Nitrates represent one of the main groups of contaminants in the Zagreb aquifer system. Some natural groundwater quality indicators can have a significant influence on their stability and mobility in the saturated zone. Correlation and multivariate statistical analyses were used to test the correlation of average values of NO 3with O 2 , ORP, pH, EC and temperature of groundwater, and to allocate observation wells that belong to the same clusters. ORP values didn`t relate to any observed variables, which is probably due to their variability which suggests changes in the oxidation-reduction conditions in the aquifer system. Principal component analysis was used for the determination of variables that are related to the nitrate concentrations and which were then used in cluster analysis. Other variables were excluded from cluster analysis. Three methods were used to perform cluster analysis, where the results calculated with Ward`s method were chosen as the most appropriate. In the end, two clusters were identified, one with smaller, and one with higher NO 3 -, O 2 and EC values. Observation wells from cluster 1 are generally located near the Sava River and have similar nitrate concentrations. Lack of other nitrogen species and moderately aerobic conditions suggest very fast nitrification in the shallow Holocene aquifer.

Persistence and evolution of trends of mean and high river flows from six hydrological stations on watercourses of the Kupa River (Kupari, Kamanje), tributary Čabranka (Zamost 2), Dobra River (Stative donje, Trošmarija), Mrežnica River (Mrzlo polje) and annual precipitation on three meteorological stations (Parg, Ogulin and Karlovac) were analysed. The paper focuses on seven indicators: annual mean flow, seasonal mean flows (winter and summer mean flows), instantaneous annual maximum flow, annual and seasonal precipitation. Analysed time series range from 1951 to 2013, and the fixed period ranges from 1984 to 2013. Time series of each indicator was scaled to standardized flow anomaly and was analysed using the Mann-Kendall Z test for monotonic trend, after which it is smoothed using LOESS algorithm. The analysis was conducted for each indicator for full record, e.g. 1951-2013, then 1952-2013 and so on until 1984-2013. Thus, the sample size varies from 63 to 30 years. The smoothed standardized flow anomaly is easily comparable among different hydrologic stations. The standardized flow anomaly on all analysed stations for all analysed indicators shows lower mean value than long-time average after mid-1980's, when fixed period starts. Further analysis of summer and winter seasonal mean flows revealed different deviation from long-term annual flow average. Trend evolution of certain indicators was proven using Mann-Kendall Z test, by plotting Z values for each iteration of start year (1951 to 1984).

Correlation and cross-spectral analysis of hydrographs was performed on the karst area of the mountainous section of the Kupa River in two time periods. Since there are karst aquifers in this area that is a part of strategic groundwater reserves of potable water, such an analysis could give better insight into the behavior of these aquifers. The functions used to describe karst aquifers are the cross-correlation function, coherence function, gain function, and phase function. The outcomes of the analysis results were very similar in the two analyzed periods, suggesting that differences in input signals between these two periods do not affect the aquifer function as a system filter. The results of this research suggest that the aquifer and overburden layer characteristics have a much stronger influence than the change of input signal on the runoff regime in the considered period.

The investigation area is located in the world-famous Dinaric karst. This study presents a conceptual model of groundwater dynamics and its interaction with surface waters, extending from the natural water retention of the Drežničko Polje to the spring zone on the far side of the Kapela Mountain range, including a description of the regional groundwater flow in the Zagorska Mrežnica spring zone. The aim of this research was to determine the possibility of an artificial enlargement of the natural retention of this karst field. Large amounts of water could be exploited in this way for the existing hydroelectric power plants of Gojak and Lešće on the Donja Dobra River. The prolonged retention of the water wave in the Drežničko Polje would extend its efficiency in regards to the production of electrical energy, and simultaneously achieve the mitigation of floods that frequently occur in the broader area of Ogulin. Photogeological analysis of the area was performed, together with geological and hydrogeological mapping, groundwater tracing, measurements of water flows in streams and springs, exploratory drilling and measurements of water levels in 26 piezometric boreholes in the Drežničko Polje. Available meteorological data from nearby weather stations (Jasenak, Drežnica and Modruš) were exploited, as well as hydrological data collected specifically for the modelling of runoff. Based on the results of the data processing, this study has determined: (1) the dynamics of the groundwater flow from the Drežničko Polje to the spring area of the Zagorska Mrežnica, (2) the dynamics of recharge and discharge of the natural retention of the Drežničko Polje; and (3) an improved interpretation of the Zagorska Mrežnica karst spring dynamics. The obtained results of groundwater flow dynamics indicate typical karst flow conditions in the Dinaric Karst, but also contain some specific features.

This paper presents basic hydrological indicators based on data from the four existing hydrological stations in the Gornja Dobra basin. The frequency and ow-duration curves of mean daily ow in the Turkovi i pro le are also de ned and presented. The dependence of the mean maximum annual speci c ows on the size of the in uential basin to the considered hydrological pro les was considered. A review of the most recent high water event in 2017 was given. KeywordsGornja Dobra, frequency and ow-duration curve, maximum speci c ow, stage-discharge curve, big water wave.