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

Lateral flow in bedrock dominates baseflow and flood flow in a catchment underlain with permeable sedimentary rock in the high‐relief Chichibu Mountains

“…More rainfall occurs in spring than in winter (Figure 2), which might indicate that rainfall in spring feeds deeper bedrock before feeding the shallower bedrock layer, and thus the relative contribution from deeper bedrock increases while the total discharge remains nearly constant. The finding that accumulated rainfall exceeding 150–200 mm is needed for significant flow to occur (Asano, Suzuki, et al, 2022) and the leaky character of Bakemonozawa demonstrated in this study (Table 1) may support the occurrence of this SiO 2 increase in spring. The other point of confusion is the decrease in SiO 2 with increasing discharge in Figure 12 despite the deeper bedrock groundwater contribution increasing during large flood peaks (Figure 10).…”

“…Water level was measured using capacitive water height recorder with tem- Water Information System, n.d.). A water intake is located approximately 200 m upstream of the Kawamata gauging location, and therefore we summed the reported discharge data and water intake data to obtain discharge at Kawamata Asano, Suzuki, et al, 2022).…”

“…During and after the storm began on 28 June 2018, with total rainfall of 211 mm and a return period of 3.23 years (Asano, Suzuki, et al, 2022), discharge increased dramatically, and daily mean stream temperature dropped from 15.5 to 11.4 C (Figure 9). Upon closer inspection, stream temperature increased by approximately 1 C, coinciding with the peak of rainfall, and then decreased by more than 3 C while discharge increased by more than 90-fold (Figure 10).…”

“…Among three nested gauge locations in Kawamata including Bakemonozawa, peak discharges and direct flows per unit area were significantly greater in larger catchments Asano, Suzuki, et al, 2022). In the smallest catchment, Bakemonozawa, 150 to 200 mm of accumulated rainfall was needed to generate significant storm flow, while 50 to 100 mm of accumulated rainfall was sufficient to generate flood flows in larger catchments, suggesting that in smaller catchments, more rainfall is needed to fill the empty spaces in the subsurface and generate surface flow.…”

“…This study characterized the contribution of the bedrock flowpath in the Bakemonozawa catchment (0.51 km 2 ) in the high‐relief Chichibu Mountains, which has permeable sedimentary bedrock of accretionary wedge. The site was selected because the bedrock flowpath contributed significantly to storage and discharge in a larger (>1–10 km 2 ) catchment in the same area (Asano et al, 2020; Asano, Kawasaki, et al, 2022; Asano, Suzuki, et al, 2022), but obtained limited information about the bedrock contribution in smaller (<1 km 2 ) catchments. Snapshot discharge measurements at 13 nested locations during baseflow within the Kawamata catchment, which includes Bakemonozawa, indicate that discharge per unit area increased by more than threefold as catchment area increased from 0.05 to 94 km 2 , demonstrated that some rainfall that infiltrates the bedrock on hillslopes in small catchments is transported downstream within the bedrock to feed baseflow (Asano et al, 2020).…”