Achieving a more realistic assessment of rockfall hazards by coupling three‐dimensional process models and field‐based tree‐ring data

Sound knowledge of the spatial and temporal patterns of rockfalls is fundamental for the management of this very common hazard in mountain environments. Process‐based, three‐dimensional simulation models are nowadays capable of reproducing the spatial distribution of rockfall occurrences with reasonable accuracy through the simulation of numerous individual trajectories on highly‐resolved digital terrain models. At the same time, however, simulation models typically fail to quantify the ‘real’ frequency of rockfalls (in terms of return intervals). The analysis of impact scars on trees, in contrast, yields real rockfall frequencies, but trees may not be present at the location of interest and rare trajectories may not necessarily be captured due to the limited age of forest stands. In this article, we demonstrate that the coupling of modeling with tree‐ring techniques may overcome the limitations inherent to both approaches. Based on the analysis of 64 cells (40 m × 40 m) of a rockfall slope located above a 1631‐m long road section in the Swiss Alps, we illustrate results from 488 rockfalls detected in 1260 trees. We illustrate that tree impact data cannot only be used (i) to reconstruct the real frequency of rockfalls for individual cells, but that they also serve (ii) the calibration of the rockfall model Rockyfor3D, as well as (iii) the transformation of simulated trajectories into real frequencies. Calibrated simulation results are in good agreement with real rockfall frequencies and exhibit significant differences in rockfall activity between the cells (zones) along the road section. Real frequencies, expressed as rock passages per meter road section, also enable quantification and direct comparison of the hazard potential between the zones. The contribution provides an approach for hazard zoning procedures that complements traditional methods with a quantification of rockfall frequencies in terms of return intervals through a systematic inclusion of impact records in trees. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural and thermal controls of rockfall frequency and magnitude within rockwall–talus systems (Swiss Alps)

Both from a systemic and natural hazard perspective, it is essential to understand the causes and frequency of rockfalls in mountain terrain and to predict the block sizes deposited at specific locations. Commonly, rockfalls are studied either retrospectively, using talus slopes, or directly by rockwall surveys. Nevertheless, our understanding of rockfall activity, particularly at the lower magnitude spectrum, is still incomplete. Moreover, the explanatory framework is rarely addressed explicitly. In this study, we investigate two rockwall–talus systems in the Swiss Alps to estimate the rockfall frequency–magnitude pattern and their key controls. We present a holistic approach that integrates deductive geotechnical and thermal investigations of the source rockwalls with abductive talus‐based explanations of rockfall volume and frequency. The rockwalls' three‐dimensional (3D) joint pattern indicates that 75% of the blocks may be released as debris fall (< 14 m³) and boulder falls (14–61 m³), which is mirrored in the corresponding talus material. Using two‐year records of near‐surface rockwall temperatures as input for a 1D heat conduction model underlines the destabilizing role of seasonal ice segregation. Deepest frost cracking of 300 cm may occur on the north‐northeast (NNE)‐exposed, snow‐rich rockwall, with peaks at the outermost surface. The synthesis of all data suggests that infrequent, large planar slides (approximately every 250 years) overlain by smaller, more frequent wedge and toppling failures (approximately every 17–50 years) as well as high‐frequency flake‐like clasts (3–6 events/year) characterize the rockfall frequency–magnitude pattern at Hungerli Peak. Here, we argue that small‐size rockfalls need more scientific attention, particularly in discontinuous permafrost zones. Our study emphasizes that future frequency–magnitude research should ideally incorporate site‐specific structural and thermal properties, rather than just focusing on climatic or meteorological triggers. We discuss how holistic rockwall–talus approaches, as proposed here, could help to increase our process understanding of rockfalls in mountain environments. Copyright © 2017 John Wiley & Sons, Ltd.     

