Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 3 - 11, doi: 10.31577/ahs-2026-0027.01.0001
Scientific Paper, English

Lynda CHEBBAH, Abdesselem KABOUR: Prediction of rainfall reduction rate in the Kébir Rhumel catchment (Algeria) using statistical tests and the Artificial Neural Networks (ANN) methods

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• The analysis of the spatio-temporal variability of precipitation in the context of climate change has become common practice, using a wide range of statistical methods, but its use for prediction in terms of values is relatively rare. The aim of this work is to highlight the climatic variability, at the scale of a catchment area, through the application of statistical tests and the Artificial Neural Network (ANN) method, in order to predict a rate of reduction of precipitation, after a break in the time series, in the Kébir Rhumel catchment (North-East Algeria). This analysis uses climate data from 1901 to 2021 (120 years). The reduction rates results obtained by the two methods ranged from 14% to 37% for the statistical method and 14% to 17% for the ANN. They were compared and mapped, leading to the conclusion that ANN is the better performing method.
  
  KEY WORDS: artificial neural network (ANN), climate variability, Kébir Rhumel, reduction rate, statistical methods
  
  Address:
  - Lynda CHEBBAH, Department of Civil Engineering and Hydraulic, Mila University Centre, Mila, Algeria; Modeling and Socio-economic Analysis in Water Sciences Laboratory, MASESE Lab., Mila, Algeria (Corresponding author. Tel.: Fax.: Email: cheblyn@yahoo.fr)
  - Abdesselem KABOUR, Department of Civil Engineering and Hydraulic, Mila University Centre, Mila, Algeria; Modeling and Socio-economic Analysis in Water Sciences Laboratory, MASESE Lab., Mila, Algeria

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 12 - 22, doi: 10.31577/ahs-2026-0027.01.0002
Scientific Paper, English

Tsetska SIMEONOVA, Milena KERCHEVA, Gergana KUNCHEVA, Margaritka FILIPOVA, Emil DIMITROV, Dimitranka STOICHEVA: Groundwater quality assessment under different anthropogenic loading in a small Bulgarian sub-basin of the Danube watershed

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• This study aims to assess the quality of groundwater under the influence of land use activities within a small sub-basin of Danube watershed in Northern Bulgaria over three monitoring periods: 2007, 2008, and 2024. A total of 22 water sources were explored, located in the mixed urbanized area of Trastenik village, Ruse district (domestic home wells) and springs and wells in forests, arable fields, and pastures, occupied by Calcic Chernozems. Different physico-chemical parameters of groundwater samples were assessed to determine their quality. The Gibbs and Piper diagrams were used to characterize the hydro-chemical dynamics of groundwater. It was concluded that the weathering of minerals and evaporation concentration were the processes controlling the groundwater chemical composition. Based on the Water Quality Index (WQI) model the groundwater sources are categorized as good to unsuitable in the urban mixed areas. It was found that the groundwater under mixed urban areas (village) was characterized with higher content of chemical elements, especially in 2007 and 2008, while after a 15-year period (in 2024) lower values of most chemical elements in the waters and a 40–50% reduction in nitrates was reported, confirming that land-use changes notably affect groundwater quality
  
  KEY WORDS: groundwater sources, WQI model, Gibbs diagram, Calcic Chernozems, Danube watershed
  
  Address:
  - Tsetska SIMEONOVA, Department of Agrochemistry, Agroecology and Farming systems, “N. Poushkarov” Institute of Soil Science, Agrotechnologies and Plant Protection, Agricultural Academy, 7 Shosse Bankya str., 1331 Sofia, Bulgaria (Corresponding author. Tel.: Fax.: Email: cecka_simeonova@abv.bg)
  - Milena KERCHEVA, Department of Physics, Erosion and Soil biota, “N. Poushkarov” Institute of Soil Science, Agrotechnologies and Plant Protection, Agricultural Academy, 7 Shosse Bankya str., 1331 Sofia, Bulgaria
  - Gergana KUNCHEVA, Department of Physics, Erosion and Soil biota, “N. Poushkarov” Institute of Soil Science, Agrotechnologies and Plant Protection, Agricultural Academy, 7 Shosse Bankya str., 1331 Sofia, Bulgaria
  - Margaritka FILIPOVA, Department of Heat, Hydraulics and Environmental Engineering, Agrarian and Industrial Faculty, University of Ruse “Angel Kanchev”, 8 Studentska str., 7017, Ruse, Bulgaria
  - Emil DIMITROV, Department of Physics, Erosion and Soil biota, “N. Poushkarov” Institute of Soil Science, Agrotechnologies and Plant Protection, Agricultural Academy, 7 Shosse Bankya str., 1331 Sofia, Bulgaria
  - Dimitranka STOICHEVA, Department of Agrochemistry, Agroecology and Farming systems, “N. Poushkarov” Institute of Soil Science, Agrotechnologies and Plant Protection, Agricultural Academy, 7 Shosse Bankya str., 1331 Sofia, Bulgaria

