Runoff dynamics and functional modeling of bedrock springs in a small granitic mountainous catchment in Hokkaido, northern Japan

Recent studies have suggested that rainwater infiltrates not only the soil layer but also the underlying bedrock in small mountainous catchments, forming a bedrock aquifer. This bedrock groundwater subsequently discharges into the soil layer, potentially affecting the initiation of shallow landslides. To clarify this influence, it is essential to understand the runoff dynamics of bedrock springs in response to precipitation. However, direct observation of bedrock spring runoff remains challenging because bedrock springs are usually covered by thick soil layers. As a result, only a limited number of studies have investigated the runoff characteristics of individual bedrock springs, and the development of runoff models for bedrock springs is still insufficient. In this study, we conducted detailed field observations in a small mountainous catchment (6.87 ha) in Hokkaido, northern Japan. The study site is underlain by granite, and bedrock layer is exposed on both banks of the stream, where springs emerge from fractures in the bedrock. As of 2025, multiple bedrock spring outlets have been identified within the catchment. Soil temperature, bedrock spring water temperature, precipitation, and bedrock spring runoff were monitored. Soil temperature and bedrock spring water temperature were continuously recorded at hourly intervals. Precipitation was measured at hourly intervals using a tipping bucket rain gauge. Bedrock spring runoff was measured by constructing small dams immediately downstream of each spring outlet and directing all spring water into triangular weirs or tipping bucket discharge gauges. In addition, soil water, bedrock spring water, and rainwater were collected for water quality analysis. Soil temperature, bedrock spring water temperature, and water quality data were used to estimate the origin of the bedrock spring water. We applied the Pw1 model, a functional model based on antecedent precipitation, to reproduce bedrock spring runoff dynamics. This model was originally proposed by Kosugi et al. (2013) to reproduce groundwater level variations that cause deep-seated landslides, using antecedent precipitation with an arbitrary half-life time and positive constants. Model parameters were optimized to maximize the Nash–Sutcliffe efficiency (NSE). Finally, we discuss the relationship between the origin of bedrock spring water and the model parameters.

Reference
Kosugi, K., Fujimoto, M., Yamakawa, Y, Masaoka, N, Itokazu, T, Mizuyama, and T, Kinoshita, A. (2013): Functional models correlating antecedent precipitation indices to bedrock groundwater levels, Journal of the Japan Society of Erosion Control Engineering, Vol.66, No.4, p.21 - 32.