多年冻土活动层浅层包气带水-汽-热耦合运移规律
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摘要
活动层水热状态直接影响多年冻土和寒区工程的稳定性。已有研究大多基于附面层理论研究冻土温度场变化,较少研究液态水和水汽运移过程及其对冻土温度场的影响。结合多年冻土活动层包气带水-热耦合迁移的物理过程和内在机制,以温度和含水率为基本变量,建立了考虑液态水和水汽相变、水分对流传热和水汽运移的土壤-地表-大气能量平衡及土壤内部水热变化的耦合模型,分析了真实野外气象条件下活动层液态水和水汽运移规律。计算结果表明:白天温度梯度水分向土壤内部运移,夜间温度梯度水分向土壤表层运移;暖季温度梯度水分以向土壤内部运移为主,冷季温度梯度水分以向地表运移为主;就全年而言,活动层各个深度处水汽运移作用大于15%,水势梯度水汽运移极小可以忽略不计,特别是浅层土壤在无降雨状态下,水分运移以温度梯度水汽运移为主;水势梯度液态水通量受降雨影响明显,在降雨事件期间和之后,液态水和水汽下渗占主导地位,降雨降低表层土壤温度、减小土壤热传导通量,有利于土壤热稳定性。
The thermal-moisture dynamics of the active layer directly affects the stability of permafrost and engineering projects in cold regions. Previous studies mainly focus on the thermal stability of permafrost based on the adherent layer theory. The migration process of liquid water and water vapor and its effects on the active layer are still lack of consideration. Considering the physical process and mechanism of liquid water-vapor transfer in unsaturated soils, a new heat and mass transfer model in saturated-unsaturated partially frozen soil was developed, in which the moisture migration in both vapor and liquid phases and heat transfer by means of conduction, convection and phase change were accommodated. The established water-vapor-heat transport model was used to analyze the water-vapor-heat transport in the shallow unsaturated zone of active layer under the in-situ meteorological conditions. The results show that the liquid water and water vapor are driven by the temperature gradient flow downward during daytime but upward at night, the liquid water and water vapor are driven by the temperature gradient flow upward in warm season but downward in cold season. Vapor water accounts for more than 15% of the water flux in the active layer and the water vapor driven by the pressure head can be neglected throughout the year. The moisture transport is mainly controlled by temperature gradient in sunny days. During and after rainfall events, rainfall infiltration is significantly enhanced and liquid water and water vapor mainly infiltrate downward. Rainfall can significantly decrease surface soil heat flux, heat conduction and soil temperature, which mitigate the permafrost degradation process.
The thermal-moisture dynamics of the active layer directly affects the stability of permafrost and engineering projects in cold regions. Previous studies mainly focus on the thermal stability of permafrost based on the adherent layer theory. The migration process of liquid water and water vapor and its effects on the active layer are still lack of consideration. Considering the physical process and mechanism of liquid water-vapor transfer in unsaturated soils, a new heat and mass transfer model in saturated-unsaturated partially frozen soil was developed, in which the moisture migration in both vapor and liquid phases and heat transfer by means of conduction, convection and phase change were accommodated. The established water-vapor-heat transport model was used to analyze the water-vapor-heat transport in the shallow unsaturated zone of active layer under the in-situ meteorological conditions. The results show that the liquid water and water vapor are driven by the temperature gradient flow downward during daytime but upward at night, the liquid water and water vapor are driven by the temperature gradient flow upward in warm season but downward in cold season. Vapor water accounts for more than 15% of the water flux in the active layer and the water vapor driven by the pressure head can be neglected throughout the year. The moisture transport is mainly controlled by temperature gradient in sunny days. During and after rainfall events, rainfall infiltration is significantly enhanced and liquid water and water vapor mainly infiltrate downward. Rainfall can significantly decrease surface soil heat flux, heat conduction and soil temperature, which mitigate the permafrost degradation process.
引文
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[19]O’NEIL K,MILLER R D.Exploration of a rigid ice model of frost heave[J].Water Resources Research,1985,21(3):281-296.