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 23 - 32, doi: 10.31577/ahs-2026-0027.01.0003
Scientific Paper, English

Kateřina HRUŠKOVÁ, Martina HOLUBECKÁ, Hana HLAVÁČIKOVÁ: From data to forecasts: Challenges of hydrological modelling in the Slovak-Ukrainian border region

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• This study presents the development and implementation of an operational hydrological forecasting system for the transboundary Uzh and Latorica River basins based on the HBV model. It demonstrates how a conceptual hydrological model can be effectively adapted for forecasting in a data-scarce transboundary region with limited observational data. Due to insufficient radar coverage and gaps in gauge measurements, meteorological forcing was derived from the ALADIN/SHMU numerical weather prediction system. The HBV model was calibrated sequentially from upstream to downstream subbasins to ensure hydrological consistency and was evaluated under a fully simulated inflow configuration reflecting real operational conditions. The results indicate very good overall model performance and reliable simulation of discharge dynamics across stations. Some limitations were identified in the representation of high flows, with a tendency toward peak attenuation. Nevertheless, the proposed framework provides a robust basis for hydrological forecasting in data-scarce transboundary basins, highlighting the importance of consistent meteorological forcing and appropriate calibration.
  
  KEY WORDS: hydrological modelling, flood forecasting, HBV model, transboundary catchment, ALADIN/SHMU
  
  Address:
  - Kateřina HRUŠKOVÁ, Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovak Republic (Corresponding author. Tel.: Fax.: Email: katerina.hruskova@shmu.sk)
  - Martina HOLUBECKÁ, Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovak Republic
  - Hana HLAVÁČIKOVÁ, Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 33 - 42, doi: 10.31577/ahs-2026-0027.01.0004
Scientific Paper, English

Lidija TADIĆ, Tamara BRLEKOVIĆ, Darko BARBALIĆ: Application of k-means clustering on flood episodes in small catchments in Croatia

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• This study presents an application of the k-means clustering method to the analysis of historical flood episodes in small catchments across the Republic of Croatia for the period 1998–2018. A total of 845 flood events with reliable and complete information were selected and described using three key parameters: flood duration, the size of affected area, and the portion of arable land within the flooded area. The results indicate that flood events in small catchments of the Danube River basin can be classified into five distinct clusters, while those in the Adriatic Sea basin are more uniform and can be grouped into two clusters. The identified clusters reveal significant differences in flood characteristics, allowing for the identification of flood types that require priority attention in flood risk management. The study demonstrates that multivariate clustering is a valuable tool for characterizing and regionalizing floods in small catchments, thereby supporting decision-making processes in flood risk management in Croatia.
  
  KEY WORDS: floods, k-means clustering, small catchments, Croatia
  
  Address:
  - Lidija TADIĆ, Faculty of Civil Engineering and Architecture Osijek, Vladimira Preloga 3, 31 000 Osijek, Croatia (Corresponding author. Tel.: Fax.: Email: ltadic@gfos.hr)
  - Tamara BRLEKOVIĆ, Faculty of Civil Engineering and Architecture Osijek, Vladimira Preloga 3, 31 000 Osijek, Croatia
  - Darko BARBALIĆ, Hrvatske vode, Ulica grada Vukovara 220, 10 000 Zagreb, Croatia

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 43 - 53, doi: 10.31577/ahs-2026-0027.01.0005
Scientific Paper, English

Pavla PEKÁROVÁ, Ján PEKÁR: Projected changes in total runoff and baseflow in the Upper Ipeľ basin upstream of the Málinec reservoir under CMIP6 climate scenarios

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• The Upper Ipeľ basin upstream of the Málinec reservoir is an important source area for drinking-water supply in central Slovakia. This study evaluates its hydrological balance, baseflow contribution, and possible future runoff development using observed hydrometeorological data from 1995–2020, baseflow estimation methods, and the conceptual BILAN model. Baseflow was estimated using the Kille method and BFI separation, while the BILAN model was calibrated for the reference period 1995–2014 and verified for 2015–2020. Future runoff was simulated for 2080–2099 under three CMIP6 Shared Socioeconomic Pathway scenarios (SSP1-1.9, SSP2-4.5, SSP5-8.5). Potential evapotranspiration was calculated by the Oudin method. The results show that the basin is highly sensitive to increasing air temperature and evapotranspiration. In the reference period, simulated runoff reached 303.3 mm, including 154 mm of baseflow and 149 mm of direct runoff. Mean annual runoff is projected to decrease to 286.7 mm under SSP1-1.9, 257.8 mm under SSP2-4.5, and 209.8 mm under SSP5-8.5, representing declines of 5.5%, 15.0%, and 30.8%, respectively. The largest reductions are expected in late summer and autumn. The results indicate a decreasing reliability of inflow to the Málinec reservoir under future climate conditions. The study also shows that in basins without groundwater observation wells, baseflow estimation combined with conceptual hydrological modelling can provide a useful basis for assessing groundwater-related runoff processes and future water availability.
  
