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Optical sensors are widely used for turbidity measurements in suspended sediment concentration studies. When conducting continuous measurements of turbidity in the field with optical sensors, results are determined according to the calibration curve (relationship between suspended sediment concentration and turbidity readings from the optical sensors). The calibration curve is developed based on the samples of the material present in and/or around the investigated stream. The concentration data are useful in water-quality-related investigations as well as for evaluating the amounts of transported (flushed, eroded) material from the catchments as suspended load. The amounts and particle sizes of transported material depend on the hydrological conditions. Usually, the particles’ size is not directly considered when developing the calibration curve. However, different particle sizes of the material from the same study site can result in different turbidity readings. Taking into account one general calibration curve for suspended sediment concentration determination can lead to misestimation of the transported material amounts. Here, the results of turbidity sensor calibration for different particle size classes are presented. Additionally, the uncertainty of the suspended material concentrations due to this effect is estimated. Further, we show how different calibration curves affect the assessment of the amount of the transported suspended load from the selected experimental catchment.

In the paper, the determination of nitrate nitrogen (NO3-N) export regimes from the Kuzlovec catchment during rainfall events is presented employing various determination methods: the ratio of coefficient of variation of NO3-N concentration and discharge (CVC/CVQ), the slope of the regression line to the points of concentration and discharge logarithms (b), both mentioned indices together, and the relative change of the NO3-N concentration during the event (Crs). According to previous analyses, the amount of exported NO3-N during rainfall events comprises approx. 30% of the total export amount, while from a temporal point of view, rainfall events represent only 10% of the time. Export regimes were identified as chemostatic (concentration does not change with discharge) and chemodynamic (concentration changes with discharge). The latter were further divided into dilution and flushing regimes. 43 rainfall events from the period April 2018–April 2020 were included in the analysis. For these events, high-frequency data of 20-minute intervals were available for concentration and discharge. All applied indices showed that the catchment responds differently from a concentration-discharge perspective and that chemodynamic behaviour is predominant, with flushing most frequently prevalent. A possible seasonal connection with regime diversity was tested using the non-parametric, statistical Wilcoxon test. A statistically significant difference was not found for any of the indices. Nevertheless, a detailed analysis of CVC/CVQ revealed that the ratio is likely related to seasonal variability in rainfall characteristics and consequently to the hydrological conditions in the catchment. Hence, it would be of great importance to include data about rainfall and hydrological properties in future studies.

Afforestation is recognized as one of the commonly used nature-based solutions for flood risk mitigation as well as for reduction of anthropogenic impacts on sediment and nutrient flushing. Processes of nutrient cycles (e.g., nitrogen cycle) are directly dependent on the amount of precipitation and its temporal and spatial distribution as water is the main transport medium and the driving force for many ecosystem processes. To understand the natural background of biogeochemical processes and transport of their products, it is therefore necessary to improve understanding of the hydrological control mechanisms, especially the formation of rainfall-runoff. Forest ecosystems without any or with negligible anthropogenic influences represent such kind of a reference, i.e. natural state in the field of nutrient flushing research. For this reason, we established an experimental monitoring system in the small, almost completely afforested Kuzlovec river catchment, Slovenia. Most of the hydrometeorological variables were measured continuously at a 20-min time step accompanied with measurements of concentrations of nitrate-nitrogen in the stream and leaf area index, which was used as an indicator of seasonal growth periods. Based on a two-year data set of various variables, we identified rainfall events for which we investigated: i) the influences of rainfall characteristics on the nitrate-nitrogen flushing, ii) the rainfall-runoff formation processes taking into account nitrate-nitrogen concentration changes during rainfall events, and iii) the role of the forest on the dynamics of the nitrate-nitrogen flushing (i.e., are there differences between the seasons). The focus of this contribution will be on the latter. We will present the most important findings obtained by using statistical analyses, such as hierarchical clustering, k-means, and principal component analysis. We believe that information obtained from such research is extremely important for improving the understanding of the processes’ controlling factors in areas with anthropogenic activities that affect the circulation and amount of nutrients exported to water bodies.

In this paper we analyse the influence of three different calculation criteria on the values of low-flow recession constants, namely the influence of the calculation method, recession curve segment length (4, 5, 6, or 7 days) and of the selected period (whole period, monthly, seasonally) for the calculation of the initial discharge, which determines the threshold on the falling limb of the hydrograph for the recession analysis. The analysis is based on daily discharge data of 11 gauging stations in the hydrogeologically non-homogenous Ljubljanica River catchment. For each of the stations, we calculated 24 values of recession constants using a different combination of criteria. Based on these values we determined the influence of each criterion on the final result using statistical tests and graphical approaches. Analysis showed that the recession constants calculated using various methods are statistically significantly different for all gauging stations. The recession constants obtained using the individual recession segment method (IRS) are on average 3.1 days higher than those calculated using the master recession curve method (MRC). The influence of the other two criteria, namely the length of the segment and the period for calculating the initial discharge, is not so obvious, since it is most likely conditioned by catchments’ characteristics influencing the time dynamics of the runoff.

