Spatio-temporal characterization of flash floods in small data-scarce watersheds of the tropical Kigezi highlands, southwestern Uganda

Abstract Flash floods are widespread natural hazards, yet detailed field-based studies remain limited, especially in tropical regions where data on their occurrence and climate knowledge are scarce. Here, we characterize flash floods across eight small watersheds in the tropical Kigezi highlands of southwestern Uganda. We trained a network of river watchers, i.e. citizens from local communities, to monitor the main river in each watershed at fixed locations over two years. Their more than 1,000 observations were paired with rainfall data from gauges located within a 2–3 kilometre radius. We identify with certainty 20 flash floods and 10 bankfull (near-flood) events. Not all watersheds experienced such events despite similar climate conditions, underscoring the dominant role of local convective rainfall over general seasonal trends. In some cases, flash floods occurred despite low recorded rainfall nearby. Conversely, 17 ‘non-flash flood’ events were also observed, i.e., intense rainfall events with no associated flood. These mismatches between the recorded rainfall and associated flash flood patterns further highlight the importance of highly localised rainfall typical of tropical climates as a flood trigger. Land use and cover also influenced flash flood patterns. Flash floods were most frequent during land preparation and planting seasons, while non-flash flood events were observed in watersheds with stronger conservation and restoration practices. In contrast, the geomorphological and lithological characteristics of the watersheds did not explain flash flood occurrence. This analysis based on real-world data from tropical Africa offers practical insights into flash flood occurrence in an understudied type of environment.