Dendrogeomorphic dating of rockfalls on low‐latitude,high‐elevation slopes: Rodadero,Iztaccíhuatl volcano,Mexico

Dynamics and rates of rockfalls have been repeatedly studied in mountain environments with archival records as well as lichenometric, radiocarbon or dendrogeomorphic approaches. In this study, we test the potential of conifers growing at a low‐latitude, high‐elevation site as a dendrogeomorphic tool to reconstruct to calendar dates associated rockfall activity. Analysis is based on tree‐ring records of Mexican mountain pine (Pinus hartwegii Lindl.) growing at timberline [~4000 m above sea level (a.s.l.)] and at the runout fringe of a north–northeast (NNE)‐facing slope of the dormant Iztaccíhuatl volcano (Mexico), which is subject to frequent rockfalls. The potential and limitations of tree‐ring data are demonstrated based on 67 rockfall impacts dated in the increment‐ring series of 24 trees since ad 1836. While findings of this paper are site‐specific, the study clearly shows the potential of dendrogeomorphic approaches in extra‐Alpine, low‐latitude environments and for the understanding of rockfall processes in space and time. Copyright © 2011 John Wiley & Sons, Ltd.     

Emergent characteristics of rockfall inventories captured at a regional scale

High-resolution rockfall inventories captured at a regional scale are scarce. This is partly owing to difficulties in measuring the range of possible rockfall volumes with sufficient accuracy and completeness, and at a scale exceeding the influence of localized controls. This paucity of data restricts our ability to abstract patterns of erosion, identify long-term changes in behaviour and assess how rockfalls respond to changes in rock mass structural and environmental conditions. We have addressed this by developing a workflow that is tailored to monitoring rockfalls and the resulting cliff retreat continuously (in space), in three-dimensional (3D) and over large spatial scales (>10⁴ m). We tested our approach by analysing rockfall activity along 20.5 km of coastal cliffs in North Yorkshire (UK), in what we understand to be the first multi-temporal detection of rockfalls at a regional scale. We show that rockfall magnitude–frequency relationships, which often underpin predictive models of erosion, are highly sensitive to the spatial extent of monitoring. Variations in rockfall shape with volume also imply a systemic shift in the underlying mechanisms of detachment with scale, leading us to question the validity of applying a single probabilistic model to the full range of rockfalls observed here. Finally, our data emphasize the importance of cliff retreat as an episodic process. Going forwards, there will a pressing need to understand and model the erosional response of such coastlines to rising global sea levels as well as projected changes to winds, tides, wave climates, precipitation and storm events. The methodologies and data presented here are fundamental to achieving this, marking a step-change in our ability to understand the competing effects of different processes in determining the magnitude and frequency of rockfall activity and ultimately meaning that we are better placed to investigate relationships between process and form/erosion at critical, regional scales. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Climatic and structural controls on Late-glacial and Holocene rockfall occurrence in high-elevated rock walls of the Mont Blanc massif (Western Alps)

In the Mont Blanc massif (European Western Alps), rockfalls are one of the main natural hazards for alpinists and infrastructure. Rockfall activity after the Little Ice Age is well documented. An increase in frequency during the last three decades is related to permafrost degradation caused by rising air temperatures. In order to understand whether climate exerts a long-term control on rockfall occurrence, a selection of paleo-rockfall scars was dated in the Glacier du Géant basin [>3200 m above sea level (a.s.l.)] using terrestrial cosmogenic nuclides. Rockfall occurrence was compared to different climatic and glacial proxies. This study presents 55 new samples (including replicates) and 25 previously-published ages from nine sampling sites. In total, 62 dated rockfall events display ages ranging from 0.03 ± 0.02 ka to 88.40 ± 7.60 ka. Holocene ages and their uncertainties were used to perform a Kernel density function into a continuous dataset displaying rockfall probability per 100 years. Results highlight four Holocene periods of enhanced rockfall occurrence: (i) c. 7–5.7 ka, related to the Holocene Warm Periods; (ii) c. 4.5–4 ka, related to the Sub-boreal Warm Period; (iii) c. 2.3–1.6 ka, related to the Roman Warm Period; and (iv) c. 0.9–0.3 ka, related to the Medieval Warm Period and beginning of the Little Ice Age. Laser and photogrammetric three-dimensional (3D) models of the rock walls were produced to reconstruct the detached volumes from the best-preserved rockfall scars (≤0.91 ± 0.12 ka). A structural study was carried out at the scale of the Glacier du Géant basin using aerial photographs, and at the scale of four selected rock walls using the 3D models. Two main vertical and one horizontal fracture sets were identified. They correspond respectively to alpine shear zones and veins opened-up during long-term exhumation of the Mont Blanc massif. Our study confirms that climate primarily controls rockfall occurrence, and that structural settings, coincident at both the massif and the rock wall scales, control the rock-wall shapes as well as the geometry and volume of the rockfall events. © 2020 John Wiley & Sons, Ltd.     

