青藏高原中、东部局地因素对地温的双重影响(Ⅰ):植被和雪盖

Dual Influences of Local Environmental Variables on Ground Temperatures on the Interior-Eastern Qinghai-Tibet Plateau(Ⅰ): Vegetation and Snow Cover

青藏高原冻土区地温既受海拔、纬度和经度（干燥度）区域地带性规律控制，同时它又受植被、雪盖、砂层、水被和地质构造等局地因素的显著影响。局地因素对地温的影响具有双重性：在不同域值条件下，它可增高或降低地温。地温随植被覆盖度减小而逐渐增高，但覆盖度减到0～20％时，地温反而降低。在青藏高原东部、南部和腹部的高山区，冷季降雪多，很多地段为稳定积雪区，雪盖厚，持续时间长，对浅层地温起保温作用；而高原腹部的高平原、河谷和盆地冷季降雪较少，雪盖薄，持续时间较短，一般保温作用微弱。当雪盖厚度超过20cm以后，保温作用即开始增强；在暖季因积雪存在时间短，雪盖薄，短期内对浅层地温起冷却作用。总之，每种局地因素迫使地温向相反方向转化阶段是一个区间值，为渐变过程。随时空尺度变化，局地因素的影响变化很大。有些地段，几种局地因素共同作用，加上活动构造和地形、地貌等的影响，使地温的时空分布和局地因素对其影响或控制变得错综复杂。因此，研究和预测地温特征和变化趋势，需要在监测植被和积雪作用的基础上进行参数选择、验证和优化。

Ground temperatures in permafrost regions on the Interior-Eastern Qinghai-Tibet Plateau are controlled by regional zonations of elevation, latitude and continentality （aridity）. They also are significantly affected by local environmental factors such as vegetation, snow cover, sand cover, and surface water/moisture conditions. However, these influences are dual, which can increase or decrease ground temperatures under certain circumstances. Ground temperatures tend to increase gradually with decreasing vegetative coverage, until the vegetation reaches the critical values of 10%～20% or becomes bare ground, and then ground temperatures tend to decrease. In mountainous regions on the eastern, southern and interior Qinghai-Tibet Plateau, snowfall is heavy in cold season, and many areas can be snow coved for a long time. The thick snow cover persists in cold season, insulating shallow soil layers from being cooled in winter. In high plains, valleys and basins in the interior of the plateau, however, snowfall is generally rare, and consequently snow cover is thin and its duration is short, resulting in weak insulation. When the thickness of snow cover exceeds 20 cm, insulative effect begins to enhance. In warm season, snow cover tends to slightly cool shallow soil layers over a short period, because of the short duration and small thickness of snow cover and the melting of snow. 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 carefully choose model parameters, and the models need to he validified and optimized on the basis of monitoring the roles of vegetation and snow cover.