The Influence of Geomorphic Heterogeneity on Streamflow Generation Processes Across Nested Tropical Andean Catchments

Understanding how streamflow is generated across spatial scales is crucial for enhancing regional hydrological models and effective water management. However, knowledge on how different geomorphic characteristics, landscape structure and subsurface properties influence streamflow remains limited, particularly in tropical Andean catchments. To address this gap, we investigated streamflow responses and generation processes across six nested tropical Andean catchments (6–91 km²), characterized by permeable fractured bedrock, interconnected lake systems, wetlands and complex geomorphology within the Quinuas Ecohydrological Observatory in southern Ecuador. Using nearly a decade of hydrometric, stable isotopes and hydrogeochemical data, we applied a multi-criteria approach that combined mixing models, transit times modelling, young water fractions estimation, and principal component analysis to identify dominant flow paths and controls on streamflow generation. Our findings reveal that, despite relatively uniform rainfall distribution across the catchments, streamflow responses are shaped by catchment structure, particularly geological controls and subsurface water storage. Small headwater catchments (6–19 km²) exhibited rapid discharge responses to rainfall, with MTTs as low as 41 weeks, where wetlands and high-altitude lakes regulated flows by slowly releasing stored water during dry periods. In contrast, larger downstream catchments (54–91 km²) showed increasing dependence on deep groundwater storage within the permeable bedrock, with MTTs exceeding 100 weeks. Young water fraction estimates further revealed that fast, well-connected flow paths persist across all catchment scales, coexisting with the progressive increase in groundwater storage and longer transit times observed downstream. The PCA revealed three groups driven by geology, topography and land cover that jointly shape MTT variability across scales. These results highlight the dominant role of geological and geomorphological variability in governing how water is generated and transported throughout complex terrain, providing new insights into structure-dependent hydrological behaviour in tropical Andean catchments.