| Studies reveal that the earth is undergoing unprecedented climate warming in the past few decades.Evidence of temperature increase has been observed obviously in the high latitude and high altitude permafrost region.The direct impacts of climate warming on permafrost are increase of ground temperature,reduction of permafrost area,thickening of the active layer,and development of thermokarst terrain.These changes have an important impact on heat and mass exchange between the atmosphere and the land surface,thus on weather and climate;vegetation and plant growth,and ecosystem as a whole.Permafrost degradation would also have significant impact on engineering constructions in cold regions,thus on local and regional economy.Most importantly,changes in active layer thickness and permafrost conditions have dramatic impact on surface and subsurface hydrological processes and water resources in cold regions.Soil moisture is one of the most important variables in permafrost change.Permafrost temperature increase,especially for relatively high temperature permafrost,would result in the increase of unfrozen water which would dramatically impact thermal,electric and mechanical properties of frozen soils.Unfrozen water plays an important role in the freezing and frozen soils.During freezing,unfrozen water is the main source for liquid water migration,which has great effect on the formation of underground ice and the impact of frost heave.Past studies focused on the indoor test about the variation of permafrost temperature and liquid water.The accurate quantitative or semi-quantitative relationship was obtained between unfrozen water content and permafrost temperature,initial soil water content,the particle size and the salinity in lab to facilitate this application in more research and engineering practice.However,any laboratory tests have limitations.The main problem in laboratory experiment is the destruction of soil structure as a result this could not represent the real situation.In order to reduce the impact of the destruction of soil structure and research the relation between unfrozen water content and permafrost temperature,soil particle size and salinity,long-term field measurement of unfrozen water content variation was implemented in this study to explore the relationship between unfrozen water content and environmental factors.This study set up hydrothermal observation field in mountain permafrost in Qilian,in order to monitoring the temporal and spatial variation characteristics of unfrozen water content in active layer,and the influence to unfrozen water content by different soil properties.The primary objective of this study is to investigate changes in unfrozen water content with temperature and soil properties mainly in the relation between unfrozen water content and permafrost temperature,soil particle size and salinity.The data and information were achieved from field measurements and laboratory experiments.Soil temperature and soil moisture were measured by using thermistor string and time domain reflectometry,respectively.Soil basic physical and chemical properties were measured at different depths.Five soil types were derived by analysing soil particle size and classifying soil texture according to the United States Department of Agriculture standard.Each soil type has an empirical equation to describe unfrozen water content as a function of soil temperatures and the effect of grain size and salt content.The relationships between frozen soil temperature and unfrozen water content and soil freezing and thawing characteristic curves were obtained under different soil types and salinity conditions.The results show that:1.The bulk density and gravel content are smaller for shallow soils than for deeper soils.Soil physical properties showed that the bulk density of the active layer and permafrost soils were 1.68g/cm~3 and 1.39g/cm~3 respectively with gravel content of about 42.31% and 39.90% respectively.Overall,soil clay and silt content is higher for soils within the active layer than for underlain permafrost soils.2.Based on laboratory experiments and soil particle distribution,five different types of soils can be classified together with soil particle accumulative distribution curves for each soil type over the study region using the USA Soil Classification System.The five different types of soils are(in the order of their particle size): silty clay,silt,sandy silt,silty sand,and sand.Overall,soils over the study area have relatively higher sand content,which may have significant effect on unfrozen water content in frozen states.3.Unfrozen water content changes exponentially with absolute value of soil temperatures in frozen ground.This is consistent with studies conducted by others in the field.Based on soil temperature changes during soil freezing and thawing,the curve of unfrozen water content with temperature can be divided into high temperature stage and low temperature stage.4.Five different soil characteristic curves of unfrozen water content change with temperature were derived and analysed through soil texture analysis.The parameters a and b,representing soil properties,can be used for model inputs,engineering design for unfrozen water description in frozen soils.The result showed the clay content,bulk density and gravel content all have significant impact on unfrozen water content in frozen soils.Unfrozen water content increases with increase of clay content,while unfrozen water content decreases with increase of bulk density and gravel content.5.Chemical analysis showed that soils over the study area is alkaline and has a high salinity,which is consistent with the arid climate.For the same soil and soil temperature,unfrozen water content is higher with higher electric conductivity value and pH values.6.The relationship between unfrozen water content and the basic parameters of soils were obtained in different soil types,which were established by the BP neural network model.We could use this relationship to study the unfrozen water variations in other areas. |
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