Land surface temperature (LST) is an essential climate variable (ECV) which can be estimated from appropriate measurements of the surface thermal infrared (TIR) radiance. LST varies on a very short time scale and closely depends on the illumination and scan angles considered. To fully exploit LST products, a method for reconstructing the temporal profile and the angular dependence at the same time is proposed here. A combined visible- thermal envelope method (VT-KDTC) is built using kernel-driven (KD) and diurnal temperature cycle (DTC) models, referring to the surface structure and thermal factors, respectively. To demonstrate the reliability of the approach, TIR data from the geostationary satellite Himawari 8 are combined with visible and near-infrared (VNIR) data from the polar orbit satellite Sentinel-3A/3B. In addition to satellite observations, a synthetic dataset from the Soil Canopy Observation, Photochemistry and Energy Fluxes (SCOPE) model is also generated. Considering an anisotropy model in addition to the DTC model leads to a method displaying a better ability to simulate LSTs with a root mean squared error (RMSE) of 0.48 K against the original satellite results, compared to only the DTC model up to 1.44 K. By utilizing the field measurements as a reference, the reconstructed results are improved with a total bias of 0.72K and an RMSE of 2.58 K. Compared to the original results without correction, approximately 41% and 10% decreases are obtained in bias and RMSE, respectively. Our proposed method can also achieve LST downscaling supported by the higher spatial resolution of VNIR data when the temperature difference is assumed to be homogeneous within the coarse pixels. Thus, a simple achievable solution can be used for temperature reconstruction to enhance the quality of the LST product.

Study region: The Northwest inland basins of China (NWC).Study focus: Terrestrial water resources, especially groundwater resources, are the main source of water for human activities and for maintaining the stability of the ecological environment in NWC. Excessive consumption of water resources will seriously affect the sustainable utilization of water resources and ecological security in this region. Therefore, it is urgent to clarify the long-term changes in water storage in this area in order to handle the pressure of future water re-sources and the natural environment. Using GRACE satellite datasets and global hydrological models (GHMs) products, this study analyzed spatiotemporal variations in terrestrial water storage anomalies (TWSA), groundwater storage anomalies (GWSA), soil moisture, snow water equivalent, and canopy interception combined anomalies (SSCA) in NWC through the application of the water balance, trend decomposition, and empirical orthogonal decomposition methods. Furthermore, the driving factors of water storage change and feasible water resource manage-ment strategies were discussed. New hydrological insights for the region: TWSA in the NWC has experienced a continuous decline over the past nearly 40 years, while SSCA has shown a weak increasing trend (0.03 cm yr-1). Since the availability of glacial retreat data (2003-2016), glacial water storage in the NWC has decreased by 0.09 cm per year, while TWSA, SSCA, and GWSA have changed at rates of -0.25, 0.02, and -0.18 cm yr-1, respectively. The North Tianshan Rivers Basin has become one of the areas with the most severe groundwater depletion in China. 2005-2010 was a turning period in the changes of TWSA, followed by widespread water loss across the NWC. Glacier and snow melt are the most important factors for the decline of TWSA in the Tianshan mountains area, and over -exploitation of groundwater by human activities is a secondary factor. For other regions, Groundwater losses remain the most significant contributor to TWSA losses. The massive loss of water storage in the Tianshan Mountains area, especially the accelerated retreat of glaciers, will affect the stable water supply to the middle and lower reaches of the oasis region, perhaps leading to increased groundwater extraction, which will threaten regional water security and sustainable development. Developing a water-saving society and implementing inter-basin water transfer arefeasible ways to alleviate the water resource crisis. Conducting a comprehensive analysis of all inland rivers in China helps to facilitate horizontal comparisons between various basins, thereby providing more comprehensive insights of water storage fluctuations. The data on water storage changes, extending back to 1980, provide a longer-term perspective on water resource changes in the region, which can contribute to enhancing water resource security and ecological environ-mental protection.

The lakes on the Qinghai-Tibetan Plateau have undergone substantial changes. As intensive cryospheric components change, the response of the lake dynamics to climatic factors, glacier-snow melting, and permafrost thawing has been complex. Based on Landsat images, meteorological data, and glacier and permafrost data, the spatial-temporal changes in the lake area on the northeastern Tibetan Plateau between 1988 and 2019 were analyzed and the driving factors behind the lake changes were further explored. The results suggest that the regional lake area increased from 1988 to 2019 at rates of 0.01-16.03 km(2)/yr. It decreased during 1988-2000, quickly increased during 2000-2012, and rapidly increased during 2012-2019. The most significant lake expansion occurred in sub-region I, which is the source region of the Yangtze River Basin. There was a sharper increase during 2012-2019 than during 2000-2012 in sub-region II (the source region of the Yellow River Basin and the Qinghai Lake Basin) and sub-region III (the Qaidam Basin). The significant lake expansion occurred about 12 years earlier in sub-region I than in sub-regions II and III. This dramatic change in the lake area was closely associated with the annual precipitation, and precipitation was the primary driving factor. Although serious glacier retreat occurred, most of the lakes in the sub-regions were non-glacier-fed lakes. The correlation between glacier ablation and the change in the lake area was poor, which suggests that glacial meltwater was not the replenishment source of most of the lakes in this region. A more accelerated increase in the active layer thickness occurred (1.90 cm/yr), which was consistent with the more rapid lake expansion, and the permafrost degradation further intensified the lake expansion.