Understanding and prediction of low-flow conditions are fundamental for efficient water resources planning and management as well as for identification of water-related environmental problems. This is problematic especially in view of water use in economic sectors (e.g., tourism) where water-use peaks usually coincide with low-flow conditions in the summer time. In our study, we evaluated various low-flow characteristics at 11 water stations in the non-homogenous Ljubljanica river catchment in Slovenia. Approximately 90% of the catchment is covered by karst with a diverse subsurface, consisting of numerous karst caves. The streams in the remaining part of the catchment have mainly torrential characteristics. Based on daily discharge data we calculated and analyzed values of 5 low-flow indices. In addition, by analyzing hydrograph recession curves, recession constants were determined to assess the catchment’s responsiveness to the absence of precipitation. By using various calculation criteria, we analyzed the influence of individual criteria on the values of low-flow recession constants. Recession curves are widely used in different fields of hydrology, for example in hydrological models, baseflow studies, for low-flow forecasting, and in assessing groundwater storages which are crucial in view of assessing water availability for planning water resources management. Moreover, in the study we also investigated the possible impact of projected climate change (scenario RCP4.5) on low-flow conditions in two sub-catchments of the Ljubljanica river catchment. For the evaluation we used the lumped conceptual hydrological model implemented in the R package airGR. For periods 2011-2040, 2041-2070, and 2071-2100 low-flow conditions were evaluated based on flow duration curves compared with the 1981-2010 period. The lowest discharges at all water stations in the Ljubljanica river catchment occur mostly during the summer months. Our results for the future show that we can expect a decrease of the lowest low-flows in the first two 30-year periods, while in the last one low-flows could increase by approx. 15%. However, the uncertainty/variability of the results is very high and as such should be taken into account when interpreting and using the results. This study demonstrates that evaluation of several low-flow characteristics is needed for a comprehensive and holistic overview of low-flow dynamics. In non-homogeneous catchments with a high karstic influence, the hydrogeological conditions of rivers should also be taken into account in order to adequately interpret the results of low-flow analyses. This proved to be important even in case of neighboring water stations.

Modeling and evaluation of the effect of afforestation on the runoff generation within the Glinščica catchment (Slovenia)Gregor Johnen1, Klaudija Sapač2, Simon Rusjan2, Vesna Zupanc3, Andrej Vidmar2, Nejc Bezak21 Radboud University Nijmegen, Faculty of Science2 University of Ljubljana, Faculty of Civil and Geodetic Engineering3 University of Ljubljana, Biotechnical Faculty Abstract:Increases in the frequency of flood events are one of the major risk factors induced by climate change that lead to a higher vulnerability of affected communities. Natural water retention measures such as afforestation on hillslopes and floodplains are increasingly discussed as cost-effective alternatives to hard engineering structures for providing flood regulation, particularly when the evaluation also considers beneficial ecosystem services other than flood regulation. The present study provides combined modelling approach and a cost-benefit analysis (CBA) of the impacts of afforestation on peak river flows and on selected ecosystem services within the Glinščica river catchment in Slovenia. In order to investigate the effects, the hydrological model HEC-HMS, the hydraulic model HEC-RAS and the flood damage model KRPAN, that was developed specifically for Slovenia, are used. It was found that increasing the amount of tree cover results in a flood peak reduction ranging from 9-13 %. Flood extensions were significantly lower for most scenarios leading to reduced economic losses. However, a 100-years CBA only showed positive net present values (NPV) for one of the considered scenarios and the benefits were dominated by the flood regulation benefits, which were higher than for example biodiversity or recreational benefits. Based on our findings we conclude that afforestation as a sole natural water retention measure (NWRM) provides a positive NPV only in some cases (i.e. scenarios) and if additional ecosystem co-benefits are considered.

In the small, forested Kuzlovec stream catchment near Ljubljana, Slovenia, nitrate flushing data were monitored in years 2018 and 2019. The amount of exported nitrates, determined based on the nitrate concentration in the stream, depends on hydrometeorological and biogeochemical conditions in the catchment. Therefore, various measuring equipment was installed in the catchment: tipping buckets for measuring precipitation amount and rainfall rates, pressure sensors in the stream for evaluation of water level and consequently of stream discharge, soil moisture sensors in three depths, a multiparameter probe for measuring the physical and chemical properties of water, including concentration of nitrates, etc. Moreover, occasionally, some discrete measurements were performed: discharge measurements using a dilution method, leaf area index measurements for determination of vegetation conditions using a plant canopy analyser, determination of soil properties including nitrate content in different soil horizons, etc.Data, which were obtained using in-situ equipment for continuous measurements, were collected or recalculated to a 20-min time step. Due to geological properties and steep slope of the terrain, the catchment is very responsive to rainfall events. This is reflected in a short time of concentration, meaning that both flood peak and nitrate concentration peak occur shortly after the rainfall event. However, concentration of nitrates in stream and consequently the amount of flushed nitrates vary on event basis. This paper presents results of analysis of different events which occurred during the monitoring in order to identify hydrometeorological and seasonal conditions on nitrate flushing.Acknowledgment: We acknowledge the financial support of the Slovenian Research Agency (research core funding No. P2-0180, and the PhD grant of the first author).

