J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 322 - 332, doi: https://doi.org/10.2478/johh-2025-0025
Scientific Paper, English

Ronald C. Garcia, Rex Victor O. Cruz, Nathaniel C. Bantayan, Canesio D. Predo, Cristino L Tiburan, Jr: Morphometric Determinants of Dry Season Flow in Small Upstream Watersheds of Mts. Banahaw – San Cristobal Protected Landscape, Philippines

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• Geomorphology influences hydrologic response behavior of watersheds but seasonal responses such as dry season flow are rarely given focused more so their availability, quality, and spatiotemporal distribution. This is crucial for sustaining hydrologic ecosystem services (HES). This study assessed the influence of watershed morphometry on dry season flow volume and its selected physicochemical characteristics. Spot measurements of stream flow and its dissolved oxygen (DO), electric conductivity (EC), total dissolved solids (TDS), and Salinity levels from 56 small upstream watersheds in the Mts. Banahaw – San Cristobal Protected Landscape (MBSCPL) was determined. Principal Component Analysis (PCA) for variable reduction of geomorphological parameters and two-stage Random Forest Regression were used to determine the drivers of dry season flow occurrence and characteristics. Results showed that small upstream watersheds can exhibit sustained dry season flow with acceptable levels of EC, TDS, and Salinity. Total stream length (Tlu), Bifurcation ratio (Rb), area, and slope were the most important variables in predicting dry season flow volume, but its model showed poor validation performance and slightly overfit (R2= 0.527, p = 0.011). Models for water quality parameters showed acceptable performance for DO, EC, Salinity, and TDS levels (R2= 0.824, 0.713, 0.623, and 0.722, respectively) with flow volume (Q2018) as the most important driver, followed by Basin relief (Bh), watershed area, and slope.
  
  KEY WORDS: Geomorphology; Morphometry; Dry season flow; Watersheds.
  
  Address:
  - Ronald C. Garcia, Department of Forestry and Environmental Science, College of Agriculture, Southern Luzon State University, Lucban, Quezon, 4328, Philippines. (Corresponding author. Tel.: Fax.: Email: rcgarcia@slsu.edu.ph)
  - Rex Victor O. Cruz, Institute of Renewable Natural Resources, College of Forestry and Natural Resources, University of the Philippines Los Banos, Laguna, 4030, Philippines.
  - Nathaniel C. Bantayan, Institute of Renewable Natural Resources, College of Forestry and Natural Resources, University of the Philippines Los Banos, Laguna, 4030, Philippines.
  - Canesio D. Predo, Institute of Renewable Natural Resources, College of Forestry and Natural Resources, University of the Philippines Los Banos, Laguna, 4030, Philippines.
  - Cristino L Tiburan, Jr, Institute of Renewable Natural Resources, College of Forestry and Natural Resources, University of the Philippines Los Banos, Laguna, 4030, Philippines.

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 333 - 353, doi: https://doi.org/10.2478/johh-2025-0026
Scientific Paper, English

Karol Kosorin: The average and closure problem of turbulence theory resolved in random space

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• The problem above marked as resolved is more than a hundred years known as the closure problem of turbulence. Extending its name follows from below presented knowledge that to be its solution successful it is necessary to find an effective averaging tool enabling one to describe and smooth down any random turbulent field without any phenomenological limitations. To convince of necessity of such tool author in the article previously proved the non-differentiability of random fields of measurable turbulence characteristics. But the decisive momentum of his solution strategy arose from the idea that randomness is an autonomous factor of physical processes and, therefore, this property can be utilized as a property of independent variables of the governing PDEs. To realize this idea author picked random frequences of turbulent fluctuations. Author then postulated the dual property as well as bifunctionality hypothesis and found suitable constitutive equations enabling him: (i) to express the instantaneous behaving of any random vector and scalar turbulent fields; (ii) to average the non-linear N–S system for the thermally known turbulent flow over the characteristic domains in the 5–D random space; (iii) to close the averaged equations systems with the set of four relationships named the Energy Distribution Equations (EDE) as the key result of the closure process. The energy invariance principle was used to find a closing equation for the energy distribution factor. The resultant EDEs were successfully verified meanwhile by comparing them with data from four independent sources of experiments made in boundary layers of wind tunnel flows of high anisotropy. This closure problem solution was obtained without the use of any auxiliary parameters or assumptions of phenomenological or experimental origin. From the nature of EDEs it follows that all turbulent mean flows are always 3–Dimensional. The use of randomness autonomy as the property of independent variables at describing turbulent flows is not limited upon Newtonian fluids.
  
