Southeastern Tibetan Research Articles

Landscape change in response to multiperiod glacial debris flows in Peilong catchment, southeastern Tibet

High-magnitude glacial debris flows in small basins in Himalayas have a significant impact on landscape. The Peilong catchment, a tributary of the Parlung Zangbo River in southeastern Tibet, was chosen as a case study of topographic response to multi-period glacial debris flows. There were no recorded debris flows in the catchment before 1983, but four large-scale glacial debris flow events with peak discharge up to 8195 m3/s blocked the river during 1983–1985 and in 2015. A combination of field survey, examination of historical records and interpretation of multi-period remote sensing images was used to assess triggering factors and geomorphic impact of the events. The results show that the debris flows during 1983 and 1985 may be attributed to seismic events in 1981 and 1982, while the event in 2015 resulted from large amount of landslide deposits caused by glacier retreat during 1993–2013 and high precipitation in 2015. In the upper-midstream broad valley, erosion and accumulation of the flows changed the channel morphology, resulting in course diversion. In the lower-midstream narrow valley, lateral erosion of debris flows induced a large number of landslides but had little impact on the channel longitudinal profile. The ability of massive glacial debris flows to change valley topography is more than ten times that of regular water flows. The landscape of the accumulation fan at the outlet of the valley is controlled by the interaction between the sediment transportation capacity of debris flows and erosional capacity of the main river. The flow transport capacity of the Peilong is greater than the delivery capacity of the Parlung Zangbo, resulting in continuous aggradation of the confluence zone.

Risk assessment of geological disasters in Nyingchi, Tibet

Abstract To reduce the adverse effects of geological disasters on Nyingchi, this article selected nine highly correlated risk assessment factors for the types of geological disasters such as landslides and debris flows and used two methods of analytic hierarchy process and fuzzy analytic hierarchy process for analysis and comparison. The spatial overlay analysis of each factor with weight is carried out in ArcGIS to make a classification map of geological hazards in Nyingchi. Risk assessment of geological disasters such as collapses, landslides, and mudslides indicates that the slope, distance from the faults, and relative elevation difference of topography have severe impact on natural disasters; at least more than 60% of the areas belong to medium-, high-, and extremely high-risk areas, and it is urgent to strengthen protection and governance; the evaluation results of the fuzzy analytic hierarchy process are more in line with the actual survey situation and are more suitable for Tibet. This research is able to provide a basis for ecological protection, ecological restoration, and geological disaster prevention and management in southeastern Tibet. It also has important reference value for the coordinated development of urban and the environmental projects.

Classification of landslides on the southeastern Tibet Plateau based on transfer learning and limited labelled datasets

ABSTRACT Rainfall and freeze-thaw landslides are common occurrences on southeastern Tibet Plateau; however, it can be difficult to categorize them via field surveys. Landslide classification can be determined using image processing methods, such as deep learning. However, the performance of classification is restricted by limited datasets resulting from small numbers of landslides. A new deep learning method is proposed herein for classifying landslides by remote sensing images using the VGG-19 and transfer learning to compensate for an insufficient number of labelled samples. Transfer learning was used to fine tune the classification model. The results of ablation experiments show that the classification model combined with transfer learning can obtain a near zero total loss value in the training process. The proposed method can achieve a lower loss value than direct learning and has a superior generalization ability. The prediction accuracies, Kappa index and F1-score of VGG-19 when using transfer learning and 30 training samples reached 98%, 0.979 and 0.982 respectively. The experiments indicate that the proposed method can be applied to landslide classification on plateaus.

Evidence for glaciation predating MIS-6 in the eastern Nyainqêntanglha Range, southeastern Tibet

Southeastern Tibet is one of the most glaciated regions on the Tibetan Plateau both at present and during the Quaternary. Numerical dating of glacial deposits has allowed the establishment of a provisional chronology of Quaternary glacial fluctuations in this region, with the oldest glaciation (Guxiang Glaciation) occurring in marine oxygen isotope stage 6 (MIS-6). During our recent field investigations, a morphostratigraphically older lateral moraine than that of the Guxiang Glaciation has been first identified, which is ~500–600 m above the Guxiang Glaciation moraine and discontinuously preserved on valley shoulders in the Bodui Zangbo River valley, eastern Nyainqentanglha Range in southeastern Tibet. Considering the moraine is best preserved at Nitong Village, here we name the glacier advance which deposited the moraine as “Nitong Glaciation”. Using electron spin resonance (ESR) technique, we dated the Nitong Glaciation moraine to 506.3±60.4 ka. Taking into account the age error and climatic conditions, we consider it most likely that the Nitong Glaciation occurred during MIS-12, although it might had happened sometime earlier.

