Peak discharges per unit area increase with catchment area in a high-relief mountains with permeable sedimentary bedrock

To improve the accuracy of flood-flow prediction under changing climate conditions, understanding the storm-flow generation at various scales in a catchment is essential. Flood-flow prediction is conducted assuming that peak discharge per unit area (hereafter referred to as peak unit discharge) decreases with catchment area, based on observations. However, this assumption may not be applicable in steep high-relief mountain catchments with permeable bedrock, although this has not been confirmed due to the difficulty of flood-flow measurement in high-relief mountains. We obtained continuous discharge data for three nested catchments with areas of 0.58 km(2), 2.2 km(2) and 94 km(2) in the high-relief Chichibu Mountains, which have permeable bedrock, and tested the hypothesis that peak unit discharge and direct runoff; flow caused by and directly following a rainfall and forms the major part of the flood hydrograph excluding base flow per unit area (hereafter referred to as unit direct runoff) were greater in larger catchments. The effect of the spatial distribution of rainfall on the spatial distribution of storm runoff was evaluated based on radar-rain gauge analyzed precipitation data after verification using ground-based rainfall data. We obtained data for 67 storms, with a depth of rainfall-event ranging from 26.5 to 231.5 mm and a return period ranging from 10(-3) to 3.2 y. During significant floods, with peak unit discharge at largest catchment was greater than 3 mm/h; peak unit discharge and unit direct runoff were always larger at larger catchment. Differences in peak unit discharge and unit direct runoff among measured catchments were often several times and sometimes more than an order of magnitude, depended on the storm events. While, the catchment mean depth of rainfall-events was up to 1.26 times greater in the largest catchment, relative to the two smaller catchments, implying that spatial variations in storm rainfall have little effect on the differences in flood flow among catchments. Our study and previous research indicate that this increasing storm runoff per unit area with catchment area is characteristic of catchments with permeable bedrock, where some rainfall that in-filtrates bedrock on hillslopes is transported downstream within the bedrock to create storm flow in larger catchments.

Automated summary

The study found that in high-relief mountain catchments, peak unit discharge and unit direct runoff increase with catchment area. Spatial variations in storm rainfall have little effect on the differences in flood flow among catchments.