Increases in the frequency of flood events are one of the major risk factors induced by climate change that lead to a higher vulnerability of affected communities. Natural water retention measures such as afforestation on hillslopes and floodplains are increasingly discussed as cost-effective alternatives to hard engineering structures for providing flood protection, particularly when the evaluation also considers beneficial ecosystem services other than flood protection. The present study provides a combined modelling approach and a cost–benefit analysis (CBA) of the effects of afforestation on peak river flows and on selected ecosystem services within the Glinscica River catchment in Slovenia. In order to investigate these effects, the hydrological model HEC-HMS, the hydraulic model HEC-RAS, and the flood damage model KRPAN, which were developed specifically for Slovenia, are used. Three scenarios were evaluated where the main difference was the extent of afforestation. It was found that increasing the amount of tree cover (i.e., ≈15–60%) results in a flood peak reduction ranging from 9 to 13%. Flood extensions were significantly lower for most scenarios leading to reduced economic losses. However, a 100-year CBA only showed positive net present values (NPV) for one of the considered scenarios, where the afforestation was considered only in floodplain areas and the prevailing benefits were those of flood protection measures, which were higher than, for example, biodiversity or recreational benefits. Additional ecosystem co-benefits that are not directly linked with flood protection are considered in case of all three scenarios.

Extreme events such as flash floods and debris flows are frequent phenomena that occur in steep torrential catchments; these kinds of events can cause notable geomorphic changes. Repeated terrestrial laser scanning (TLS) surveys were performed in a steep forested catchment of the Kuzlovec torrent (drainage area ~ 0.7 km2) in central Slovenia, where a ~ 200-m long section of the torrent was scanned in 2013, 2014, and 2015. The main aim of this study was to perform the geomorphic response detection in the torrent due to hydro-meteorological events of different magnitudes. After applying several pre-processing steps, digital terrain models (DTMs) with a cell resolution of 5 cm were produced. The geomorphic change detection was performed using the DTM of Difference approach (DoD). Several above-average flow events occurred in the period from 2013 to 2015 (some of them can be regarded as floods). The 2014 August extreme flash flood that was initiated by the rainfall event with a return period exceeding 100 years, where maximum 1-minute rainfall intensities were up to 288 mm/h, led to erosion rates of an order of magnitude higher than average annual erosion rates. Moreover, the analysis of the geomorphic changes shows that the August 2014 flash flood caused intense sediment transport processes that resulted in the changes at the location of the main stream channel thalweg and reduced channel roughness. The unit stream power for the scanned section of the torrent was assessed to be approximately 500 W/m2 during this extreme event. This is above the thresholds that were suggested to differentiate between the situations where significant geomorphic changes can occur and the situations where geomorphic changes are not notable.

The karst aquifer of the Ljubljanica River catchment, which has numerous springs and sinks, presents an interesting environment for studying hydrogeological processes. This study aims to explore the behavior of U isotopes and to evaluate their use as tracers of hydrogeochemical processes as an alternative to classical geochemical tracers (i.e., physicochemical parameters, elemental ratios, and alkalinity) involved in water&ndash, rock interactions and water flow in this karst water system. Basic hydrochemical parameters, as well as the spatiotemporal variations of total U concentrations, 234U/238U activity ratios, and &delta, 238U values, were monitored in water samples from springs and sinks under different hydrological conditions. The bedrock as the source of dissolved and detrital U was also analyzed. Multi-collector inductively couple plasma-mass spectrometry results reveal variations of the 234U/238U activity ratios, which are consistently negatively correlated with the discharge at most analyzed sites. Large 238U/235U isotope fractionation occurred during bedrock weathering, and the large variability of the measured &delta, 238U values is seemingly unrelated to the lithological characteristics of the bedrock or discharge. Our results confirm that 234U/238U activity ratios in water can be used as a tracer for studying changes in groundwater flows and the mixing of waters of different origins under different hydrological conditions.

