Integrating hillslope erosion monitoring with tracer-based characterization of water sources in a small Alpine catchment

Hydrological and erosion processes in mountainous catchments are often influenced by climatic, geologic, and anthropogenic factors. Recent studies show that significant erosion in the European Alps is caused by extreme events, such as floods, windthrows, and avalanches. However, studies integrating the monitoring of hillslope erosion with environmental tracers to investigate the origin of overland flow (OVF) are rare. Thus, this study aims to: i) examine the temporal geomorphologic changes of an eroded experimental hillslope (including sediment estimation) in the Bridge Creek Catchment (BCC, eastern Italian Alps); and ii) compare the OVF generating in erosion risk and non-prone to erosion areas using environmental tracers (i.e., δ²H and δ¹⁸O, electrical conductivity, and major ions).

The BCC catchment covers an area of 0.14 m², with an elevation range from 1931 to 2515 m a.s.l. Geomorphic changes in two eroded hillslopes (Area 1: 910 m²; Area 2: 484 m²) near the lower part of BCC were analyzed using the Difference of Digital Surface Models (DoD) reconstructed from four high-resolution DSMs obtained by Unmanned Aerial Vehicle between August 2023 and October 2024. Additionally, three sediment collectors (SCs) were installed at the hillslope toe to quantify the amount of sediment deposited between late June and November 2024. Water samples for environmental tracer analysis were collected from stream water, spring water, shallow groundwater, OVF from the hillslope, rainfall, and saturated riparian zone between June 2023 and November 2024. Further, soil leaching tests with different durations (24 and 48 hours) were conducted to assess ion enrichment in water mixed with eroded soil.

The DoD analysis showed that topsoil removal particularly along the rill networks. Furthermore, shallow erosion caused by nival abrasion from snow movement was observed during the April 2024 survey. Soil losses from the experimental hillslope in BCC (Area 1 with three SCs) were estimated to be approximately 3.9 tons for the monitoring period, originating from an effective contributing area of 610 m² (out of 910 m²). The OVF from erosion-prone hillslope had a less negative isotopic composition, reflecting the isotopic signatures of recent precipitation events. In contrast, water sources from non-erosion risk areas depicted a depletion in heavy isotopes compared to summer rainwater. Stream water, spring water, groundwater, and saturated OVF in riparian zones exhibited less variability in isotopes compared to OVF from the eroded hillslope and rainfall. These findings highlighted the influence of snowmelt and summer rainfall on hillslope erosion.

Acknowledgements: This study was carried out within the Agritech National Research Center and received funding from the European Union Next-Generation EU (PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR) – MISSIONE 4 COMPONENTE 2, INVESTIMENTO 1.4 – D.D. 1032 17/06/2022, CN00000022). This abstract reflects only the authors’ views and opinions; neither the European Union nor the European Commission can be considered responsible for them.

Keywords

Alpine hillslopes, soil erosion, digital surface model, overland flow, environmental tracers