Operating Principles of Soil-Moisture Sensors and the Role of Capacitive Soil-Moisture Sensors in Landslide Early Warning Worldwide and in Vietnam

Rainfall-induced landslides are among the most damaging geohazards worldwide and in Vietnam, particularly in mountainous areas characterized by steep terrain, highly weathered soil profiles, and extreme rainfall. In this context, monitoring the moisture state of soil is highly significant because soil moisture directly reflects infiltration, the degree of saturation, and the reduction of matric suction prior to slope instability. This paper reviews the operating principles of soil-moisture sensors and analyzes the role of capacitive soil-moisture sensors in landslide early-warning systems worldwide and in Vietnam. In principle, capacitive sensors estimate volumetric water content through changes in the dielectric permittivity of the soil-water medium; because water has a much higher dielectric constant than the dry soil phase, the measured capacitance signal can be converted into soil moisture after appropriate calibration. International studies show that incorporating soil-moisture data into early-warning systems that previously relied only on rainfall thresholds can improve predictive performance, with antecedent saturation and event-scale increases in saturation being highly meaningful variables for landslide risk. Capacitive sensors have received particular attention because they enable real-time measurement, are low cost, consume little power, and can be readily integrated with wireless sensor networks and IoT platforms; they are therefore suitable for wide-area monitoring systems or high-risk locations. However, the accuracy of this sensor type still depends strongly on soil-specific calibration and is affected by temperature, salinity, soil texture, and installation conditions. In Vietnam, landslide early-warning approaches have developed from rainfall-based models and susceptibility mapping toward real-time monitoring systems; selected case studies, such as Nam Dan, have integrated soil-moisture sensors with pore-water pressure, displacement, and rainfall sensors to improve warning capacity. Nevertheless, precise location-specific warning remains challenging because of limitations in geological, topographic, and real-time field observation data. The paper concludes that capacitive soil-moisture sensors are a promising component of modern landslide early-warning systems, but they should be deployed in a multi-parameter framework that combines rainfall, pore-water pressure, slope displacement, and predictive modeling to increase reliability under Vietnamese conditions.