The cryosphere is defined by the presence of frozen water in its many forms: glaciers, ice caps, ice sheets, snow, permafrost, and river and lake ice. In the extended Hindu Kush Himalaya (HKH) region, including the Pamirs, Tien Shan and Alatua, the cryosphere is a key freshwater resource, playing a vital and significant role in local and regional hydrology and ecology. Industry, agriculture, and hydroelectric power generation rely on timely and sufficient delivery of water in major river systems; changes in the cryospheric system may thus pose challenges for disaster risk reduction in the extended HKH region.

The complementary relationship (CR) of evapotranspiration allows the estimation of the actual evapotranspiration rate (ET_a) of the land surface using only routine meteorological data, which is of great importance in the Tibetan Plateau (TP) due to its sparse observation network. With the highest in situ automatic climate observation system in a typical semiarid alpine steppe region of the TP, the wind function of Penman was replaced by one based on the Monin‐Obukhov Similarity theory for calculating the potential evapotranspiration rate (ET_p); the Priestley‐Taylor coefficient, α, was estimated using observations in wet days; and the slope of the saturation vapor pressure curve was evaluated at an estimate of the wet surface temperature, provided the latter was smaller than the actual air temperature. A symmetric CR was obtained between the observed daily actual and potential evapotranspiration. Local calibration of the parameter value (in this order) is key to obtaining a symmetric CR: α, wet environment air temperature (T_wea), and wind function. Also, present symmetric CR contradicts previous research that used default parameter values for claiming an asymmetric CR in arid and semiarid regions of the TP. The effectiveness of estimating the daily ET_a via symmetric CR was greatly improved when local calibrations were implemented. At the same time, an asymmetric CR was found between the observed daily ET_a and pan evaporation rates (E_pan), both for D20 aboveground and E601B sunken pans. The daily ET_a could also be estimated by coupling the E_pan of D20 aboveground and/or E601B sunken pan through CR. The former provided good descriptors for observed ET_a, while the latter still tended to overestimate it to some extent.

Quantitative estimation of actual evapotranspiration (ET_a) by in situ measurements and mathematical modeling is a fundamental task for physical understanding of ET_a as well as the feedback mechanisms between land and the ambient atmosphere. However, the ET_a information in the Tibetan Plateau (TP) has been greatly impeded by the extremely sparse ground observation network in the region. Approaches for estimating ET_a solely from routine meteorological variables are therefore important for investigating spatiotemporal variations of ET_a in the data‐scarce region of the TP. Motivated by this need, the complementary relationship (CR) and Penman‐Monteith approaches were evaluated against in situ measurements of ET_a on a daily basis in an alpine steppe region of the TP. The former includes the Nonlinear Complementary Relationship (Nonlinear‐CR) as well as the Complementary Relationship Areal Evapotranspiration (CRAE) models, while the latter involves the Katerji‐Perrier and the Todorovic models. Results indicate that the Nonlinear‐CR, CRAE, and Katerji‐Perrier models are all capable of efficiently simulating daily ET_a, provided their parameter values were appropriately calibrated. The Katerji‐Perrier model performed best since its site‐specific parameters take the soil water status into account. The Nonlinear‐CR model also performed well with the advantage of not requiring the user to choose between a symmetric and asymmetric CR. The CRAE model, even with a relatively low Nash‐Sutcliffe efficiency (NSE) value, is also an acceptable approach in this data‐scarce region as it does not need information of wind speed and ground surface conditions. In contrast, application of the Todorovic model was found to be inappropriate in the dry regions of the TP due to its significant overestimation of ET_a as it neglects the effect of water stress on the bulk surface resistance. Sensitivity analysis of the parameter values demonstrated the relative importance of each parameter in the corresponding model. Overall, the Nonlinear‐CR model is recommended in the absence of measured ET_a for local calibration of the model parameter values.

Identifying water and snow cover/glaciers (SCG) accurately is of great importance for monitoring different water resources in the Tibetan Plateau. However, discriminating between water and SCG remains a difficult task because of their similar spectral characteristic according to the physical principles of remote sensing. To efficiently distinguish different kinds of water resources automatically, here we proposed two new indices including: (i) the normalized difference water index with no SCG information (NDWI_ns) to extract lake water and suppress SCG: and (ii) the normalized difference snow index with no water information (NDSI_nw) to extract SCG and suppress lake water. Both new water and snow indices were tested in the Tibetan Plateau using Landsat series, showing that the overall accuracies of NDWI_ns and NDSI_nw were in the range of 94.6–97.0% and 94.9–97.0% in mapping the lake water from SCG and mapping the SCG from lake water, respectively. Further comparisons suggest that these new two indices improved upon the previous normalized difference snow index/modified normalized difference water index (NDSI/MNDWI) in mapping the water body and SCG. While the present study only focuses on the validation over certain areas in Tibetan Plateau, the newly proposed NDWI_ns and NDSI_nw have the potential for better monitoring the lake water and snow/glacier areas over other cold regions around the globe.

Abstarct: We used atmospheric air-temperature data from the Chinese radiosonde network to analyze changes in freezing-level heights (FLHs) during the past 48 years and studied their impacts on the cryosphere. We examined radiosonde time-series data from 92 selected Chinese radiosonde network stations. Generally, FLH exhibited a latitudinal zone, declining from the south. The FLH trend during 1958–2005 showed spatial inhomogeneity, most uniform distributions during autumn, and significant upward trends. Temporal variability of FLH in eastern China was briefly associated with El Niño Southern Oscillation events, but the causes of FLH changes in western China require further investigation. FLH in western and northern China has mostly increased since 1958, and might be considered a possible indicator of cryospheric change during the second half of the 20th century. There were significant correlations between FLH and changes in snow cover, glaciers and permafrost.

Hoh Xil is an uninhabited extremity secluded on the Tibetan Plateau hinterland. A complete mapping of ground motion variation in Hoh Xil is essential for in-depth understanding of terrain’s responses to climate change on the Tibetan Plateau. However, the inaccessibility and extremely harsh environment impeded extensive field investigations on landform alteration and its formative process. Such difficulty can be resolved by Interferometric Synthetic Aperture Radar (InSAR), which enables a broad detection of subtle permafrost motions at millimeter precision. This study, for the first time, accomplished a Multi-temporal InSAR (MT-InSAR) deformation mapping from 2015 to 2020 in Hoh Xil, with a wide coverage of about 200,000 km ² . 1,592 Sentinel-1 images were processed based on the small baseline subset (SBAS) technique. The results show that Hoh Xil was experiencing dynamic permafrost disturbances. Thawing permafrost with both the linear subsidence rate higher than 2 mm/yr and the periodic amplitude over 2 mm was primarily detected in areas of flat or gentle slopes. The InSAR cumulative deformation is highly correlated with permafrost thawing depth. Significant lag times were identified between seasonal oscillation of InSAR deformation and land surface temperature (LST). Thermokarst landforms of retrogressive thaw slumps and thermokarst lakes broadly formed and dynamically evolved as a consequence of permafrost degradation. Particularly, widespread thawing permafrost characterized by the spatial clustering of thermokarst lakes appeared to occur in areas adjacent to large lakes. The discovered dynamic permafrost disturbances in Hoh Xil manifested even the secluded Tibetan Plateau hinterland was facing the threat of climate change.