  KEY WORDS: Randomness as autonomous factor; 5–D random space; Energy distribution equations; Mathematical expectations; Energy distribution factor; Tensor of anisotropy.
  
  Address:
  - Karol Kosorin, Institute of Hydrology, SAS, Bratislava, Slovakia. (Corresponding author. Tel.: Fax.: Email: k.kosorin@gmail.com)

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 354 - 367, doi: https://doi.org/10.2478/johh-2025-0027
Scientific Paper, English

Igor Leščešen, Constantin Mares, Milan Josić, Biljana Basarin: Unraveling the impact of large-scale climate circulations on the maximum discharges in the middle and lower Danube basin

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• Understanding the relationships between large-scale climate circulation and precipitation and river discharge at the local level is of central importance for deciphering hydroclimatological processes. Annual maximum river discharges in Europe are more sensitive to the variability of atmospheric circulation than mean discharges. However, there is a knowledge gap regarding the specific influence of atmospheric patterns on maximum discharge dynamics in the Middle and Lower sections of the Danube basin, especially in Serbia and Romania. In this study, historical climate data and the influence of atmospheric patterns (Greenland-Balkan Oscillation index, Mediterranean Oscillation index and North Atlantic Oscillation index) were analysed to understand their impact on hydrological extremes. The results showed significant correlations between discharge and atmospheric oscillations, in particular a statistically significant decreasing trend of maximum monthly discharge at Orsova station (trend: –0.0548, p-value: 0.0305) in the period 1961–2018. Correlation analyses showed significant positive correlations between runoff and atmospheric indices in different time periods and stations, underlining the complexity of these relationships. In addition, GEV analysis underlined the stability of maximum discharges at Bezdan and Bogojevo stations over time, with notable instability observed at Orsova and Ceatal Izmail stations. These results emphasize the importance of considering both temporal and spatial variations in river discharge dynamics when assessing the impact of climate change on hydrological extremes.
  
  KEY WORDS: Danube; Maximum discharges; Climate change impacts; Atmospheric oscillations; Extreme value analysis.
  
  Address:
  - Igor Leščešen, Institute of Hydrology, Slovak Academy of Sciences, Dubravska cesta 9, 841 04 Bratislava.
  - Constantin Mares, Institute of Geodynamics, Romanian Academy, 19-21 J.L. Calderon st., 020032 Bucharest-37 Romania.
  - Milan Josić, Department of Geography, Tourism and Hotel Management, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, Novi Sad, Serbia. (Corresponding author. Tel.: Fax.: Email: milan.josic2000@gmail.com)
  - Biljana Basarin, Department of Geography, Tourism and Hotel Management, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, Novi Sad, Serbia.

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 368 - 377, doi: https://doi.org/10.2478/johh-2025-0028
Scientific Paper, English

Mojca Šraj, Nejc Bezak, Andrej Vidmar: Changes in annual maximum discharge occurrence in Slovenia during the period 1961–2023

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• The study compares changes in annual maximum (AM) discharge occurrence at 33 gauging stations in Slovenia for the period 1961-2023. The entire period was divided into two sub-periods, namely the period 1961-1990 and the period 1991-2023. The frequency of AM occurrence per day of the year was calculated for all stations under consideration, and the kernel density estimate was calculated for the two periods. The findings reveal that, except for one gauging station, the AM discharge has occurred later during the recent three decades (1991-2023) than during the first 30-year period (1961-1990). The shift ranged from one to 35 days. This shift could be mainly attributed to higher air temperatures in the spring and summer months, which intensify precipitation events. At four stations, the day of maximum density shifted from spring to autumn and, at one station, from autumn to spring. All of these stations are located in the eastern part of the country. In contrast, gauging stations in Alpine regions of the western part of the country show smaller shifts, attributed to reduced snow accumulation and earlier snowmelt.
  
