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Changes in the hydrological regime are widely investigated using a variety of approaches. In this study, we assess changes in annual and seasonal flow characteristics based on a probabilistic representation of the seasonal runoff regime at the daily time scale. The probabilistic seasonal runoff pattern is constructed by determining quantiles from marginal distributions of daily flows for each day within the year. By applying Fourier transformation on the statistics of the daily flow partial series, we obtain smooth periodical functions of distribution parameters over the year and consequently of the quantiles. The main findings are based on the comparison of the dry, average, and wet hydrologic condition zones as defined by the daily flow quantiles of selected probabilities. This analysis was conducted for ten catchments in Serbia by considering changes between two 30-year nonoverlapping periods, 1961–1990 and 1991–2020. It was found that the relative change in runoff volume is the most pronounced in the extreme dry condition zone in the winter season (−33% to 34%). The annual time shift is the largest in the dry and average condition zones, ranging from −11 to 12 days. The applied methodology is not only applicable to the detection of hydrologic change, but could also be used in operational hydrology and extreme flow studies via drought indices such as the Standardized Streamflow Index.

Flood flow frequency analysis is performed on the series of hydrological observations. The observations introduce uncertainties in the analysis results, inherited from limited data sets, operational difficulties in high flow measurements, flood flow estimates in the rating curve extrapolation zone, as well as different mechanisms of runoff generation. Difficulties in defining flood flow probabilistic model arise in the presence of outliers (significantly larger or smaller value compared to the remaining data set values). The paper provides an overview of methods for the detection of outliers from traditional practice to the latest research results, as well as procedures for outlier treatment in the further statistical analysis, focusing especially on the recommendations from the recently published Bulletin 17C in the USA. The assumptions about the population, procedures of outlier detection, the treatment of transformed data sets, and potential difficulties in the application are highlighted. Statistical tests and procedures are applied to flood data at 70 hydrological stations in Serbia. Analysis of flood flow frequency in the presence of outliers is performed by the methods of the annual maxima and peaks over threshold (POT). The examples include both upper and lower outliers. Comments are given on the differences in the results from the applied outlier detection and treatment procedures. Recommendations to the practitioners are given in the conclusions in order to reduce uncertainty in the flood flow frequency analysis in the presence of outliers., Statistička analiza velikih voda se radi na nizovima hidroloških osmatranja, koja u rezultate ove analize unose neizvesnosti u pogledu ograničene dužine osmatranja, teškoća u merenju vrlo velikih protoka i njihove procene iz zone ekstrapolovanih krivih protoka, kao i zbog različitih mehanizama generisanja oticaja. Definisanje probabilističkog modela velikih voda je otežano i zbog postojanja izuzetaka u nizovima, tj. značajno velikih ili malih vrednosti u odnosu na većinu. U radu se daje pregled postupaka i statističkih testova za otkrivanje izuzetaka od standardnih do najnovijih, kao i načini postupanja sa izuzetnim vrednostima. Najveća pažnja posvećena je otkrivanju i tretmanu donjih izuzetaka, koji u velikoj meri utiču na raspodelu a time i na usvojene računske velike vode. Poseban osvrt je dat na najnovije preporuke iz američkog Biltena 17C. Komentarišu se pretpostavke o raspodeli populacije, procedure testiranja, postupanje sa transformisanim nizovima i ukazuje na moguće poteškoće u primeni. Sama primena testova i procedura prikazana je u praktičnom delu, gde se statistička analiza velikih voda u prisustvu izuzetaka sprovodi za karakteristične primere sa hidroloških stanica u Srbiji na nizovima formiranim metodama godišnjih maksimuma i prekoračenja preko praga. Primeri obuhvataju i gornje i donje izuzetke. Komentarišu se razlike u rezultatima prilikom primene različitih procedura u prisustvu izuzetaka i predlaže niz iskustvenih mera kako bi ocena velikih voda bila što pouzdanija.

