1981-2017年西藏“一江两河”流域5cm地温及其界限温度时空变化特征

利用西藏“一江两河”流域9个气象站点1981-2017年逐日5 cm地温资料,采用线性回归、Mann-Kendall非参数检验等方法,分析了该流域5 cm地温及其界限温度的时空分布、突变特征,并探讨了地温变化率与经纬度、海拔高度之间的关系。结果表明：“一江两河”流域年、季平均5 cm地温总体呈自西向东递增分布,并随海拔升高而降低。1981-2017年流域月平均5 cm地温均呈显著升高趋势,升温率为0.23~0.98 ℃/10a,以4月最大,7月最小。年平均5 cm地温以0.58 ℃/10a的速率显著升高,各季地温也都趋于上升,其中春季升温率最大,夏季最小。5 cm地温≥ 12 ℃表现为初日提早、终日推迟、持续日数延长、积温增加的年际变化趋势。同样,≥ 14 ℃界限温度也有类似的变化,但变幅比≥ 12 ℃的要大。在10年际变化尺度上,流域年、季平均5 cm地温表现为逐年代际升高的变化特征。5 cm地温≥ 12 ℃和≥ 14 ℃界限温度在21世纪前10年呈初日提早、持续日数延长和积温偏多的态势。M-K检验显示,除夏季外,其他三季平均5 cm地温均发生了气候突变,其中春季和秋季的突变点分别出现在2004年和2005年,而冬季发生在1997年;年平均5 cm地温在2003年出现了突变。5 cm地温≥ 12 ℃初日的突变点在2004年,终日发生突变时间较晚,为2014年;持续日数突变点较早,在1997年;积温在2005年发生了突变。而5 cm地温≥ 14 ℃界限温度的突变点发生在2004年前后。相对于气温的变化,5 cm地温的升温幅度更大,突变时间较晚。

Spatial-temporal change of mean soil temperature and its critical temperature at 5 cm depth in the region of the Yarlung Zangbo River and its two tributaries of Tibet during 1981-2017

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.