Abstract
Changes in the freeze–thaw cycles of shallow soil have important consequences for surface and subsurface hydrology, land–atmosphere energy and moisture interaction, carbon exchange, and ecosystem diversity and productivity. This work examines the shallow soil freeze–thaw cycle on the Tibetan Plateau (TP) using in–situ soil temperature observations in 0–20 cm soil layer during July 1982–June 2017. The domain and layer averaged beginning frozen day is November 18 and delays by 2.2 days per decade; the ending frozen day is March 9 and advances by 3.2 days per decade; the number of frozen days is 109 and shortens by 5.2 days per decade. Altitude and latitude combined could explain the spatial patterns of annual mean freeze–thaw status well. Stations located near 0 °C contour line experienced dramatic changes in freeze–thaw cycles as seen from subtropical mountain coniferous forest in the southern TP. Soil completely freezes from surface to 20-cm depth in 15 days while completely thaws in 10 days on average. Near-surface soil displays more pronounced changes than deeper soil. Surface air temperature strongly influences the shallow soil freeze–thaw status but snow exerts limited effects. Different thresholds in freeze–thaw status definition lead to differences in the shallow soil freeze–thaw status and multiple-consecutive-day approach appears to be more robust and reliable. Gridded soil temperature products could resolve the spatial pattern of the observed shallow soil freeze–thaw status to some extent but further improvement is needed.
Highlights
As a sensitive indicator of climate change, shallow soil freeze– thaw cycles (SSFTC for short) affect the water and energy flux interactions between land surface and atmosphere (Zhang and Armstrong 2001)
The annual mean beginning frozen day (BFD) averaged over the 0–20 cm depth ranges from the earliest Day 113 (October 21) at Wudaoliang to the latest Day 175 (December 22) at Luolong with a regional average of Day 141 (November 18) (Figs. 2a–b)
BFDs occur primarily in the sparsely vegetated central Tibetan Plateau (TP) and the Qilian Mountains with high elevation, while late BFDs are found over the dry and low Qaidam Basin, and the relatively densely vegetated northeast and southern TP (Fig. 2b). This can be seen from the distributions of annual mean BFD against vegetation types presented in Fig. 2c which shows that low– biomass types such as Alpine meadow (AM), Alpine grassland (AG), Alpine shrubland (AS), and Temperate desert (TD) correspond to early BFDs while the high-biomass Subtropical mountain coniferous forest (SMCF) corresponds to late BFDs, with Temperate forest grassland (TFG) lying in between
Summary
As a sensitive indicator of climate change, shallow soil (usually within 1 m depth below surface) freeze– thaw cycles (SSFTC for short) affect the water and energy flux interactions between land surface and atmosphere (Zhang and Armstrong 2001). Changes in SSFTC have important consequences for land surface and subsurface hydrological processes (e.g., Wang et al 2009; Cheng and Jin 2013; Cuo et al 2015; Zheng et al 2018; Smith et al 2019), soil microbial processes (Urakawa et al 2014), carbon exchange (Turetsky et al 2019), vegetation growth and ecological processes (Van Wijk et al 2003; Beer et al 2007; Kreyling et al 2008; Cuo et al 2016). SSFTC adds complexity to the hydrological cycles due to the intrinsic connections of both processes to groundwater (Bookhagen 2012)
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