Geophysical constraints on the nature of lithosphere in central and eastern Tibetan plateau

The structure and rheology of lithosphere are fundamental to understanding lithospheric deformation operating within the Tibetan plateau and hence holds significant implications for continental dynamics. Yet, some of these aspects remain less well constrained in Tibet. In this study we construct new models of isotropic-average shear-wave velocity and radial anisotropy from previously published Rayleigh-wave and Love-wave phase velocities at periods of 8–143 s. Integrating with other previously published geophysical data, we demonstrate that a weak mid-to-lower crust is pervasive within the plateau, as directly inferred from seismically low velocity and high conductivity at these depths along with high Moho temperature. When combining with the observation of positive radial anisotropy patterns (Vsh > Vsv), we speculate that this weak mid-to-lower crust could probably flow towards southeastern Tibet, whereas in the Qinling Orogen this crustal flow is absent. This weak mid-to-lower crust in Tibet could decouple upper crust and lower crust/upper mantle, accommodating active deformation in response to the Indian-Asian convergence. Additionally, our results show that the subduction front of Indian lithospheric mantle (LM) could reach to the Bangong-Nujiang-Suture (BNS) in central Tibet and to the Jingsha-River-Suture (JRS) in eastern Tibet. We find no expected high velocity beneath northern Tibet to support southward subduction of the Asian LM. Instead, the northern Tibet is characterized with a continuous low-velocity-anomaly from Moho to 200 km. Coupled with the observations of shallow Curie-point depths and high Moho temperature, we infer that the Tibetan upper mantle in the north is hotter than that in the south, probably resulting from mantle upwelling due to either the Indian LM subduction, or thickened Tibetan lithosphere delamination with some contributions of shear heating and radioactive heating from the thickened Tibetan crust. This study highlights the nature of lithosphere, associated dynamic processes and the overall deformation within the Tibetan plateau.

Contrast between metamorphic and ore-forming fluids in the Ailaoshan belt, southeastern Tibet: New constraints on ore-fluids source for its orogenic gold deposits

The source of ore-forming fluids for orogenic gold deposits is still disputed with models ranging from derivation from metamorphism of continental crust to derivation from metasomatized mantle lithosphere. This dispute is addressed by this study of the Ailaoshan orogenic gold belt, southeastern Tibet, whose evolution involved intense metamorphism on the eastern side of the Ailaoshan shear zone. Both the regional metamorphic barren quartz veins and gold-related quartz veins in this belt are genetically related to movement on the Ailaoshan shear zone and thus formed over the same limited age span. Microthermometry, fluid compositions and stable isotope analyses conducted on both metamorphic fluids extracted from regional metamorphic quartz veins and ore-forming fluids collected from auriferous quartz veins at Zhenyuan and Jinchang gold deposits reveal significant differences. These include: (1) regional metamorphic quartz veins contain three types of fluid inclusions, including CO2-rich, aqueous-CO2 and aqueous types. The CO2-bearing inclusion types are more abundant than those in auriferous quartz veins; (2) those fluids from barren metamorphic veins with high CO2 content (up to 45 mol%) are typified by low-medium temperature (217–418 °C), low salinity (2–8 wt% NaCl) and variable pressure (28–440 MPa), and belong to the NaCl-CO2-H2O system. In contrast, the ore-forming fluids, which exhibit lower temperature (mainly 170–270 °C), higher salinity (mainly 5–13 wt% NaCl), and lower pressure (20–282 MPa and 14–44 MPa for Zhenyuan and Jinchang, respectively), belong to the NaCl-H2O-CO2 system; (3) the metamorphic fluids contain high Ca2+ and Mg2+ in liquids and CO2, N2 and CO in gases, whereas the ore-forming fluids are enriched in K+ in liquid and H2O, CO2, CH4 in gases; (4) the pyrites from regional metamorphic quartz veins have δ34S values ranging from 2.08 to 2.86‰ (mean = 2.50‰), indicating a single sulfur source of metamorphic fluids, thus contrasting with the broad range of δ34S values of ore sulfides. These distinctions negate a genetic relationship between crustal metamorphism and gold mineralization and demonstrate that ore-fluids that deposit orogenic gold deposits, at least in the Ailaoshan belt cannot be derived or evolved from crustally-derived regional metamorphic fluids. This provides a new line of evidence for the derivation of ore-forming fluids from a sub-crustal, metasomatized mantle lithosphere source for at least some orogenic gold deposits.

