Variability of Hydraulic Conductivity in Frozen Ground Zone: A Case Study

Permafrost and seasonally frozen ground greatly affect the groundwater discharge, and this intensified discharge could facilitate the transport of dissolved solids, nutrients and carbon cycle in cold and arid regions. The seasonally frozen ground is regarded as an active layer near the surface land in which the ice-liquid water content is varied seasonally with temperature. The hydraulic conductivity (K) of the active layer, as a controlling factor to determine groundwater discharge, is varied by ice-liquid water content, and variations in the density of liquid water versus ice and hard to be quantified in regional scale. In this study, we use Qinghai-Tibet Plateau (QTP) as a representative case to quantify the seasonal variation of K of active layer driven by temperature. QTP is the source area of most major rivers of China including Yellow River, Yangtze River, Lancang River, Yarlung Zangbo River, which is covered with permafrost and seasonally frozen ground. The air temperature and ground temperature data and active layer thickness data from ten meteorological stations are used to construct a vertical temperature change model with multiple scenarios to calculate the K in active layers of Yarlung Zangbo River Basin in QTP. According to different freeze-thaw conditions, the whole period of the active layer is divided into four periods, including thawing period, all-thawed period, freezing period and all-frozen period. We obtain seasonal variation of K at a depth of 0-100 cm of the active layer at each meteorological station during four periods. Our results show that at freezing period and thawing period, the value of K ranges between 10 m/d and 10^-3m/d. Our estimates provide the important basis for seasonal groundwater discharge evaluation, and these findings suggest the effects of climate change on hydrological processes in cold and arid regions.