Linking Rainfall Simulation and Single-Ring Measurements on a Swiss Alpine Hillslope: A Novel Framework for Surface Runoff Estimation

Surface runoff in alpine catchments is sensitive to rainfall intensity, resulting in substantial variations in runoff coefficients across rainfall scenarios. Although this dependency is well documented in the literature, it is often neglected in practical applications by assuming a fixed runoff coefficient for all events. Moreover, quantitative assessment of infiltration processes is time-consuming, and operational hazard-assessment methods commonly rely on qualitative infiltration assumptions rather than in situ measurements. In this study, we test a new framework that links simple field infiltration tests with the estimation of event-dependent surface runoff coefficients. To this end, we compared single-ring infiltrometer measurements (SR) with 1 m2 rainfall-simulation experiments (RSEs) across four land-use types: meadow, afforestation, broadleaf forest, and spruce forest. This study’s novelty lies in the cross-scale integration and explicit evaluation of established methods for operational hazard assessment, conducted in an alpine catchment near Brienz, Switzerland. The results show that directly upscaling surface runoff coefficients based on field-measured saturated hydraulic conductivity values provides a solid basis for estimating their order of magnitude, although discrepancies can be large depending on the approach used and the rainfall intensity considered. At i=140 mm h−1, RSE-derived mean apparent infiltration ranged from i=94.7 mm h−1 in spruce forest to i=123.3 mm h−1 in mixed broadleaf forest; the lowest individual plot value was i=56.8 mm h−1 for spruce forest. The Stone model fitted the trench-level RSE data with R2≥0.94, whereas the two-parameter Bonetti inversion reached a parameter bound in 9 of 12 trenches and did not yield physically interpretable Kfs distributions. Relative to RSE-Stone at i=140 mm h−1, SR-Bonetti predicted approximately 2–7 times higher surface runoffs for the three non-spruce land uses, while agreement was obtained only for the sampled spruce site. We discuss several potential reasons for the discrepancy between the RSE and SR results, including disturbance of the organic layer during SR measurements, seasonal variability in soil conditions and differences in the number of repetitions. Although the proposed framework provides a feasible approach to improving the quantitative assessment of flash-flood hazards in alpine catchments smaller than 10 km2, further research is needed to validate its results and enhance its practical applicability.