Effects of slope aspect on altitudinal pattern of soil C:N:P stoichiometry in alpine forest of Tibet

Knowledge of altitudinal patterns in soil C, N and P distribution is important for understanding biogeochemical processes in mountainous forests, yet the influence of slope aspects on soil stoichiometry has been largely neglected in previous studies. In this paper, a total number of 150 topsoil samples at four altitudes (3700, 3900, 4100, 4380 m a.s.l.) on sunny and shady slopes of Sygera mountains in the Southeastern Tibet were collected. Soil C, N and P contents, and pH, were measured. Soil temperature, moisture and richness of plant species were investigated at each sampling site. The results showed that: 1) in sunny slope, soil C, N and P concentrations increased with the increase in altitude, whereas soil C:N, C:P, and N:P decreased along the altitudinal gradient on s. Soil moisture was the main regulator of soil nutrition and stoichiometric ratios. 2) In shady slope, soil C and N contents had no significant difference along the altitudinal gradient except the higher values at low altitude, whereas soil P increased first and then decreased. Soil C:N increased with the increase in altitude, whereas C:P and N:P decreased first and then increased. Soil temperature and species richness were the main factors influencing soil nutrition and stoichiometric ratios. 3) Decoupling of soil C:N:P stoichiometry was observed in shady slope owing to changes in soil pH and temperature. 4) The rich contents of soil C and P were observed at two slopes along the altitudinal gradient, and high capacity of N supply existed at the topsoil in shady slope. These results suggested that slope aspect plays an important role in shaping the altitudinal pattern of soil C:N:P stoichiometry in mountainous forests.

ESR Chronology of Bedrock Fault Activity in Carbonate Area: Preliminary Results from the Study of the Lijiang-Xiaojinhe Fault, Southeastern Tibet, China

Carbonated rocks constitute one of the main lithologies of the southeastern Tibet area, China, a tectonically very active zone. However, due to the lack of suitable dating materials, it is difficult to carry out chronological studies of the local tectonic evolution in such carbonate areas. In the present study, electron spin resonance (ESR) method had been applied on the dating of carbonates heated during fault activity of the Lijiang-Xiaojinhe (LX) Fault, an important active fault located in the northwest of Yunnan Province. Clear displaced landforms show that the fault has undergone strong late-Quaternary activity. During the fault activity, the heat produced by friction lead to the melting of the frictional surface of the rocks, and the melting can attenuate or zero the ESR dating signal of carbonate. The aim of the present paper was to check the ability of carbonate use to chronologically identify fault activity using electron spin resonance (ESR) method. The results showed the last fault activity of the LX fault was dated by ESR about 2.0±0.2 ka ago, in agreement with historical and radiocarbon data. Hence ESR can be if necessary a practicable dating alternative method for the study of fault activity chronology in carbonate rock area.

