Processes of Soil Thawing-Freezing and Features of Ground Temperature and Moisture at D105on the Northern Tibetan Plateau

Base on the ground temperature and water content data obtained in CAMP/Tibet,1st January,2002 to 31 December,2005,the features of ground temperature,moisture and soil freezing-thawing process at D105 in the northern part of Tibetan Plateau are analyzed.The results show that ground temperature above 40-cm depth existed evident daily variation,and the phase of soil temperature lagged obviously with depth.The annual maximum ground temperature was in August and September;while the minimum was in January and February.The multiannual climate variation could be recognized down to the depth of 185 cm.Soil began to freeze and thaw downwards all from the ground surface.Soil freezing velocity increased with depth,but soil thawing velocity deceased with depth.During the period of freezing,soil temperature was low at the top and high at the bottom.However,soil temperature distribution during the period of thawing was opposite to that during soil freezing period.Above the 60-cm depth,soil water content had an obvious fluctuation during the period of thawing,suggesting that frequent water and heat exchange existed between soil above the depth of 60-cm and atmosphere.The process of soil thawing-freezing were affected by ground average temperature and the daily variation of ground temperature;soil water content greatly influenced the process of soil thawing-freezing,distribution of soil heat and allocation of surface energy.Consequently,soil freezing-thawing process was affected by coupled water and heat.