Understanding the temporal variability of the nutrient transport from catchments is essential for planning nutrient loss reduction measures related to land use and climate change. Moreover, observations and analysis of nutrient dynamics in streams draining undisturbed catchments are known to represent a reference point by which human-influenced catchments can be compared. In this paper, temporal dynamics of nitrate-nitrogen (NO3-N) flux are investigated on an event basis by analysing observed lag times between data series. More specifically, we studied lag times between the centres of mass of six hydrological and biogeochemical variables, namely discharge, soil moisture at three depths, NO3-N flux, and the precipitation hyetograph centre of mass. Data obtained by high-frequency measurements (20 min time step) from 29 events were analysed. Linear regression and multiple linear regression (MLR) were used to identify relationships between lag times of the above-mentioned processes. We found that discharge lag time (LAGQ) and NO3-N flux lag time (LAGN) are highly correlated indicating similar temporal response to rainfall. Moreover, relatively high correlation between LAGN and soil moisture lag times was also detected. The MLR model showed that the most descriptive variable for both LAGN and LAGQ is amount of precipitation. For LAGN, the change of the soil moisture in the upper two layers was also significant, suggesting that the lag times indicate the primarily role of the forest soils as the main source of the NO3-N flux, whereas the precipitation amount and the runoff formation through the forest soils are the main controlling mechanisms.

This study attempts to assess the consistency of various low-flow indices evaluated at 12 water stations in the Ljubljanica river catchment, Slovenia, which is due to its high diversity of hydrogeological properties recognized as a non-homogenous catchment. Daily discharge data were used in the study for evaluating low-flow statistics (baseflow index, discharges from flow duration curves, mean annual minimums), seasonal indices, and analysis of hydrograph recession curves. The indices were compared at each station and their values were interpreted in terms of the connection between surface water and groundwater as well as the extent of contribution of delayed sources to surface water. Although all low-flow statistics point to the same stations in terms of the greatest and the lowest contribution of water from groundwater, we identified inconsistencies at some water stations. The hydrograph recession analysis and seasonal indices support the conclusions drawn from low-flow statistics analysis, with the seasonality ratio lower than 1 at all stations, meaning that summer low-flow conditions prevail across the catchment. However, at the torrential stream Gradascica, an inconsistency was found with recession constants that could be attributed to the inappropriate choice of computational parameters. We assessed the overall consistency heat map visualisation. Although it shows consistency between low-flow indices, some variations demonstrate that in case of hydrologically and geologically heterogeneous catchments, an analysis of several low-flow indices is needed in order to obtain a holistic overview of low-flow conditions and to avoid bad decisions in water resources management due to inappropriate consideration of low-flow characteristics.

During a weather front that passed over large parts of Slovenia on 18.9.2007, extreme rainfall events were triggered causing several severe flash floods with six casualties. Out of 210 municipalities in Slovenia, 60 were reporting flood damages, and the total economic flood damage was later estimated at close to 200 million Euro; highest damage was claimed by Železniki municipality in NW Slovenia. The main purpose of the study presented in this paper was to put together available meteorological and hydrological data in order to get better insight into temporal and spatial dynamics and variability of the flash flood event along the Selška Sora River flowing through the town of Železniki. The weather forecast by the Environmental Agency of the Republic of Slovenia (ARSO) lead to early warning of floodings but has underestimated rainfall amounts by a factor of 2. Also meteorological radar underestimated ground rainfall as much as by 50%. During that day, in many rainfall gauging stations operated by ARSO in the area under investigation, extreme rainfall amounts were measured, e.g. 303 mm in 24 h or 157 mm in 2 h. Some of the measured rainfall amounts were the highest registered amounts in Slovenia so far. Statistical analysis using Gumble distribution was performed and rainfall return periods were estimated. When assessing rainfall return periods, a question of the sampling error as a consequence of short rainfall records used was raised. Furthermore, measured rainfall data were used to reconstruct hydrographs on selected water stations along the Selška Sora River. The cumulative areal precipitation for the Selška Sora River catchment upstream of Železniki amounted to 219 mm, while the modeled effective precipitation used to simulate the hydrograph peak was only 57 mm. The modeled direct runoff coefficient therefore amounts to 0.26. Surprisingly low value is mainly caused by the applied unit hydrograph method that seeks to meet the peak discharge rather than hydrograph volume. However, the spatial distribution of the rainfall in the area was highly variable and present spatial positioning of rain gauges is obviously inadequate for proper representation of the actual spatial amount of rainfall. The study confirmed that post-flood investigation should focus on discharges and hydrological response of the catchment rather than simply analyzing statistical characteristics of rainfall.