Spatio-temporal response mechanism of physiological characteristics of Abies georgei var. smithii to altitude

Abies georgei var. smithii (Abies George I varsmithorr. II (Viguie et Gaussen) Cheng et l.) belongs to Abies of Pinaceae, which is distributed in the southeast of Tibet, southwest of Sichuan and northwest of Yunnan. In this study, Abies georgei var. smithii in Sejila Mountain of southeastern Tibet was studied, the contents of Malondialdehyde (MDA), proline (pro), hydrogen peroxide (H2O2), soluble sugar and chlorophyll as well as the activities of superoxide dismutase (SOD) and superoxide dismutase (SOD) in the leaves, twigs, tree stems and roots of Abies georgei var. smithii were measured at different altitudes in the growing and non-growing seasons, single factor analysis of variance and multiple comparison were used to process the data. The results showed that the contents of Malondialdehyde (MDA), proline (pro) and hydrogen peroxide (H2O2) in the roots of Abies georgei var. smithii were significantly higher than those of other tissues in both growing and non-growing seasons, the content of chlorophyll in the leaves increased at first and then decreased with the elevation gradient, on the contrary, the ratio of chlorophyll a/b (CHLA/CHLB) increased with the elevation gradient. The activities of Peroxidase (POD) and superoxide dismutase (SOD) were the lowest. When the altitude was more than 4300m, the activities of POD and SOD decreased. The results showed that the physiological resistance of Abies georgei var. smithii decreased when the elevation was over 3900m, which was not beneficial to the growth and development of Abies georgei var. smithii, therefore, the distribution of Abies georgei var. smithii along the higher elevation gradient was limited by the unfavorable environmental factors at high altitude.

Present‐Day Kinematics in Southeastern Tibet Inferred From GPS Measurements

AbstractQuantifying the regional kinematics is critical for understanding the deformation mechanisms of the Tibetan Plateau. In this study, we construct an elastic block model using a dense GPS velocity field to derive the fault slip rates in SE Tibet. We uncover good agreement between GPS and late Quaternary slip estimates, suggesting that the GPS slip rates estimated from our model can represent the long‐term fault motions in SE Tibet. Specifically, our model shows that the slip along the Xianshuihe fault increases from west to east, which may result from the initiation of the Longriba fault. The Litang, Zhongdian, and Xiaojiang faults and the faults bounding the Lijiang block exhibit extensional motions, in agreement with the regional tectonics and earthquake focal mechanisms. In the regions north of the Red River fault whose kinematics can be described through the rotations of subblocks with small residuals, the fault motions agree with focal mechanism patterns and tectonic block models. In regions south of the Red River fault, the right‐lateral shearing among the Red River, Lancang and Sagaing faults appears to explain the first‐order deformation features. However, the relatively large residuals and complex tectonic setting suggest that various other factors, such as asthenospheric flow and the subduction of the Burma plate may also influence the deformation. Thus, a single model extended to interpret all the geodynamic features in SE Tibet, especially south of the Red River fault, may be overly simple, and the extent to which each factor contributes to this deformation requires further study.

Artificial Neural Network-based prediction of glacial debris flows in the ParlungZangbo Basin, southeastern Tibetan Plateau, China

Accurate prediction on geological hazards can prevent disaster events in advance and greatly reduce property losses and life casualties. Glacial debris flows are the most serious hazards in southeastern Tibet in China due to their complexity in formation mechanism and the difficulty in prediction. Data collected from 102 glacier debris flow events from 31 gullies since 1970 and regional meteorological data from 1970 to 2019 in ParlungZangbo River Basin in southeastern Tibet were used for Artificial Neural Network (ANN)-based prediction of glacial debris flows. The formation mechanism of glacial debris flows in the ParlungZangbo Basin was systematically analyzed, and the calculations involving the meteorological data and disaster events were conducted by using the statistical methods and two layers fully connected neural networks. The occurrence probabilities and scales of glacial debris flows (small, medium, and large) were predicted, and promising results have been achieved. Through the proposed model calculations, a prediction accuracy of 78.33% was achieved for the scale of glacial debris flows in the study area. The prediction accuracy for both large- and medium-scale debris flows are higher than that for small-scale debris flows. The debris flow scale and the probability of occurrence increase with increasing rainfall and temperature. In addition, the K-fold cross-validation method was used to verify the reliability of the model. The average accuracy of the model calculated under this method is about 93.3%, which validates the proposed model. Practices have proved that the combination of ANN and disaster events can provide sound prediction on geological hazards under complex conditions.