  KEY WORDS: Timing of annual maximum (AM) discharge; Seasonality; Seasonal change; Slovenia.
  
  Address:
  - Mojca Šraj, University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, SI-1000 Ljubljana, Slovenia. (Corresponding author. Tel.: Fax.: Email: mojca.sraj@fgg.uni-lj.si)
  - Nejc Bezak, University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, SI-1000 Ljubljana, Slovenia.
  - Andrej Vidmar, University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, SI-1000 Ljubljana, Slovenia.

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 378 - 395, doi: https://doi.org/10.2478/johh-2025-0029
Scientific Paper, English

Nanes Hassanin Elmasry, Mohamed Kamel Elshaarawy: Interpretable Stacked Gradient-Boosting Models for Predicting the Discharge Coefficient of Elliptical Side Orifices

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• This study introduces a novel machine learning framework to accurately predict the discharge coefficient (Cd) of elliptical side orifices (ESOs). A cleaned experimental dataset consisting of 575 entries, refined using the Interquartile Range (IQR) method to remove outliers was employed. Five key dimensionless input variables were used to predict Cd: relative crest height (W/B), relative orifice width (a/B), relative orifice height (b/B), relative upstream height (y₁/B), and upstream Froude number (F1). Four advanced Bayesian-optimized base models: Extreme Gradient Boosting (BO-XGB), LightGBM (BO-LGB), CatBoost (BO-CGB), and Histogram-based Gradient Boosting (BO-HGB) were integrated within a stacked ensemble architecture. A meta-learner based on Multiple Linear Regression (MLR) linearly combined these predictions to form the final Stacked Model (SM-MLR). Among the base models, the BO-CGB model achieved the best validation performance, with R²=0.8884, RMSE=0.0100, and MARE=0.0155. The final SM-MLR model outperformed all base learners and prior models, reaching R²=0.920, RMSE=0.0086, and MARE=0.0122. Model interpretation using Shapley Additive Explanations (SHAP) and Partial Dependence Plots (PDPs) revealed that a/B and b/B were the most influential. PDP analysis highlighted a consistently positive influence of a/B and a nonlinear but stabilizing trend for b/B. In contrast, W/B exhibited a strong negative linear effect on Cd, while y₁/B and F1 showed more complex, nonlinear behaviors. These nonlinear and geometry-dependent relationships reinforce the fact that the hydraulic behavior of ESOs is not adequately captured by classical side-orifice theory. Accordingly, this study provides a comprehensive ML-based framework tailored to this geometry, and the analysis offers new theoretical insight into how ESO geometric ratios govern lateral outflow mechanics, addressing a key gap in hydraulic modeling of non-rectangular side orifice. To support practical application, a user-friendly graphical user interface (GUI) was developed, enabling engineers to estimate Cd in real time based on the input parameters. Overall, the proposed stacked ensemble approach significantly advances both the theoretical understanding and predictive accuracy of ESO discharge behavior, offering a robust and practical tool for modern hydraulic design.
  
  KEY WORDS: Bayesian optimization; Discharge; Elliptical orifice; Gradient Boosting; Machine learning; Prediction.
  