Rockfalls and rockfall inventory data: Some observations from surprise valley,Jasper National Park,Canada

The major controls of rockfall activity are divided into two interacting groups. (a)Climatic factors which, through their control of temperatures and the availability and state of water, are primary controls of rockfall trigger mechanisms. (b) Geologic factors which, via cliff form (plan, profile, dissection etc.) and the character and availability of materials, influence the type, spatial distribution and intensity of rockfall activity. Detailed examination of these controls suggest that both seasonal and daily patterns of rockfall activity can vary markedly over a very small area. The implications of this variability for the design and interpretation of rockfall inventories are discussed using sample data from Surprise Valley, Jasper National Park, for the period from May-October 1969. On a seasonal basis rockfall activity showed a major spring peak with secondary maxima in the fall and associated with major summer storms. Two sites of differing aspect and morphology were studied in more detail revealing marked differences in the mean hourly frequency (0·94:0·20), mean hourly probability (0·40:0·16, hours with rockfall/hours observed) and daily pattern of rockfall activity. The west facing site showed equal hourly probability of rockfalls from 10⁰⁰ to 20⁰⁰ hr whereas the east facing slope had a greater hourly probability when it was in the sun (11⁰⁰ 14⁰⁰hr) than in the shade (14⁰⁰-19⁰⁰hr). The differences in rockfall frequency and probabilities reflect the physical characteristics of the individual sites whereas the daily pattern of rockfalls is related to microclimate. These results suggest that if future inventory studies are to make a significant contribution, they must be carefully designed field experiments in which. (i) Data are restricted to the study or comparison of single continuously observed sites. (ii) Study sites and/or data periods are carefully selected to isolate and investigate specific controls e.g. the influence of cliff form, aspect, periods of rainfall etc. on rockfall patterns. (iii) Good on-site microclimate data are available. Also comparison of frequency measures based on the arithmetic mean should be avoided since the distribution of rockfalls per hour closely follows a Poisson distribution with occasional high values which unduly influence the value of the arithmetic mean. It is suggested that rockfall probabilities, based on the binary decision of whether or not a rockfall occurs in a given period, are a more useful measure for daily patterns of rockfall activity.     

Evaluation of meteorological controls of reconstructed rockfall activity in the Czech Flysch Carpathians

Rockfall is an important process in the final sculpturing of escarpments and scree slopes that originate in bedrock landslides in the Flysch Carpathians. The spatio‐temporal characteristics of rockfall activity were studied at four localities representative of old landslides in the highest part of the Czech Flysch Carpathians (Moravskoslezské Beskydy Mountains). Historical activity, chronology, and spatial context of rockfall activity were reconstructed using dendrogeomorphic techniques and rockfall rate index (RR). A total of 1132 increment cores from 283 trees growing in the rockfall transport and accumulation zones enabled the dating of 989 rockfall events. Reconstruction of a 78‐year‐long RR chronology suggests similar rockfall histories and trends at all study sites, indicating the existence of major common factors driving rockfall dynamics in the region. Temporal analysis and correlation of the RR series obtained with monthly mean temperatures, numbers of days with temperature transitions through 0 °C and monthly precipitation totals show that meteorological characteristics have evident but variable influence on rockfall activity. The most important factor is the effect of freeze–thaw cycles throughout the year, supplemented by low temperatures, especially during autumn. The influence of precipitation totals is of lesser importance. Copyright © 2011 John Wiley & Sons, Ltd.     

