Tihomir Frangen

Thermal imaging was utilized across three hydrothermal systems in Croatia for preliminary screening of low-temperature surface hydrothermal manifestations, particularly subthermal springs (13–20 °C). This study assessed the effectiveness of this remote sensing method for initial geothermal surveys and monitoring of surface geothermal anomalies. The methodology integrated GIS tools (Esri, Redlands, CA, USA) to identify potential zones of subthermal water outflow using thermal orthomosaics derived from the photogrammetric reconstruction of thermal infrared images. Thermal images were acquired using a fixed-wing eBee Plus RTK unmanned aerial vehicle (UAV) (senseFly Ltd., Cheseaux-sur-Lausanne, Switzerland) equipped with a senseFly thermoMap camera (senseFly Ltd., Cheseaux-sur-Lausanne, Switzerland) and processed using Pix4Dmapper software. Due to COVID-19 restrictions, the intended methodology was simplified by discarding temperature calibration. Temperature calibration of thermal images was performed only for a few smaller areas to address discrepancies between measured ground temperatures and temperature values depicted in the maps. The methodology was validated near the Daruvar hydrothermal system, where a thermal anomaly corresponding to the subthermal spring location was visually detected, demonstrating the applicability of the method for surface investigation of low-temperature geothermal systems. No new subthermal or thermal springs were discovered. In total, 45.35 km² was surveyed over 9 days, making this a viable and low-cost methodology for preliminary exploration. However, limitations, including the narrow time window for thermal data acquisition, regulatory requirements for drone flights, and subjectivity of the methods used, were identified.

In Northern Croatia, the stability of slopes is increasingly compromised by a combination of anthropogenic pressures, seismic activity, and climate-driven changes in precipitation patterns. This study presents an integrated, multi-level investigation of the complex, composite Hum na Sutli landslide to characterize its failure mechanism, identify cascading triggering factors, and provide a quantitative basis for impact assessment and mitigation plan development. By reviewing the existing relevant (geo) data, information on the landslide’s historical background and triggering factors was gathered. Material properties were determined in the field and confirmed via laboratory tests. With the integration of new data and multilevel methodology, including unmanned aerial vehicle (UAV) derived light detection and ranging data (LiDAR) data and Electrical Resistivity Tomography (ERT), the characterization of a landslide type was conducted, and an initial landslide map and model were created. Analyzing precipitation data from over the last 25 years provided insights into the area’s changing precipitation trends, highlighting the importance of continuous monitoring of this site. The presented research results for the Hum na Sutli landslide provide a viable basis for mitigation plan creation. Furthermore, laboratory results establish a correlation in landslide susceptibility between two regional units: the Golubovec and Vrbova formations, based on their similar clay-silt-sand compositions and observed failure mechanisms. The research presented here highlights the benefits of multi-level data analysis, emphasizing the integration of existing data with new high-resolution remote sensing data in order to develop a rapid and reliable initial landslide model.

In the Plitvice Lakes National Park, several hydrogeological catchments can be distinguished, but their boundaries are not clearly defined. This study focused on the Matica River catchment area, which covers the main contributors to the lake system and its overall water balance. An initial assessment indicated that the Matica River catchment is among the most vulnerable areas due to the anticipated land-use expansion related to agriculture and tourism. This research provides critical hydrogeological data supporting sustainable management in response to the increasing extremes of floods and droughts induced by climate change. Two separate campaigns (March 2023 and April 2025) were carried out, each involving three simultaneous tracer injections using different fluorescent dyes. The results of earlier tracer tests were evaluated; furthermore, a hydrochemical analysis of the spring water offered valuable insights into subsurface processes and anthropogenic impacts. Tracing in the southwest clarified the boundary between the Plitvice Lakes and Una River catchments. In the Homoljac polje, the tracer responses highlighted a triple junction between the Plitvice Lakes, Gacka, and Una River catchments. In the southeastern Brezovac polje, the boundary between the Crna Rijeka and Bijela Rijeka catchments was delineated in detail.

This study aims to improve the accuracy of small-scale landslide susceptibility maps (LSMs) by comparing two critical terrain factors—slope and relief energy. Slope is commonly used in LSMs, but its values are significantly sensitive to the spatial resolution of digital elevation models (DEMs). Although some studies have also addressed the effect of DEM resolution on relief parameters, direct comparisons between slope and relief energy remain limited. This research examines how these factors perform at different DEM resolutions and compare them to identify the most effective predictor for small-scale LSMs. Using the frequency ratio method, two LSM scenarios were evaluated: one using slope alongside geological units, and another using relief energy instead of slope, with various neighborhood distances. The study was conducted over a 29,785 km² area in the Pannonian part of Croatia. The findings indicate that relief energy is more stable across different DEM resolutions and enhances the accuracy of LSMs, particularly in large and geologically diverse regions. These results suggest that relief energy may serve as a more reliable factor for small-scale LSMs, offering practical implications for improving landslide risk prediction and land management strategies.

Climate change is anticipated to have predominantly negative impacts on human habitats in the coming decades, leading to significant environmental transformations, including an increase in landslide frequency. This paper introduces a novel approach to evaluating the susceptibility of geological units (GUs) to landslides, based on an analysis of the energy relief index. The method is applied to Pannonian Croatia, specifically the North Croatian Basin and the Hrvatsko Zagorje Basin, where landslides are the predominant mass movement process. Previous attempts to create landslide susceptibility maps for Croatia did not adequately analyze landslide inventories. Our approach utilizes three key datasets: landslide inventory, geological maps, and relief energy, to clearly identify which GUs are most frequently in an unstable equilibrium state. The findings highlight that the youngest isolated Miocene unit (M₇) has the highest landslide susceptibility. This method shows significant promise by enabling precise identification of landslide-prone GUs and offering the potential for integration into the production of small-scale landslide susceptibility maps, particularly for determining weighting factors for GUs within specific study areas. However, it is crucial to ensure that the landslide inventory accurately represents the entire landslide population in the study area.

