J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 74 - 83, doi: .
Scientific Paper, English

Pavla Pekárová, Zbyněk Bajtek, Ján Pekár, Pavol Miklánek, Igor Leščešen, Tomáš Borároš: From decadal runoff depletion to daily streamflow cycles: The role of riparian evapotranspiration in a forested catchment

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• This study investigates the role of evapotranspiration in the hydrological balance of the small forested Vydrica catchment (Slovakia) by linking long-term climatic trends with short-term streamflow dynamics. A 60-year mass balance analysis (1961–2020) was integrated with high-frequency streamflow observations across nine severe summer drought events (1992–2025). The long-term analysis revealed a paradoxical trend: despite a slight increase in precipitation, annual runoff significantly decreased. This decline is driven by rising air temperatures and increased evaporative demand, causing altered seasonal runoff distribution and more severe summer low flows. The short-term event analysis demonstrated that diurnal streamflow fluctuations are strongly controlled by available baseflow. During moderate droughts, active riparian vegetation generates distinct diurnal streamflow cycles with a typical six-hour lag behind maximum air temperature. However, during extreme low-flow periods, the diurnal amplitude flattens and evapotranspiration losses drop by an order of magnitude, indicating that the groundwater table has fallen below the effective root zone. The findings prove that climate-driven long-term warming continuously depletes baseflow reserves, progressively restricting the daily transpiration capacity and resilience of riparian forests.
  
  KEY WORDS: Evapotranspiration; Hydrological balance; Small forested catchment; Diurnal streamflow fluctuations; Low flows.
  
  Address:
  - Pavla Pekárová, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic. (Corresponding author. Tel.: Fax.: Email: pekarova@uh.savba.sk)
  - Zbyněk Bajtek, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic.
  - Ján Pekár, Comenius University in Bratislava, Faculty of Mathematics, Physics, and Informatics, Mlynská dolina, 842 48 Bratislava, Slovak Republic.
  - Pavol Miklánek, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic.
  - Igor Leščešen, Institute of Hydrology SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic.
  - Tomáš Borároš, Slovak Hydrometeorological Institute, Jeséniova 17, P. O. BOX 15, 833 15 Bratislava 37, Slovak Republic.

J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 84 - 97, doi: .
Scientific Paper, English

Jaromír Říha, Stanislav Kotaška: Backward analysis of the Bílá Desná dam break outflow

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• In September 1916, the Austro-Hungarian Empire (today the Czech Republic) experienced the failure of the embankment dam Bílá Desná (historically named Weiße Desse), which at that time was the most disastrous dam failure in central Europe. In this study, available information on the failure of the Bílá Desná dam was gathered and analysed. The initiating cause of the Bílá Desná dam failure was an internal erosion. For the backward analysis of the dam failure, the method of analogy, empirical formulae, and three numerical models, AREBA, DL Breach and HEC-RAS were used. Due to uncertainty in the reservoir volume, the solution was carried out for the originally determined (during the dam design) and newly determined reservoir volumes. The analysis indicates the peak breach outflow range between about 360 and 810 m3/s. The most realistic and probable peak outflow discharge is about 520 m3/s. The comparison of individual methods indicate that the AREBA numerical model using Opendata (2024) provides the most realistic failure hydrograph.
  
  KEY WORDS: Bílá Desná; Dam; Breach; Internal erosion; Backward analysis.
  
  Address:
  - Jaromír Říha, Brno University of Technology, Faculty of Civil Engineering, Institute of Water Structures, Veveří 331/95, 602 00 Brno, Czech Republic. (Corresponding author. Tel.: Fax.: Email: riha.j@vutbr.cz)
  - Stanislav Kotaška, Brno University of Technology, Faculty of Civil Engineering, Institute of Water Structures, Veveří 331/95, 602 00 Brno, Czech Republic.