From geotechnical analysis to quantification and modelling using LiDAR data: a study on rockfall in the Reintal catchment,Bavarian Alps,Germany

In the year 2007, enhanced rockfall activity was observed within the scarp of a 500 BP rockslide in the Reintal catchment (Northern Calcareous Alps, Germany); the largest of a series of events took place in August, when almost 50000 m³ of rock were detached from the subvertical rock face and deposited on a talus cone. In this case study, we focus on three aspects of rockfall research: first, we compile detailed geomorphological and geotechnical findings to explain the causes of the recent events. The results of laboratory tests and stability estimations suggest that rockfall activity will persist in the future as the old rockslide scarp still contains unstable rock masses. Second, we use digital elevation data from a pre‐event airborne LiDAR survey (ALS) and post‐event terrestrial laserscanning (TLS) to quantify landform changes and the mass balance of the rockfall event(s). The widespread availability of ALS elevation data provides a good opportunity to quantify fresh events using a comparatively inexpensive TLS survey; this approach is complicated by uncertainties resulting from the difficult coregistration of ALS and TLS data and the specific geometric problems in steep (ALS) and flat (TLS) terrain; it is therefore limited to at least medium‐sized events. Third, the event(s) is simulated using the results of the LiDAR surveys and a modified GIS‐based rockfall model in order to test its capability of predicting the extent and the spatial distribution of deposition on the talus cone. Results show that the model generally reproduces the process domain and the spatial distribution of topographic changes but frequently under‐ and over‐estimates deposition heights. Copyright © 2011 John Wiley & Sons, Ltd.     

Sidewall erosion: Insights from in situ-produced 10Be concentrations measured on supraglacial clasts (Mont Blanc massif,France)

Sidewall erosion because of rockfalls is one of the most efficient erosional processes in the highest parts of mountain ranges; it is therefore important to quantify sidewall erosion to understand the long-term evolution of mountainous topography. In this study, we analyse how the ¹⁰Be concentration of supraglacial debris can be used to quantify sidewall erosion in a glaciated catchment. We first analyse, in a glaciated catchment, the cascade of processes that move a rock from a rockwall to a supraglacial location and propose a quantitative estimate of the number of rockfalls statistically mixed in a supraglacial sand sample. This model incorporates the size of the rockwall, a power law distribution of the size of the rockfalls and the mean glacial transport velocity. In the case of the Bossons glacier catchment (Mont Blanc massif), the ¹⁰Be concentrations obtained for supraglacial samples vary from 1.97 ± 0.24 to 23.82 ± 1.68 × 10⁴ atoms g⁻¹. Our analysis suggests that part of the ¹⁰Be concentration dispersion is related to an insufficient number of amalgamated rockfalls that does not erase the stochastic nature of the sidewall erosion. In the latter case, the concentration of several collected samples is averaged to increase the number of statistically amalgamated rockfalls. Variable and robust ¹⁰Be-derived rockwall retreat rates are obtained for three distinct rockfall zones in the Bossons catchment and are 0.19 ± 0.08 mm year⁻¹, 0.54 ± 0.1 mm year⁻¹ and 1.08 ± 0.17 mm year⁻¹. The mean ¹⁰Be retreat rate for the whole catchment (ca. 0.65 mm year⁻¹) is close to the present-day erosion rate derived from other methods. © 2019 John Wiley & Sons, Ltd.     

