Unveiling Rainfall-Runoff Dynamics Across Spatial Scales: Insights from a Tropical Montane Andean Catchment

Tropical montane catchments play a critical role in regional hydrological processes. However, rainfall-runoff dynamics and their change across spatial scales remain understudied. Understanding these processes is essential for water resource management in the face of climate change and human activities. To address this knowledge gap, we conducted a multi-scale study of six sub-catchments in a lake-dominated tropical Andean system (~3200 – 4400 m a.s.l.), ranging from 6.19 km² to 90.7 km² over 10 years. We applied a multi-criteria approach including hydrometric data analysis, simple mixing models (MM) to determine geographical sources of streamflow contributions, mean transit time (MTT) estimations, and regression analyses to identify the key drivers of catchment behavior. Our results showed that runoff coefficients exhibited higher values on the headwater catchments suggesting that lakes regulate discharge primarily at small scales. Mixing model results showed that streamflow contributions stemmed from rainfall, wetland soil water, and groundwater from varying depths, with smaller catchments heavily influenced by wetlands and larger ones relying on groundwater recharge. Notably, groundwater contributions were higher below 3442 m a.s.l. suggesting a significant discharge area at this elevation. Discharge MTTs ranged from a few weeks in the smallest catchments to more than one hundred weeks in the largest, while groundwater MTTs extended from one hundred up to two hundred weeks, reflecting contributions from deeper zones. Regression analyses revealed that mean catchment slope, among other factors, significantly influenced hydrological behavior. This study demonstrates the value of a multimethod, multiscale approach for understanding rainfall-runoff dynamics in tropical montane systems. Our findings emphasize the regulatory role of headwater lakes at small scales and the previously underestimated role of deep groundwater contributions in larger catchments, providing a foundation for improved hydrological modeling and sustainable water management strategies in the tropical Andes.