J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 98 - 112, doi: .
Scientific Paper, English

Gaetano Caltabellotta, Dario Autovino, Massimo Iovino, Vincenzo Bagarello: A four-year monitoring of beerkan infiltration rates in a sandy-loam soil

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• Long-term investigations on temporal variability of infiltration rates are important for characterizing the porous medium but they are challenging and infrequent. Infiltration rates were measured in a sandy-loam soil by performing 16 beerkan infiltration runs on 14 sampling dates during a nearly four-year period. The means of the initial infiltration rates (i0) varying by approximately four times, were less variable than the means of both the final (if) and the mean (imed) infiltration rates, varying by 13 and nine times, respectively. The means of all considered parameters decreased as the antecedent soil water content (i) increased but the i effect was stronger for i0 (coefficient of determination, R2 = 0.84) than both if and imed (R2 = 0.55-0.68). With reference to these last two parameters, i effects were significant for the lowest measured values (R2 = 0.63-0.76) but not for the highest ones (R2 = 0.12-0.13). Particularly small if and imed values were obtained in initially very wet soil conditions. The antecedent soil water content can be expected to strongly control infiltration rates in a sandy-loam soil. Sampling the widest possible range of i values is recommended to obtain the most complete information about temporal variability of infiltration rates.
  
  KEY WORDS: Single-ring infiltration; Soil hydrodynamic behaviour; Time variations; Early- and late-time infiltration rates; Soil water content effects.
  
  Address:
  - Gaetano Caltabellotta, Department of Agricultural, Food and Forest Sciences, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy.
  - Dario Autovino, Department of Agricultural, Food and Forest Sciences, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy. (Corresponding author. Tel.: Fax.: Email: dario.autovino@unipa.it)
  - Massimo Iovino, Department of Agricultural, Food and Forest Sciences, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy.
  - Vincenzo Bagarello, Department of Agricultural, Food and Forest Sciences, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy.

J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 113 - 124, doi: .
Scientific Paper, English

Mustafa Çakir, Gizem Nazli Ural, Mükerrem Oral, Okan Oral, Mesut Yilmaz: Explainable AI and Ensemble Machine Learning Analysis of River Flow Dynamics: Influence of Key Climatic Variables (Temperature, Humidity, Precipitation)

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• Accurate short-term river flowrate forecasting is essential for flood risk mitigation and sustainable water management. However, many machine learning (ML) applications in hydrology lack strict temporal validation and interpretability, limiting operational reliability. This study develops a reproducible and explainable framework for one-day-ahead daily flowrate forecasting in the Eşen River Basin (Türkiye) using hydro-meteorological data (2017-2022) from one flowrate station and four meteorological monitoring stations.The workflow integrates the Box-Cox transformation, lag-based feature engineering (up to 3 days, reflecting short-term hydrological memory), Boruta feature selection, and strictly time-aware rolling validation (2017-2021 training; 2022 independent test). Classical time-series models (ARIMA, TBATS), interpretable baselines (DT, LR), and advanced ML algorithms (RF, GBM, XGBoost, SVM, ANN) were benchmarked using RMSE, MAE, R², NSE, and KGE. Ensemble tree-based models consistently outperformed classical and baseline approaches in magnitude-sensitive metrics. XGBoost achieved the highest predictive accuracy (R² = 0.864; NSE = 0.864; RMSE = 6590 dm³ s⁻¹). Although TBATS yielded the highest KGE (0.865), ensemble models better captured nonlinear dynamics and flowrate variability. SHAP and LIME analyses revealed that short-term flow lags dominate predictive structure, while precipitation and temperature exert regime-dependent influence. The complete workflow is openly deployed via a reproducible R-Shiny environment. The results demonstrate that explainable ensemble learning, combined with strict temporal validation, provides a reliable and transparent framework for operational hydrological forecasting.
  
  KEY WORDS: Daily flowrate forecasting; Ensemble learning; Explainable AI; SHAP; LIME; Time-aware validation; Hydrology.
  