[20]VAN GENUCHTEN M T.A closed-form equation for predicting the hydraulic conductivity of unsaturated soils[J].Soil Science of American in Journal,198044(5):892-898.
[21]MUALEM Y.A new model for predicting the hydraulic conductivity of unsaturated porous media[J].Water Resources Research,1976,12(3):513-522.
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[24]徐敩祖,王家澄,张立新.冻土物理学[M].北京:科学出版社,2010.XU Xiao-zu,WANG Jia-cheng,ZHANG Li-xin.Frozen soil physics[M].Beijing:Science Press,2010.
[25]ZHOU Jia-zuo,LI Dong-qing.Numerical analysis of coupled water,heat and stress in saturated freezing soil[J].Cold Regions Science and Technology,2012,72:43-49.
[26]BANIMAHD S A,ZAND-PARSA S.Simulation of evaporation,coupled liquid water,water vapor and heat transport through the soil medium[J].Agricultural Water Management,2013,130:168-177.
[27]CAMILLO P J,GURNEY R J.A resistance parameter for bare-soil evaporation models[J].Soil Science,1986,141(2):95-105.
[28]张明礼,温智,薛珂,等.降水对北麓河地区多年冻土活动层水热影响分析[J].干旱区资源与环境,2016,30(4):159-164.ZHANG Ming-li,WEN Zhi,XUE Ke,et al.The effects of precipitation on thermal-moisture dynamics of active layer at Beiluhe permafrost region[J].Journal of Arid Land Resources and Environment,2016,30(4):159-164.
[2]WEN Z,NIU F J,YU Q H,et al.The role of rainfall in the thermal-moisture dynamics of the active layer at Beiluhe of Qinghai-Tibetan Plateau[J].Environmental Earth Science,2014,71(3):1195-1204
[3]赵林,程国栋,李述训,等.青藏高原五道梁附近多年冻土活动层冻结和融化过程[J].科学通报,2000,45(11):1205-1210.ZHAO Lin,CHENG Guo-dong,LI Shu-xun,et al.Thawing and freezing processes of active layer in Wudaoliang region of Tibetan Plateau[J].Chinese Science Bulletin,2000,45(11):1205-1210.
[4]HARLAN R L.Ananalys of coupled heat-fluid transport in partially frozen soil[J].Water Resources Research,1973,9:1314-1323.
[5]O’NEIL K,MILLER R D.Exploration of a rigid ice model of frost heave[J].Water Resources Research,1985,21(3):281-296.
[6]LAI Y M,PEI W S,LI S Y,et al.Study on theory model of hydro-thermal-mechanical interaction process in saturated freezing silty soil[J].International Journal of Heat and Mass Transfer,2014,78:805-819.
[7]张明礼,温智,薛珂,等.青藏铁路多年冻土区润湿地段斜坡路基温度与变形分析[J].岩石力学与工程学报,2016,35(8):1677-1687.ZHANG Ming-li,WEN Zhi,XUE Ke,et al.Temperature and deformation analysis on slope subgrade with rich moisture of Qinghai-Tibet railway in permafrost regions[J].Chinese Journal of Rock Mechanics and Engineering,2016,35(8):1677-1687.
[8]ZHOU J,WEI C,LI D,et al.A moving-pump model for water migration in unsaturated freezing soil[J].Cold Regions Science and Technology,2014,104(8):14-22.
[9]ZHANG S,TENG J,HE Z,et al.Importance of vapor flow in unsaturated freezing soil:A numerical study[J].Cold Regions Science&Technology,2016,126:1-9.
[10]SAKAI M,TORIDE N,SIMUNEK J.Water and vapor movement with condensation and evaporation in a sandy column[J].Soil Science Society of America Journal,2009,73(3):707-717.
[11]SAITO H,SIMUNEK J,MOHANTY B P.Numerical analysis of coupled water,vapor,and heat transport in the vadose zone[J].Vadose Zone Journal,2006,5(2):784-800.
[12]AN N,HEMMATI S,CUI Y.Numerical analysis of soil volumetric water content and temperature variations in an embankment due to soil-atmosphere interaction[J].Computers&Geotechnics,2017,83:40-51.