  KEY WORDS: BILAN model, Upper Ipeľ basin, Málinec reservoir, SSP1-1.9, SSP2-4.5, SSP5-8.5, future runoff
  
  Address:
  - Pavla PEKÁROVÁ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic (Corresponding author. Tel.: Fax.: Email: pekarova@uh.savba.sk)
  - Ján PEKÁR, Comenius University in Bratislava, Faculty of Mathematics, Physics, and Informatics, Department of Applied Mathematics and Statistics, Mlynská dolina, 842 48 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 54 - 65, doi: 10.31577/ahs-2026-0027.01.0006
Review, English

Bruna Nogueira REZENDE, Vassiliki Terezinha Galvao BOULOMYTIS, Sergio Nascimento DUARTE, Antonio Carlos ZUFFO: Evaluation of peak flows using the combined SCS-CN Dimensionless Unit Hydrograph method in small tropical rural catchments

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• Accurate peak runoff prediction in small catchments remains difficult due to limited data and the sensitivity of hydrological parameters to land use and climate. This study evaluated the combined SCS‑CN method and Dimensionless Unit Hydrograph (DUH) in three rural catchments in São Paulo, Brazil. Weighted Curve Number (CN) values were derived from high‑resolution GIS land use data, yet the method overestimated maximum hourly flows by 5–12 times compared to observations, raising concerns about oversizing hydraulic structures when CN values are applied without calibration. The findings confirm that CN and the initial abstraction ratio (λ) are not universal constants but vary with land use, soil, and rainfall. In tropical catchments, high rainfall intensities and rapid land‑use change make CN especially sensitive, while in temperate regions, seasonal vegetation and agricultural practices also affect CN and λ. These results underscore the need for regional calibration and context‑specific adjustments to ensure reliable flood risk assessment and sustainable water management.
  
  KEY WORDS: Curve number, calibration, land use and land cover, climate conditions, flood management, peak flow estimation
  
  Address:
  - Bruna Nogueira REZENDE, Department of Biosystems Engineering, College of Agriculture “Luiz de Queiroz” (ESALQ), University of Sao Paulo (USP), Piracicaba, Sao Paulo, Brazil (Corresponding author. Tel.: Fax.: Email: bruna.n.rezende@usp.br)
  - Vassiliki Terezinha Galvao BOULOMYTIS, Faculty of Civil Engineering, Federal Institute of Science, Education and Technology of Sao Paulo (IFSP), Caraguatatuba, Sao Paulo, Brazil (Corresponding author. Tel.: Fax.: Email: vassiliki@ifsp.edu.br)
  - Sergio Nascimento DUARTE, Department of Biosystems Engineering, College of Agriculture “Luiz de Queiroz” (ESALQ), University of Sao Paulo (USP), Piracicaba, Sao Paulo, Brazil
  - Antonio Carlos ZUFFO, School of Civil Engineering, Architecture and Urban Design, State University of Campinas (UNICAMP), Campinas, Sao Paulo, Brazil

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 66 - 71, doi: 10.31577/ahs-2026-0027.01.0007
Scientific Paper, English

Noureddine MAKHLOUFI: A hybrid hydrological approach, correcting HEC-HMS conceptual biases using XGBoost and satellite soil moisture data

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• This study proposes a novel hybrid modeling framework coupling the physically-based model HEC-HMS with a machine learning algorithm XGBoost, acting as an error-correction post-processor for the Tikzal basin in Algéria. The data-driven model integrates instantaneous precipitation, conceptual simulated streamflow, and antecedent soil moisture derived from satellite remote sensing (GLDAS). The results demonstrate that XGBoost algorithm substantially increased the Kling-Gupta Efficiency (KGE) from 0.648 to 0.832, while raising the Nash-Sutcliffe Efficiency (NSE) to 0.822 and reducing the Root Mean Square Error (RMSE) to 0.107. Unlike conventional Bagging approaches that tend to smooth out variance, XGBoost successfully corrected the baseflow biases of HEC-HMS without compromising the extrapolation of peak flows. It reveals that the hybrid model maintains strong physical consistency, and leverages satellite soil moisture to dynamically modulate runoff based on the basin's saturation state.
  
