青藏高原深层土壤热扩散率的时空分布特征

Spatio-temporal Distribution of Thermal Diffusivity in Deep Soil in Qinghai-Tibetan Plateau

青藏高原是地气相互作用相对活跃的地区,深入了解青藏高原土壤热扩散率的变化,才能正确计算地表能量平衡进而准确认识青藏高原对全球和区域气候变化的影响。根据青藏高原1980—2001年39个观测站点实测的0.8 m和3.2 m土壤温度资料,利用热传导对流法结合最小二乘法拟合求得各站点的土壤热扩散率,并分析了土壤热扩散率的时空变化规律。结果表明,1980—2001年期间青藏高原土壤热扩散率在20世纪90年代以前波动较大,20世纪90年代以后波动较小。青藏高原东部地区的深层土壤热扩散率从春季至夏季增大,夏季至秋季减小,秋季至冬季减小;夏季最大值出现在青、川、甘三省的交界处,土壤热扩散率的值为8×10-6 m^2s^(-1),冬季最大值为5.1×10-7 m^2s^(-1);而除东部以外的青藏高原其他地区的土壤热扩散率,春季至夏季减小,夏季至秋季增加,秋季至冬季减小,该区域土壤热扩散率的变化范围为1.2×10-7 m^2s^(-1)~9.2×10-7 m^2s^(-1)。土壤热扩散率的多年平均最大值出现在青海省和甘肃省西南部以及四川西部的青藏高原东部地区,土壤热扩散率的极值为6.4×10-6 m^2s^(-1)。最小值出现在祁连山地区,土壤热扩散率为1.2×10-7 m^2s^(-1),中部地区为相对高值区。

【Objective】The Qinghai-Tibet Plateau is an area relatively active in ground-air interaction. Only through understanding in depth variation of soil thermal diffusivity in the Qinghai-Tibet Plateau, can the surface energy balance on the Plateau be correctly calculated and the impact of the Plateau on global and regional climate change be understood in depth.【Method】Based on the data of soil temperatures at 0.8 m and 3.2 m deep in soil profiles accumulated in the 39 observation sites in the Qinghai-Tibet Plateau during the years from 1980 to 2001,thermal diffusivity of each observation site was worked out through fitting of the heat conduction convection method coupled with the least square method, and spatio-temporal variation of soil thermal diffusivity figured out.The heat conduction convection method encompasses heat conduction and thermal convection in calculating soil thermal conductivity. The amplitude and phase was obtained through fitting of the measured soil temperatures at the two soil depth with the least squares method, and substituted into the formula to yield soil thermal diffusivities.【Result】Results show that from 1980 to 2001, soil thermal diffusivity of the Qinghai-Tibet Plateau appeared to be on a declining trend. It fluctuated quite sharply before the 1990 s and less in the 1990 s. As affected by climate factors, soil properties and soil moisture content, soil thermal diffusivity vary in spatial distribution. Soil thermal diffusivity in the deep soil in the eastern part of the Qinghai-Tibet Plateau increased from spring to summer and decreased from summer to autumn and from autumn to winter. It peaked in summer up to 8×10^-6 m^2s^-1 in summer at the junction of Qinghai Province, Sichuan Province and Gansu Province and in winter up to 5.1×10^-7 m^2s^-1.In the rest of the Plateau, soil thermal diffusivity decreased from spring to summer, increased from summer to autumn and decreased again from autumn to winter, varying in the range of 1.2×10^-7 m^2s^-1 to 9.2×10^-7 m^2s^-1.For years the maximum mean monthly value of soil thermal diffusivity wasobserved in the eastern part of the Qinghai-Tibet Plateau,including Qinghai,Southwest of Gansu and West Sichuan,being 6.4×10^-6 m^2s^-1,whereas the minimum was in the region of Qilian Mountains,being 1.2×10^-7 m^2s^-1, and the mean monthly soil thermal diffusivity was relatively highin the central part of the Plateau. As affected by geographical location, soil properties, soil moisture content and number of snow coverage days, the central and eastern parts of the Plateau, to the northeast part of Nagqu, are sorted as alpine frigid humid climate zone, where the soil thermal diffusivity is high due to high precipitation and high soil water content, andthe area of the Qilian Mountains is as alpine frigid semi-arid climate zone, where the soil thermal diffusivity is low varying in the range of 1×10^-7～4.8×10^-7 m^2s^-1 because it is a permafrost region low in soil water content. In most parts of the Qinghai-Tibet Plateau, soil thermal diffusivity varies in the magnitude order of10-7 m^2s^-1, and in the central and eastern parts abeingrelatively high-valued regions, it does in the magnitude order of 10-6 m^2s^-1.【Conclusion】All the findings in this study demonstrate that soil thermal diffusivity varies sharply temporally and spatially, as affected by a complicated variety of factors,which merit further studies.