In the case of ungauged catchments, different procedures can be used to derive the design hydrograph and design peak discharge, which are crucial input data for the design of different hydrotechnical engineering structures, or the production of flood hazard maps. One of the possible approaches involves using a hydrological model where one can calculate the design hydrograph through the design of a rainfall event. This study investigates the impact of the design rainfall on the combined one-dimensional/two-dimensional (1D/2D) hydraulic modelling results. The Glinščica Stream catchment located in Slovenia (central Europe) is used as a case study. Ten different design rainfall events were compared for 10 and 100-year return periods, where we used Huff curves for the design rainfall event definition. The results indicate that the selection of the design rainfall event should be regarded as an important step, since the hydraulic modelling results for different scenarios differ significantly. In the presented experimental case study, the maximum flooded area extent was twice as large as the minimum one, and the maximum water velocity over flooded areas was more than 10 times larger than the minimum one. This can lead to the production of very different flood hazard maps, and consequently planning very different flood protection schemes.

Summary The simple dynamical system approach was implemented to analyze, explain and simulate streamflow fluxes in diverse seasonal hydrological conditions within the forested Padež stream catchment in SW Slovenia. The catchment is characterized by the flushing, torrential hydrological response conditioned by the flysch geological settings of a low hydraulic conductivity. Consequently, the streamflow formation is not controlled solely by the deeper subsurface catchment storage but is also strongly influenced by the rainfall–runoff that bypasses the deeper subsurface part of the total catchment storage. Therefore, fast component of the streamflow is identified using two-component hydrograph separation; the component recession behavior is described by a separate sensitivity function and used in a simple model to simulate the streamflow. According to the simulation results, the Padež stream catchment behaves primarily like a deeper subsurface storage-dependent system during most of the hydrological conditions. When rainfall intensities increase (rainfall intensities close to 10 mm/h or higher), triggering of the secondary streamflow formation mechanism described by separate, bypassing flow sensitivity function becomes evident and causes fast hydrograph formation with steeply rising and falling limbs. To be able to implement the modeling concept for streamflow predictions, the rainfall losses, most likely associated with interception losses not covered under the potential evapotranspiration calculation, would have to be more thoroughly analyzed through rainfall interception measurements. Our study shows the possible way that two hydrological concepts, the streamflow recession analysis and the two-component hydrograph separation based on relatively easily measurable tracers, such as electrical conductivity, could be combined for analyzing streamflow fluxes.

Soil degradation is a major environmental problem in many parts of the world, including Slovenia. The spatially distributed WATEM/SEDEM model can be used to identify the most critical parts of the catchment with regard to soil erosion. Five Slovenian (Central Europe) catchments with inhomogeneous topography, land use, geological conditions, hydro-meteorological properties and sizes (catchment areas between 1 and 2000 km2) were modeled with calibrated parameters, while the WATEM/SEDEM model was calibrated with an automatic parameter estimation procedure, which is model independent. Both direct and indirect information regarding sediment yields, including turbidity measurements, daily suspended sediment concentration observations and bed load observations, were used for the WATEM/SEDEM model’s calibration. A detailed rainfall erosivity (R) factor map, which was constructed from 5-min rainfall data from 31 pluviographic meteorological stations, was used as one of the inputs for the WATEM/SEDEM model. The calculated mean annual soil loss was between 0.3 and 7.4 t/ha/year, and the sediment delivery ratio (SDR) ranged from 0.07 to 0.22 for 5 modeled catchments. The results indicate that the SDR decreases with increasing catchment area; however, the ratio between the average sediment yield and mean soil erosion obviously depends on many other factors, e.g., topography, climatic and geological conditions. The parcel trap efficiency parameter for forests had the greatest influence on the WATEM/SEDEM model’s outputs in all five case studies.

The paper presents a review of structural measures that were taken to cope with floods in some cities along the Danube River, such as Vienna, Bratislava, and Belgrade. These cities were also considered as case studies within the KULTURisk project. The structural measures are reviewed and compared to each other according to the type, duration of application, the return period of the design flood event, how the project measures are integrated into spatial planning and the problems that occur in the flood defences today. Based on this review, some suggestions are given on how to improve the flood risk management in flood-prone areas.

Karst systems can be generally characterised by their high hydrological heterogeneities related mainly to highly variable permeabilities, which can significantly change over small spatial scales. This makes tracing and quantification of water flow pathways an extremely demanding task. In this study we present an analysis of hydrological characteristics of a complex karst system, the Ljubljanica river catchment in central Slovenia. Spatially distributed data on stable isotope composition (δ18O, δ2H) of precipitation as inputs, and of several karst springs/sinks as outputs, were obtained. These data were used to identify spatial and seasonal patterns and hydrological behaviour of the karst system in contrasting hydrological conditions. The intensive mixing of continental and Mediterranean air masses over the Ljubljanica river catchment makes the precipitation source identification difficult. However, the results of the precipitation isotopic composition analysis indicate a spatial pattern that could be recognised also in the δ18O and δ2H values of the karst springs and sinks. Along the prevailing karst conduits, the spatial differences in the δ18O and δ2H values decreased. The mean transit time (MTT) estimates using δ18O or δ2H as tracers were similar, with those for the main karst conduits and tributaries ranging between 0.34 and 0.74 years. Such a relatively short MTT could be expected for karst catchments without extensive deep groundwater storage. The fraction of young water (Fyw) for the whole catchment was 0.28, meaning that more than one-quarter of the total discharge was younger than approximately 2.3 months (assuming that the catchment transit time is described by exponential distribution). Small differences in the MTT over different parts of the karst catchment area might indicate intensive mixing and homogenisation of water along the underground conduits. However, the catchment’s homogenisation strongly depends on the preceding hydrological conditions; the differences in the isotope composition can be identified during low-flow conditions, which might indicate the dominant influence of the local recharge of the karst springs.