  KEY WORDS: Error-correction; GLDAS soil moisture; HEC-HMS model; Hybrid hydrological modeling; XGBoost algorithm
  
  Address:
  - Noureddine MAKHLOUFI, Department of Hydraulics, Faculty of Civil Engineering and Architecture, University of Hassiba Benbouali, 02180 Chlef, Algeria (Corresponding author. Tel.: Fax.: Email: n.makhloufi@univ-chlef.dz)

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 72 - 83, doi: 10.31577/ahs-2026-0027.01.0008
Scientific Paper, English

Chehla SAADOUN, Abdesselem KABOUR, Lotfi ZEGHADNIA: Hydrological response to climate change: Dam inflow dynamics in the Côtier Constantinois East and Medjerda-Mellegue basins

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• In a context of increasing climate change, this study evaluates the impact of climate variability on water resources. To achieve this, it analyses annual inflows were analysed at 15 stations located upstream of dams in the Côtier (Coastal) Constantinois East and Medjerda-Mellègue watersheds between 1958 and 2024. Several statistical methods were employed, including Pettitt's breakpoint test, the Mann-Kendall trend test, Sen's slope, the Standardised Runoff Index (SRI) and Innovative Trend Analysis (ITA). The results reveal a significant and persistent decrease in annual inflows, with breaks in the data identified as early as the 1970s in the south and from the 1990s onwards in the north. Reductions observed range from 38% to 47% in the north, exceeding 70% in the south. The SRI index indicates a predominance of drought periods in the south, whereas the north experiences a more balanced cycle of wet and dry periods. The Mann–Kendall test and Sen’s slope estimator consistently indicate significant decreasing trends in annual inflows at most stations. This is further supported by the Innovative Trend Analysis (ITA), which confirms a generalized decline across the study area. These results emphasise the clear impact of climate change on regional hydrological regimes and the necessity of adapting water resource management strategies.
  
  KEY WORDS: climate change, Constantinois East Coastal watershed, Medjerda-Mellegue watershed, inflows, Pettitt test, Mann-Kendall test, Innovative Trend Analysis, Standardized Runoff Index
  
  Address:
  - Chehla SAADOUN, Mohamed-Chérif Messaadia University, Souk Ahras, Laboratory of Modeling and Socio-Economic Analysis in Water Science – MASESE Lab., Algeria (Corresponding author. Tel.: Fax.: Email: : c.saadoun@univ-soukahras.dz)
  - Abdesselem KABOUR, Mila University Centre, Laboratory of Modeling and Socio-Economic Analysis in Water Science – MASESE Lab., Algeria
  - Lotfi ZEGHADNIA, Mohamed-Chérif Messaadia University, Souk Ahras, Laboratory of Modeling and Socio-Economic Analysis in Water Science – MASESE Lab., Algeria

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 84 - 93, doi: 10.31577/ahs-2026-0027.01.0009
Scientific Paper, English

Radoslav SCHÜGERL: Analysis of aquatic vegetation impact on the lowland river during growing season

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• Vegetation growth drastically influences water levels and flow patterns in lowland rivers. As soon as plants start to grow in spring, the flow resistance increases to reach a maximum in early summer. The presence of vegetation in stream ecosystems is highly dynamic in both space and time. Discharge calculations are based on velocity measurements, which deliver important information about structural characteristics of the river, presence of vegetation, etc. Removing the vegetation allows for determining vegetation density as well as for studying the influence of plants on flow distribution and patterns. It is shown that the resistance coefficient, influenced by the amount of biomass, is an important parameter for the hydraulic characteristics of the river. The presence of aquatic vegetation and their blockage characteristics are studied and a relation between discharge, biomass and resistance coefficient is set up. A digital photography technique is used to map aquatic vegetation cover, which has a very high spatial and a flexible temporal resolution. A digital camera mounted on a handheld telescopic pole is used. The method is successfully applied in lowland river (the Malina River in Slovakia).
  
  KEY WORDS: aquatic vegetation, biomass, growing season, hydraulic resistance, lowland river
  
  Address:
  - Radoslav SCHÜGERL, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic (Corresponding author. Tel.: Fax.: Email: schugerl@uh.savba.sk)

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 94 - 99, doi: 10.31577/ahs-2026-0027.01.0010
Scientific Paper, English

Dana PAVELKOVÁ, Milan GOMBOŠ, Andrej TALL, Branislav KANDRA: Effect of different gypsum application rates on saturated and unsaturated hydraulic conductivity of silty clay soil in field conditions

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• This study aimed to quantify the effect of gypsum application (CaSO₄·2H₂O) at rates of 0, 2, and 4 t ha⁻¹ on the saturated and unsaturated hydraulic conductivity of silty clay soil under field conditions. Unsaturated hydraulic conductivity K(h) was measured using a disk permeameter at pressure heads of −5 to −9 cm, while saturated hydraulic conductivity Ks was determined using a Guelph permeameter by the “One Head, Combined Reservoir” method. Gypsum application had a significant positive effect on both types of conductivity. At the rate of 4 t ha⁻¹, Ks increased approximately fourfold (from 11.7 to 52.0 cm d⁻¹). An increase in K(h) was observed at all pressure heads, with the highest relative increase (527%) at hw = −9 cm. The results confirm that the application of 2–4 t ha⁻¹ of gypsum represents an effective way to improve the structure and infiltration properties of silty clay soils, even 2.5 years after application.
  
