青藏高原多年冻土区积雪对沼泽、草甸浅层土壤水热过程的影响

Abstract

The average altitude of Tibetan Plateau is over 4000 m above sea level with vast areas of permafrost.The interaction between permafrost and the atmosphere is realized by the dynamic water-heat process within the active layer.The snowy condition on the earth surface directly impacts on the variations of the thickness of the active layer and the annual mean soil temperature in the permafrost area.Thus,the shallow soil water-heat process with or without snow cover is a very important uncertain factor in the water and energy circulation processes of the permafrost areas in Tibetan Plateau.In typical alpine meadow and swamp,the active soil moisture and temperature at different depth of layer were observed to reveal the influences of the snow cover changes on the soil water-heat processes.The results show that the thawing and freezing started-time of active soil were later and the duration of freezing were longer with snow cover than that without snow both in alpine meadow and swamp areas.With the snow cover,the active soil temperature variation rate decreased while the soil moisture variation rate increased.The snow cover plays important roles in restraining the soil temperature change and b accelerating the moisture change in active soil layer.The impacts of snow cover on soil temperature and moisture are greater in autumn and summer than that in winter and spring,and the influence is more dominant in melting process than in freezing process.Snow cover results in reducing the annual maximum soil temperature and rising the annual minimum soil temperature in alpine swamp.On the contrary,both the annual maximum and minimum soil temperature increase under snow cover in alpine meadow.By comparing the temperature differences between the annual maximum and minimum temperature of swamp and meadow soil with or without snow cover,it shows that the snow cover has greater influence on swamp soil temperature than on meadow soil.Some parameters,such as time of melting(Ts),duration of melting and increasing(Tr),the range of soil moisture content increasing(ΔWs),time of freezing(Td),duration of freezing(Tz) and the range of soil moisture content falling(ΔWd),were selected to objectively reflect the influence of snow cover on soil moisture during the freezing process.The results show that the snow cover has more influence on soil moisture in melting and rising process than that in freezing and falling process,and has a more influence on swamp soil moisture than on meadow soil.