Abstract:

Based on daily mean soil temperature at 5-cm depth from 9 meteorological stations in the Yarlung Zangbo River and its two tributaries in Tibet from 1981 to 2017, the spatial-temporal distribution and climate abrupt characteristics of the mean soil temperature and its critical temperature at 5-cm depth have been comprehensively analyzed using the methods of linear regression and Mann-Kendall test. In particular, the dependence of the change rates of mean soil temperature at 5-cm depth has been explored on the altitudes and longitudes as well. The results show that the annual and seasonal mean soil temperature at 5-cm depth increases gradually from west to east but decreases with the altitude. During 1981-2017, the monthly mean soil temperature exhibits a significantly increasing trend with a rate of 0.23-0.98 ℃/10a, with a peak value occurring in April and the trough value in July. Also, the annual mean soil temperature exhibits a significant upward trend at a rate of 0.58 ℃/10a. Noticeably, the maximum increasing rate occurs in spring and the minimum in summer. In terms of the critical soil temperature ≥ 12 ℃ at 5-cm depth, the first day occurred much earlier, whereas the terminal day was postponed, the duration and elevated accumulated temperature were prolonged. Similarly, the critical soil temperature ≥ 14 ℃ exhibits a similar pattern albeit a larger amplitude. As for the trend over decadal timescale, the annual and seasonal mean soil temperature at 5-cm depth in the watershed investigated here exhibits a pronounced increasing trend. The selection of ≥ 12 ℃ (14 ℃) critical soil temperature at 5-cm depth points to the predated first day, prolonged duration and increased cumulated temperature in the first decade of the 21st century. The M-K mutation test shows that the abrupt change of seasonal mean soil temperature at 5-cm depth in spring and autumn occurred in 2004 and 2005, respectively, whereas the abrupt changes occurred in the winter of 1997, and the mutation of annual mean soil temperature was found in 2005. Furthermore, it was also found that abrupt change point occurred in 2004 for the first day of critical soil temperature ≥ 12 ℃, and a later abrupt point happened in 2014 for the terminal day, as compared to the duration in 1997, and the accumulated temperature occurred in 2005. By comparison, the abrupt change point for the first day, the terminal day, the duration and the accumulated temperature for the critical soil temperature ≥ 14 ℃ at 5-cm depth occurred around 2004. Compared to the variation in air temperature, soil temperature at 5-cm depth from 1981 to 2017 had a larger increasing rate of temperature and but a delayed abruption change point.

Key words: variation trend, climate abrupt, mean soil temperature at 5-cm depth, critical temperature, the Yarlung Zangbo River and its two tributaries