Abstract. Selin Co, located within permafrost regions surrounded by glaciers, has exhibited the greatest increase in water storage among all the lakes on the Tibetan Plateau over the last 50 years. Most of the increased lake water volume has been attributed to increased precipitation and the accelerated melting of glacier ice, but these processes are still not sufficient to close the water budget with the expansion of Selin Co. Ground ice meltwater released by thawing permafrost due to continuous climate warming over the past several decades is regarded as another source of lake expansion. This study presents the first attempt to quantify the water contribution of ground ice melting to the expansion of Selin Co by evaluating the ground surface deformation. We monitored the spatial distribution of surface deformation in the Selin Co basin using the small baseline subset (SBAS) interferometric synthetic aperture radar (InSAR) technique and compared the results with the findings of field surveys. Then, the ground ice meltwater volume in the watershed was calculated based on the cumulated settlement. Finally, this volume was compared with the lake volume change during the same period, and the contribution ratio was derived. SBAS-InSAR monitoring during 2017–2020 illustrated widespread and large subsidence in the upstream section of the Zhajiazangbu subbasin, where widespread continuous permafrost is present. The terrain subsidence rate was normally between 5 and 20 mm a−1, indicating rapid ground ice loss in the region. The ground ice meltwater was released at a rate of ∼57×106 m3 a−1, and the rate of increase in lake water storage was ∼485×106 m3 a−1 during the same period, with ground ice meltwater contributing ∼12 % of the lake volume increase. This study contributes to explaining the rapid expansion of Selin Co and equilibrating the water balance at the watershed scale. More importantly, the proposed method can be extended to other watersheds underlain by permafrost and help in understanding the hydrological changes in these watersheds.

Soil moisture (SM) products presently available in permafrost regions, especially on the Qinghai–Tibet Plateau (QTP), hardly meet the demands of evaluating and modeling climatic, hydrological, and ecological processes, due to their significant bias and low spatial resolution. This study developed an algorithm to generate high-spatial-resolution SM during the thawing season using Sentinel-1 (S1) and Sentinel-2 (S2) temporal data in the permafrost environment. This algorithm utilizes the seasonal backscatter differences to reduce the effect of surface roughness and uses the normalized difference vegetation index (NDVI) and the normalized difference moisture index (NDMI) to characterize vegetation contribution. Then, the SM map with a grid spacing of 50 m × 50 m in the hinterland of the QTP with an area of 505 km × 246 km was generated. The results were independently validated based on in situ data from active layer monitoring sites. It shows that this algorithm can retrieve SM well in the study area. The coefficient of determination (R2) and root-mean-square error (RMSE) are 0.82 and 0.06 m3/m3, respectively. This study analyzed the SM distribution of different vegetation types: the alpine swamp meadow had the largest SM of 0.26 m3/m3, followed by the alpine meadow (0.23), alpine steppe (0.2), and alpine desert (0.16), taking the Tuotuo River basin as an example. We also found a significantly negative correlation between the coefficient of variation (CV) and SM in the permafrost area, and the variability of SM is higher in drier environments and lower in wetter environments. The comparison with ERA5-Land, GLDAS, and ESA CCI showed that the proposed method can provide more spatial details and achieve better performance in permafrost areas on QTP. The results also indicated that the developed algorithm has the potential to be applied in the entire permafrost regions on the QTP.

Abstract. Serling Co lake, surrounded by permafrost and glacier-occupied regions, has exhibited the greatest increase in water storage over the last 50 years among all the lakes on the Tibetan Plateau. However, increases in precipitation and glacial melting are not enough to explain the increased water volume of lake expansion. The magnitude of the contribution of thawing permafrost to this increase under climate warming remains unknown. This study made the first attempt to quantify the water contribution of ground ice melting to the expansion of Serling Co lake by evaluating the ground surface deformation. We monitored the spatial distribution of surface deformation in the Serling Co basin using the SBAS-InSAR technique and compared it with the findings of field surveys. Then, the ground ice meltwater volume in the watershed was calculated based on the long-term deformation rate. Finally, this volume was compared with the lake volume change during the same period, and the contribution ratio was derived. SBAS-InSAR monitoring during 2017–2020 illustrated widespread and large subsidence in the upstream section of the Zhajiazangbu subbasin, where widespread continuous permafrost is present. The terrain subsidence was normally between 5 and 20 mm/a, indicating rapid ground ice loss in the region. The ground ice meltwater reached 56.0 × 106 m3/a, and the rate of increase in lake water storage was 496.3 × 106 m3/a during the same period, with ground ice meltwater contributing 11.3 % of the lake volume increase. This study is especially helpful in explaining the rapid expansion of Serling Co lake and equilibrating the water balance at the watershed scale. More importantly, the proposed method can be easily extended to other watersheds underlain by permafrost and to help understand the hydrologic changes in these watersheds.

