Variation characteristics and impact factors of the depth of zero annual amplitude of ground temperature in permafrost regions on the Tibetan Plateau

The accurate judgment on the depth of zero annual amplitude (ZAA) of ground temperature has significant implications for estimating permafrost evolution and determining the lower-boundary depth used in climate models, land surface models or hydrological models. Based on the analysis of ground temperatures observed in 15 typical monitoring boreholes on the Tibetan Plateau, the primary variation characteristics and impact factors on the depth of ZAA are revealed and a simplified method for calculating the depth of ZAA is put forward. The results showed that the average ZAA of low-temperature permafrost is 4.6 m more than the average one of high-temperature permafrost; the ZAA decreases with the increase of permafrost temperature, but there are several boreholes with high soil moisture content bored in low-temperature permafrost regions being having relatively smaller ZAAs. Due to the effect of dynamic of heat and water and freezing and thawing process occurred in the active layer, ground temperature changing depth has no significant correlation with annual ground temperature variation in the shallow layer (0.5 m in depth), but has significant positive correlation with temperature annual range near the permafrost table. The differences of thermo-physical parameters among boreholes at different regions are the main cause of significant regional difference and variation of ZAA; the ground water content, temperature, texture and water phase state play important roles in the variation of the thermo-physical parameters of geological media. Annual variation depth of ground temperature has no significant correlation with the shallow ground temperature changes (less than 0.5 m in depth), but has significantly positive correlation with annual temperature range near the permafrost table; the differences in thermal diffusivity of stratum medium are the main reason for annual ground temperature changing depth with regional difference; the change in soil moisture content, temperature, texture and phase state of water is an important factor affecting thermo-physical properties.