Heavy Rainfall Patterns and High Streamflow Dynamics in the Southern Ecuadorian Andes

Ecuador’s southern tropical Andes are heavily impacted by intense tropical moisture, leading to flash floods and landslides. Here, we provide insights into the characteristics of the heavy rainfall triggering high streamflow in this area. High spatiotemporal resolution (500 m and 5 min) estimates from a high-elevation X-band weather radar (6 years, recording 1632 rainfall events) were analyzed for the small-scale rainfall properties, and a Lagrangian approach was employed to detect the moisture trajectories and sources. Employing a statistical model to analyze the rainfall space–time– intensity structures, rainfall was classified and characterized into three intense spatially clustered (convective) and one spatially homogenous (stratiform) rain types, which differ in their advection properties, intensities, and spatial structure. Tracking the rainfall trajectories, we found a predominant pathway for air masses to reach the study area, with the majority originating from the eastern flank of the Andes through the north Amazon basin (63.5%), followed by the high Andes Mountains (29%) and coastal plains (3.9%), whereas only a small proportion stems from the Pacific Ocean (3.6%). The major focal area from which air masses originate is only 250 km east of the study region. Analyzing high streamflow in the Tomebamba catchment, we estimated that convective rainfall types with a minimum duration of 2 h and intensity of 23 mm h²¹ originating mostly from the Andes and the north Amazon basin can potentially trigger high runoff events, with peaks ranging between 100 and 200 m³ s²¹. Rainfall characteristics and moisture sources are crucial for nearby catchments in tropical–temperate climates, aiding in weather forecasting and short-term flood warnings.