Hillslope-scale variability in seasonal frost depth and soil water content investigated by GPR on the southern margin of the sporadic permafrost zone on the Tibetan Plateau

Abstract:

Ground temperature data show that permafrost has recently been absent at a site on the southern edge of the sporadic permafrost zone on the Tibetan Plateau (TP). A detailed survey of seasonal frost depth (SFD) and soil water content (SWC) here is significant for understanding the hydrological response to thawing permafrost. However, little is known about the spatial heterogeneity of SFD and SWC at the hillslope-scale in this vulnerable permafrost region. Thus, high-frequency ground-penetrating radar (GPR) was applied to a field site that varied in terms of topography (slope, aspect and elevation) and surface environments (vegetation cover and stream presence). The GPR data and accompanying field observations of gravimetric water content and frost depth revealed a spatial variation at the hillslope-scale in SFD and SWC, and indicated that topography, vegetation and stream distribution significantly influence the patterns observed. The average SFD was much deeper along the north-facing slope, compared to the south-facing slope in early May and its thickness varied considerably with altitude along each slope. An increase in the extent of vegetation cover correlated with decreasing SFD. The SWC at shallow depth was higher along the south-facing slope than along the north-facing slope in the beginning of the thawing period. For slopes of both aspects, the SWC vertical profiles exhibited a similar variability, with SWC decreasing with depth, but at different rates. This study demonstrates that GPR provides an appropriate method for quantifying soil water content at hillslope scale on the Tibetan Plateau.