Conceptual hydrologic models are commonly applied for flow forecasting, estimation ofdesign flows and assessment of climate change impact on water resources. Therefore,reliability of hydrologic simulations obtained by employing these models is crucial.However, these simulations are fraught with uncertainties, which stem, inter alia, fromparameter estimates. The parameter estimates are affected by data errors, objectivefunctions and optimisation algorithm employed for model calibration, but also byproperties of the calibration period. Namely, model calibration over different periods mayresult in quite different parameter estimates because parameter optimality does not holdoutside the calibration period. This temporal variability of optimal parameter estimatesyields deterioration in model performance outside the calibration period. Therefore,variability of optimal parameter estimates is major issue when it comes to application ofhydrologic models, because these models are primarily used for runoff simulationsoutside the calibration period.In this Thesis temporal variability in parameters of the 3DNet-Catch model is analysed.The AMALGAM algorithm, aimed at multi-objective optimisation, is applied for modelcalibration. The model is calibrated in dynamic manner, over all 1- to 25-year longcalibration periods, with one water year prior to every calibration aimed at model warmup.Prior ranges of the parameters and settings for the optimisation algorithms (e.g.population size, mutation probability, etc.) are kept constant through all simulations forgiven catchment. The analysis of temporal variability in model parameters is based on thenon-dominated, or Pareto-optimal sets, which are selected subsequent to the optimisationof the initially sampled population of parameter sets. Impact of combination of objectivefunctions used for model calibration and model structural complexity on temporalvariability in the Pareto-optimal parameters is also examined in this research. To isola, Концептуални хидролошки модели су нашли широку примену у израдихидролошких прогноза и предикција, и у анализи утицаја климатских промена наводне ресурсе. Стога је поузданост симулација добијених применом ових моделавеома важна. Међутим, у хидролошким симулацијама постоје неизвесности, којепотичу и од оцена параметара модела. На оцене параметара модела утичу грешке уподацима, избор критеријумских функција и оптимизационог алгоритма, али икарактеристике калибрационог периода. Наиме, калибрација модела токомразличитих периода даће различите оцене параметара, зато што параметри који суоптимални током једног периода не морају бити оптимални изван њега. Последицаваријабилности оптималних параметара у времену је и лошија ефикасност моделатј. мање поуздане симулације ван калибрационог периода. Имајући у виду да сехидролошки модели користе за хидролошке симулације ван калибрационогпериода, за њихову примену кључно је изучавање променљивости оптималнихпараметара модела током времена.У овој дисертацији анализиран је утицај калибрационог периода на оценепараметара хидролошког модела 3DNet-Catch. За калибрацију модела коришћен јесавремени алгоритам за вишекритеријумску оптимизацију AMALGAM, којипредставља комбинацију неколико глобалних оптимизационих алгоритама.Хидролошки модел је калибрисан на свим периодима дужине од једне до 25узастопних хидролошких година, уз једну хидролошку годину намењену„загревању“ модела. Овакав приступ је назван „динамичка“ калибрација модела.Почетни опсези параметара, као и подешавања за оптимизациони алгоритам (нпр.број чланова популације, вероватноћа мутације и др.) исти су за све калибрационепериоде за разматрани слив. Након оптимизације параметара издвојена су тзв.међусобно недоминантна решења (Парето оптимални скупови параметара илискупови са Парето фронта), на основу којих је вршена анализа променљивости...

In most developed countries, practices and research results are translated into standards or guidelines for flood flow assessment, usually with the compulsory application in the design and exploitation of hydraulic structures and systems. This paper briefly presents the draft guidelines for flood flow statistical analysis in gauged basins based on methods of statistical hydrology, intended for practitioners in Serbia. It explains the motivation for composing national guidelines and gives insight into contemporary trends in flood flow assessment in a few selected countries. The content of the draft guidelines is shown, as well as the main conclusions of the expanded research in terms of the proposed methodology for detection and handling of exceptional values. The proposed recommendations for modernization of existing, common engineering practices are highlighted. The paper presents the guidelines at the end of the first from three planned research phases, with the primary aim to encourage public debate which, in pair with the professional review, would lead to the adoption of the guidelines., U većini razvijenih zemalja, iskustva iz prakse i rezultati istraživanja, pretočeni su u standarde ili preporuke za proračun velikih voda, najčešće sa obaveznom primenom u projektovanju ili eksploataciji hidrotehničkih objekata i sistema. U ovom radu ukratko je prikazan nacrt preporuka za proračun velikih voda na izučenim slivovima metodama statističke hidrologije, koji je namenjen domaćoj stručnoj javnosti. Objašnjena je motivacija za izradu preporuka u Srbiji i dat je uvid u savremene tendencije u standardizaciji proračuna velikih voda u nekoliko odabranih zemalja. Prikazan je sadržaj nacrta preporuka, a priloženi su i glavni zaključci proširenog istraživanja u pogledu predložene metodologije za otkrivanje i postupanje sa izuzetnim vrednostima. Sumirane su predložene konkretne preporuke za osavremenjavanje postojeće, uobičajene inženjerske prakse. U radu se stručnoj javnosti predstavlja predlog teksta preporuka nastao posle prve od tri predviđene faze istraživanja, sa prevashodnim ciljem da podstakne javnu raspravu koja bi, uz stručnu recenziju, dovela do usvajanja preporuka.