  KEY WORDS: gypsum, hydraulic conductivity, disk permeameter, Guelph permeameter, soil structure
  
  Address:
  - Dana PAVELKOVÁ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic (Corresponding author. Tel.: Fax.: Email: pavelkova@uh.savba.sk)
  - Milan GOMBOŠ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Andrej TALL, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Branislav KANDRA, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 100 - 109, doi: 10.31577/ahs-2026-0027.01.0011
Scientific Paper, English

Zbyněk BAJTEK, Igor LEŠČEŠEN: Trends in discharge and thermal regimes of mountain streams in the contact zone of the Western and High Tatra Mountains

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• This study analyzes the dynamics and long-term changes in the hydrological and thermal regimes of mountain streams located at the boundary between the Western and High Tatra Mountains. The primary objective is to evaluate trends in average monthly discharges and water temperatures (Tw) over a 12-year period (2008–2020) in the context of observed climate variability. The research utilizes data from five selected monitoring stations (e.g., Belá River, Studený Creek) and builds on high-resolution data from the representative Jalovecký Creek catchment. The Mann-Kendall test was applied to detect the statistical significance of changes in both discharge and temperature series. Our results indicate a noticeable shift in the seasonal distribution of runoff, characterized by an earlier onset of spring peaks due to accelerated snowmelt and a general reduction in snow water equivalent. Furthermore, a rising trend in water temperature was identified, particularly during the summer months, which correlates closely with increasing air temperatures and changes in catchment land cover. A key methodological focus of the study is the validation of historical morning observations (Tw7) against modern daily averages (Tw), confirming their reliability for long-term monitoring in high-gradient mountain environments. The findings underscore the increasing thermal sensitivity of alpine-type streams, influenced by both climatic drivers and the natural decline of forest cover.
  
  KEY WORDS: mountain hydrology, discharge trends, water temperature, Western Tatras, Mann-Kendall test, climate change
  
  Address:
  - Zbyněk BAJTEK, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic (Corresponding author. Tel.: Fax.: Email: bajtek@uh.savba.sk)
  - Igor LEŠČEŠEN, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 110 - 121, doi: 10.31577/ahs-2026-0027.01.0012
Scientific Paper, English

Milan ONDERKA, Silvia KOHNOVÁ, Katarína MIKULOVÁ, Viera RATTAYOVÁ: Combining elicited prior with new observations improves frequency estimates of rare rainfall events

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• The selection of exceedance probabilities for rainfall extremes depends on the importance of the infrastructure and the consequences of its failure. Estimating quantiles of short-duration rainfall extremes is challenging due to the limited length of available instrumental records, especially in the region of very low probabilities. Bayesian approaches have been proposed as an alternative to classical frequency methods, as they allow the incorporation of additional sources of information through prior distributions, thereby improving inference from relatively short data series. The aim of this study is to improve the estimation of quantiles of short-duration rainfall extremes (15 and 30-minute durations) focusing on the region of low probabilities, by combining modern observations from the Hurbanovo station (1966–2009) with expert-derived historical quantiles from the period 1901–1965 within a Bayesian framework. The goal of this study is to demonstrate how incorporating expert information as prior knowledge can supplement the relatively short instrumental record and reduce the uncertainty of extreme rainfall quantile estimates, particularly for low annual exceedance probabilities (p~0.01). The results indicate that the width of the credible intervals for these quantiles is substantially reduced when additional expert knowledge is incorporated into the prior.
  
  KEY WORDS: Rainfall, extremes, quantiles, Bayesian inference, elicitation, expert knowledge, Hurbanovo
  
  Address:
  - Milan ONDERKA, Department of Land and Water Resources Management, Faculty of Civil Engineering Slovak University of Technology in Bratislava, Radlinského 11, 810 05 Bratislava, Slovak Republic; Regional Climatological Institute, 900 68 Plavecký Štvrtok 794, Slovak Republic (Corresponding author. Tel.: Fax.: Email: milan.onderka@stuba.sk)
  - Silvia KOHNOVÁ, Department of Land and Water Resources Management, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Radlinského 11, 810 05 Bratislava, Slovak Republic
  - Katarína MIKULOVÁ, Slovak Hydrometeorological Institute, Jeséniova 17, Bratislava, Slovak Republic
  - Viera RATTAYOVÁ, Slovak Hydrometeorological Institute, Jeséniova 17, Bratislava, Slovak Republic; Regional Climatological Institute, 900 68 Plavecký Štvrtok 794, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 122 - 130, doi: 10.31577/ahs-2026-0027.01.0013
Scientific Paper, English

Márta KOCZKA BARA, Yvetta VELÍSKOVÁ, Marek SOKÁČ, Valentín SOČUVKA: Seasonal changes in selected water quality parameters in a small and a large water supply reservoir