  Address:
  - Nanes Hassanin Elmasry, Civil Engineering Department, Faculty of Engineering, Horus University-Egypt, New Damietta 34517, Egypt (Corresponding author. Tel.: Fax.: Email: nhassanin@horus.edu.eg)
  - Mohamed Kamel Elshaarawy, Civil Engineering Department, Faculty of Engineering, Horus University-Egypt, New Damietta 34517, Egypt (Corresponding author. Tel.: Fax.: Email: melshaarawy@horus.edu.eg)

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 396 - 403, doi: https://doi.org/10.2478/johh-2025-0030
Scientific Paper, English

Jimmy Clifford Oppong, Martin Bartuška, Jan Frouz Frouz: How Well Bucket Lysimeters Correspond with Whole-catchment Runoff and its Chemistry: A Case Study of Artificial Experimental Catchments at a Post-mining Site

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• This study evaluated efficiency of bucket lysimeters for measuring water fluxes and ion transport in four hydrologically isolated experimental catchments representing reclaimed (levelled and planted by alder) and unreclaimed (wave like topography, unvegetated) post-mining sites near Sokolov, Czech Republic. Weekly measurements of leachate from lysimeters and surface/subsurface runoff from experimental catchments, in which lysimeters were installed, were collected from 2021 to 2024. Ion concentrations (Ca²⁺, Na⁺, Li⁺, NH₄⁺, K⁺) were quantified using ion-selective electrodes. Upscaled estimates showed higher accuracy at the unreclaimed site (R² = 0.81 for total runoff, R² = 0.88 for evapotranspiration) than at the reclaimed site (R² = 0.72 and R² = 0.77). Lysimeter leachate explained surface runoff variance at unreclaimed (R² = 0.75) and reclaimed (R² = 0.47) sites, but was not predictive for subsurface flow. Among ions, Li⁺ showed the highest predictive capacity (R² = 0.44 - 0.56), while NH₄⁺ showed consistent patterns across sites. K⁺, Na⁺, and Ca²⁺ showed variable transport influenced by soil and vegetation development. Lysimeters captured surface water fluxes and evapotranspiration but did not represent subsurface flow or solute transport well. Better lysimeter performance at the unreclaimed site suggests that vegetation development reduces hydrological predictability during ecosystem recovery.
  
  KEY WORDS: Bucket lysimeter; Leachate; Ions; Surface and subsurface flow; Experimental Catchment; Evapotranspiration.
  
  Address:
  - Jimmy Clifford Oppong, Charles University, Faculty of Science, Institute for Environmental Studies, Benátská 2, 128 00, Prague, Czechia. (Corresponding author. Tel.: Fax.: Email: oppongj@natur.cuni.cz)
  - Martin Bartuška, Charles University, Faculty of Science, Institute for Environmental Studies, Benátská 2, 128 00, Prague, Czechia. Institute of Soil Biology and Biogeochemistry, Biological Centre of the Academy of Sciences of the Czech Republic, Na Sádkách 7, 370 05 České Budějovice, Czechia.
  - Jan Frouz Frouz, Charles University, Faculty of Science, Institute for Environmental Studies, Benátská 2, 128 00, Prague, Czechia. Institute of Soil Biology and Biogeochemistry, Biological Centre of the Academy of Sciences of the Czech Republic, Na Sádkách 7, 370 05 České Budějovice, Czechia. Institute of Soil Biology and Biogeochemistry, Biological Centre of the Academy of Sciences of the Czech Republic, Na Sádkách 7, 370 05 České Budějovice, Czechia.

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 404 - 411, doi: https://doi.org/ 10.2478/johh-2025-0031
Scientific Paper, English

Agnieszka Ziernicka-Wojtaszek, Robert Wojtaszek: Variation in runoff in Poland in relation to precipitation and air temperature

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• The study presents variation in river runoff in Poland in relation to precipitation and air temperature. The annual runoff value was analysed in average, warm wet, warm dry, cold dry, and cold wet years, categorized on the basis of average annual air temperature and annual precipitation totals. In addition, a statistical mathematical model of annual runoff was developed as a function of input variables – temperature and precipitation in the warm half and cold half of the year. Statistically significant differences were demonstrated between the runoff values for warm dry years, average years, and cold wet years.
  
  KEY WORDS: Precipitation; Temperature; Hydrological water balance; Runoff; Poland.
  