A Deep Learning Method for Mapping Glacial Lakes from the Combined Use of Synthetic-Aperture Radar and Optical Satellite Images

Glacial lakes (GLs), a vital link between the hydrosphere and the cryosphere, participate in the local hydrological process, and their interannual dynamic evolution is an objective reflection and an indicator of regional climate change. The complex terrain and climatic conditions in mountainous areas where GLs are located make it difficult to employ conventional remote sensing observation means to obtain stable, accurate, and comprehensive observation data. In view of this situation, this study presents an algorithm with a high generalization ability established by optimizing and improving a deep learning (DL) semantic segmentation network model for extracting GL contours from combined synthetic-aperture radar (SAR) amplitude and multispectral imagery data. The aim is to use the high penetrability and all-weather advantages of SAR to reduce the effects of cloud cover as well as to integrate the multiscale and detail-oriented advantages of multispectral data to facilitate accurate, quantitative extraction of GL contours. The accuracy and reliability of the model and algorithm were examined by employing them to extract the contours of GLs in a large region of south-eastern Tibet from Landsat 8 optical remote sensing images and Sentinel-1A amplitude images. In this study, the contours of a total 8262 GLs in south-eastern Tibet were extracted. These GLs were distributed predominantly at altitudes of 4000–5500 m. Only 17.4% of these GLs were greater than 0.1 km2 in size, while a large number of small GLs made up the majority. Through analysis and validation, the proposed method was found highly capable of distinguishing rivers and lakes and able to effectively reduce the misidentification and extraction of rivers. With the DL model based on combined optical and SAR images, the intersection-over-union (IoU) score increased by 0.0212 (to 0.6207) on the validation set and by 0.038 (to 0.6397) on the prediction set. These validation data sufficiently demonstrate the efficacy of the model and algorithm. The technical means employed in this study as well as the results and data obtained can provide a reference for research and application expansion in related fields.

Reconstructing the incision of the Lancang River (Upper Mekong) in southeastern Tibet below its prominent knickzone using fluvial terraces and transient tributary profiles

The Lancang River (Upper Mekong) is a key large river in Asia with a prominent knickzone over 500 km distance (27.5‐30.7°N), as it descends the southeastern margin of the Tibetan Plateau. To explore the role of tectonics in regulating the river downcutting at regional and local scales, we reconstructed the rate and spatial pattern of incision within and below the knickzone. We combined the investigation of fluvial terraces with tributary river profile analysis for a 300-km reach near Yunlong below the knickzone (25–27.5°N). We conducted field mapping and dating of strath terraces at the heights of 60–380 m above the trunk river. The overlying terrace deposits comprise fluvial conglomerates and overbank sands as well as tributary alluvial fan conglomerates. K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating results suggest terrace formation during the Middle Pleistocene and the incision rate varying at <0.59–0.22 mm/yr. The river profiles of most tributaries in the study area are in a transient state with a knickpoint separating an upstream relict graded profile and a downstream steepened reach. These tributary profiles suggest river incision of ~50–1800 m, showing a spatial trend of increase towards the north. Knickpoint elevation and channel steepness of tributaries show a similar spatial pattern of northward increase. We suggest that the spatial variation in river incision along the Lancang River near Yunlong is likely to have been regulated by differential rock uplift in southeastern Tibet. Using the K-feldspar pIRIR dating method, we rectified the incision rate within the knickzone to vary at <0.63–0.28 mm/yr. New dating results thus suggest that terrace-derived incision rates along the Lancang River are similar to regional erosion rates on million-year and millennial time scales in terms of magnitude and spatial pattern. Coupled erosion rates on various time scales along the Lancang River and spatial pattern of river incision near Yunlong supports a primary role of tectonic forcing in shaping the southeastern Tibetan landscape.

