Dual Influences of Local Environmental Variables on Ground Temperatures on the Interior-Eastern Qinghai-Tibet Plateau(II):Sand-layer and Surface Water Bodies

Ground temperatures in permafrost regions on the Qinghai-Tibet Plateau are controlled by regional zonations of elevation,latitude and continentality(aridity).They are also significantly affected by local environmental factors such as vegetation coverage,snow cover,sand layers,and surface water/moisture conditions.However,these influences are dual in that they can increase or decrease ground temperatures under certain circumstances.Temperatures of soils beneath sand dunes and thick(10~20 cm) sand layers are warmer than those of adjacent areas without sand coverage.However,temperatures of soils beneath shallow(10~20 cm) sand layers are cooler than those of adjacent areas without sand coverage.Within a same geomorphological unit,temperatures of soils in the paludified areas or wetlands are generally the lowest,favoring the formation and protection of permafrost.Freshwater lakes and ponds and middle to large rivers tends to increase ground temperatures and to deepen the depth of maximum thaw in summer,resulting in various taliks.However,the salty lakes tend to cool or protect cryopeg because of its conductive water-layer structure.In summary,the influence of each local environmental factor on ground temperatures could reverse in direction at a certain range.That is a gradual process depending on the timing and locality.In some areas,in addition to the impacts from the active tectonics,topography and geomorphology,the influences of several local factors superimpose and further complicate the impacts of local factors on ground temperatures.Therefore,research and forecast of the features and changing trends of ground temperatures need to select model parameters and the models need to be validified and optimized on the basis of monitoring the roles of sand coverage and surface water bodies.