Monitoring soil moisture with Cosmic Ray Neutron Sensing (CRNS) at a slow-moving landslide using cosmic-ray neutron sensing in Lower Austria

Soil moisture is a key controlling factor in landslide processes, as it directly influences soil strength, cohesion, and pore pressure dynamics. Elevated moisture levels, particularly during prolonged or intense rainfall, reduce frictional resistance and shear strength, thereby increasing slope instability and landslide susceptibility. The state of Lower Austria is especially prone to landslides due to its geological setting, dominated by mechanically weak Flysch and Klippen Zone formations composed of interbedded limestones and deeply weathered materials. These conditions, in combination with hydrological drivers, land-use changes, and anthropogenic influences, result in a high predisposition to slope failures.

Reliable monitoring of soil moisture is therefore essential for improving the understanding of both predisposing and triggering factors of landslides. Recent advances in monitoring technologies, such as Cosmic-Ray Neutron Sensing (CRNS), enable spatially averaged soil moisture measurements that overcome the limitations of conventional point-scale sensors. CRNS provides direct estimates of water content integrated over a footprint with a horizontal radius of several tens of metres and a penetration depth of some tens of centimetres, offering a representative measure of near-surface soil moisture at the hillslope scale.

In this study, CRNS is deployed at the slow-moving Hofermühle landslide in Lower Austria to evaluate its suitability for long-term landslide monitoring over a three-year period. CRNS-derived soil moisture estimates are analysed in conjunction with data from time domain reflectometry (TDR) sensors and piezometers to investigate contrasting hydrological response behaviours during extreme events, including the September 2024 event and the subsequent development of hydrological conditions in the slope. These observations are further related to horizontal displacement rates derived from inclinometer measurements. All datasets are interpreted in the context of local geological and hydrological settings to assess the added value of footprint-scale soil moisture observations for capturing spatially integrated moisture dynamics relevant to slope stability. These findings explore the potential of CRNS to support the development of landslide monitoring strategies by bridging the gap between point-scale and hillslope-scale hydrological observations. Further monitoring is planned to be carried out with the static CRNS as well as mobile rover applications at other study sites in Lower Austria.