Southeastern Tibetan Plateau serves as the dominant sand contributor to the Yangtze River: Evidence from Pb isotopic compositions of detrital K‐feldspar

AbstractThere is as yet no consensus as to the dominant source of sediment transported to the oceans: the steep tectonically active highlands, or the gently sloping lowlands? In this study, new Pb isotopic analyses of detrital K‐feldspar have been combined with published data to constrain sand supply in the Yangtze River, one of the largest fluvial sediment transport systems on Earth. Tributaries draining southeastern Tibet have predominantly unradiogenic K‐feldspar sand grains. K‐feldspar in the middle Yangtze comprises c. 50% of grains of this kind, confirming that the southeastern Tibetan tributaries are the dominant sediment contributors. K‐feldspars characteristic of tributaries of the middle‐lower reaches are not strongly represented in the trunk Yangtze, indicating that they contribute little sediment. Our study demonstrates that tectonic uplift in southeastern Tibet is the primary control on sand supply. This is consistent with quartz 10Be data and with bulk‐sand petrography, indicating a major contribution of detritus from steep mountainous terrain.

Proximate mechanisms responsible for random mating by size in the Himalayan toad Duttaphrynus himalayanus

Abstract Exploring the mechanisms that affect mating pattern with respect to body size has implications for understanding the evolution of sexual selection. Theory predicts that the absence of a relationship between female body size and fecundity, unbiased operational sex ratio, and a short breeding season will lead to random mating by body size in anuran amphibians. We tested these predictions in the Himalayan toad Duttaphrynus himalayanus inhabiting southeastern Tibet. Our study did not detect any correlation between female body size and number of eggs laid, nor was there a significant difference in the sex ratio of toads captured from the breeding site. In addition, the toads were reproductive for only a short period, from late April to early May (typical of an explosively breeding species). As expected, we detected a weak but not significant relationship between body size of amplexing males and females. Our results revealed no apparent size-assortative pairing in the study population of the Himalayan toad and may contribute to an increasing body of literature on mating patterns in relation to body size in animals with indeterminate growth.

Geographic patterns of Lucanus (Coleoptera: Lucanidae) species diversity and environmental determinants in China.

Clarifying the geographic patterns of species diversity and the determinant factors can provide essential information for species conservation and management. Stag beetles (Coleoptera: Lucanidae) of Lucanus are important saproxylic insects and can be used for biomonitoring forests. Most of Lucanus species are facing conservation concerns due to their limited distribution and fragmented habitats, particularly in China, which has the richest species diversity of this genus. The distribution patterns of species diversity of Lucanus at large spatial scales remain portly understood. We studied the distribution patterns of Lucanus and its environmental and geographic determinants in China. Distribution data for 72 species and subspecies were examined. All these species are distributed in southern China except for Lucanus maculifemoratus dybowskyi, which is mainly distributed in north China. The hotspot for Lucanus in China is southeastern Tibet. Our study indicated that the species richness of Lucanus in China was shaped by the precipitation of the wettest and driest month, net primary productivity, digital elevation model, and latitude at a large scale. These variables collectively explained 56.2% of the variation in species richness; precipitation contributed the most (44.1%). Our results provide valuable insights to improve the conservation of Lucanus and can contribute to furthering our understanding of the biogeography of stag beetles in China.

How Continuous Are the “Relict” Landscapes of Southeastern Tibet?

Pervasive low-relief, high-elevation surfaces separated by incised canyons are common across the Southeastern margin of the Tibetan Plateau and have been used to define the nature of crustal deformation that drove plateau growth. A common assumption is that these surfaces were once part of a continuous low-relief paleotopography that has undergone surface uplift and dissection. Recent research, however, has questioned this assumption and the derived geodynamic models, which suggests that these surfaces formed in situ through drainage network reorganization and the piracy of upstream drainage area. Here, we test the continuity of the low-relief surfaces across SE Tibet using a new inversion scheme that also illuminates the nature of conflicting hypotheses. Our analysis is based on combining the local information contained in maps of normalized channel steepness with the more distributed and integrated information contained in maps of normalized landscape response time. This allows us to model the formation of a hypothetical landscape prior to rock uplift and dissection. We find that large variations in channel steepness are required along the trunk channels within the inferred paleotopography. This is inconsistent with a low-relief surface prior to surface uplift and indicates that a surface interpolated between remnants cannot be used to robustly measure geodynamic processes in space and time. Furthermore, our inverse framework highlights many different solutions to this ill-posed problem and thus provides an explanation as to why the topography alone cannot be used to provide a unique solution to the debate.