It is not clear how projected climate change will impact the hydrological functioning of complex catchments that have significant karst characteristics. Therefore, in this paper we focused on the investigation of the low- and high-flow characteristics of the karst Ljubljanica River catchment. One smaller (51 km2) and one larger (1135 km2) catchment were selected in order to investigate the projected climate change impact on the hydrological conditions. For the investigation of the hydrological situation in the future, we used a lumped conceptual hydrological model. The model was calibrated using past measured daily data. Using the calibrated model, we investigated the impact of five different climate models outputs for the moderately optimistic scenario (RCP4.5). We investigated the situation in next 30-years periods: 2011&ndash, 2040, 2041&ndash, 2070, and 2071&ndash, 2100. Several low and high-flow indices were calculated and compared. The results indicate that a summer precipitation decrease (i.e., 2011&ndash, 2070) could lead to lower low-flow values for the investigated areas, which could increase the vulnerability of karst areas. Thus, additional focus should be given to water resource management in karst areas. On the other hand, mean flow could increase in the future. The same also applies for the high-flows where flood frequency analysis results indicate that a climate adaptation factor could be used for the hydrotechnical engineering design. However, differences among investigated models are large and show large variability among investigated cases.

During two consecutive years the monitoring of rainfall nitrate (NO3-) and ammonium (NH4+) concentrations, combined with high-frequency measurements of streamwater NO3- concentrations, provided insight into the mechanisms controlling the dissolved inorganic nitrogen (DIN) budgets of the forested Padež stream catchment in SW Slovenia. During both years, the catchment's annual wet atmospheric DIN deposition (12.6 and 13.8kg-N/ha) exceeded the annual DIN export (10.7kg-N/ha and 8.8kg-N/ha). The analysis of the monthly DIN budgets discloses seasonal patterns. In winter and early spring, the catchment behaves as a net source of DIN, whereas during late spring, summer, and early fall the catchment generally behaves as a net sink of the atmospheric DIN. Due to large evapotranspiration in the growth period, most of the DIN remains in the catchment and is flushed during the dormant season. Despite high evapotranspiration and consequently low runoff coefficients characteristic for the Mediterranean climate, large rainfall events in the growth period can cause intensive washout of DIN from the catchment. At a monthly basis, the DIN flushing might exceed the catchment's DIN retention capacity and the catchment might behave as a net source of DIN. Therefore, the hydrological conditions have the ability to shift the catchment's role in regulating the DIN budgets. The high responsiveness of the streamwater NO3- concentration in the growth period indicates a large pool of DIN in the forest soils which can become mobile by runoff formation. Wet atmospheric DIN deposition at the Padež catchment is considerable; highest DIN inputs can be expected when air masses approach the catchment from the southwest to southeast where there are extensive urban areas along the Italian, Slovenian and Croatian coast. However, the Padež catchment does not appear to be approaching N saturation, presumably due to strong internal N cycle in the forest soils.

Summary Rainfall kinetic energy–intensity relationships for rainfall erosivity estimation were established on the basis of raindrop size distribution measurements performed in the Brkini hilly area in southwest Slovenia, a transitional area between the Mediterranean and continental climate conditions. A set of measurement instruments, an optical disdrometer coupled with a rain gauge, was installed on two locations: Koseze and Kozjane. The data set contained raindrop size distribution and rainfall intensity measurements of the rainfall events that occurred during a one-year period in the 2008/2009 season. The rainfall intensities obtained with the rain gauges were used for event rainfall depth control measured by the disdrometers and testing the established kinetic energy–intensity ( KE–I ) relationships. Two exponential KE – I relationships were established for each measuring site, for 1-min and 5-min rainfall intensity data, respectively. Their performances were tested and compared with the performances of other KE – I relationships proposed by authors throughout the world, which could be recognised as relatively suitable for kinetic energy estimation in the Mediterranean area. The comparison included 11 KE – I relationships expressed with linear, linear-log or exponential formulations. The analyses exposed the rainfall intensity overestimation by the disdrometers and the limited use of the tipping bucket rain gauge for kinetic energy estimation under such climate conditions. According to our results, the established KE – I relationship for 5-min intensity data in Koseze is recommended to be used while estimating rainfall kinetic energy in the Mediterranean part of Slovenia. As input data, rainfall intensities measured with precise weighing rain gauges, as those installed in the Slovenian meteorological network, should be used. However, when using rainfall intensity data obtained with a tipping bucket rain gauge of lower accuracy or insufficient intensity range, the exponential relationship of Coutinho and Tomas (1995) is expected to deliver better kinetic energy estimations.

