Variations in soil temperature at BJ site on the central Tibetan Plateau

The temporal and spatial variation in soil temperature play a significant role in energy and water cycle between land surface and atmosphere on the Tibetan Plateau. Based on the observed soil temperature data (hourly data from 1 January 2001 to 31 December 2005) obtained by GAME-Tibet, the diurnal, seasonal and interannual variations in soil temperature at BJ site (31.37° N, 91.90° E; 4509 a.s.l.) near Naqu in the central Tibetan Plateau were analyzed. Results showed that the average diurnal variation in soil temperature at 4 and 20 cm depth can be described as sinusoidal curve, which is consistent with the variation of solar radiation. However, the average diurnal variation in soil temperature under 60 cm was very weak. The average diurnal amplitude in soil temperature decreased by the exponential decay function with the increase of soil depth (R2=0.92, p<0.01). It is demonstrated that the average diurnal maximum soil temperature decreased by the exponential decay function with the increase of soil depth (R2 =0.78, p<0.01). In contrast, the average diurnal minimum soil temperature increased by the exponential grow function with increasing of soil depth (R2=0.86, p<0.01). There were a linear negative correlation between the average annual maximum Ts and soil depth (R2=0.96, p<0.01), a logarithmic function relationship between the average annual minimum soil temperature and soil depth (R2=0.92, p<0.01). The average seasonal amplitude in soil temperature followed the exponential decay function with the increase of soil depth (R2=0.98, p<0.01). The mean annual soil temperature in each layer indicated a warming trend prominently. During the study period, the mean annual soil temperature at 4, 20, 40, 60, 80, 100, 130, 160, 200 and 250 cm depth increased by 0.034, 0.041, 0.061, 0.056, 0.062, 0.050, 0.057, 0.051, 0.047 and 0.042 °C /a, respectively.