The evolution of map development has been shaped by advancing techniques and technologies. Nevertheless, field and remote mapping with cabinet data analysis remains essential in this process. Geological maps are thematic maps that delineate diverse geological features. These maps undergo updates reflecting changes in the mapped area, technological advancements, and the availability of new data. Herein, a geological assessment example focused on enhancing mapped data using digitally processed historical (legacy) aerial images is presented for a case study in the Dinarides karst area in Croatia. The study area of Bribirske Mostine is covered by the Basic Geological Map of Yugoslavia (BGMY) at a 100,000 scale, which was developed during the 1960s. As the BGMY was developed 60+ years ago, one of its segments is further analyzed and discussed, namely, faults. Moreover, applying modern-day technologies and reinterpretation, its data, scale, presentation, and possible areas of improvement are presented. Georeferenced digital historical geological data (legacy), comprising BGMY, archive field maps, and aerial images from 1959 used in BGMY development, are reviewed. Original faults were digitalized and reinterpreted within the geographic information system with the following conclusions: (i) more accurate data (spatial positioning) on faults can be gained by digitally processing aerial photographs taken 60+ years ago with detailed review and analysis; (ii) simultaneously, new data were acquired (additional fault lines were interpreted); (iii) the map scale can be up-scaled to 1:25,000 for the investigated area of Bribirske Mostine; and (iv) a newly developed map for the Bribirske Mostine study area is presented.

Tracer testing is the only method in karst hydrogeology that can definitively determine whether a particular site belongs to a watershed of a particular karst spring. Therefore, it is an essential technique for delineating groundwater basins in karst areas. The availability of tracer test results is often limited due to the complicated and relatively expensive application of this approach, especially for large regional watersheds. The Croatian part of the Dinaric karst region extends for several hundred kilometers along the Adriatic coast and consists almost entirely of highly karstified carbonate rocks. The groundwater basins in these areas almost never match the surface morphology of the terrain. In practice, all available results of previous surveys are often used to define watersheds, regardless of the methodology and age of their implementation. This is also true for the earlier delineations of the Gacka River watershed, a regional karst basin in the Croatian Dinaric karst. However, tracer testing methods, especially the accuracy of tracer determination and monitoring, have improved significantly during this time. In order to assess the reliability of past tracing results in this significant karst basin, we reviewed reports of previous tracer tests. More recent tests, in particular the most recent multitracer injection test with continuous tracer detection on the major springs, produced high-quality data that allowed us to assess the reliability of the findings from prior research. A number of large karst springs with partially overlapping subcatchments feed the Gacka River. After discarding unreliable tracing data, we reevaluated the subcatchments of the main springs as well as the characteristics of the regional groundwater flow patterns throughout the basin, which is particularly important for water quality protection measures of the springs. The Gacka River basin is used as a case study to emphasize the importance of thoroughly assessing the reliability of previous tracing data before using them in regional analyses.

Badlands are specific landforms characterized by intense denudation processes. Their occurrence is mainly associated with clay-like materials and semi-arid and Mediterranean climates. This study presents the badland inventory for the Paleogene Pazin Basin located on the Istrian peninsula in Croatia. A total of 5,381 badland polygons, with a total area of 10.25 km², were manually outlined, based on the visual interpretation of orthophotos at a scale of 1:5,000. The badlands in Istria are represented as small and isolated landforms and are exclusively associated with flysch and flysch-like materials, which cover a significant part of the study area (46%). The analysis of badland inventory shows that badlands are spatially not equally distributed but that their abundance is increasing from NW to SE part of the analyzed area. Additionally, the results of the spatial analysis indicate the positive relationship between badland occurrence and relative relief, which is presented via the relief-badland index.

Our study focuses on a sizeable transboundary karst catchment in Croatia and Bosnia and Herzegovina, extending over 2000 km². A complex underground conduit system and extreme karst forms heterogeneity are the main characteristics of the area in question. Since determining the boundary of such a large and complex catchment is difficult, we used different kinds of data sets, of which the most relevant are the available geological data, hydrochemical data, hydrological data, and tracing tests data, to divide the regional catchment into six subcatchments. We also examined past archived reports and carried out new hydrological investigations of several major and minor springs. Our research results in a hydrogeological map that can be used as a base for establishing site-specific groundwater protection zones, for water balance calculations and the planning of new research in this area, especially the ones regulating combined cross-border efforts to prevent groundwater contamination and ensure sufficient drinking water.

Vis Island is situated in southern Croatia. The island is mostly composed of karstified carbonate rocks and belongs to the Dinaric karst region, which is a locus typicus of karst landforms. Located far from the mainland, Vis island has maintained a successful water supply from its own karst aquifer for decades. Hydrogeological research has been undertaken to protect this excellent karst aquifer by establishing sanitary protection zones and to explore the possibility of increasing the pumping yield. New groundwater velocity data obtained via a tracer test were in accordance with the rock mass hydraulic conductivity calculated from previous pumping tests. The hydrochemical interpretation indicated several different phenomena, from carbonate and sulfate rock dissolution to seawater mixing with groundwater. A conceptual model of the island’s aquifer was improved, and two main catchments were delineated according to tracer test results, and connected with the geological setting, hydrochemical data, and new climatological insights. Such an approach is applicable for similar karst aquifers, in which topographic and hydrogeological divides usually do not coincide.