Effect Of Active Layer Freeze-Thaw Process On The Dynamic Of Supra-Permafrost Groundwater In The Permafrost Region Of The Qinghai-Tibet Plateau Heartland

The Qinghai-Tibet Plateau is the highest-altitude,low-latitude frozen soil area with complex hydrogeological conditions in the world,the groundwater in which area is affected by frozen soil greatly,its distribution and movement conditions are very particular in these regions.Supra-permafrost groundwater is the most active type of groundwater in the frozen soil areas.It has very frequent interactions with the atmosphere and surface water mainly in spring and summer,which plays an important role in the plateau hydrological cycle.With global warming,groundwater dynamics can be regarded as one of the indicators of the Plateau response to climate change,and is an extremely important research direction of the Plateau permafrost hydrology.Studies have shown that the reason why the supra-permafrost groundwater has obvious seasonal characteristics is that the permeability of the soil also has a similar seasonal change process.Regarding the study of groundwater dynamics,the soil hydraulic coefficient has to be one of the key parameters considered not only controls the movement of groundwater and solute,but also affects the accuracy of water resources assessment in the basin.The research on the dynamics of supra-permafrost groundwater and the change of soil permeability can help to accurately describe the temporal and spatial variation of groundwater dynamics in the cold region and the seasonal phase transition process of the permafrost active layer.Therefore,we carried out related research on hillsides with typical alpine meadow and near-river zones in a small watershed of the Fenghuo Mountain in the source area of the Yangtze River.We observed the water level dynamics of supra-permafrost groundwater during the freeze-thaw process of the active layer and the pumping test was carried out to determine the change of the hydraulic coefficient of the active layer during the freez-thaw process.Finally we analyzed the influencing factors causing the spatialdifference of water level dynamics and hydraulic coefficient,the result shows that:(1)Driven by soil temperature,the trend of water level dynamics and hydraulic coefficient of all observed test points is consistent with the same process of the active layer.The water level dynamics with the ground temperature of each soil layer have significant nonlinear Boltzmann functional relationship in the gradual freeze period,in which the groundwater level responds more to shallow ground temperature changes than shallower ones.The hydraulic coefficient shows a great correlation with the ground temperature,and the response of the hydraulic coefficient to the temperature change is more sensitive than the water level dynamics during the boundary period between the melting and freezing of the active layer,which explains when the process of temperature gradual decline,the groundwater flow movement showed a significant weakening trend.(2)The difference in replenishment conditions has a significant impact on the water dynamics of supra-permafrost groundwater,and the activity of the groundwater flow also causes different energy distributions,which also has a significant effect on soil permeability.Among them,the existence of grade will form a water potential difference,and the hydraulic property of the test point located under the low slope position is superior to the high slope position because the recharge condition is better.Similarly,the hydraulic coefficient of the low slope position is higher than the high slope position.The existence of the grade difference also caused the point on the high slope position to respond more strongly to the supply of atmospheric precipitation.There is also a very significant correlation between river water and the dynamics of supra-permafrost groundwater.Although the groundwater dynamics in the near channel are more sensitive to the arrival of the freeze period,its hydraulic properties are significantly better than the slopes during the freeze-thaw process.(3)The difference between the slope orientation and the vegetation coverage of the test sites impact on the water level dynamics and soil permeability is through the different energy exchange process.Solar radiation is the main driving factor for the increase of ground temperature.The active layer on the sunny slope absorbs more energy than the shady slope,which leads to different process of freeze-thaw cycle of the active layer.According to the observation results,the melting of the sunny slope precedes the shady slope for 29 days,and during the gradual freezing period,the water level and the hydraulic coefficient decrease less than the shady slope.The coverage of vegetation hindered the exchange of energy between the soil and theatmosphere.During the freeze process,the soil permeability with good vegetation coverage decreased more slowly.