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• Water reservoirs are one of the most important sources of drinking water for the population. The effects of climate change can impact not only the quantity (floods, droughts) but also the quality of water, for example by altering the concentration of certain substances present in the water. Rising air temperatures can affect water temperature and, consequently, the ecosystem and water quality in water reservoirs. This article examines seasonal changes in selected water quality indicators in the Rozgrund and Klenovec water reservoirs (Slovakia). Based on their storage capacity, Rozgrund is a small water reservoir and Klenovec is a large water reservoir. Over a two-year period, water quality indicators were measured in situ in these reservoirs during the spring, summer, and autumn seasons. Measurements were taken at multiple vertical profiles across the reservoir, and at each profile at various depths down to the reservoir bottom. The following water quality parameters were monitored: water temperature, dissolved oxygen concentration, pH, and specific electrical conductivity of the water. The depth-dependent variation of the monitored parameters across seasons in both reservoirs was analyzed. The results showed that, within a single monitoring campaign (season), the measured values were generally similar at every monitoring profile in the reservoir. Within a single profile, depth stratification of the monitored parameters was observed, which was most pronounced during the summer season. The values of individual parameters varied significantly across different seasons. These findings applied to both the large and small water reservoir. The most interesting results were those for dissolved oxygen concentration, particularly during the summer season, when these values, after gradually decreasing toward the bottom, began to increase again at certain depth horizons. Such increased dissolved oxygen concentrations at greater depths indirectly indicate the presence of cyanobacteria in these horizons. This study provided important information on the spatial distribution and stratification of water quality parameters in reservoirs. These results help evaluate and understand processes related to water quality development in water bodies. The results can also support the better management of these important sources of drinking water.
  
  KEY WORDS: water reservoirs, drinking water supply, water quality parameters
  
  Address:
  - Márta KOCZKA BARA, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic (Corresponding author. Tel.: Fax.: Email: bara@uh.savba.sk)
  - Yvetta VELÍSKOVÁ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Marek SOKÁČ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Valentín SOČUVKA, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 131 - 139, doi: 10.31577/ahs-2026-0027.01.0014
Scientific Paper, English

Natália GAŠPARÍKOVÁ, Tatiana KALETOVÁ, Marek SOKÁČ: Zero-flow occurrence analysis in the stream of Slovakia in 2022

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• The analysis of dry periods on watercourses in the territory of the Slovak Republic in the hydrological year 2022 identifies the duration and intensity of minimum and zero flows at 22 gauging stations where flows ≤ 0.001 m³ s⁻¹ were recorded; 15 were selected for detailed analysis. The study focused on the temporal and spatial distribution of dry periods and their relationships with climatic, geological, topographic, and anthropogenic factors. The flow data were supplemented with information on slope, elevation and land use of the corresponding catchment. Data on average monthly temperatures and rainfall totals from the nearest meteorological stations were used to assess the influence of climatic conditions. The results highlight the vulnerability of streams to climatic extremes and the importance of a detailed assessment of hydrological and landscape parameters in identifying areas at risk. The analysis showed a pronounced seasonality of drought occurrence, with the highest frequency in the summer months, corresponding to the so-called summer type of low water availability. This finding can serve as a basis for further research in drought monitoring and water resources management.
  
  KEY WORDS: zero flows, streams, hydrological drought, watercourses
  
  Address:
  - Natália GAŠPARÍKOVÁ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic; Institute of Landscape Engineering, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture in Nitra, Tulipánová 7, 949 76 Nitra, Slovak Republic (Corresponding author. Tel.: Fax.: Email: gasparikova@uh.savba.sk)
  - Tatiana KALETOVÁ, Institute of Landscape Engineering, Faculty of Horticulture and Landscape Engineering, Slovak University of Agriculture in Nitra, Tulipánová 7, 949 76 Nitra, Slovak Republic
  - Marek SOKÁČ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 140 - 148, doi: 10.31577/ahs-2026-0027.01.0015
Scientific Paper, English

Viera RATTAYOVÁ, Milan ONDERKA, Zuzana NÉMETOVÁ, Jozef PECHO, Kamila HLAVČOVÁ: Impact of uncertainty in solar radiation and wind speed on FAO-56 reference evapotranspiration estimates