[13]ZHANG Ming-li,WEN Zhi,XUE Ke,et al.A coupled model for liquid water,water vapor and heat transport of saturated-unsaturated soil in cold regions:Model formulation and verification[J].Environmental Earth Sciences,2016,75(8):1-19.
[14]ZENG Y J,SU Z B,WAN L,et al.A simulation analysis of the advective effect on evaporation using a two-phase heat and mass flow model[J].Water Resource Research,2011,47(10),W10529.doi:10.1029/2011WR010701.
[15]ZENG Y J.Coupled dynamics in soil:Experimental and numerical studies of energy,momentum and mass transfer[M].Heidelberg:Springer,2012.
[16]滕继东,贺佐跃,张升,等.非饱和土水气迁移与相变:两类“锅盖效应”的发生机制及数值再现[J].岩土工程学报,2016,38(10):1813-1821.TENG Ji-dong,HE Zuo-yue,ZHANG Sheng,et al.Moisture transfer and phase change in unsaturated soils:Physical mechanism and numerical model for two types of‘Canopy Effect’[J].Chinese Journal of Geotechnical Engineering,2016,38(10):1813-1821.
[17]吴紫汪,程国栋,朱林楠,等.冻土路基工程[M].兰州:兰州大学出版社,1988.WU Zi-wang,CHENG Guo-dong,ZHU Lin-nan,et al.Permafrost subgrade engineering[M].Lanzhou:Lanzhou University Press,1988.
[18]谭贤君,陈卫忠,伍国军,等.低温冻融条件下岩体温度-渗流-应力-损伤(THMD)耦合模型研究及其在寒区隧道中的应用[J].岩石力学与工程学报,2013,32(2):239-250.TAN Xian-jun,CHEN Wei-zhong,WU Guo-jun,et al.Study of thermo-hydro-mechnical-damage(THMD)coupled model in the condition of freeze-thaw cycles and its application to cold region tunnels[J].Chinese Journal of Rock Mechanics and Engineering,2013,32(2):239-250.
[19]O’NEIL K,MILLER R D.Exploration of a rigid ice model of frost heave[J].Water Resources Research,1985,21(3):281-296.
[20]VAN GENUCHTEN M T.A closed-form equation for predicting the hydraulic conductivity of unsaturated soils[J].Soil Science of American in Journal,198044(5):892-898.
[21]MUALEM Y.A new model for predicting the hydraulic conductivity of unsaturated porous media[J].Water Resources Research,1976,12(3):513-522.
[22]CASS A G,CAMPBELL G S,JONES T L.Enhancement of thermal water vapor diffusion in soil[J].Soil Science Society of America Journal,1984,48(1):25-32.
[23]CAMPBELL G S.Soil physics with BASIC:Transport models for soil-plant systems[M].New York:Elsevier,1985.
[24]徐敩祖,王家澄,张立新.冻土物理学[M].北京:科学出版社,2010.XU Xiao-zu,WANG Jia-cheng,ZHANG Li-xin.Frozen soil physics[M].Beijing:Science Press,2010.
[25]ZHOU Jia-zuo,LI Dong-qing.Numerical analysis of coupled water,heat and stress in saturated freezing soil[J].Cold Regions Science and Technology,2012,72:43-49.
[26]BANIMAHD S A,ZAND-PARSA S.Simulation of evaporation,coupled liquid water,water vapor and heat transport through the soil medium[J].Agricultural Water Management,2013,130:168-177.
[27]CAMILLO P J,GURNEY R J.A resistance parameter for bare-soil evaporation models[J].Soil Science,1986,141(2):95-105.
[28]张明礼,温智,薛珂,等.降水对北麓河地区多年冻土活动层水热影响分析[J].干旱区资源与环境,2016,30(4):159-164.ZHANG Ming-li,WEN Zhi,XUE Ke,et al.The effects of precipitation on thermal-moisture dynamics of active layer at Beiluhe permafrost region[J].Journal of Arid Land Resources and Environment,2016,30(4):159-164.