Integration of upward GPR and water content reflectometry to monitor snow properties

We adopt upward ground-penetrating radar (up-GPR) and water content reflectometry sensors to monitor the seasonal behaviour of snow density. Upward ground-penetrating radar permitted to observe at a single fixed station the time-lapse response of the electromagnetic signal at the main frequency of 1500 MHz, with the antenna radiating upward from the soil toward the snow surface. Measurements have been performed in a test site on Italian Alps (at an elevation of about 2100 m above sea level) during the 2014–2015 winter season at an interval of 30 minutes. The data processing of radar data involved the travel-time picking and the conversion into snow depth and density. Water content reflectometry measurements have been useful in order to calibrate the radar response and retrieve information on the presence of liquid water content. The integration of upward ground-penetrating radar and water content reflectometry technology allows us to infer snow high and layering, snow density changes during the winter season, and a preliminary estimate of the liquid water content. For snow in dry condition, we are able to estimate density values through mixing rules or polynomial formula. Snow density varies during the season in a range between 250 kg/m3 and 450 kg/m3; the results are in good agreement with the results of the ground truth. For snow in wet condition, the residuals of the electrical permittivity, after a trend removal on the original water content reflectometry data, permitted to estimate liquid water content in the range between 3% and 5%, during some periods of the winter season, according to warmer climate condition. Snow layering and densification processes are monitored by the response of upward groundpenetrating radar: fast phenomena such as wetting front infiltration can be also pointed out even if they appear challenging if other observations are not available (e.g., monitoring with water content reflectometry).