Spatiotemporal Variability of Rainfall‐Streamflow Dynamics in Mountainous Tropical Environments: Insights From Agricultural Catchments of Northwest Rwanda

Accurate rainfall–streamflow observations are essential for understanding runoff generation and flood hazards in mountainous regions. Yet such information remains scarce in many areas, especially for small and medium‐sized catchments in tropical environments of developing countries. This study develops high‐resolution rainfall–streamflow datasets and evaluates the spatiotemporal variability of rainfall–runoff dynamics to assess the applicability of the rational method, from its empirical formulation to its physical interpretation, for flood‐hazard management in the Nyamutera (44 km 2 ) and Gaseke (109 km 2 ) catchments of the mountainous Mukungwa watershed in northwestern Rwanda. To address data scarcity, cost‐effective stream monitoring stations were installed to record flow depths, complemented by three automatic rain gauges and two weather stations that record rainfall and other weather parameters at 15‐min intervals. Periodic discharge measurements were conducted to establish stage–discharge rating curves, which were used to derive continuous streamflow records from April 2022 to May 2023. Analysis of 40 storm‐event responses revealed marked contrasts between the catchments: Nyamutera produced higher runoff coefficients (0.05–0.40; mean = 0.20; annual mean = 0.18) than Gaseke (0.02–0.35; mean = 0.10; annual mean = 0.11), reflecting its steeper slopes and lower storage capacity. Design runoff coefficients for the 100‐year event were 0.55 and 0.51, and recorded peak discharges reached 131 and 122 m 3 /s, respectively. These values reflect Nyamutera's faster, more concentrated flow pathways, in contrast to the more attenuated response observed in Gaseke. The results highlight differences in runoff behaviour between the two catchments, likely reflecting their contrasting topography, soil properties and storage capacity. Although the monitoring network captured most dynamics, uncertainties remain in monitoring very low flows, extreme peaks and high rainfall variability. This study suggests additional monitoring techniques that could help capture these conditions. The developed approach provides a practical, transferable framework for rainfall‐streamflow characterisation in similarly data‐limited mountainous environments.