Seasonal and diurnal variations of in-stream NO3-N concentration oscillations were studied through high-frequency measurements of streamwater’s physical, chemical parameters (in-stream NO3-N concentration, water temperature, dissolved oxygen, pH) and hydrometeorological variables (stream discharge, solar radiation) under hydrologically stable conditions. The study was carried out in 2006, within the 42 km2 forested Padež stream watershed in the southwestern part of Slovenia, which is characterized by distinctive hydrogeological settings (flysch) and climate conditions (transitional area between the Mediterranean and continental climate). Fine temporal data resolution (15-min interval) enabled identification of the factors responsible for seasonal variability in the diurnal pattern of the streamwater NO3-N concentrations versus seasonal and diurnal behavior of meteorological and other water chemistry constituents. The observed seasonal variability of in-stream NO3-N daily oscillations indicates the important role of primary production uptake, particularly during seasons when deciduous vegetation is dormant and light levels in the stream are high. Highest daily NO3-N concentration amplitudes (0.3 mg/l-N) and daily changes in the NO3-N flux (0.4–0.5 g/s-N) were observed in spring; the NO3-N concentration oscillations in summer showed a considerably smaller effect of the in-stream uptake (maximum NO3-N daily concentration amplitude 0.1 mg/l-N; daily change in the NO3-N flux 0.02 g/s-N). Seasonal shifts in the timing of daily maximum (up to 6 h) and minimum NO3-N concentrations (between 1 and 3 h) provided some additional indications of seasonal changes in the in-stream primary production uptake and its relation to the terrestrial component of the forested watershed.

Summary The variability of hydrologically induced mobilization of nitrate during sequences of hydrological events in different seasons was studied through high-frequency measurements of streamwater nitrate concentrations accompanied by a detailed hydrometeorological monitoring system. The study was carried out in 2006, within the 42 km 2 forested Padež watershed in the southwestern part of Slovenia, which is characterized by distinctive flushing, an almost torrential hydrological regime influenced by impermeable flysch geological settings. More than 15 recorded hydrographs which, in the hydrological and biogeochemical sense, differed substantially, disclosed a highly variable but at the same time a strong linkage between hydrological and biogeochemical controls of nitrate exports from the spatial perspective of a watershed. During most of the hydrographs, with the exception of early spring rainfall events, a positive relationship between the nitrate concentration and discharge was observed with peak nitrate concentrations having a time delay in the order of a few hours after the hydrograph peaks. Peak nitrate concentrations in periods of rainfall events span from 3.5 mg/l-N in late spring to 14 mg/l-N in the case of the autumn hydrograph. However, the dilution effect as a consequence of high event water contributions was observed in certain events. The role of specific hydrological events on the nitrate mobilization proved to be important as the size of the accumulated nitrate pool available for mobilization was large throughout most of the hydrographs. The biogeochemical environment of the forest soils presumably drastically alters the size of the available nitrate pool at the studied watershed and, together with the specific hydrological conditions, shifts the watershed from the flush-limited state in the late spring, summer and autumn to the source-limited state in winter and early spring.

Stream flow regimes are determined by watershed characteristics: climate, geology, topography, soil, vegetation and human activities. In the process of urbanisation, natural land surfaces are replaced by man made artificial coverage, such as paved roads, parking lots and roofs, which usually also implies vegetation clearing and soil compaction. Gutters, drains and storm sewers are built to accelerate the conveyance of runoff to stream channels, thus affecting the drainage system. The impact of urbanisation is complex and affects different elements of the hydrological cycle. The commonly observed hydrological responses of the watershed to urbanization are increased volume and peak of floodwaters. Concerning the ecological status of stream water, the intensified rainfall runoff induces increased pollution risks and diminishes the value of the stream water body as a habitat, especially during dry periods. In order to improve the flood safety, the regulations of the stream channel have further devaluated the ecological role of the urban streams. The magnitude of the impact is usually enlarged with the decrease in the stream size. The present paper aims at presenting the results of a two-year study monitoring the impacts of the urban environment on the watershed of the Glinscica stream situated in the central part of Slovenia. The study area of 19.3 km 2 represents a great complexity in terms of the land use pattern. The watershed was equipped with three rainfall stations, a Doppler velocity meter and a water quality multiprobe. In a short period of time more than 10 thunderstorm events were recorded and analyzed. The hydrological response of the watershed was analyzed and, interestingly, it did not show the “typical” urban impact on the runoff processes. The main water quality parameters such as temperature, pH, TDS, ORP, conductivity, dissolved oxygen and especially the concentrations of nitrate and ammonium, were measured to obtain an insight into seasonal and short time dynamics of the water quality. The results show substantial seasonal and along-the-channel variations of concentration of dissolved oxygen, nitrate and ammonium content due to biochemical processes in the channeled stream. The continuous tracing of nitrate and ammonium showed significant influence of stream regulation works on short time variations of the measured water quality parameters.