基于无人机倾斜摄影的强震区公路高位危岩崩塌形成机制及稳定性评价

强震区公路高位危岩崩塌具有极高隐蔽性和危害性,传统的接触式勘察方法难以有效调查震后位于公路两侧高陡斜坡体上的危岩崩塌体。提出一种基于无人机的倾斜摄影测量技术,该技术采用无人机超低空采集高位危岩体的高清影像数据,构建三维空间模型,从而提取危岩体特征参数,为危岩体稳定性分析提供数据支撑。利用无人机对某高速公路危岩崩塌地质灾害路段进行调查,对无人机倾斜摄影测量成果进行分析,明确调查区19处危岩崩塌体特征和崩塌成因机制,在此基础上评价典型崩塌体稳定性。并且使用RocFall软件模拟分析典型危岩体崩落运动轨迹,研究高位危岩崩塌对公路的危险性。研究成果对强震区山区公路高位危岩崩塌勘察、稳定性评价工作具有重要的参考价值。     

Dating of rockfall damage in trees yields insights into meteorological triggers of process activity in the French Alps

Rockfall release is a rather unpredictable process. As a result, the occurrence of rockfall often threatens humans and (infra)structures. The assessment of potential drivers of rockfall activity therefore remains a major challenge, even if the relative influence of rainfall, snowmelt, or freeze–thaw cycles has long been identified in short-term monitoring projects. In the absence of longer-term assessments of rockfall triggers and possible changes thereof, our knowledge of rockfall dynamics remains still lacunary as a result of the persisting scarcity of exhaustive and precise rockfall databases. Over the last decades, several studies have employed growth disturbances (GDs) in tree-ring series to reconstruct rockfall activity. Paradoxically, these series were only rarely compared to meteorological records. In this study, we capitalize on the homogeneity of a centennial-old reforestation plot to develop two reconstructions – R1 including only growth suppressions, and R2 based on injuries – with limited biases related to decreasing sample size and changes in exposed diameters back in time. By doing so, our study also and quite clearly highlights the large potential that protection forests have in terms of yielding reliable, multidecadal rockfall reconstructions. From a methodological perspective, we find no synchronicity between R1 and R2, as well as an absence of meteorological controls on rockfall processes in R1. This observation pleads for a careful selection of GDs in future reconstructions. In terms of process dynamics, we demonstrate that summer intense rainfall events (>10 mm day⁻¹) are the main drivers for rockfall activity at our study site. Despite the stringency of our detection procedure, correlations between rockfall activity and meteorological variables remain comparable to those reported in previous studies, as a result of the complexity and multiplicity of triggering factors. We therefore call for a more systematic coupling of tree-ring analysis with rockfall and microclimatic monitoring in future studies. © 2020 John Wiley & Sons, Ltd.     

Downvalley fining of hillslope sediment in an alpine catchment: implications for downstream fining of sediment flux in mountain rivers

The size distributions of sediment delivered from hillslopes to rivers profoundly influence river morphodynamics, including river incision into bedrock and the quality of aquatic habitat. Yet little is known about the factors that influence size distributions of sediment produced by weathering on hillslopes. We present results of a field study of hillslope sediment size distributions at Inyo Creek, a steep catchment in granitic bedrock of the Sierra Nevada, USA. Particles sampled near the base of hillslopes, adjacent to the trunk stream, show a pronounced decrease in sediment size with decreasing sample elevation across all but the coarsest size classes. Measured size distributions become increasingly bimodal with decreasing elevation, exhibiting a coarse, bouldery mode that does not change with elevation and a more abundant finer mode that shifts from cobbles at the highest elevations to gravel at mid elevations and finally to sand at low elevations. We interpret these altitudinal variations in hillslope sediment size to reflect changes in physical, chemical, and biological weathering that can be explained by the catchment's strong altitudinal gradients in topography, climate, and vegetation cover. Because elevation and travel distance to the outlet are closely coupled, the altitudinal trends in sediment size produce a systematic decrease in sediment size along hillslopes parallel to the trunk stream. We refer to this phenomenon as ‘downvalley fining.’ Forward modeling shows that downvalley fining of hillslope sediment is necessary for downstream fining of the long-term average flux of coarse sediment in mountain landscapes where hillslopes and channels are coupled and long-term net sediment deposition is negligible. The model also shows that abrasion plays a secondary role in downstream fining of coarse sediment flux but plays a dominant role in partitioning between the bedload and suspended load. Patterns observed at Inyo Creek may be widespread in mountain ranges around the world. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

