Tracing hydrological connectivity and biogeochemical interactions across scales through isotope-enabled water quality modelling frameworks

As droughts become more common, they affect the local availability of water, but also alter the quality and ecology of streams. The complex interactions between landcover change, hydrological partitioning and water availability as well as water quality are difficult to quantify, especially at different temporal and spatial scales. Tracers can help test and constrain hydrological models and reveal new insights into the relationships between water fluxes, storage and ages. Here, we present insights from integrated isotope and water quality monitoring and modelling approaches on pathways, transformations and catchment responses. We coupled stable water isotopes into a water quality modelling framework to simulate 30-years of NO₃-N dynamics. The isotope-aided model effectively constrained hydrological processes and mapped the (dis)connection of different flow paths involved in NO₃-N transport. We use such tracer-aided modelling framework to investigate, quantify and visualise ecohydrological fluxes and dynamics of water storage, pathways and ages across different scales as well as the effects of connectivity between landscapes and riverscapes. Results also highlight the role of transient hydrological states in nutrient cycling on top of landscape characteristics. Hydrological connectivity controls N transformations by regulating soil moisture and determining available NO₃-N for processing from upstream inflows. Hydrological pathways determine where, when, and which NO^­₃-N storages are connected. At our groundwater-dominated study catchment, subsurface flows are the primary pathway, but transition to near-surface flow occurred in specific riparian “hot spots” due to development of soil saturation along flow paths, resulting in flashy stream NO₃-N peaks. Our findings underscore the necessity of considering hydrological connectivity in nutrient modelling and management planning, which can be revealed by tracer-aided modelling. Such integrated modelling frameworks provide robust science-based evidence for policy makers allowing quantitative assessment of landuse effects on connectivity, water availability and quality as well as effective communication with stakeholders.