Abstract. Permafrost has been warming and thawing globally, with subsequent effects on the climate, hydrology, and the ecosystem. However, the permafrost thermal state variation in the northern lower limit of the permafrost zone (Xidatan) on the Qinghai–Tibet Plateau (QTP) is unclear. This study attempts to explore the changes and variability in this permafrost using historical (1970–2019) and future projection datasets from remote-sensing-based land surface temperature product (LST) and climate projections from Earth system model (ESM) outputs of the Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5, CMIP6). Our model considers phase-change processes of soil pore water, thermal-property differences between frozen and unfrozen soil, geothermal flux flow, and the ground ice effect. Our model can consistently reproduce the vertical ground temperature profiles and active layer thickness (ALT), recognizing permafrost boundaries, and capture the evolution of the permafrost thermal regime. The spatial distribution of permafrost and its thermal conditions over the study area were controlled by elevation with a strong influence of slope orientation. From 1970 to 2019, the mean annual ground temperature (MAGT) in the region warmed by 0.49 ∘C in the continuous permafrost zone and 0.40 ∘C in the discontinuous permafrost zone. The lowest elevation of the permafrost boundary (on the north-facing slopes) rose approximately 47 m, and the northern boundary of discontinuous permafrost retreated southwards by approximately 1–2 km, while the lowest elevation of the permafrost boundary remained unchanged for the continuous permafrost zone. The warming rate in MAGT is projected to be more pronounced under shared socioeconomic pathways (SSPs) than under representative concentration pathways (RCPs), but there are no distinct discrepancies in the areal extent of the continuous and discontinuous permafrost and seasonally frozen ground among SSP and RCP scenarios. This study highlights the slow delaying process of the response of permafrost in the QTP to a warming climate, especially in terms of the areal extent of permafrost distribution.

Abstract. Permafrost has been warming and thawing at a global scale with subsequent effects on the climate, hydrological, ecosystem and engineering system. However, the variation of permafrost thermal state in the northern lower limit of the permafrost zone (Xidatan) on the Qinghai–Tibetan Plateau (QTP) is unclear. To evaluate and project the permafrost changes, this study simulated the spatiotemporal dynamics of this marginal permafrost historically (1970–2019) based on the detailed investigation and monitoring datasets from 1987 in this study region, improved remote sensing-based Land Surface Temperature product (LST) and climate projections from Global Climate Model (GCM) outputs of Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5, CMIP6). Our model takes into consideration of phase change processes of soil pore water, thermal property difference between frozen and thawed soil, geothermal flux flow, and ground ice effect. The results indicate that 1) our model can consistently reproduce the ground temperature field and active layer thickness (ALT), is superior in recognizing permafrost boundaries, and would realistically capture the evolution of the permafrost thermal regime, 2) spatial distribution of permafrost and its thermal conditions over the study area were controlled by elevational with a strong influence of slope aspects, 3) from 1970 to 2019, the regional averaged means annual ground temperature (MAGT) had warmed by 0.49 °C in the continuous permafrost zone and 0.40 °C in the discontinuous permafrost zone, and the lowest elevation of permafrost boundary (on north-facing slopes) rose approximately 47 m, as well as the northern boundary of discontinuous permafrost has approximately retreated southwards 1~2 km, while the lowest elevation of permafrost boundary remains unchanged for continuous permafrost zone, 4) the warming rate in MAGT is projected to be slighter higher under Shared Socioeconomic Pathways (SSPs) than that of Representative Concentration Pathways (RCPs), but no distinct discrepancies in the areal extent of the continuous, discontinuous permafrost and seasonally frozen ground among SSP and RCP scenarios. This study highlights the slow delaying process in the response of mountain permafrost to a warming climate, especially in terms of the areal extent of permafrost distribution.