  Address:
  - Mustafa Çakir, Iskenderun Vocational School of Higher Education, Iskenderun Technical University, Iskenderun, Türkiye.
  - Gizem Nazli Ural, Faculty of Fisheries, Akdeniz University, Antalya, Türkiye.
  - Mükerrem Oral, Kemer Faculty of Maritime, Akdeniz University, Antalya, Türkiye.
  - Okan Oral, Faculty of Engineering, Akdeniz University, Antalya, Türkiye.
  - Mesut Yilmaz, Faculty of Fisheries, Akdeniz University, Antalya, Türkiye. (Corresponding author. Tel.: Fax.: Email: myilmaz@akdeniz.edu.tr)

J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 125 - 136, doi: .
Scientific Paper, English

D.A.L. Leelamanie, Morihiro Maeda: Time-dependent contact angle and repellency persistence in water-repellent Japanese forest soils in relation to droplet geometry and line tension

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• Water repellency (WR) restricts the spontaneous wetting of soil and lowers the affinity between soil and water. Studies focused on temporal alteration of droplet contact angles on hydrophobic soil substrates and their relation to persistence of WR are highly limited. This study examined time-dependent contact angle and WR persistence considering geometry and line tension droplets using six hydrophobic forest soils. Soils were collected from Japanese cedar (Cryptomeria japonica, CED1, CED2), Japanese cypress (Chamaecyparis obtusa, CYP1, CYP2), and cedar/cypress mixed (MIX1, MIX2) forests. The time-dependence of apparent and geometric soil-water contact angles (θA and θG, respectively) and repellency persistence, using the water drop penetration time (WDPT), were determined. In all tested soils, both θA and θG showed a gradual continuous decline with increasing time to reach zero within a period of ~5–6 min, with strong negative exponential correlations (R2 ≥ 0.90). The θA showed a strong positive linear correlation with θG (R2 > 0.99; p < 0.05), indicating the preservation of a fair spherical segment shape of droplets during spreading irrespective of the possible roughness on granular soil surfaces. The line tensions at the periphery of the contact circle of the water droplet on the tested hydrophobic soil surfaces were in a range of 200–300 μJ m–1, which was within the previously reported order-of-magnitude range (10–5–102 μJ m–1). The time-dependent nature of the contact angle in the present study can most likely be attributed to water molecule adsorption processes or amphiphilic molecular reorientation processes. The soil-water contact time until contact angle receded below 90° (CT-θ90°) was not equivalent to WDPT, emphasizing that the time-dependent decline of contact angle would not correspond with water infiltration into the soil matrix. Still, the strong positive linear correlation between CT-θ90° and WDPT (R2 = 0.926; p < 0.05) indicated a close relation between these two parameters.
  
  KEY WORDS: Droplet geometry; Line tension; Repellency persistence; Soil water repellency; Time-dependence of contact angle.
  
  Address:
  - D.A.L. Leelamanie, Department of Soil Science, Faculty of Agriculture, University of Ruhuna, Mapalana, Kamburupitiya 81100, Sri Lanka. Graduate School of Environmental and Life Science, Okayama University, 3-1-1 Tsushima-Naka, Kita-Ku, Okayama 700-8530, Japan. (Corresponding author. Tel.: Fax.: Email: leelamanie@soil.ruh.ac.lk)
  - Morihiro Maeda, Graduate School of Environmental and Life Science, Okayama University, 3-1-1 Tsushima-Naka, Kita-Ku, Okayama 700-8530, Japan.

J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 137 - 148, doi: .
Scientific Paper, English

Alirahm Rahimpour, Hossein Afzalimehr, Saeid Okhravi, Mohammad Nazari-Sharabian, Moses Karakouzian: Flow structure in a local meander bend of a gravel-bed river

