Field monitoring of cyclic freezing process: effect of air temperature on frozen layer thickness

Freezing is an important factor in soil degradation. In order to predict soil erosion in mountainous areas, the freezing process and the frequency and extent of freezing must be understood. This study aims to identify a relationship between air temperature and a depth of freezing, taking into account soil water content. The soil is considered to be frozen when all the free water within the pores freezes which, according to our numerical model, mainly occurs at temperatures slightly below 0°C. On the other hand, the temperature is not homogeneous in the soil as it is driven by the process of heat diffusion from the soil-atmosphere interface to depth, controlled by soil thermal conductivity and heat capacity, which depend on ice and water contents. As a consequence, the relationship between air temperature and the thickness of the frozen layer is not direct, and the relevance of using air temperature as a measure of frozen depth is to be evaluated.

A small N-S-oriented claystone ridge in an Eastern Pyrenees badland is being monitored. A series of thermometers, water content sensors, and specific heat sensors are collecting data in 5-minute intervals on both sides of the ridge. The data show an attenuation of temperature oscillation with an increasing depth and a time delay of the surface temperature propagation. The differences in the soil temperatures on the north and south side are moreover showing the importance of solar radiation in the process. These observations are further integrated into a procedure allowing for the analysis of possible ad-hoc relationships between current and past air temperature and depth of the frozen layer.