A scree slope rockfall model

Certain observed characteristics of scree slopes; namely concavity of profile, straight slope angle less than the angle of repose, and good size sorting of particles, are not consistent with an angle of repose model for accumulation. An alternative model is proposed based upon rockfall and surface stone movement and is tested against experimental data of particle movement in the field. It is found that the mechanical model of stone movement generated adequately explains the motion of particles on scree slopes and that it is in keeping with the characteristics of many screes. The static features of some Isle of Skye screes were also measured and the straight-concave slope form with good downslope sorting of material, characteristic of the rockfall process, was found to be present.     

地震引发滚石灾害及其基本特征研究

通过对已有地震资料的搜集、整理和综合分析,研究了地震滚石灾害的致灾方式、诱发环境条件及发生模式,总结得出地震引发滚石灾害的基本特征,提出了解决地震滚石灾害的可行性方法和思路,以期能为相关研究工作提供参考.     

The use of ballistic trajectory and granular flow models in predicting rockfall propagation

The term rockfall is often used ambiguously to describe various mass movement processes. Here we propose more precise terminology based on the physical nature of the moving mass, differentiating between two distinct types of rockfall: fragmental rockfall and rock mass fall. For both rockfall types, the current knowledge of the mechanisms controlling propagation of the mass movement are described, showing how these mechanisms can be simulated with different modelling approaches. However, we point out that almost no development has been realized concerning dynamic behaviour of the transitional processes between these two end‐member rockfall types. Some simplified means of dealing with these complications are suggested, but we emphasize that a considerable amount of fundamental methodological development remains necessary. Copyright © 2012 John Wiley & Sons, Ltd.     

Erosional power in the Swiss Alps: characterization of slope failure in the Illgraben

Landslides and rockfalls are key geomorphic processes in mountain basins. Their quantification and characterization are critical for understanding the processes of slope failure and their contributions to erosion and landscape evolution. We used digital photogrammetry to produce a multi‐temporal record of erosion (1963–2005) of a rock slope at the head of the Illgraben, a very active catchment prone to debris flows in Switzerland. Slope failures affect 70% of the study slope and erode the slope at an average rate of 0.39 ± 0.03 m yr¯¹. The analysis of individual slope failures yielded an inventory of ~2500 failures ranging over 6 orders of magnitude in volume, despite the small slope area and short study period. The slope failures form a characteristic magnitude–frequency distribution with a rollover and a power‐law tail between ~200 m³ and 1.6 × 10⁶ m³ with an exponent of 1.65. Slope failure volume scales with area as a power law with an exponent of 1.1. Both values are low for studies of bedrock landslides and rockfall and result from the highly fractured and weathered state of the quartzitic bedrock. Our data suggest that the magnitude–frequency distribution is the result of two separate slope failure processes. Type (1) failures are frequent, small slides and slumps within the weathered layer of highly fractured rock and loose sediment, and make up the rollover. Type (2) failures are less frequent and larger rockslides and rockfalls within the internal bedded and fractured slope along pre‐determined potential failure surfaces, and make up the power‐law tail. Rockslides and rockfalls of high magnitude and relatively low frequency make up 99% of the total failure volume and are thus responsible for the high erosion rate. They are also significant in the context of landscape evolution as they occur on slopes above 45° and limit the relief of the slope. Copyright © 2012 John Wiley & Sons, Ltd.     

