reproduce discharge (KGE ~ 0.45) and stream DOC (KGE ~ 0.69) dynamics. Spatially distributed DOC simulations were independently compared using point DOC measurements for different soil types across the catchment, which allowed for identifying DOC production hot spots and hot moments. Results showed higher hydrological connectivity between slopes and valleys with increasing precipitation. Wetter conditions also favoured DOC production (wet month = 82 mg L⁻¹, dry month = 5 mg L⁻¹) and transport to the stream network (DOC concentrations: during events ~15 mg L⁻¹, during baseflows ~4 mg L⁻¹). Our results also suggest that minor changes in meteorological conditions directly affect páramo soil water dynamics and biogeochemistry. Knowledge of when and where DOC production in mountain catchments is greatest is important for water managers to understand when they make decisions about water security, especially considering climate change predictions for the Andean region.