Rapid permafrost degradation is observed in northern regions as a result of climate change and expanding economic development. Associated increases in active layer depth lead to thermokarst development, resulting in irregular surface topography. In Central Yakutia, significant areas of the land surface have been deteriorated by thermokarst; however, no mitigation or land rehabilitation efforts are undertaken. This paper presents the results of numerical modeling of the thermal response of permafrost to changes in the active layer hydrothermal regime using field data from the village of Amga, Republic of Sakha (Yakutia), and mathematical analysis. The results suggest that restoring a thick ice-enriched layer will require increasing the pre-winter soil moisture contents in order to increase the effective heat capacity of the active layer. Snow removal or compaction during the winter is recommended to maximize permafrost cooling. The thickness of the restored transition layer varies from 0.3 to 1.3 m depending on soil moisture contents in the active layer. The modeling results demonstrate that damaged lands can be restored through a set of measures to lower the subsurface temperatures. A combination of the insulating layer (forest vegetation) and the high heat capacity layer (transition layer) in the atmosphere–ground system would be more effective in providing stable geocryological conditions.

In areas underlain by ice complex deposits, degradation of the protective layer activates cryogenic processes, leading to surface disturbance, alterations in ground heat and moisture regimes, and loss of permafrost landscape stability. Disturbed sites with shallow polygonal-wedge ice are especially vulnerable. Pre-winter irrigation is considered a method for the controlled modification of heat and moisture transfer in the upper part of ice-rich deposits of the ice complex in Central Yakutia. The method is intended to accelerate the formation of a transitional (protective) layer in areas disturbed by thermokarst. This study aimed to assess, through coupled modeling of heat and moisture transfer, the optimal pre-winter irrigation rate that creates conditions for the formation of an ice-saturated protective layer without over-wetting the seasonally thawed layer. To achieve this, the study draws on meteorological data from the Amga station and thermal and moisture monitoring data from experimental sites in the Amga River valley. The model was implemented in COMSOL Multiphysics and includes a heat transfer equation accounting for phase transitions using the apparent heat capacity method, as well as the Richards equation for variably saturated flow. A series of prewinter irrigation scenarios ranging from 0 to 500 mm was calculated, with control of the transition to a quasi-steady-state regime. Under the considered conditions, the optimal pre-winter irrigation rate ensuring the formation of an ice-saturated protective layer was found to be no more than W = 300 mm, depending on the initial moisture conditions of the seasonally thawed layer. However, at W = 100 and 200 mm, the infiltration front does not reach the lower part of the seasonally thawed layer, whereas at W = 400 and 500 mm, the amount of unfrozen water begins to increase and the temperature of the underlying permafrost rises. Therefore, these results can be used in planning permafrost reclamation measures in areas with ground ice development, including the ice complex, and in selecting pre-winter irrigation regimes.