A data mining, regression tree algorithm M5 was used to review the role of mutual hydrological and seasonal settings which control the streamwater nitrate flushing during hydrological events within a forested watershed in the southwestern part of Slovenia, characterized by distinctive flushing, almost torrential hydrological regime. The basis for the research was an extensive dataset of continuous, high frequency measurements of seasonal meteorological conditions, watershed hydrological responses and streamwater nitrate concentrations. The dataset contained 16 recorded hydrographs occurring in different seasonal and hydrological conditions. Based on predefined regression tree pruning criteria, a comprehensible regression tree model was obtained in the sense of the domain knowledge, which was able to adequately describe most of the streamwater nitrate concentration variations (RMSE=1.02 mg/l-N; r=0.91). The attributes which were found to be the most descriptive in the sense of streamwater nitrate concentrations were the antecedent precipitation index (API) and air temperatures in the preceding periods. The model was most successful in describing streamwater concentrations in the range 1–4 mg/l-N, covering large proportion of the dataset. The model performance was little worse in the periods of high streamwater nitrate concentration peaks during the summer hydrographs (up to 7 mg/l-N) but poor during the autumn hydrograph (up to 14 mg/l-N) related to highly variable hydrological conditions, which would require a less robust regression tree model based on the extended dataset.

Experimental watersheds are critical to the advancement of hydrological science. By setting up three experimental watersheds, Slovenia also obtained its grounds for further development of the science and discipline. In the Dragonja experimental watershed the studies are focused on the afforestation of the watershed in a mediterranean climate, on the Reka river the water balance in a partly karstic area is examined, and on the case of the Glinscica stream the implications of the urban environment are studied. We have obtained valuable experience and tested new measuring equipment on all three experimental watersheds. Measurements and analysis on the experimental watersheds improved the current understanding of hydrological processes. They resulted in several PhD Theses, Master Theses and scientific articles. At the same time the experimental watersheds provide support to the teaching and studying process.

Understanding of interactions between hydrological and biogeochemical responses of catchments on rainfall events is usually unclear from periodic measurements and requires tracing of the temporal dynamics of the processes. Smaller streams reflect strong connections between hydrological processes of the rainfall runoff formation and biogeochemical processes in the catchment; consequently, the responsiveness of the streamwater chemistry to changed hydrological states is very high. The study was carried out in 2007, within the 42 km2 forested Padež catchment in the southwestern part of Slovenia, which is characterized by distinctive flushing, an almost torrential hydrological regime influenced by impermeable flysch geological settings. Recorded hydrographs which, in the hydrological and biogeochemical sense, differed substantially, disclosed a highly variable, but at the same time a strong linkage between hydrological, biogeochemical and particular topographic controls of nitrate exports from the spatial perspective of a studied catchment. The role of specific hydrological events on the nitrate mobilization proved to be important as the size of the accumulated nitrate pool available for mobilization was large throughout the observed hydrographs. The biogeochemical environment of the forest soils presumably significantly affects the size of the available nitrate pool in the studied catchment.

Suspended sediment load (SSL) observations are usually less frequent than precipitation and river discharge measurements; therefore a reliable procedure is needed for the estimation of SSL. One year of precipitation, SSL, and discharge measurements at 20-min intervals were performed at the Kuzlovec torrent in Slovenia. The Frank copula was selected to construct an event-based model using the following variables: precipitation sum (P), peak discharge (Q), and SSL. The idea was to estimate the SSL based on the measured P and Q. The proposed model was additionally tested using the daily data from the Gornja Radgona station on the Mura River, for which 29 years of data were available and where Khoudraji-Liebscher copulas were used. The estimated SSL values using the copula were compared with different regression models. The proposed copula model yielded meaningful SSL estimates. Some performance criteria and tests indicated that the copula model gives a better fit to the measured data than other tested methods.

Climate changes have a high impact on river discharges and therefore on floods. There are a few different methods we can use to predict discharge changes in the future. In this paper we used the complex HBV model for the Vipava River and simple correlation between discharge and precipitation data for the Soča River. The discharge prediction is based on the E-OBS precipitation data for three future time periods (2011–2040, 2041–2070 and 2071–2100). Estimated discharges for those three future periods are presented for both rivers. But a special situation occurs at the confluence where the two rivers with rather different catchments unite, and this requires an additional probability analysis.