  Address:
  - Agnieszka Ziernicka-Wojtaszek, Department of Ecology, Climatology and Air Protection, Faculty of Environmental Engineering and Land Surveying, University of Agriculture in Krakow, al. Mickiewicza 24/28, 30-059 Krakow, Poland. (Corresponding author. Tel.: Fax.: Email: agnieszka.ziernicka-wojtaszek@urk.edu.pl)
  - Robert Wojtaszek, Department of Hydraulic Engineering and Geotechnics, Faculty of Environmental Engineering and Land Surveying, University of Agriculture in Krakow, al. Mickiewicza 24/28, 30-059 Krakow, Poland.

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 412 - 442, doi: https://doi.org/10.2478/johh-2025-0032
Scientific Paper, English

Harshvardhan, Deo Raj Kaushal: Equilibrium scour pattern around tandem piers at an intermediate below-critical inflow velocity

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• Scouring around bridge piers is one of the major reasons for bridge failure. A total of 20 experiments of scour around tandem piers were conducted in an open channel, at a subcritical inflow velocity (U/Uc=0.67) by placing an equal or smaller diameter circular bridge pier downstream of an existing pier with relative diameters in the range 1.00 to 0.67, at varied normalized spacings in the range 4 to 9.33, and detailed measurements of the equilibrium scour beds were taken. Separate scour holes with distinct deposition between the piers was observed in the majority of cases. Highest reinforcement effects were observed for pier combinations with Dd/Du=0.90 and 0.80, resulting in larger scour hole dimensions at the upstream pier, compared to isolated condition. The spacing corresponding to the minimum scour hole dimensions at the downstream pier was observed in the range L/Du=4 to 9.33 for each pier combination and it decreased with the decrease in diameter of the downstream pier. Variations for the normalized equilibrium scour depth, lateral and longitudinal extents of the scour hole, scour surface area, and scour hole volume as a function of normalized spacing for different ratios of downstream and upstream pier diameters, are presented in the form of graphs and equations.
  
  KEY WORDS: Inline bridge piers; Scour; Volume; Area; Longitudinal Extent; Lateral Extent; Unidentical Piers; Equilibrium scour; Low inflow velocity.
  
  Address:
  - Harshvardhan, Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India. Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India.
  - Deo Raj Kaushal, Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India. (Corresponding author. Tel.: Fax.: Email: kaushal@civil.iitd.ac.in)

J. Hydrol. Hydromech., Vol. 73, No. 4, 2025, p. 443 - 456, doi: https://doi.org/10.2478/johh-2025-0033
Scientific Paper, English

Murodil Madaliev, Zokhidjon Abdulkhaev, Mumin Nabiev, Abdurakhim Dustmatov: Flow past one and two adjacent square cylinders based on a two-fluid turbulence model

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• This paper presents an improved version of the Spalding two-fluid turbulence model developed by Malikov. The model is applied to numerically simulate turbulent flows past one and two square cylinders at a Reynolds number of Re = 47,000. The SIMPLE algorithm with a semi-implicit scheme and second-order accuracy is used to solve the equations. The code developed by the authors was tested on a two-dimensional benchmark problem of flow past a single square cylinder and then applied to crossflow past two square cylinders located side by side. The ratio of the distance between the cylinders, T/d, varied from 0 to 5, which allowed us to identify three characteristic flow regimes: single, slot (interference), and antiphase synchronous. The simulation results were compared with experimental data and showed good agreement in terms of pressure distribution, vortex wake structure, and aerodynamic force coefficients. Comparison with RANS and LES models confirmed that Malikov's model provides comparable accuracy with significantly lower computational costs.
  
  KEY WORDS: Navier–Stokes equations; Separated flow; Control volume method; Two-fluid model; SIMPLE algorithm.
  
  Address:
  - Murodil Madaliev, Fergana State Technical University, Fergana, Uzbekistan, 150100.
  - Zokhidjon Abdulkhaev, Fergana State Technical University, Fergana, Uzbekistan, 150100. (Corresponding author. Tel.: Fax.: Email: zokhidjon@fstu.uz)
  - Mumin Nabiev, Fergana State Technical University, Fergana, Uzbekistan, 150100.
  - Abdurakhim Dustmatov, Fergana State Technical University, Fergana, Uzbekistan, 150100.