Description
FIG. 1: Sensore HOBO Pendant® Temperature/Light Data Logger
The presence of snow on the ground plays an important role in influencing the soil thermal regime and energy exchange at the soil-atmosphere interface. Snow, as is well known, is an excellent insulator and limits soil heat loss, resulting in a higher average annual soil temperature than air temperature. Snow cover, in terms of thickness, persistence on the ground throughout the year, etc., is certainly one of the main factors driving periglacial processes in Alpine environments and determining the presence/absence of permafrost. There are very precise instruments that allow the monitoring of the presence and evolution of the seasonal snowpack (weather stations with sonar nivometer), or it is possible to monitor the snowpack by making manual measurements directly in situ. These methods are expensive, require a considerable logistical effort and, in alpine environments, can expose the operator to considerable risks.
FIG. 2: Dettaglio dell’installazione dei sensori
For these reasons, in many Alpine sites, continuous monitoring of the snow cover is not possible, and it is therefore difficult to obtain information on this important factor influencing periglacial processes. Several authors have developed a method involving the use of low-cost, temperature-recording data loggers, which are arranged vertically on a rod and allow the snow thickness to be monitored by comparing the temperatures measured in the atmosphere with those measured below the snow surface. This method, despite some limitations, is a good compromise between relatively low cost and detectable data, and has been successfully used in Canada. The method, which was also tested by our working group, was sometimes found to be unreliable, probably due to some problems related to the ability of such sensors to monitor snow types with different characteristics from those where the method mentioned above was tested.
FIG. 3: Operazioni di manutenzione alla stazione di monitoraggio installata nel sito di Cima Uomo
The Geological Service of the Autonomous Province of Trento, in conjunction with the Universities of Padua and Pavia, has therefore devised and tested a new method for monitoring snow cover based on the use of sensors that record both temperature and light intensity, which in addition to offering greater reliability in our environments they can also be used in smaller numbers for snow poles and allow the detection of even the smallest thickness of snow on the ground (5 cm), something that the use of thermometers alone does not allow, as the snow cover requires a thickness of approximately 30 cm in order to significantly isolate the thermometer from the thermal conditions in the air.
The monitoring system consists of HOBO Pendant® Temperature/Light Data Loggers (Fig. 1), which are waterproof data loggers that record light intensity and temperature data. The temperature measurement range is from -20°C to 70°C (with a resolution of 0.14°C to 25°C and an accuracy of ± 0.53°C from 0°C to 50°C), while for light intensity, the measurement range is from 0 to 320000 lux. The sensors are installed along a larch pole by means of metal supports, covered with a wooden support (Fig. 2) in order to keep the logger horizontal throughout the winter. The sensors were placed along the pole every 40 cm from 0 cm up to 120 cm. The sensors installed on the pole were oriented to the north to reduce the effect of heating on the snow around the pole. The sampling interval used is 2 hours for both the T° and brightness data, in order to allow data to be recorded for the entire winter period. A temperature sensor is then positioned 200 cm above the ground with a corresponding passive solar screen to monitor the air temperature; a second temperature sensor is positioned a few centimetres below the ground to monitor the thermal state of the ground and thus to have a further indirect data on the presence of permafrost at the study site.
FIG. 4: Grafico della copertura nevosa al suolo nel sito di monitoraggio di Cima Uomo, relativo alla stagione invernale 2017-2018, confrontato con il dato analogo registrato dalle vicine stazioni di rilevamento di Meteotrentino situate a Passo Valles ed a Passo S. Pellegrino. Una copertura nevosa omogenea di spessore superiore al metro isola il terreno dagli scambi termici con l’atmosfera
To date, three "snow poles", structured as follows, have been installed in three permafrost monitoring sites in Trentino: these are three rock glaciers, two located in the Presanella Group (in Val d'Amola and south of the Maroccaro pass) and one in the Eastern Dolomites, in the vicinity of Cima Uomo (Seppi et al., 2014; Seppi et al. 2019).
The three rods have been installed at the above-mentioned sites since the 2013-2014 winter season and data download and maintenance operations are carried out annually (Fig. 3). The processing of the data recorded by the sensors makes it possible to obtain a graph of snow thickness trends with an accuracy of 5 cm (Fig. 4).
The monitoring system described was tested at the snow field of the Avalanche Centre in Arabba (ARPAV Veneto) during the 2014-2015 winter season, providing excellent correspondence with the actual measurements.