Does size matter? Scaling of runoff generation processes across nested tropical Andean catchments

Understanding how runoff generates across spatial scales is crucial for enhancing regional hydrological models and effective water management. However, knowledge on how different geomorphic characteristics and landscape structure influence streamflow remains limited, especially in tropical Andean catchments. To address this gap, we investigated runoff responses and streamflow generation across six nested tropical Andean catchments (6 to 91 km 2 ), characterized by the presence interconnected lakes, wetlands, and complex geomorphology, within the Quinuas Ecohydrological Observatory in southern Ecuador. Using nearly a decade records of hydroclimatic, stable isotopes, and hydrogeochemical data, we employed a multi-criteria approach that combined mixing models, transit times modeling, and principal component analysis (PCA) to study runoff responses and to identify dominant flow paths influencing streamflow generation. Our findings reveal that, despite relatively uniform rainfall distribution across the catchments, runoff responses are shaped by catchment features, including geomorphology, land cover, 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, slowly releasing water during dry periods. In contrast, larger downstream catchments (54 – 91 km2) became increasingly dependent on groundwater contributions, with MTTs exceeding 100 weeks. The PCA revealed three groups driven by topography, geology, and land cover that jointly shape MTT variability. Our results highlight the importance of catchment geomorphological variability in controlling runoff generation, thereby addressing a significant knowledge gap in páramo hydrology and providing new insights into the role of spatial scale in the hydrology of tropical Andean catchments.