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• Accurate meteorological observations are essential for reliable estimation of reference evapotranspiration (ET₀) using the FAO-56 Penman-Monteith model. In practical applications, several input variables, particularly solar radiation, are often not directly measured but estimated, introducing uncertainty into ET₀ calculations. This study investigates how uncertainties in wind speed (u₂) and shortwave solar radiation (Rs) propagate into ET₀ estimates when applying the standard FAO-56 formulation. Controlled linear perturbations were used to simulate systematic measurement errors and extreme deviations, and the effect of replacing daily observations with long-term monthly mean values was also evaluated. Model sensitivity was assessed using linear sensitivity analysis, Monte Carlo simulations, and variance-based Sobol indices, enabling quantification of both the direct and interaction effects of input variables on ET₀ variance. Linear perturbation experiments showed a nearly proportional response of ET₀ to variations in solar radiation, reflecting its role in determining net radiation. In contrast, the response to wind speed was weaker and slightly nonlinear. Variance-based decomposition identified maximum air temperature as the dominant contributor to ET₀ variability, due to its influence on multiple components of the Penman-Monteith equation. Vapor pressure also showed a notable contribution, while wind speed influenced ET₀ primarily through interaction effects. The results indicate that moderate uncertainties in wind speed and solar radiation inputs may have a limited impact on ET₀ estimates under the analyzed conditions. Notably, air temperature, although is not expressed explicitly in the PM equation, emerged as the dominant contributor to ET₀ variability through its influence on multiple equation components. These findings suggest that, for applications relying on estimated or remotely sensed data, careful attention to temperature accuracy is more critical than minimizing uncertainty in solar radiation, while wind speed improvements provide modest but non-negligible reductions in ET₀ uncertainty.
  
  KEY WORDS: AO56 Penman-Monteith evapotranspiration, sensitivity analysis, Sobol method, reference evapotranspiration
  
  Address:
  - Viera RATTAYOVÁ, Slovak Hydrometeorological Institute, Jeséniova 17, 831 01 Bratislava, Slovak Republic; Regional Climatological Institute, 900 68 Plavecký Štvrok 794, Slovak Republic (Corresponding author. Tel.: Fax.: Email: v.rattayova@regioclim.eu)
  - Milan ONDERKA, Faculty of Civil Engineering, Slovak University of Technology, Radlinského 11, 811 05 Bratislava, Slovak Republic; Regional Climatological Institute, 900 68 Plavecký Štvrok 794, Slovak Republic
  - Zuzana NÉMETOVÁ, Institute of Hydrolgy SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Jozef PECHO, Regional Climatological Institute, 900 68 Plavecký Štvrok 794, Slovak Republic
  - Kamila HLAVČOVÁ, Faculty of Civil Engineering, Slovak University of Technology, Radlinského 11, 811 05 Bratislava, Slovak Republic

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 149 - 159, doi: 10.31577/ahs-2026-0027.01.0016
Scientific Paper, English

Aqila Rheva Latifah IBRAHIM, Lintang Nur FADLILLAH: Suspended sediment load and discharge relationships: Insights from long-term monitoring in the Bedog River, Indonesia

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• This study aims to analyze the correlations between water level and river discharge, as well as between discharge and suspended sediment. Estimates of the total suspended sediment load in Bedog Watershed, Yogyakarta, Indonesia are provided as well. The Bedog Sub-watershed, located on the southern slope of Mount Merapi wasonly slightly affected by the 2010 eruption. Data were obtained through continuous water level monitoring and depth-integrated suspended sediment sampling, with field observations conducted during the rainy season (November 2024–April 2025). Statistical analysis was conducted using Pearson correlation. Strong positive correlations were identified during both the rising (r=0.95) and falling (r=0.99) phases of the hydrograph, as well as between discharge and suspended sediment during the ascending (r=0.97) and descending (r=0.93) phases. Estimated annual suspended sediment load from 2015 to 2024 ranged from 3,360.59 to 4,550.29 tons year⁻¹, with sediment yield classified as very low, ranging from 0.291 to 0.393 tons ha⁻¹ year⁻¹
  
  KEY WORDS: hydrograph, landuse change, river sediment, sediment yield, total suspended sediments (TSS)
  
  Address:
  - Aqila Rheva Latifah IBRAHIM, Department of Environmental Geography, Faculty of Geography, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
  - Lintang Nur FADLILLAH, Department of Environmental Geography, Faculty of Geography, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia (Corresponding author. Tel.: Fax.: Email: lintang.n.f@ugm.ac.id)

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 160 - 170, doi: 10.31577/ahs-2026-0027.01.0017
Scientific Paper, English

Abdesselam BELKESSAM, Fatma Zohra BOURAS, Amal LOUNIS, Dalila HANK: Hydrological modeling of a small agricultural watershed using the SWAT model: The case of the Zaiane River, Algeria

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• Sustainable water resource management requires a good understanding of the elements of water balance. However, measuring many of these elements in the field is difficult, and the use of modeling tools is essential. The main objective of this study was to develop a monthly hydrological model to calculate the water balance of a small agricultural watershed (Oued Zaiane) and evaluate the capacity and effectiveness of the model. We opted for a methodology based on combining the free geographic information system QGIS with the agro-hydrological model Soil and Water Assessment Tool (SWAT) to perform geospatial analyses and simulate hydrological functions at a watershed scale. The model calibration phase was conducted using the SWAT-CUP tool with an adjustment algorithm (SUFI2) to optimize the sensitivity and uncertainty analysis parameters. The model was run from 2013 to 2020 with a calibration period of 2015 to 2019. The results obtained after calibration showed a satisfactory simulation after evaluating the model's performance using the following statistical coefficients: coefficient of determination (R²) and Nash–Sutcliffe efficiency coefficient (NSE). The monthly calibration of the flow rate yielded good values for coefficients such as R²=0.78 and NSE=0.55, confirming SWAT as an effective tool for simulating the hydrology of agricultural watersheds.
  