For summer-accumulation-type glaciers, the glaciological literature is lacking studies on determining the snow line altitude (SLA) from optical images at the end of the summer as an indicator of the equilibrium line altitude (ELA). This paper presents a workflow for extracting the SLA from Landsat images based on the variation in the albedo with the altitude in the central line area of glaciers. The correlation of >0.8 at the 99% confidence level between the retrieved SLAs with ELAs derived from the interpolation of ground-based, mass balance measurements indicated that the workflow can be applied to derive the SLA from end-of-summer satellite data as an indicator of ELA. The ELA was under-estimated by the calculated SLA. The relationship between the end-of-summer SLA and the ELA depends on the intensity of glacier melting. Subsequently, the workflow was applied to the seven glaciers in the Eastern Tien Shan Mountains, and a time series of the SLA was obtained using 12 end-of-summer Landsat scenes from 1994 to 2016. Over the whole study period, a mean SLA of 4011.6 +/- 20.7 m above sea level (a.s.l.) was derived for the seven investigated glaciers, and an increasing SLA was demonstrated. The increase in SLAs was consistent for the seven glaciers from 1994 to 2016. Concerning the spatial variability, the east-west difference was prominent, and these differences exhibited a decreasing trend. The average SLA of each glacier is more influenced by its morpho-topographic variables. The interannual variations in the average SLA are mainly driven by the increasing summer air temperature, and the high correlation with the cumulative summer solid precipitation reflects the characteristics of the summer-accumulation-type glaciers.

Glaciers are a reliable freshwater resource in arid regions of West China and the vulnerability of its changes is closely related to regional ecosystem services and economic sustainable development. Here, we took the Qilian Mountains as an example and analyzed the spatiotemporal characteristics of glacier changes from 1998 to 2018, based on remote sensing images and the Second Chinese Glacier Inventory. We estimated the basic organizational framework and evaluation index system of glacier change vulnerability from exposure, sensitivity and adaptability, which covered the factors of physical geography, population status and socio-economic level. We analyzed the spatial and temporal evolutions of glacier change vulnerability by using the vulnerability evaluation model. Our results suggested that: (1) Glacier area and volume decreased by 71.12 +/- 98.98 km(2) and 5.59 +/- 4.41 km(3), respectively, over the recent two decades, which mainly occurred at the altitude below 4800 m, with an area shrinking rate of 2.5%. In addition, glaciers in the northern aspect (northwest, north and northeast) had the largest area reduction. Different counties exhibited remarkable discrepancies in glacier area reduction, Tianjuan and Minle presented the maximum and minimum decrease, respectively. (2) Glacier change vulnerability level showed a decreasing trend in space from the central to the northwestern and southeastern regions with remarkable differences. Vulnerability level had increased significantly over time and was mainly concentrated in moderate, high and extreme levels with typical characteristics of phases and regional complexity. Our study can not only help to understand and master the impacts of recent glacier changes on natural and social aspects but also be conducive to evaluate the influences of glacier retreat on socio-economic developments in the future, thus providing references for formulating relevant countermeasures to achieve regional sustainable development.

This article attempts to predict the spatiotemporal changes of permafrost in the Headwater Area of the Yellow River (HAYR) on the northeastern Qinghai-Tibet Plateau, Southwest China by using field monitoring and numerical models. Permafrost in the HAYR is categorized into four types: low- and high-ice-content high-plain permafrost and low- and high-ice-content alpine permafrost. According to these permafrost types, changes in permafrost temperature were calculated by coupling a geometric model with the soil thermal conduction model. Based on the calculation results, this paper evaluates the changes of permafrost in the HAYR over the past 50 years and predicts the change trends of permafrost in the HAYR under the scenarios of RCP2.6, RCP6.0, and RCP8.5 for possible climate change in 2050 and 2010 from the Intergovernmental Panel on Climate Change Fifth Assessment Report. The results show that (a) in the process of permafrost degradation, the same permafrost type at different degradation stages results in different modes and rates of increasing temperature. The response of permafrost to climate change differs in various degradation stages of permafrost; (b) from 1972 to 2012, the areal extent of permafrost degradation was 1,056 km(2), resulting from a sharp air temperature increase after the 1980s. By 2050, the areal extent of permafrost degradation into seasonal frost is similar under the three scenarios of climate change. The areal extent of permafrost degradation is 2,224, 2,347, and 2,559 km(2) or 7.5%, 7.9%, and 8.6% of the total area in the HAYR, respectively. In RCP2.6, the areal extent of permafrost degradation into seasonal frost by 2100 would be approximately 3,500 km(2) greater than that by 2050. In RCP6.0, the areal extent of permafrost degradation by 2100 would be 10,000 km(2) or 32.9% of the total area in the HAYR. In RCP8.5, the area of permafrost degradation by 2100 would be 18,492 km(2) or 62.2% of the total area in the HAYR; (c) the active layer thickness (ALT) in the HAYR would increase significantly. The average of the ALT was 1.51 m by 1972 and 2.01 m by 2012, respectively. Under the RCP2.6, RCP6.0, and RCP8.5 scenarios, the basin-wide average of ALT would be 2.21, 2.40, and 3.08 m by 2050 and 2.78, 4.07, and 4.39 m by 2100, respectively.