Magnitude frequency study of rockfall in Co. Antrim,N. Ireland

Detailed observations of rockfall were kept on a weekly basis over a two year period. The results indicate that there is both a seasonal distribution of rockfall with maxima in February-March and November–December, and continuous small scale rockfall throughout the year. There appears to be some connection between rockfall and frost processes, while the peculiar geotechnical properties of the lava flows in county Antrim, in particular the presence of microfractures, would seem to be the main underlying cause for the observed activity.     

Correction of TRMM 3B43 monthly precipitation data over the mountainous areas of Peru during the period 1998–2007

In an attempt to estimate the spatial and temporal behaviour of rainfall over the mountainous areas of the Peruvian Andes, a new in situ monthly rainfall dataset has been collected (1998–2007) and compared with Tropical Rainfall Measuring Mission (TRMM) 3B43 monthly precipitation data for regions located above 3000 m. The reliability of the TRMM 3B43 data varies depending on the root mean squared error ratio (%RMSE) and correlation coefficient. Because of the discrepancy between the two datasets, the use of additive and multiplicative correction models is proposed for the TRMM 3B43 data. In the Peruvian mountain ranges, these correction models better approximate TRMM rainfall monthly values, as already verified for annual values. Copyright © 2011 John Wiley & Sons, Ltd.     

Monitoring the 2004 andesitic block-lava extrusion at Volcán de Colima,México from seismic activity and SO2 emission

The 2004 andesitic block-lava extrusion at Volcán de Colima, México was accompanied and followed by numerous seismic signals associated with rockfalls, pyroclastic flows and explosive events. We analyze temporal variations in the number of rockfalls and explosions, the seismic signal duration of rockfalls and the energy of the explosion and compare this with both the rate of magma discharge and SO₂ emission. Characteristics of seismic signals and SO₂ emission are compared with those observed during the 1998–1999 Volcán de Colima block-lava extrusion. For both eruptions, the explosive activity was low during the lava extrusion and increased after its termination. The variation in the daily number and the total duration of rockfall seismic signals gives a good reflection of the development of the lava emission process. An increase in magmatic degassing (SO₂ flux) was observed some days before the onset of lava extrusion. The degassing strongly decreased some days before the peak in the rate of the 1998–1999 lava emission but reached its peak together with the peak in the rate of the 2004 lava emission. These features of seismic activity and SO₂ emission demonstrate that they are good tools for monitoring the extrusion process.     

Response of low flows under climate warming in high‐altitude permafrost regions in western China

The increase in low flows (winter discharge and minimum monthly discharge), caused primarily by permafrost degradation, is common in high‐latitude permafrost regions, whereas the dynamics of low flows in high‐altitude permafrost regions remain largely unknown. Long‐term discharge data from 28 unregulated catchments in western China were analysed, and the findings showed that winter discharge/minimum monthly discharge significantly increased (p ≤ 0.1) in 82/82%, 55/64%, and 0/0% of the catchments in the higher‐latitude mountain permafrost regions (Tienshan Mountains), mid‐latitude mountain permafrost regions (Qilian Mountains), and mid‐ to low‐latitude plateau permafrost regions (the source regions of the Yangtze and Yellow rivers), respectively. The differences in permafrost type and the distribution of permafrost and alpine cold desert (which is similar to tundra) were found to be the main causes for the different responses in the low flows. The rate of change of low flows (winter discharge and minimum monthly discharge) was negatively and linearly correlated with permafrost coverage when coverage was less than 40% of the catchment area, whereas the low flows changed only slightly when the permafrost coverage exceeded 40%. A significant thickening of the active layer increased the low flows in the lower permafrost‐covered catchments, which are dominated by warm permafrost. However, in the higher permafrost‐covered catchments with cold permafrost and a cold climate, only an increase in permafrost temperature (without a notable thickening of the active layer) occurred, resulting in non‐significant changes in low flows.