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• This study investigates the spatial variability of flow structure in a natural meandering bend of the Kheir-Abad River, a gravel-bed mountain stream in southwestern Iran, with emphasis on velocity distribution, boundary shear stress, and secondary circulation. Field measurements were conducted along a 270 m reach (arc angle 113°) across nineteen cross-sections, revealing a highly heterogeneous flow field characterized by a central high-velocity core and reduced velocities near the banks. Comparison with HEC-RAS simulations shows that the model captures the general spatial organization of velocity and shear stress, but produces a smoothed representation and underestimates localized variations, particularly in downstream sections where field data indicate pronounced acceleration and peak shear stress values (up to 84.1 Pa). The observed shear stress distribution deviates from classical outer-bank dominance, with maximum values frequently occurring in the central channel and downstream regions, reflecting the influence of coarse bed roughness, complex secondary flow structures, and thalweg-controlled momentum concentration. Analysis of the secondary flow coefficient (C) supports this interpretation, with peak values coinciding with zones of increased velocity and shear stress, indicating intensified momentum exchange and enhanced near-bed transport capacity. Overall, the results demonstrate that flow in gravel-bed meanders is governed by strongly non-uniform and three-dimensional processes that cannot be fully captured by simplified hydraulic assumptions, and that integrating field measurements with numerical modeling provides a robust framework for interpreting flow dynamics and their morphodynamic implications.
  
  KEY WORDS: Flow Structure; Meander bend; Gravel-bed river; Shear stress; Secondary flow.
  
  Address:
  - Alirahm Rahimpour, Faculty of Civil Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran.
  - Hossein Afzalimehr, Faculty of Civil Engineering, Iran University of Science and Technology, Tehran 16846-13114, Iran. (Corresponding author. Tel.: Fax.: Email: hafzali@iust.ac.ir)
  - Saeid Okhravi, Institute of Hydrology, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, 84104, Slovakia. (Corresponding author. Tel.: Fax.: Email: saeid.okhravi@savba.sk)
  - Mohammad Nazari-Sharabian, Department of Engineering, Central Connecticut State University, New Britain, CT 06050, USA.
  - Moses Karakouzian, Department of Civil and Environmental Engineering and Construction, University of Nevada, Las Vegas, NV 89154, USA.

J. Hydrol. Hydromech., Vol. 74, No. 2 - Early view, 2026, p. 149 - 160, doi: .
Scientific Paper, English

Dimos Touloumidis, Efstathios K. Oikonomou: Rethinking Design Rainfall Estimation under Non-Stationary Extremes and Imperfect Reanalysis Data

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• Design rainfall return levels are critical inputs for engineering applications such as urban drainage design, flood mitigation, resilient transport infrastructure and climate-sensitive planning. However, their estimation is significanlty challenged by changing rainfall regimes, which can weaken the stationarity assumptions traditionally used in design rainfall analysis. A second challenge is data availability: design estimates rely on long gauge records that are often spatially sparse, while reanalysis products such as ERA5-Land provide spatial continuity but may poorly represent point-scale extremes, especially in convectively active and topographically complex settings. This study develops a reproducible GEE workflow to jointly examine these challenges by assessing temporal changes in extreme precipitation return levels and evaluating the reliability of ERA5-Land for design-oriented rainfall screening. Annual maxima from ERA5-Land daily precipitation are modelled using Gumbel and GEV distributions, with the workflow demonstrated globally and evaluated locally in Thessaloniki, Greece, against a long in-situ gauge record for return periods of 5, 10, 50 and 100 years. Results show that ERA5-Land systematically underestimates gauge-based design rainfall, with discrepancies increasing toward rarer events; consequently, gauge-to-ERA5-Land correction factors range from 1.17 to 1.29 under Gumbel and from 1.13 to 1.46 under GEV. A reference-to-recent period comparison further indicates substantial gauge-inferred intensification, including an approximately 33% increase in the 100-year Gumbel return level, while ERA5-Land shows little corresponding change. The proposed framework is reproducible and transferable for screening applications, but correction factors remain site-specific and require local gauge validation before engineering use.
  
  KEY WORDS: Extreme precipitation; Design rainfall; Extreme value theory; Correction factors.
  
  Address:
  - Dimos Touloumidis, Information Systems and e-Business Laboratory, Department of Applied Informatics, School of Information Sciences, University of Macedonia, 546 36 Thessaloniki, Greece. Centre for Research and Technology Hellas, Hellenic Institute of Transport, 57001 Thermi, Greece. (Corresponding author. Tel.: Fax.: Email: touldimos@uom.edu.gr)
  - Efstathios K. Oikonomou, School of Rural and Surveying Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece.