Timing of flood in mountainous rivers is strongly determined by watershed scale regardless of topography
Researchers at University of Tsukuba have effectively collected data on water level fluctuations during flood events in a meso-scale mountainous watershed, known for its significant high-relief topography. Such observations were previously scarce due to difficulties in data collection in these regions. The analysis reveals that the timing of flood propagation is predominantly determined by the watershed's overall size, overshadowing the effects of topographical differences.
Tsukuba, Japan—Understanding the dynamics of rainfall-runoff in mountainous terrains is vital for refining predictions of sediment-related hazards, which are becoming increasingly critical under the influence of climate change. Despite extensive flood monitoring in various mountainous locales globally, comprehensive data from areas characterized by high-relief have been limited primarily to smaller watersheds, typically under 1 km2, featuring moderate river discharges. Meso-scale watersheds, spanning approximately 1-10 km2 and noted for their steep gradients and substantial sediment and water flows, present substantial data collection challenges.
In this investigation, the team monitored seven neighboring watersheds within the Ikawa Experimental Forest, operated by the University of Tsukuba in Shizuoka Prefecture, Japan. This area typifies the large, high-relief mountainous landscapes of Japan. The introduction of non-contact ultrasonic water-level gauges minimized the risk of equipment loss and enabled accurate measurements in locations where river channel topography fluctuates minimally, thus affecting data reliability.
Analysis of the collected data focused on the "peak lag time"—the interval between the zenith of rainfall and the corresponding peak in runoff. This metric is crucial for understanding the rainfall-runoff relationship. The study found that larger watersheds exhibited longer peak lag times. Comparison with data from lower-relief areas indicated that watersheds of similar sizes show comparable peak lag times, suggesting that watershed scale is a more significant factor in determining flood timing than the degree of topographical relief. This finding implies that floods in steep terrains can advance downstream as rapidly as those in flatter areas.
The insights from this research illuminate aspects of the rainfall-runoff process in high-relief meso-scale mountainous watersheds, areas where detailed data have been notably lacking. These findings are anticipated to enhance the accuracy of forecasts for sediment-related disasters significantly.