Spatio-temporal variability in debris-flow activity: a tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736

Past debris-flow activity on the forested cone of the Geisstriftbach torrent (St. Niklaus, Valais, Swiss Alps) was assessed from growth disturbances in old conifer trees, providing a much improved record of past events. The study of 633 tree-ring sequences sampled from 252 European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Silver birch (Betula pendula Roth.) trees allowed reconstruction of 53 debris-flow events since AD 1736. The spatial analysis of trees affected during particular events on the geomorphic map allowed for a spatial representation of individual events and a reconstruction of four flow patterns. Based on our results and Siegfried maps, we believe that before the formation of a dogleg near the cone apex in the late 1890s, debris flows preferentially used the channels located in the west-southwestern part of the Geisstriftbach cone. This study contributes to our understanding of debris-flow processes on cones and provides an example of how dendrogeomorphic techniques may help in the reconstruction and understanding of debris flows in Alpine area

The days of plenty might soon be over in glacierized Central Asian catchments

Despite the fact that the fast-growing population of Central Asia strongly depends on glacial melt water for fresh water supply, irrigation and hydropower production, the impact of glacier shrinkage on water availability remains poorly understood. With an annual area loss of 0.36 to 0.76%, glaciers are retreating particularly fast in the northern Tien Shan, thus causing concern about future water security in the densely populated regions of Bishkek and Almaty. Here, we use exceptionally long in-situ data series to run and calibrate a distributed glacio-hydrological model, which we then force with downscaled data from phase five of the Climate Model Intercomparison Project CMIP5. We observe that even in the most glacier-friendly scenario, glaciers will lose up to two thirds (−60%) of their 1955 extent by the end of the 21st century. The range of climate scenarios translates into different changes in overall water availability, from peak water being reached in the 2020s over a gradual decrease to status quo until the end of the 21st century. The days of plenty, however, will not last much longer, as summer runoff is projected to decrease, independent of scenario uncertainty. These results highlight the need for immediate planning of mitigation measures in the agricultural and energy sectors to assure long-term water security in the densely populated forelands of the Tien Shan

Debris-flow activity along a torrent in the Swiss Alps: Minimum frequency of events and implications for forest dynamics

This study reports on a tree-ring-based reconstruction of geomorphic activity and illustrates impacts of such processes on tree germination along a debris-flow torrent in the Swiss Alps. Analysis included the identification of growth disturbances and the assessment of germination dates for 28 trees along the channel of the Geisstriftbach torrent (Valais, Swiss Alps). Provided that recolonizing trees indicate the minimum time elapsed since the last deposition, germination dates suggest that a devastating debris-flow event in the 1880s had cleared the surface and scoured the currently active channel. This interpretation is supported by two topographic maps showing a dislocation of the channel. Analyzing the age structure of trees along the channel in more detail, we observe higher tree ages with increasing distance from the cone apex. In addition, dendrogeomorphic methods allowed for the reconstruction of 13 debris-flow events between AD 1913 and 2006. In combination with geomorphic mapping, the spatial distribution of trees affected by individual events was assessed and a minimum frequency of previous debris-flow events reconstructed. Although the present study was based on a limited set of tree-ring records, it illustrates that tree-ring analysis in combination with cartographic methods holds much promise for dating minimum ages of surfaces cleared by destructive events as well as for determining the spatio-temporal impacts of past debris-flow activity

Spatio-temporal variability in debris-flow activity: A tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736

ISSN:1661-8734ISSN:1661-872

Climate change impacts on glaciers and runoff in Tien Shan (Central Asia)

Climate-driven changes in glacier-fed streamflow regimes have direct implications on freshwater supply, irrigation and hydropower potential. Reliable information about current and future glaciation and runoff is crucial for water allocation, a complex task in Central Asia, where the collapse of the Soviet Union has transformed previously interdependent republics into autonomous upstream and downstream countries. Although the impacts of climate change on glaciation and runoff have been addressed in previous work undertaken in the Tien Shan (known as the ‘water tower of Central Asia’), a coherent, regional perspective of these findings has not been presented until now. Here we show that glacier shrinkage is most pronounced in peripheral, lower-elevation ranges near the densely populated forelands, where summers are dry and where snow and glacial meltwater is essential for water availability. Shifts of seasonal runoff maxima have already been observed in some rivers, and it is suggested that summer runoff will further decrease in these rivers if precipitation and discharge from thawing permafrost bodies do not compensate sufficiently for water shortfalls

Contrasting responses of Central Asian rock glaciers to global warming

While the responses of Tien Shan glaciers – and glaciers elsewhere – to climatic changes are becoming increasingly well understood, this is less the case for permafrost in general and for rock glaciers in particular. We use a novel approach to describe the climate sensitivity of rock glaciers and to reconstruct periods of high and low rock glacier activity in the Tien Shan since 1895. Using more than 1500 growth anomalies from 280 trees growing on rock glacier bodies, repeat aerial photography from Soviet archives and high-resolution satellite imagery, we present here the world's longest record of rock glacier movements. We also demonstrate that the rock glaciers exhibit synchronous periods of activity at decadal timescales. Despite the complex energy-balance processes on rock glaciers, periods of enhanced activity coincide with warm summers, and the annual mass balance of Tuyuksu glacier fluctuates asynchronously with rock glacier activity. At multi-decadal timescales, however, the investigated rock glaciers exhibit site-specific trends reflecting different stages of inactivation, seemingly in response to the strong increase in air temperature since the 1970s