Revision of existing methodologies for generating monthly-flow series at ungauged basins based on multivariate nonlinear correlation has led to a simple two-parameter model. While the existing methodology used hydrological, meteorological and geomorphologic input data, the proposed model requires hydrological and geomorphologic input data only. The proposed methodology requires formation of separate pools of donor catchments for model parameter estimates. The proposed two-parameter model and improvement in the sphere of homogeneous region identification were verified using 195 runoff data sets from hydrologic stations in Serbia in the 1961-2005 period, divided into three non-overlapping 15-year periods. Nash-Sutcliffe's model efficiency coefficient (NSE) was used to assess the: (1) quality of proposed model with identified model parameters; (2) quality of a nonlinear multivariate equation for standard normal variables estimation with identified model parameters; (3) quality of proposed model with model parameter estimates. Generated time-series statistics and nonlinear multivariate equation quality are also evaluated. Five model calibration and validation results are shown. Generated flow series variation coefficient is the best replicated statistics with relative absolute error less than 10%.

This paper summarizes findings on the flood flow estimation in ungauged catchments during the last decade in Serbia. The method used for synthetic unit hydrograph ise well known in hydrological practice as SCS method. Attention is paid to regional relations between characteristics of the basin and hydrograph elements, with emphasis to time of concentration and basin lag time. A medium basin with gauged runoff data is selected for application of studied methodology. Flood flows are also estimated by conventional statistical methods. Flood flow estimation results are compared and discussed for 20, 50 and 100 year return period. Calculation procedures are carried out for available rainfall data. Effective rainfall is calculated by SCS CN Method.

This paper addresses the problem of defining appropriate characteristic which leads to homogeneous regions identification, required for application of multiple nonlinear regression methodology for interpolation of hydrologic data from gauged to ungauged basins. The underlying methodology of multiple nonlinear regression is based on theory of standardization and normalization of correlation. In order to obtain synthetic runoff series at ungauged basins, one of the key steps is estimation of spatial correlation function. There are theoretical and empirical correlation functions. While empirical correlation function is estimated according to correlation coefficients of runoff series at gauged basins and their mutual distance, its pair - theoretical correlation function is merely a shift of empirical correlation function. This shift is made to achieve correlation coefficient of one for zero distance. However, this shift is acceptable only if performed in homogeneous region comprising of basins which behave in a hydrologically similar way. A basin characteristic that describes hydrological behavior similarity is examined among morphologic characteristic, basin proximity, and runoff time series statistics. There are two criteria for homogeneity control. They refer to empirical and theoretical spatial correlation function, and eventually identify donor catchments for ungauged basin. Study area for research is the territory of Republic of Serbia without provinces of Vojvodina and Kosovo. Basins up to area of 1000 km2 are taken into consideration. Within the period 1961-2005, three sub-periods of 15 years each are examined for mean monthly runoff time series simulation potential at ungauged basins.

The paper presents in detail application of the peak over threshold (POT) method for flood flow estimation for the Toplodolska river, gauge station Topli Do. In addition, design flood flows are estimated by means of the synthetic unit hydrograph methodology, and the Limit Intensities Theory. The basis for flood estimation is the 14-year flow record at the Topli Do station and the daily precipitation maximal record. The nearby hydrologic station Visočka Ržana on the Visočica River is used as the control station for calculated flows. Flood flow estimation results are also shown compared to the flood flow envelopes for the territory of Serbia. Computations represent a part of a hydrologic study of water captures for small hydropower plant in the village of Topli Do., U radu je detaljno prikazana primena metode parcijalnih serija za proračun velikih voda Toplodolske reke u profilu V.S. Topli Do. Računske velike vode ocenjene su još i metodom sintetičkog jediničnog hidrograma - aproksimacija trouglom i teorijom graničnih intenziteta. Korišćeni su podaci osmatranja proticaja na V.S. Topli Do, trajanja 14 godina i osmatranja visine dnevnih padavina. Kao kontrolna stanica usvojena je V.S. Visočka Ržana na reci Visočici. Rezultati ocene velikih voda prikazani su u odnosu na anvelope specifičnih oticaja velikih voda reka Srbije. Proračuni su vršeni u okviru hidrološke studije za profile zahvata planirane male hiroelektrane (MHE) u ataru sela Topli Do.