Pure Appl. Geophys. Ó 2016 Springer International Publishing Pure and Applied Geophysics DOI 10.1007/s00024-016-1348-2 The Triglav Glacier (South-Eastern Alps, Slovenia): Volume Estimation, Internal Characterization and 2000–2013 Temporal Evolution by Means of Ground Penetrating Radar Measurements 1,3 2 3 4 COSTANZA DEL GOBBO, RENATO R. COLUCCI, EMANUELE FORTE, MICHAELA TRIGLAV Cˇ EKADA, 5 and MATIJA ZORN Abstract—It is well known that small glaciers of mid latitudes 1. Introduction and especially those located at low altitude respond suddenly to climate changes both on local and global scale. For this reason their monitoring as well as evaluation of their extension and volume is Small glaciers and glacierets, unlike the larger ice essential. We present a ground penetrating radar (GPR) dataset bodies are important indicators of short term varia- acquired on September 23 and 24, 2013 on the Triglav glacier to tions in the climate system, both on local and on identify layers with different characteristics (snow, firn, ice, debris) global scale due to their fast response to climate within the glacier and to define the extension and volume of the actual ice. Computing integrated and interpolated 3D using the changes (Kuhn 1995; Hughes et al. 2006). The very whole GPR dataset, we estimate that at the moment of data small glaciers are particularly sensitive to topocli- 2 3 acquisition the ice area was 3800 m and the ice volume 7400 m . matic conditions (Hughes and Woodward 2009), and Its average thickness was 1.95 m while its maximum thickness was slightly more than 5 m. Here we compare the results with a pre- thus they often exhibit large mass balance fluctua- vious GPR survey acquired in 2000. A critical review of the tions even in a very short time (Hughes 2008, 2010). historical data to find the general trend and to forecast a possible Although these glaciers are very small they still have evolution is also presented. Between 2000 and 2013, we observed relevant changes in the internal distribution of the different units to be considered because they are so numerous that at (snow, firn, ice) and the ice volume reduced from about 35,000 m3 regional scale, they can contain a significant amount to about 7400 m3. Such result can be achieved only using multiple of ice. In such context, if we consider just all glaciers GPR surveys, which allow not only to assess the volume occupied larger than 1 km2, this could result in errors in the by a glacial body, but also to image its internal structure and the actual ice volume. In fact, by applying one of the widely used order of ±10 % in the regional estimations (Bahr and empirical volume-area relations to infer the geometrical parameters Radic´ 2012). In the European Alps, to obtain evalu- of the glacier, a relevant underestimation of ice-loss would be ations with smaller errors, certainly essential for achieved. regional water resource quantification and exploita- 2 Key words: 3D GPR, 4D analysis, ice melting, time moni- tion, an inventory of all ice bodies down to 0.01 km , toring, climate changes, Triglav glacier, Slovenia, South-eastern or even smaller, would be necessary (Bahr and Radic´ Alps. 2012; Pfeffer et al. 2014). In addition, the alpine cryosphere represents an important drinking water reserve as well as a key factor in the landscape evolution and biodiversity conservation. Differences 1 Present Address: Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52f, 6020 Innsbruck, between small glaciers, glacierets, ice patches and Austria. snow patches are in fact based on their dynamics 2 Department of Earth System Sciences and Environmental (motion or non motion), internal materials (ice, firn, Technologies, ISMAR-CNR, Viale R. Gessi 2, 34123 Trieste, Italy. snow, folds) and genesis (glacial or nival). For this 3 Department of Mathematics and Geosciences, University of Trieste, Via Weiss 2, 34128 Trieste, Italy. E-mail: [email protected] reason, it is increasingly crucial to understand not 4 Geodetic Institute of Slovenia, Jamova 2, 1000 Ljubljana, only the extension, but even more important, the Slovenia. internal structure and evolution of such smaller ice 5 Anton Melik Geographical Institute, Research Centre of the Slovenian Academy of Sciences and Arts, Gosposka ulica 13, 1000 masses for a correct evaluation of their characteristics Ljubljana, Slovenia. and genesis (Serrano et al. 2011). C. Del Gobbo et al. Pure Appl. Geophys. The Triglav glacier (46°22042.52N 13°50011.86E, Alps and also in Slovenia. Its location and the sur- Fig. 1), known in Slovenian as Zeleni sneg (Green rounding morphologies ensure a fairly high insulation snow), lies at an altitude between about 2400 and rate especially during summer. It has been regularly 2500 m.a.s.l. on the north-eastern slope of Mount measured, observed, and monitored since 1946 by the