  KEY WORDS: Hydrological modeling, SWAT, GIS, Watershed, Oued Zaiane
  
  Address:
  - Abdesselam BELKESSAM, Agricultural Water Management Laboratory (LMEA), National Higher School of Agronomy (ENSA), ES16.03, Dept. of Agricultural Engineering, Algiers, Algeria (Corresponding author. Tel.: Fax.: Email: Abdesselam.belkessam@edu.ensa.dz)
  - Fatma Zohra BOURAS, Laboratory of Integrated Improvement of Plant Production (C2711100), Algiers, Algeria (Corresponding author. Tel.: Fax.: Email: f-zohra.bouras@edu.ensa.dz)
  - Amal LOUNIS, Agricultural Water Management Laboratory (LMEA), National Higher School of Agronomy (ENSA), ES16.03, Dept. of Agricultural Engineering, Algiers, Algeria
  - Dalila HANK, Agricultural Water Management Laboratory (LMEA), National Higher School of Agronomy (ENSA), ES16.03, Dept. of Agricultural Engineering, Algiers, Algeria

Acta Hydrologica Slovaca, Vol. 27, No. 1, 2026, p. 171 - 182, doi: 10.31577/ahs-2026-0027.01.0018
Scientific Paper, English

Viera RATTAYOVÁ, Milan ONDERKA, Zuzana NÉMETOVÁ, Andrej TALL, Branislav KANDRA, Katarína HRČKOVÁ, Jozef GUBIŠ: Seasonal differences between FAO-56 crop evapotranspiration and lysimeter measurements in Slovakia: The role of soil water availability

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• Crop evapotranspiration is a key variable in agricultural water management and hydrological studies. In Central Europe, direct evapotranspiration measurements remain scarce due to the limited availability of lysimeter and eddy covariance stations. This study quantified the seasonal difference between crop evapotranspiration (Etc) estimated using the FAO 56 Penman-Monteith method with climatically adjusted crop coefficients and actual evapotranspiration (Eta) measured by weighing lysimeters at two research stations in Slovakia, Borovce and Petrovce nad Laborcom, with particular focus on the role of seasonal water availability and antecedent dry periods as controls on this difference. The comparison revealed substantial seasonal variability in the Etc-Eta relationship. At Borovce, daily weighted absolute percentage error (WAPE) reached 38.1%, decreasing to 31.6% at the monthly scale. The largest discrepancies occurred during summer, when water availability became the dominant limiting factor. A post-rainfall analysis showed that the differences between Etc and Eta increased systematically with the duration of rain-free periods, with mean daily BIAS increasing from 0.27 mm one day after rainfall to more than 1.1 mm after four consecutive dry days. Analysis of satellite derived soil moisture data (ESA CCI SM) confirmed that near-surface soil moisture was the strongest predictor of daily Etc-Eta differences (Pearson R=−0.556, p<0.001), fully mediating the effect of antecedent dry periods and identifying a soil moisture threshold of SM*=0.262 m³ m⁻³ below which FAO-56 Etc systematically overestimates lysimeter Eta. The seasonal pattern of differences was consistent across both stations. The results demonstrate that the agreement between FAO-56 Etc and lysimeter-measured Eta is strongly controlled by seasonal conditions and soil moisture availability. While Etc provides a reasonable approximation of actual evapotranspiration under well-watered conditions, it systematically overestimates Eta during dry periods.
  
  KEY WORDS: evapotranspiration, lysimeter, FAO-56 Penman–Monteith, soil water availability, antecedent rainfall
  
  Address:
  - Viera RATTAYOVÁ, Slovak Hydrometeorological institute, Forecast and Warnings centre, Jeséniova 17, P. O. BOX 15, 833 15 Bratislava, Slovak Republic; Regional Climatological Institute, 900 68 Plavecký Štvrtok 794, Slovak Republic (Corresponding author. Tel.: Fax.: Email: v.rattayova@regioclim.eu)
  - Milan ONDERKA, Faculty of Civil Engineering, Slovak University of Technology, Radlinského 11, 81105 Bratislava, Slovak Republic; Regional Climatological Institute, 900 68 Plavecký Štvrtok 794, Slovak Republic
  - Zuzana NÉMETOVÁ, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Andrej TALL, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Branislav KANDRA, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic
  - Katarína HRČKOVÁ, Research Institute of Plant Production, National Agricultural and Food Centre, Bratislavská cesta 122, Piešťany, Slovak Republic
  - Jozef GUBIŠ, Research Institute of Plant Production, National Agricultural and Food Centre, Bratislavská cesta 122, Piešťany, Slovak Republic