Application and comparison of different ensemble learning machines combining with a novel sampling strategy for shallow landslide susceptibility mapping

The existence of shallow landslide brings huge threats to the human lives and economic development, as the Lang County, Southeastern Tibet prone to landslide. Landslide susceptibility mapping (LSM) is considered as the key for the prevention of hazard. The primary goal of the present study is to assess and compare four models: classification and regression tree, gradient boosting decision tree (GBDT), adaptive boosting-decision tree and random forest for the performance of landslide susceptibility modeling. Firstly, a landslide inventory map consisting of 229 historical shallow landslide locations was prepared and the same number of non-landslide points was determined by k-means clustering. Secondly, 12 conditioning factors were considered in the landslide susceptibility modeling. The prediction performance of the four models were estimated by fivefold cross validation and relative operating characteristic curve (ROC), area under the ROC curve (AUC) and statistical measures. The results showed that the GBDT performed best in the training and validation dataset, with the highest prediction capability (AUC = 0.986 and 0.940), highest accuracy value (95.3% and 88.1%) and highest kappa index (0.904 and 0.772), respectively. Therefore, the GBDT was considered to be the most suitable model and applied to the whole study area for LSM. The results of this study also demonstrate that the performance can be enhanced with the use of ensemble learning. The sampling strategy of non-landslide points can be improved by combining with clustering analysis which are more reasonable.

Risk assessment of Sichuan–Tibet Highway susceptible to debris flows at Suotong Basin, southeastern Tibet

The planned Sichuan–Tibet Highway will pass through many debris flow-prone areas. Suotong catchment with a drainage area of 38.03 km2, a tributary of Parlung Zangbo River in the Bomi County of Tibet, has not experienced large-scale debris flows since 1991. Field survey and image interpretation show that glacial till in the catchment has volume of approximately 160 mm3 and is the primary source of loose materials for debris-flow reoccurrence. Once large glacial debris flows occur, they will cause harm to the highway. A comprehensive method including engineering geology, hydraulics, and computational mathematics is developed to quantitatively assess the debris-flow risk to the highway. The magnitude and peak discharge of glacial debris flows of a 100 year return period are calculated using empirical relationships in this region. The peak velocity at the outlet can reach 6.75 m/s, and the peak discharge is 6238 m3/s for such a return period. The Saint–Venant equations governing debris-flow movements are numerically solved by finite difference method. The simulated results are close to those by empirical methods. The hazard value at the alluvial fan is as high as 24.7 according to the numerical simulation. Two routing schemes (one goes through the tunnel and other pass the bridge) are compared with each other. Although the bridge scheme is cheaper than the tunnel scheme, the bridge’s section cannot fully discharge debris flows within the 100-year return period. Therefore, the tunnel scheme is safer and more strongly recommended than the bridge scheme.

Patterns and trends in grain self-sufficiency on the Tibetan Plateau during 1985–2016

Capacity for grain self-sufficiency on the Tibetan Plateau (TP) is an important basis for ensuring social stability and regional sustainability. Thus, based on county-level statistical data for population, grain production and consumption, we analyzed patterns and trends in grain supply and demand at regional, provincial, and county levels on the TP between 1985 and 2016. We applied two indices to evaluate capacity for grain self-sufficiency and found that the regional average self-sufficiency rate increased quickly by 1.97%/a since 1989, reaching 173.03% on the plateau over the period between 2010 and 2016. This indicates that grain supply in this region is able to fully meet demand. In addition, all provinces apart from Xinjiang exhibited similar increasing trends, attaining grain self-sufficiency during 2010–2016. Furthermore, 59% of counties attained grain self-sufficiency over this period, mainly distributed in southern Tibet, in the Sichuan-Tibet junction area, and in eastern Qinghai Province. A number of gaps in grain supply and demand occurred within the headwater regions of the Yangtze and Yellow rivers as well as on the Qiangtang Plateau. Grain self-sufficiency significantly increased over the study period in 36% of counties, mainly distributed in the agricultural areas of southeastern Tibet and in eastern Qinghai. Across the whole plateau, capacity for grain self-sufficiency substantially increased between 1985 and 2016, although serious spatial imbalances remain.