Triglav (2864 m a.s.l.), the highest peak in the Julian Anton Melik Geographical Institute of the Research Triglav Glacier Evolution by Means of GPR b Figure 1 (e.g., 2009, 2011), and since 2003 the ice was no Location map of the study area (a) and a detail around Mount longer outcropping like in the previous seasons Triglav (b). The blue area corresponds to the limit of the surface covered by snow (with some snow free patches inside) at the (Gabrovec et al. 2013). moment of the 2013 GPR survey. The yellow dots show the Unlike such long and detailed historical data on position of the GPR profiles. In c a photo taken from the helicopter the areal extent, no precise information is available on September 23, 2013 with the approximated limit of the snowfield as in (b) and the location of the tectonic discontinuity about the volume of the Triglav glacier and of its later discussed in the text. On the bottom of the photo, several evolution through time. This is indeed a crucial typical glacial morphologies (roche mountonne´e) are apparent aspect for any water equivalent estimation and for realistic forecasts about the future evolution of any Centre of the Slovenian Academy of Sciences and glacial body (Bahr et al. 2015). Such problem affects Arts (Gabrovec et al. 2014), although information the glaciology because even if there are several about the area evolution derived by different pho- empirical equations and correlations between the area togrammetric techniques are available since 1897 and volume extension of glaciers, many pitfalls are (Triglav Cˇ ekada et al. 2014). The Triglav glacier has reported, especially for smallest size glaciers (Bahr some similarities in terms of altitude, latitude and et al. 2015). Moreover, even if the volume changes size with other small glaciers and ice patches in the with time can be estimated by both photogrammetric Julian Alps (Colucci 2016). All these ice bodies are and LiDAR techniques (e.g., Triglav Cˇ ekada and generally located on the north facing slopes and Gabrovec 2013), the overall volume of frozen mate- develop on carbonate rocks which are typically rials (and their characteristics) are a more challenging characterized by high albedo owing to light-colored issue. The use of ground penetrating radar (GPR) rock types (Hughes 2007). techniques in glaciological studies at different scales In 1946, when scientific campaigns on the Triglav has a quite long history due to the low electric con- glacier started, it covered an area of 0.144 km2 ductivity of frozen materials, which allows to reach (Verbicˇ and Gabrovec 2002). During the Little Ice investigation depths that would be difficult to reach Age (LIA) the area of the glacier was about 0.4 km2 otherwise (e.g., Arcone 1996). GPR surveys were in (Gabrovec et al. 2014) and it still had an extension of fact traditionally applied to image the ice stratigra- about 0.3 km2 at the beginning of the 1900s and phy, measure the snow/ice thickness and evaluate the values within the range 0.10–0.27 km2 until the end volume of glaciers. On the other hand, GPR studies of the 1970s (Triglav Cˇ ekada et al. 2014). After that focusing on time monitoring of subsurface evolution period the Triglav glacier showed a dramatic and (i.e., 4-D analyses) are still challenging due to logistic continuous shrinking, mainly due to the higher sum- problems and to the varying topography, which mer temperatures and dryer winters. Since the end of makes quite difficult to compare data acquired in the 1970s the glacier had no more movement also different periods. In recent years, some examples on testified by the absence of crevasses (Gabrovec et al. ice caps and glaciers of different size both with ter- 2013). In the last decades, the average summer tem- restrial and airborne surveys have been provided peratures increased significantly (Tosˇic´ et al. 2016) (Machguth et al. 2006; Saintenoy et al. 2013; Colucci and the glacier undergone a marked thinning espe- et al. 2015), but the full potential of GPR for cially in the middle part, because the snow tends to glaciological monitoring is probably still unexploited. accumulate in the lower part of the glacier, where it The main purpose of this work is to estimate the can remain until the next season. Since the mid 1980s volume of the Triglav glacier and evaluate its internal the ice body is divided in some isolated ice patches structure determining the actual presence of ice and separated by rocky outcrops. This is a typical its volume through a dense high resolution GPR behavior of the recession of glaciers resulting in a dataset. Such new information is compared with data transition from ice bodies into ice patches, with already available for the Triglav glacier and for the melting on nonmoving residual ice masses (Serrano other glaciers of the Eastern Alps to make more et al.